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1. Problem Definition Supporting Document

1.1 Annotated Bibliography

There have been numerous iterations of the ME 3281 lab kit spanning the last decade. While none of

them completely embody the ideal lab kit, each iteration has evolved and improved upon the last. It is

very important to carefully and thoroughly research these previous lab kits so past mistakes can be

avoided and efforts are not duplicated. This research mainly focused on the mechanical components

of the lab kit, namely the motor, mass, springs, and position sensors.

Much of the research effort involved narrowing down the abundance of resources compiled by

previous senior project groups. The previous groups also documented past work so there are many

sources and documents to review and eliminate when the information is redundant. Many of these

sources are academic publications and product descriptions.

[1] Durfee, Li and Waletzko, June, 2004 “Take-home Lab Kits for System Dynamics and

ControlsCourses”. Proceedings of the 2004 American Controls Conference.

This document provides information about the design of the first generation of the take-home kit. It is

a 4th order system composed of multiple springs, masses, and a damper.

Knowing the history of this project has provided guidance in the design of the 2013 iteration. This

information has allowed the team to learn from mistakes, and benefit from strengths.

[2] Durfee, Li and Waletzko, June, 2005 “At-home System and Controls Laboratories.”

Proceedings of the 2005 American Society of Engineering Educators Conference.

This document covers more information regarding the 1st generation lab kit. It provides further insight

into the motivation and implementation of the original iteration of this project.

This source was used as background information for 2013 take-home kit design.

[3] Waletzko, D. December, 2005 “Distributed Laboratory Modules for System Dynamics and

Controls Courses”, A Plan A Master’s Thesis, University of Minnesota, Institute of

Technology.

Waletzko’s thesis was another helpful source providing design details about the 2nd generation kit. It

is a comprehensive documentation describing its implementation. The rotary system used in this kit is

used as the basis for the 2013 iteration.

Gaining insight from as many different designs and revisions of this kit provided guidance for new kit

design.

[4] ME4054 Work, Available:

http://www.me.umn.edu/dlab/ME4054work.html [Accessed: March 3, 2013]

This site provides information about another kit design, done by senior design students in the Spring

of 2003. It details each component of the kit.

This was another design which contributed to lessons learned and decisions for the new kit.

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[5] Jouaneh, M. and Palm, W., June, 2009 "System Dynamics Take-Home Laboratory Kits", In

Proceeding of the 2009 ASEE Annual Conference, Austin, TX.

This document describes the take-home lab kit developed by Rhode Island University. It contains 4

different kinds of lab manuals and gives information about kit design and use.

There is no known patent surrounding a kit of this type, so the design team was able to use the ideas

implemented in other kits for the benefit of the 2013 design.

[6] Quanser Inc, Available:

http://www.quanser.com/flippers/Rotary/2012/ [Accessed: March 3, 2013]

Quanser Inc produces a commercial lab kit that teaches system dynamics and controls concepts.

This source provided some ideas that could be used in the take-home kit design, such as the concept

of interchangeable components that may be used so students can understand their effects on system

performance.

[7] Educational Control Product (ECP), “Industrial Plant Emulator”, Available:

http://www.ecpsystems.com/controls_emulator.htm. [Accessed: April 5, 2009].

Educational control product is a kit used in industry or an educational setting. It gives details on

interchangeable components as well as basic concepts of spring and mass configurations to our kit.

This is yet another example of a source which was used to refine the design of the 2013 take-home

kit.

[8] Distributed Laboratories, Available: http://www.me.umn.edu/dlab/ [Accessed: March 6,

2013]

Reference [8] is a central index of the efforts that have been made towards designing and providing a

take-home ‘distributed’ lab kit to students. It indexes the people involved, initial proposal, relevant

progress and publications, as well as various other software and files for download. This is the place

to begin when understanding the background of this subject.

The distributed laboraties page was used to understand the background and various iterations that the

take-home kit has undergone. The decisions made in the past, as well as lessons learned, are a helpful

guide towards the new 2013 kit design.

[9] Ace Controls International, "Rotary Dampers," Available:

http://ace-ace.com/wEnglisch/pages/Produkte/index.php?IdTreeGroup=268&navid=1

[Accessed: March 6, 2013]

Rotary dampers are used to provide damping in a rotary system. Ace Controls has a comprehensive

selection of rotary dampers and provides detailed spec sheets for each model.

Off-the-shelf commercial dampers were initially considered for use in the take-home kit. Ace

Controls was used as one of the starting points for the rotary damper search.

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[10] Sparkfun Electronics "Optical Detector / Phototransistor – QRd1114," Available:

https://www.sparkfun.com/products/246 [Accessed: March 6, 2013]

Sparkfun provides an extensive selection of electronics components for the hobbyist. Detailed

information and project ideas are also provided for many types of components. The site is a valuable

starting point for new and undefined projects. Sparkfun was used to seek out and understand

components that could be used for position detection. Once a component type was discovered, an

effort was made to find a cheaper, commercial oriented source.

[11] Hirozumi, S., Takeaki, K., and Tok Bearing Co., Ltd. (2005). U.S. Patent No. 6121526.

Washington, DC: U.S. Patent and Trademark Office.

Patent 6121526 is one of the first and foremost patents regarding rotary dampers. It provides details

on principles of operation, as well as overall packaging.

In an effort to design a simple damper from scratch, it was necessary to understand the physical

principles behind operation of existing rotary dampers. Some of these ideas were used to consider a

simple home-made damper used in the kit.

[12] Mabuchi Motor Co LTD. RF-370CA-15370 Motor Curve. 2013. Chart. Jameco, Belmont,

CA. Web. 4 Mar 2013.<http://www.jameco.com/Jameco/Products/ProdDS

/238473.PDF>.

This website contains motor parameters and schematic for the Jameco 238473 motor used in the take-

home kit.

[13] Close, Charles, Dean Frederick, and Jonathan Newell. Modeling and Analysis of Dynamic

Systems. 3rd Edition. Hoboken: John Wiley and Sons, 2001. Print.

System dynamics and controls textbook used in the ME 3281 class at the University of Minnesota.

Used as a reference for all systems and controls equations and modeling techniques.

1.2 Patent Search

Objectives

Our design project is a take-home lab kit for use in the ME3281: System Dynamics and

Control course. The kit will consist of an electric DC motor, mass, spring, and damper. When

assembled, this kit will be used to create a first and second order system. This kit will interface with

MATLAB using an Arduino microcontroller.

All the components required for this system will be off-the-shelf with the exception of the

custom printed circuit board (PCB) and a modified gear on the motor shaft. There are likely no

individual parts that are patentable, but the design of the system as a whole could be patentable. Also

the programming for the Arduino/MATLAB interface will be custom. The final lab kit package could

have value on the free market to be used as an educational tool purchased by other universities or

hobbyists.

Search Criteria

An extensive patent search was done to determine if any similar mass, spring, damper system

simulation units have been previously patented. Google’s patent search, USPTO, and

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www.freepatentsonline.com were used. Various combinations of search words such as “mass,”

“spring,” “damper,” “control system,” “position control,” “speed control,” “plant,” and “feedback.”

Findings

No patents applicable to our lab kit were found. However, there are some similar devices

currently produced by commercial companies. Two examples of such kits are the Model 220:

Industrial Plant Emulator manufactured by Educational Control Products and the SRV02 Base Unit

manufactured by Quanser. Both of these companies were contacted by our project team only to find

that neither of these devices carry any U.S. patents.

The Model 220: Industrial Plant Emulator is used to study control of systems. The

mechanism allows for varying drive and load inertias by changing the mass mounted to a rotating disc

on both the drive motor shaft and on a shaft connected by rubber belts. This allows the user to

experiment how changing gear ratios changes system dynamics. This kit does not have any energy

storage devices such as a spring (besides mass momentum) nor does it have energy dissipation by

damping, both of which are essential requirements for the ME3281 lab kit.

The SRV02 Base Unit is similar in function to the Model 220. It also contains an electric

motor to rotate a mass by spinning a series of gears. It also neglects spring and damping mechanisms.

Even though both of these kits are professionally engineering and manufactured, they fall far short of

meeting the needs required for a successful ME3281 lab kit.

1.3 User Need Research

To help determine user needs, a survey was developed to give to students who had previously taken

the class. The survey had four questions:

1. Do you feel that ME 3281 would benefit from a take home kit?

2. Do you have an Arduino Microcontroller?

3. Would you be willing to pay around $50 for such a kit in addition to your textbook?

4. From the following list, select all topics that you feel could benefit from an interactive

physical demonstration.

a. System Modeling

b. Step Response

c. Frequency Response

d. PID Control

To simplify the survey process, the survey was sent to every student in ME 4054W, as all students in

the class should have already taken ME 3281. There were 84 responses; the results of the survey are

shown in Table 1-1.

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Question Yes (%)

1.) Do you feel that ME 3281 would benefit from a take home kit? 73.81%

2.) Do you have an Arduino Microcontroller? 86.90%

3.) Would you be willing to pay around $50 for such a kit in addition

to your textbook? 39.29%

4.) From the following list, select all topics that you feel could

benefit from an interactive demonstration.

a) System Modeling 67.86%

b) Step Response 59.52%

c) Frequency Response 71.43%

d) PID Control 84.52%

Table 1-1: User Need Survey Results

The intent of the first question was to determine if there was a need for the product. From the

survey results, it can be seen that 73.8% of students felt that the class could have benefited from a lab

component, verifying that the market exists.

The second question was used to validate our intent to use the Arduino Microcontroller from

the ME 2011 class as the lab kit controller. As can be seen from Table 1-1, 86.9% of the class has an

Arduino of some form.

The previous kit models have all cost around $50 a piece. The survey results from question 3

show that only 39.29% of students are willing to pay such a cost. This number increases slightly to

51.6% when calculated using only the students who felt the lab kit would be beneficial. From these

results, it is clear that low cost is a very important design criterion.

The last question was used to determine what topics would be most beneficial to cover in the

lab. From the results it can be seen that PID control is the most requested topic, followed by

Frequency Response, System Modeling, and Step Response. These results will be used to design the

lab curriculum, focusing more on areas that the students felt were confusing.

These survey results were combined with needs determined from our own experiences from

ME 3281 to form a list of user needs. The importance rankings were based on the survey response

data, as well as our own intuition from previous experience in the class. This list is provided in Table

1-2.

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# Customer Need Importance Source

1 Will effectively teach ME3281

Curriculum 5 Survey

2 Will be reasonably priced 4 Survey

3 Will integrate Arduino microcontroller 3 Survey

4 Will be easy to construct 2 Team List

5 Will be reasonably sized 2 Team List

7 Will be reliable and rugged 4 Team List

9 Will collect position data accurately 4 Team List

10 Will operate safely 5 Team List

11 Software is intuitive and easy to set up 4 Team List

12 Will utilize Matlab user interface 3 Team List

13 Software incorporates platform

independence 4 Team List

14 Will determine velocity accurately 4 Team List

Table 1-2: Ranked List of User Needs

1.4 Concept Alternatives

With regard to the physical makeup of the take-home kit, there are 4 subsystems that require

concept consideration and selection. Additionally, there are also multiple high level system

configurations to be considered. Lastly, a robust software package must be designed. The

TAKEHOME team has considered various proposals of each system and has made a selection based

on objective criteria available for each. Past experiences and lessons learned have weighed heavily

into this process, and will be leveraged for the most effective and highest quality overall outcome.

Type of System

In the most general sense, the system to be replicated with the ME3281 take-home kit is a

traditional mass-spring-damper system. This is a second-order system with two energy storing

elements; the mass and the spring. Within this, two types of systems can be created: linear and

rotational.

A linear system was created as a part of 2004 work on the take-home kit project. This was

modeled as a ‘quarter-car’ linear translational model, with 2 springs, 2 masses, and a damper. The

system representation is shown in Figure 1-1. The top mass and spring can be removed to convert it to

second order. This system requires alignment and control of the directional movement of the mass,

and a method of converting the rotational motion of the motor into linear translation, which introduce

complexities.

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Figure 1-1: Quarter car model translational system

A rotational system has been the most common method to create a simple mass-spring-

damper system, both in take-home kit project work, as well as commercially available systems. This

system is easier to produce, as the output shaft of the motor can be directly coupled to the system to

drive input. A system representation is shown below in Figure 1-2. However, there are challenges in

creating a linear damping component, as well as spring configuration, which will be discussed in

subsequent subcomponent sections.

Figure 1-2: Rotational system with motor torque input

Spring Alternatives

The spring selection has been an area of trouble for prior revisions of the take-home kit. A

rubber band was wrapped around the mass and a stationary post to provide a spring force about the

central shaft. See Figure 1-3 for illustration from the 2009 4054 project report. This design was

advised against by project advisor Professor Durfee. The rubber bands are subject to significant

degradation, causing spring constant changes, as well as breakage. This potentially leaves the student

without a spring element.

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Figure 1-3: Wrapped rubber band spring configuration

The next spring concept considered was that of a metal torsional spring. This type of spring

is specifically designed for rotational systems, provides a very linear spring force, and could be

packaged easily in the take home kit system. However, this type of spring is very difficult to find in

spring rates low enough to be suitable for this kit, and is designed to only be operated in one direction

of rotation. See Figure 1-4 for illustration of spring configuration

Figure 1-4: Torsion Spring, pulley, motor shaft configuration

In order to keep the kit simple and affordable, it is desirable to attempt to incorporate more

traditional coiled linear springs into the rotational system. An initial idea was to use two springs that

are mounted at points 180 degrees apart on the pulley, and to fixed positions on the lab kit base. This

configuration is shown in Figure 1-5. This concept has the benefit of simple, compact construction.

However, there is concern over the spring coils ‘binding’ as they come into contact with the pulley,

resulting in severe rotational limits. There is also a lack of perfect linearity as one spring relaxes by a

different distance as the opposite spring extends during rotation.

To resolve the drawbacks of the previous concept while still integrating linear coil springs,

another design is considered. This concept is achieved by using a single mount point on the pulley,

and cable to alternatingly pull springs from a fixed mount point on the unit baseF. This design allows

the use of very cheap, small and readily available coil springs. Rotation is limited around 90

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degrees, and requires a mount point some distance out from the pulley. Exact mount distance depends

on the relaxed spring length. See Figure 1-6 for illustration of this concept.

Figure 1-5 (left): 180 degree separated spring configuration, mounting points shown in red Figure 1-6 (right): Shared mount configuration with cable, mounting points shown in red

Damper Alternatives

Damper selection and incorporation has been a recurring problem in past 3281 take-home kit

revisions. Various ideas have been considered, but ultimately thrown out for various reasons, in

favor of using internal system friction as the damping agent. This is a less than ideal solution, so

damper selection is once again revisited. Friction is a less than ideal damping method, as the damping

term is linearly dependent on the angular velocity of the system, which introduces a non-linearity and

causes non-ideal system behavior. Taking into consideration the fact that this kit is meant to closely

approximate an ideal system for the benefits of student learning, friction must not be allowed as the

only damping component.

The first type of damping component considered is one of the many commercially available

offerings. The principle behind the operation of such devices is the internal use of a rotating disc

submerged in viscous fluid, along with various valves and vanes to direct fluid flow. This has the

end result of very linear damping with respect to rotational velocity. However, these components

suffer from a few drawbacks. The low availability of dampers small enough for this application, as

well as a high cost (around $3.50 each), makes them less than ideal. Additionally, these units are rated

for very low rotational speeds; 10 cycles per minute. See Figure 1-7, below, for a patent schematic

of a rotational damper.

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Figure 1-7: Patent schematic of a common type of rotational damper

Using similar general principles behind the viscous rotational damper, another concept is

proposed that replicates a similar behavior in a simple and inexpensive way. A small fan blade

could be incorporated that sits inside a cup of water or other viscous fluid. When spun by the motor,

this blade will produce a damping effect. This damping force is due to a drag force generated by the

blade against the liquid, quantified by Equation 1-1.

𝑓𝑑 = 12⁄ 𝜌𝑈2𝐶𝑑𝐴

Equation 1-1: Drag Force

As shown, this force varies quadratically with velocity, which again represents a non-

linearity. However, the effect of this term can be reduced by using a fluid significantly more dense

than air (water, oil) and by limiting velocity ‘U’ to very small values. This will replicate a linear

damping factor over the limited breadth of operation of the take-home kit. See Figure 1-8 for a rough

physical layout of this concept. One noticeable impact of this design is the necessity to mount the

motor with the shaft facing downwards.

Figure 1-8: Proposed liquid damper concept

Lastly, a damping component which is unavoidable and must be incorporated, is the internal

motor friction. As stated previously, this is a non-ideal, non-linear damping force that is apparent

when a motor is accelerated from a stop, requiring above a threshold level of current before the shaft

begins to spin. This is a source of damping, however, and will be quantified with motor

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characterization. Efforts will be made to ensure this does not greatly deter from ideal system

performance.

Mass Alternatives

Prior Designs of the take-home kit have incorporated a 1/2” bore off-the-shelf shaft collar as

the primary rotational system mass. While cheap and effective, this mass relies on a magnet to be held

to the rotating pulley, and allows for no flexibility to view the effect of mass changes on system

response. In its place, another alternative presently under consideration is the use of stacked

washers. These are a very cheap alternative that would allow for many levels of mass configuration.

However, a new method must be designed to secure them and prevent movement and sliding, which

will create inconsistencies in system performance.

Position/Velocity Sensor Alternatives

One of the most important parts of this kit is the ability to detect rotational position. Along

with this requirement, a new requirement has been introduced for the 2013 take-home team to also

add velocity awareness to the kit. Velocity is the first derivative of position, so any sensor which

measures position can be used to derive velocity in software.

The 2006 take-home kit revision incorporated a potentiometer to detect position. To do this,

one potentiometer terminal is provided a voltage signal, one is provided ground, and the wiper is the

output signal. This is used in a manner of voltage divider circuit, with output voltage varying with

position. This solution is reliable and performs well, however, there are a few disadvantages. First,

it requires a geared, belt, roller, or other type of indirect connection to the motor, as it is not easily

packaged into the motor’s rotational axis. That connection is sensitive and problematic. Second, to

obtain a quality, low-friction potentiometer that is not limited by number of turns, it is quite expensive

(around $15 each). Lastly, the potentiometer is large and with moving parts, does have the potential

to eventually fail. See Figure 1-9 for the 2006 configuration of the potentiometer.

Figure 1-9: Prior potentiomer configuration, with gear pulley

One proposed concept to provide position measurement is through the use of a

phototransistor. An example of this type of component is shown in Figure 1-10. This device emits

infrared light, and measures reflections of the signal. It could be used in conjunction with a wheel

that incorporates evenly spaced black/white color transition, or with a wheel which has evenly spaced

holes around the circumference directly above the mounting point of the sensor. Doing this will

provide a measurement best used for velocity, but can be integrated to find position. This solution is

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simple and cheap. However, it comes with a few drawbacks; primarily the lack of built-in position

measurement, which will create a more complicated programming sequence and larger error. The

second major drawback is resolution which is limited by the total number of black/white transitions or

holes.

Figure 1-10: Phototransistor (courtesy of sparkfun.com)

A final position/velocity concept under consideration is that of a Hall Effect type of system.

This system uses a magnet, which is mounted to and rotates on the central shaft, and two analog Hall

Effect sensors, similar in size and cost to the phototransistor, which are mounted some distance away

and located with 90 degrees of rotational separation. See Figure 1-11 for a schematic of this

configuration. An alternative could be to mount the magnet offset some distance from the axis of

rotation, which triggers a Hall Effect sensor as it rotates. This is undesirable, however, due to the

negative impacts on rotational inertia of the unit and the creation of vibration during operation.

The Hall Effect sensors make a measurement on the strength of the magnetic field created by

the magnet. As it rotates, the magnetic field’s magnitude changes and polarity reverses, which is

captured by the sensors. The use of two sensors provides precise position information about the

rotational position of the system, as well as measurement sensitivity throughout the range of rotation.

This is a standard, well known way of making position measurement, and has been tested to perform

well.

This concept is very promising as it is very cheap, with the sensors themselves priced at

around $1 each. The magnet being relatively cheap as well. It is reliable and robust, as the sensors

incorporate no moving parts and are very accurate. All that is required is solid mounting of the

magnet to the rotational shaft, and a precise mounting points and orientation of the sensors.

Figure 1-11: Hall effect sensor configuration, with north and south magnet poles in blue and red,

respectively

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Software Package Alternatives

The selection and creation of a software package, by which the student will perform the lab

tasks and view system output, is a vital part of the success of this take-home lab kit. In the past, the

desktop software that the student uses to interact with the physical system was implemented in Visual

Basic and pre-compiled before being run by the student. This created many compatibility problems

with various operating systems and software environments. To rectify this, the 2009 iteration of the

take-home kit made use of a graphical user interface implemented in MATLAB. As part of the initial

requirements for the 2013 revision, it was requested that the software be implemented in a platform-

independent software package, which makes installation and use on student home computers less

problematic. In addition, it is requested that the software be easily updated and enhanced, by TA’s or

professors, through the use of straight- forward implementation and thorough documentation. Lastly,

the use of the Arduino as the hardware control device limits the ‘server’ software to only being

Arduino-compatible C-code.

The first software package concept is using a web-based user interface to interact with the

serial port to which the Arduino is connected. There is no straightforward way to do serial

communication via HTML or JavaScript, so a local web server would have to be developed which

interacts with a serial driver to control output with the Arduino. This solution is convoluted and

complicated, and probably isn’t easily developed in the time allotted for the semester.

The second software solution consists of interaction with the Arduino through serial

communication in MATLAB Simulink. This would allow the software to be implemented in the

function-block style Simulink development environment, which generates the MATLAB side, and

Arduino server-side code to execute the Simulink model. This is a reasonable alternative and allows

easy enhancements and changes to the software and user interface. However, professor Durfee

expressed concern with adding another level of abstraction onto the software package by introducing

Simulink. However, using MATLAB as the user interface software is very viable; students may

obtain an educational license for MATLAB for free from the university, and this software is already

used in ME3281 for homework assignments. Using it for the take-home lab exercises would provide

seamless integration with current curriculum. MATLAB is also platform independent and would be

less sensitive to different operating systems and system environment differences.

Similar to the previous option, the final option is to write a custom MATLAB user interface

and/or modify the 2009 MATLAB software to fulfill requirements for the 2013 implementation. This

combines all the benefits of using MATLAB with the simplicity of eliminating Simulink from the

equation. This software will be well documented and laid out for easy updates and changes in the

future. It will be combined with custom Arduino server code that will control the hardware and

provide position and velocity feedback to the user interface for graphing and analysis. It will be

possible to export data to a CSV file for manipulation in other software packages. However, one

downfall is the reduced performance of the serial communication with the Arduino due to the

overhead of the MATLAB environment. However, with careful benchmarking and testing, the impact

of this could be minimized to produce a useable, flexible user interface for the take-home kit.

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1.5 Concept Selection

System Type

Selection Criteria Rotational Linear

Cost 0 0

Complexity 0 0

Size 0 -

Custom Components 0 -

Component Selection

Motor 0 -

Spring 0 +

Mass 0 +

Damper 0 +

Position 0 -

Net Score 0 -1

Table 1-3: System Type Selection

In Table 1-3, the rotational system, because it was used in the past, is set as the benchmark. The cost

criterion refers to the price to purchase the kit as a whole, including consideration to all components.

Because cheaper is better, points are awarded for a lower cost. Complexity, size, and number of

components are to be minimized, and points awarded to this end.

Using a Linear type of system eliminates some of the complexities and challenges of

rotational; specifically with regard to spring and damper implementation. However, the size and

number of custom components required to allow smooth motion of a linear system is also a

formidable challenge. One of the custom components required is a necessary type of cam drive in

order to turn the rotational motion of the motor into linear motion. In the end, a linear system also

makes a PID control lab, as well as velocity control, quite difficult. Both alternatives have strong

points, but in the end, staying with a rotational system makes the most sense.

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Spring

Selection Criteria Rubber

Band

Torsion

Spring

Split

Mount

Coil

Spring

Shared

Mount Coil

Spring

Cost 0 - - -

Size 0 + + -

Linearity 0 0 - +

Range of Motion 0 - - 0

Working Life 0 + + +

Mounting 0 - + +

Net Score 0 -1 0 1

Table 1-4: Spring Selection

The criteria used in Table 1-5 include a few that are to be minimized; cost and size. To be

maximized are working life, linearity, and range of motion. The mounting category awards points to

the solutions that are easiest to mount.

Using the rubber band spring configuration as the baseline, the other spring options offer

some attractive benefits. The torsion spring, however, must eliminated due to the inability to rotate in

both directions. It does seem physically possible, but the manufacturer recommends against it, and

likely would lead to non-linear spring force. The shared mount coil spring offers all the benefits of the

split mount coil spring, but also improves by allowing a larger range of motion, and better linearity.

While it suffers from size, this configuration is determined to be the best moving forward.

Damper

Selection Criteria Internal

Friction

Rotary

Damper

Liquid

Fan

Damper

Cost 0 - 0

Size 0 0 -

Linearity 0 + +

Cycle Speed 0 - +

Damping

Coefficient 0 + +

Net Score 0 0 2

Table 1-5: Damper Selection

The damper selection criteria includes cost and size which award points for a decrease from

the internal friction benchmark. Linearity, cycle speed, and damping coefficient are increased when

points are awarded.

It is concluded that internal friction is unacceptable as the only source of system damping,

due to its non-linearity. There are various forms of commercially available rotary dampers, but in the

end this option must be eliminated due to the restriction of 10 rotation cycles per minute; it is desired

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to have the take-home kit operate at higher frequencies than that. This leaves only the liquid fan

damper, which offers a great opportunity for hands-on education about damping for ME3281 students.

This type of damping is not quite ideal, but will be sufficient and helpful for the purposes of this kit.

Position/Velocity Sensor

Selection Criteria Potentiometer Phototransistor Hall Effect

Cost 0 + +

Size 0 - 0

Accuracy 0 - +

Position 0 - +

Velocity 0 + 0

Simplicity 0 + -

Net Score 0 0 2

Table 1-6: Position/Velocity Sensor Selection

The criteria used in Table 1-7 include accuracy, velocity, and simplicity, which award points

to an increase from the benchmark potentiometer. Size and cost are to be minimized. The position and

velocity criterion refer to the capability for accurate measurement of those metrics by the design under

consideration.

The potentiometer that has been used in past revisions of the take-home kit has been a good

solution to position sensing. However, there is room for improvement with regard to cost and size.

Use of a phototransistor and trigger wheel offer a cheap, simple alternative, however, this is mostly

only good for velocity measurement and suffers from a lack of resolution. The 90 degree separated

Hall Effect sensors with shaft mounted magnet offer a great overall solution; good accuracy and

resolution, precise rotational position measurement. Even though this will require more programming

and a computational derivative to be calculated to find velocity, it is the solution that will be

implemented in the 2013 take-home lab kit.

Software Package

Selection Criteria MATLAB Simulink Web-Based

Cost 0 0 0

Simplicity 0 - -

Ease of Use 0 - 0

Ease of Maintenance 0 0 -

Platform Independence 0 0 -

Ease of Troubleshooting 0 0 -

Relation to Course 0 0 -

Net Score 0 -2 -4

Table 1-7: Software Selection

The criteria used to evaluate the software are mostly self-explanatory. Ease of maintenance

refers to the difficulty involved with fixing bugs and making changes, which is also related to ease of

troubleshooting. Relation to course awards points for using packages that are used elsewhere in

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coursework, and platform independence awards points to solutions that can be run on a larger number

of operating systems.

The 2009 version of the take-home kit implemented a MATLAB User interface, used with a

USB driver to communicate with the PIC board. This approach is used as a baseline for 2013

selection. Simulink offers a good off-the shelf method for developing a control program with both

Arduino server-side and a user interface. However, this introduces another layer of abstraction to the

software, and if configuration is necessary for the Arduino code that is generated, it could prove

difficult. A web-based approach would be very easy to use and could be run on a host computer

without any special software (other than a web browser), however there would be many other issues

such as the necessity of a web server or other middleware to interact with the serial port and the

difficulty with troubleshooting or modifying such a setup. Lastly, this solution would not be exempt to

compatibility issues due to the wide range of browsers and versions that might be installed on a host

computer, would exponentially complicate implementation, and ultimately require an ‘approved

browser list’ which would have to be tested and updated over time.

In the end, it makes the most sense to stay with a MATLAB user interface, as it is the most

platform independent, would be easy to modify and troubleshoot, is free to use and very familiar to

students, and ties in well with other MATLAB assignments in ME3281. The features implemented by

the 2009 group can be leveraged and extended to provide a full feature set for the take-home kit.

2. Design Description Supporting Documents

2.1 Manufacturing Plan

2.1.1 Manufacturing Overview

Great consideration was taken in the design of this lab kit to meet two main criteria: low cost

and simplicity. To keep the costs low, only two pieces of the kit are custom made, the shaft coupler

and the PCB. The rest of the kit utilizes off-the-shelf parts that are mass produced, widely available,

and low cost.

The pulley assembly is the centerpiece of the whole kit, and is responsible for providing a

means for mounting the mass, springs and magnet on to the motor. This assembly consists of a shaft

coupler, an off the shelf pulley and a magnet. The shaft coupler is a one inch long, plastic cylinder

with a 5/64 inch hole drilled in one end, to a depth of 3/8 inch. The magnet is then attached to the

coupler, at the end with the hole, with glue, and the pulley is attached likewise to the coupler, just

above the magnet. When the coupler is glued to the motor shaft, the magnet is in the optimal position

for use with the Hall Effect sensors. In addition, the pulley is in position to be used as the mounting

point for the springs.

The springs are mounted by wrapping a string around the pulley. The string is cut to a

precise length and has one end tied to each spring. The springs are then attached to two mounting

points on the PCB. The string is held to the pulley by first double wrapping it around the pulley, and

then securing it with a tight rubber band. Above the pulley the remainder of the shaft is used to mount

the washers. The washers are secured by a tight press fit connection and also by a rubber band

wrapped on the shaft above them. This connection ensures that the washers do not slip during

operation of the kit.

The design of this assembly is created with the principle of cost reduction, in mind. The

vertical stacking orientation of all of the components minimizes the amount of space that the kit

occupies and therefore reduces the size of the PCB to which the components are mounted, and the

18

size of the box needed to contain the kit in its entirety. All of this supports the goal of reducing the

cost of the kit, and maintaining a standard of simplicity.

The custom PCB contains only a few holes to mount the motor, legs, and springs, along with

traces for the Hall Effect sensors’ power, ground, and signal terminals. Precise spacing is used in the

Hall Effect sensor holes, to maintain a correct distance to the magnet. These are shown in Figure 2-2,

as the 3-hole groupings above and to the left of the motor through-hole and attachment holes. The

spring attachment points are provided as holes on the bottom of the figure, and attachment holes are

provided at each corner, for the legs of the kit. This is a simple design, requiring only a 1-layer PCB,

though more layers may be used. The configuration also acts as the lab kit base, further reinforcing

the low cost and simple design philosophy. The PCB has been designed using the ExpressPCB

software from www.expresspcb.com which allows for easy and intuitive PCB creation, instant quotes,

and fast order turnaround.

2.1.2 Part Drawing

The main component, the pulley, is displayed below in Figure 2-1.

Figure 2-1: Line Drawing of the Mass System

The PCB schematic is displayed below in Figure 2-2.

19

Figure 2-2: PCB Schematic

All other components of the lab kit are off-the-shelf that require no product drawings, e.g.

bolts, washers, screws, rubber bands, etc.

2.1.3 Bill of Materials

Description Vendor Part # Quantity Cost Ea.

Total Cost

PCB ExpressPCB N/A 1 $3.00 $3.00

Pulley McMaster 3434T38 1 $2.05 $2.05

Plastic Shaft QuickParts N/A 1 ? ?

Magnet Amazing Magnet

H250F - DM 1 $3.01 $3.01

Spring McMaster 9654K412 2 $0.67 $1.34

Washers McMaster 98032A492 5 $0.04 $0.18

Hall Effect Sensors DigiKey 620-1433-ND 2 $1.20 $2.40

H-Bridge DigiKey L293DNEE4-ND

1 $2.56 $2.56

Motor Jameco 238473 1 $2.39 $2.39

Powersupply (12 V, 1A) DigiKey 237-1455-ND 1 $5.92 $5.92

Powersupply Jack DigiKey SC1313-ND 1 $0.98 $0.98

Wire – 22 AWG, 1 ft DigiKey A3051R-100-ND

5 $0.35 $1.73

Glue McMaster 66185A11 1 $2.47 $2.47

Rubberband McMaster 12205T92 2 $0.01 $0.01

20

Bolt – #8-32 x 1.75” McMaster 90272A204 4 $0.07 $0.29

Bolt – #8-32 x 1” McMaster 90272A199 1 $0.05 $0.05

Nuts #8-32 McMaster 90480A009 5 $0.01 $0.07

Rubber Caps McMaster 6448K75 4 $0.16 $0.64

Fishing Line – 1 ft McMaster 944T5 1 $0.01 $0.01

Machine Screws – M3.0x0.5 – 5 mm

McMaster 94879A114 2 $0.07 $0.14

4" x 6" Index cards Amazon B005EDXAFK 2 $0.03 $0.06

Wooden Dowel McMaster 97015K13 1 $0.19 $0.19

Vinyl Tubing - 3/16" dia, 1" length McMaster 5233K53 1 $0.02 $0.02

Box – 3”x3”x2” McMaster 21225T81 1 $0.56 $0.56

Capacitor (.1uF) Jameco 25523 1 $0.15 $0.15

Capacitor (10uF) Jameco 10882 2 $0.15 $0.30

Total: $30.09

Optional (assume already own)

Arduino Uno DigiKey 1050-1024-ND

1 $27.83 $27.83

Breadboard DigiKey 700-00012-ND

1 $3.49 $3.49

Total: $61.41

Table 2-1: Bill of Materials

2.1.4 Manufacturing Procedure

The PCB acts as the base for mounting many parts of the kit. 1.75 inch long bolts act as legs,

of which there is one in each corner. They are attached directly to the PCB with a nut. The rubber caps

are then placed over the end of the bolts providing a good non-slip footing for the kit. Two 1 inch bolt

are secured in the opposite direction from the other bolts, projecting from the top of the PCB. The

motor is mounted to the PCB as well, using plastic machining screws. The Hall Effect sensors are

mounted and soldered in the pins surrounding the motor shaft. Refer to Figure 2-3 for the overall

layout of the kit.

21

Figure 2-3: Overall Component Layout

The pulley assembly is made by gluing the magnet and pulley to the shaft coupler. The shaft

coupler is a 1 inch long, ¼ inch diameter plastic dowel with a 5/64 inch hole drilled to a depth of 3/8

inch at one end. The dowel is cut to size with a band saw and the hole is made on a lathe with a center

bore. The magnet is glued flush to the end of the coupler containing the hole and the pulley is glued

just above the magnet. This puts the magnet in the proper location to be used with the Hall Effect

sensors. The washers, used as the mass, are then placed on top of the pulley around the remainder of

the protruding coupler shaft. These washers are then secured by tightly wrapping a rubber band

around the shaft above them. The pulley assembly is secured to the motor shaft with glue, providing a

strong connection. The function of the pulley is to act as a mounting point for the springs. Each spring

is tied to the end of a length of string which is wrapped around the groove of the pulley. The other

ends of the springs are then secured to the bolts that extend out of the top of the PCB. Another rubber

band is then wrapped tightly around the string and pulley, securing the string to the pulley firmly and

preventing any slipping.

With the kit assembled, the electrical circuits are then assembled. The electrical components

are contained on a breadboard, including a power jack and H bridge. The Arduino microcontroller is

wired to the H bridge which is then wired to the motor along with power from the jack. The sensors

are the only electrical component not contained on the breadboard, because they must be in close

proximity to the magnet. Wires will extend from the pins to which the sensors are attached to the

breadboard and, in turn, to the Arduino.

22

2.2.1 Implementation plan

The ME3281 take-home kit has some unique implementation requirements, as it is designed

to be used in an educational environment as a supplement to class lectures and other coursework. As a

result, the educational institution (in this case, the University of Minnesota Mechanical Engineering

department) must purchase enough supplies for each student, and assemble ‘kits’ that may be sold for

student assembly and use.

It is intended that kits will be ordered in batches of 250, so UMN must make an order well in

advance to the beginning of the semester. Work must be done to then organize the large batches into

individual packaged kits with the sufficient numbers of each part. This will require basic organization

of materials, but also some other operations, such as cutting the fishing line into correct lengths. In

addition, some number of Arduino microcontrollers and breadboards must also be obtained for student

purchase in the case that they do not already have them.

The next step is distribution. Students must have a system in place to pick up and pay for the

main kit, along with a breadboard and Arduino, if necessary. This will vary from school to school, but

often times a department has a ‘supply depot’ which has the systems in place to provide and bill a

student for required course supplies.

In order for each student to make use of the take-home kit, a few other setup steps are

required. The student must do the physical assembly of the kit. This will require basic construction

and connection of kit components using some basic tools; a Philips head screwdriver, and hex key.

Just a few solder joints need to be made, which will require the student to either own a soldering iron,

or have the ability to use a department provided iron to solder the small number of connections that

need to be made. Detailed instructions are provided to make this as seamless as possible, and it is

recommended that teaching assistants or the professor be available for help and troubleshooting.

Some basic physical tests will verify proper construction.

Lastly, the student must load the software. The Arduino must be connected to the host PC,

and the executable file must be loaded and deployed to the Arduino using the IDE available on the

web. Matlab must be installed, if it is not already. Most schools are able to provide the student version

for free or very cheap. Once this is accomplished. The Matlab ‘.m’ file can be opened and executed to

proceed with the lab exercises. These functions have been outlined in section 4.2.2 in Volume 1.

When these steps are completed, the kit is now in the hands of the student and ready to be

used to complete course assignments.

23

2.2.2 Process Drawings

Overview

Figure 2-4: Overview Process Drawing

Hardware

Figure 2-5: Hardware Process Drawing

Software

Figure 2-6: Software Process Drawing

Software

Hardware

Experiment Results

Acquiring Lab kit

Part Assembling

• Breadboard

• PCB

• Arduino

Testing

Downloading and installing

•Matlab

•Arduino drivers

•IDE

Downloading software for

the lab kit

Connecting Arduino with

computer

Uploading program to

ArduinoTesting

Combine

d

24

2.2.3 Component List

Component Manufacturer Part# Description

Mechanical

Parts

Pulley McMaster 3434T38 Part that loads spring system and

mass system

Magnet Amazing

Magnet 5856K4

Part that causes change of

magnetic field

Shaft Collar N/A N/A Mechanical fastener

Spring McMaster 9654K412 Spring system of the lab kit

Washers McMaster 98032A492 Mass system of the lab kit

Motor Jameco 238473 Mechanical Power source

Rubber band McMaster 12205T92 Part that prevents slip between

Fishing line and pulley

Bolt – #8-32 x 1.75” McMaster 90272A204 N/A

Bolt – #8-32 x 1” McMaster 90272A199 N/A

Nuts #8-32 McMaster 90480A009 N/A

Rubber Caps McMaster 6448K75 Part that prevents movement of the

lab kit while lab kit is operating

Fishing Line – 1 ft McMaster 944T5 Part that extends the spring to

connect with the pulley

Nylon Machine

Screws – M3.0x0.5 –

5 mm

McMaster 94879A114 N/A

Box – 3”x3”x2” McMaster 21225T81 Part that contains the lab kit for

convenience

Electrical

Parts

PCB Express PCB N/A Part that wires all the components

Hall effect sensor DigiKey 620-1433-ND

Part that measures displacement of

the system by detecting change of

magnetic field

H-Bridge DigiKey L293DNEE4-

ND

Part that enables a voltage to be

applied across a load in either

direction

Power supply(12V,

1A) SparkFun TBA Electric Power Source

Power supply jack SparkFun TBA N/A

Wire – 22 AWG, 1 ft DigiKey A3051R-100-

ND N/A

Arduino Uno DigiKey 1050-1024-

ND Part that the program is loaded in

Breadboard DigiKey 700-00012-

ND N/A

Capacitor (.1uF) Jameco 25523 N/A

Capacitor (10uF) Jameco 10882 N/A

Tools Soldering Iron N/A N/A For soldering hall effect sensors

and wires on the PCB Solder N/A N/A

25

Software

Arduino Environment N/A N/A Enables PC to upload program

code to an Arduino board

Matlab N/A N/A Enables the lab kit to

communicate with PC

Table 2-2: Component List

2.2.4 Implementation procedure

To install the hardware, perform the following steps:

1. Unpack the box that contains all the hardware components. Check to ensure all components

are included (refer to BOM).

2. Solder the two Hall Effect sensors on the specified locations on PCB.

3. Mount the motor to the PCB and affix the motor with 2 screws.

4. Solder two wires to the motor.

5. Put wires into the Pin1, Pin2, +5, GND holes and solder the wires.

6. Put 4 leg bolts into the each hole in the corners and a leg bolt into the spring leg hole.

7. Attach shaft adapter to motor shaft, leaving approximately 1/8" between collar and board,

using a small drop of glue applied to the adapter hole.

8. Press fit/glue magnet to the shaft adapter.

9. Press fit/glue pulley to shaft adapter. Wait sufficient time for glue to dry.

10. Hook one end of spring to mount point on PCB. Hook other to string loop. Repeat for other

spring.

11. Connect two springs with string that winds the pulley (Detach the spring system when you

perform the velocity PID experiment).

Figure 2-7: PCB pin diagram

12. Put the H-bridge on the breadboard. Complete H-bridge circuit shown in Figure 2-5.

26

13. Wire up PCB and breadboard shown in Figure 2-5.

14. Wire the Arduino to the breadboard as shown in Figure 2-5.

15. Connect the power jack to the breadboard.

Figure 2-8: Circuit diagram

Figure 2-9: Circuit schematic diagram

27

To install and run the software, perform the following steps:

1. Download and install Matlab, which can be obtained directly from Matlab or from your

educational institution. Information on the student version can be found at the following link:

http://www.mathworks.com/academia/student_version/

2. Download and install the Arduino drivers and IDE appropriate for your model. Instructions

can be found at the following location for Windows:

http://arduino.cc/en/Guide/windows

3. Connect the Arduino microcontroller to the lab kit, as specified in the instructions.

4. Connect the Arduino to the host PC.

5. Open and deploy the provided “DlabServer.ino” file to the Arduino using the IDE. To do this,

open the file and click the ‘Upload’ button on the top tool bar. This is shown in white in

Figure 2-6.

Figure 2-10: Arduino software upload

6. Open Matlab, browse to the location of the provided ‘dlab.m’ file and execute it.

7. Select the correct communications port for the Arduino from the intro screen, shown in Figure

2-7, below.

28

Figure 2-11: Matlab intro screen

8. There will be a brief pause, then the intro screen will indicate it is connected to the Arduino.

Once that is complete, select and proceed with individual lab exercise modules. See Figure 2-

8, below.

Figure 2-12: Module selection screen

3. Evaluation Supporting Documents

3.1 Evaluation Reports

Position Accuracy

Introduction:

For the lab kit to function properly, the sensors must be accurate, and sensitive enough to

detect small changes in position. Hall effect sensors were chosen due to their non-mechanical nature

so as not to change the system dynamics by adding friction.

29

Method:

To test the sensing elements, a prototype of the kit was assembled. Position measurements

were then taken every 𝜋/8 radians (22.5°), and the actual angle compared with the measured angle.

Results:

The results of this experiment are shown in Figure 3-1.

Figure 3-1: Hall effect positional accuracy

As can be seen above, the measurements are very accurate, having a very linear relationship.

A plot of the errors is shown below in Figure 3-2.

Figure 3-2: Hall effect positional errors

0

1

2

3

4

5

6

0 2 4 6

Me

asu

red

Th

eta

(ra

dia

ns)

Actual Theta (radians)

Hall Effect Positional Accuracy

Measured

Actual

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0 1 2 3 4 5 6

Sen

sor

Erro

r

Actual Theta (radians)

Sensor Errors

30

Note that the errors are random and don’t follow a trend, again validating the linearity of the

sensor measurements. The average error is found to be ±0.067 radians (3.8°).

Discussion:

This sensor configuration is found to be suitable for the lab kit. The sensor measurements are

linear and accurate down to ±0.067 𝑟𝑎𝑑𝑖𝑎𝑛𝑠.

Frequency Response

Introduction:

It is important to be able to characterize the system dynamics of the lab kit. This can be done

by sending a known input signal to the system and then comparing that to the output signal. This is

called frequency response. In a linear system with a constant input, the output signal will not vary

with time. Also, the amplitude of the output will double if input signal doubles when the system is

linear. The lab kit has been designed using components with linear or nearly linear characteristics.

Methods:

The system dynamics of the lab kit has been determined theoretically by using the provided data

specification sheets on the components used and by experimentation. The complete dynamics can be

seen in section 2 of volume I of this design report. The magnitude of the frequency response of the

system is described below in equation 1.

𝑀(𝜔) =𝐺

𝑘√(1 −𝜔2

𝜔𝑛2)

2

+ (2𝜁𝜔𝜔𝑛

)2

(1)

The variables are defined as follows: 𝐺 is a gain constant. This is needed to scale the conversion of

the torque to a voltage. It must be determined experimentally. 𝐽 is the total system inertia, 𝑘 is the

spring constant found, and 𝑏 is the damping coefficient. The input frequency is 𝜔. The natural

frequency 𝜔𝑛, and the damping ration 𝜁 are defined below in equations 2 and 3 respectively.

𝜔𝑛 = √

𝑘

𝐽 (2)

𝜁 =𝑏

√4𝑘𝐽 (3)

Using equation 1, a theoretical bode plot can be produced. This plot displays how the lab kit should

respond to a known input for the bandwidth of interest. This bode plot is seen below in Figure 3-3.

31

Figure 3-3: Theoretical bode plot

It should be noted that a gain of 1/25 should have originally been used to develop the theoretical

transfer function. This gain is necessary to scale back the function due to the conversion of the torque

to a voltage. Converting this gain to dB gives -28 dB and would shift the plot in Fig. 3-3 down by that

amount.

The strategy to determine the experimental frequency response of the lab kit is to send a constant

input signal and sweep across the frequency range of interest. This frequency range is from 1 Hz – 20

Hz. The output signal level is then measured and compared to the input signal. These results can then

be represented as a Bode plot and analyzed.

A frequency response software module was programmed. This module was used to bench test the

frequency response of the lab kit by first inputting the correct values of J, k, B, and G to describe the

system. Then 1 Hz was chosen as the frequency of the input signal, and the signal was sent to the lab

kit. The software model then plotted the output magnitude in decibels. Next the frequency was

increased in increments of approximately 0.5 Hz, sent to the lab kit, and the output magnitude was

added to the existing plot. This method was repeated up through 20 Hz.

32

Results:

The results of the experiment can be seen below in Figure 3-4.

Figure 3-4: Experimental Bode Plot

The solid blue line is the theoretical system response, and the green circles represent each

experimental result.

Discussion:

A gain of 25 was added during this experiment to allow the experimental data to match up correctly

with the theoretical plot. This gain would not have been needed if the gain for the torque to voltage

conversation was added initially.

The experimental results are only about 5 dB off before it peaks. The experimental resonant frequency

is approximately 2 Hz larger than the theoretical value, showing an excellent correspondence with

theory. After the peak, the experimental response closely aligns with the theoretical response.

These results are very impressive. The lab kit uses very small components, especially the inert mass.

Because of these small components, even slight errors in calculations and measurements can greatly

change results. It can now safely be assumed that the system’s transfer function accurately describes

the system.

33

PID Control

Introduction:

PID control is a concept that many students have a hard time understanding. This lab kit can

perform PID control to attain a precise rotational position. To reinforce proper learning, it is important

that the practical results match that given by theory. The lab kit has many obstacles in the way of

producing good results, such as non-linear friction and negative magnet interactions. However,

reasonable results are possible.

Methods:

To test PID control, three terms in PID control were tested separately. First, 𝐾𝑝, proportional

gain is tested. To test proportional gain, integral gain, 𝐾𝑖 and derivative gain, 𝐾𝑑 are set to 0. To

verify that 𝐾𝑝 works properly, 𝐾𝑝 changed to 0, 15, 30, and 45. The reference was changed to see

how the kit tracks the reference. If the results is matched with the theoretical trend, it can be

concluded that 𝐾𝑑 works well. The next experiment is to test integral term, 𝐾𝑖. In this experiment,

𝐾𝑝 was fixed to 15 and 𝐾𝑑 was fixed to 0. By changing 𝐾𝑖, it was possible to visualize the effect of

𝐾𝑖. If the trend of the results is the same as the theoretical trend, 𝐾𝑖 works well. The final experiment

is to test 𝐾𝑑. This experiment was performed with the same method as one used in the previous

experiment. 𝐾𝑝 and 𝐾𝑖 were fixed to 15 and 5. Then, 𝐾𝑑 was changed to 10 and 20. The trend of

how the kit tracks the reference will become apparent after following this procedure. This trend was

compared to the theoretical trend to make sure that 𝐾𝑑 value works. If all these experiments satisfy

the criteria, it can be conclude that PID control works well.

Results:

The first experiment is to test proportional gain. Figure 3-5, 3-6, 3-7, and 3-8 shows

proportional control with different gains.

Figure 3-5: 𝐾𝑝 = 0, 𝐾𝑖 = 0, 𝐾𝑑 = 0

34

Figure 3-6: 𝐾𝑝 = 15, 𝐾𝑖 = 0, 𝐾𝑑 = 0

Figure 3-7: 𝐾𝑝 = 30, 𝐾𝑖 = 0, 𝐾𝑑 = 0

Figure 3-8: 𝐾𝑝 = 45, 𝐾𝑖 = 0, 𝐾𝑑 = 0

35

The green line is the reference input and the blue line is the output of the motor.

Theoretically, the output of a controller is 𝐾𝑝 multiplied times error. Using only proportional control

can be expected to create steady-state error. As 𝐾𝑝 increases, the corrective response of the motor

should also increase. The figures demonstrate that this does, in fact, happen. In Figure 3-5, there are

smaller changes in output as compared to the output in Figure 3-8. And all the cases cannot reach the

reference input. This is well matched with the theoretical trend.

The next experiment is to test 𝐾𝑖 term. Figure 3-9 and 3-10 show the reference input and the

output of the motor. The output of the motor, in every case, becomes the same as the reference input.

However, there are some small differences in the trend.

Figure 3-9: 𝐾𝑝 = 15, 𝐾𝑖 = 4, 𝐾𝑑 = 0

Figure 3-10: 𝐾𝑝 = 15, 𝐾𝑖 = 6, 𝐾𝑑 = 0

Theoretically, when the integral gain exists, the output can reach the reference input because

it eliminates the steady state error by integrating the error. When the integral gain is high, output

reaches the steady state fast. The experimental results are matched with the theoretical trend.

36

The last experiment is to test a derivative gain. Figure 3-11 and 3-12 show the effects of a

derivative gain.

Figure 3-11: 𝐾𝑝 = 15, 𝐾𝑖 = 5, 𝐾𝑑 = 2

Figure 3-12: 𝐾𝑝 = 15, 𝐾𝑖 = 5, 𝐾𝑑 = 10

Theoretically, the derivative gain improves settling time and stability since this predicts a

system behavior. So the fluctuation after overshoot decreased. Figure 3-11 and 3-12 shows the fact.

During the process of this experiment, it was discovered that the shaft magnet has some

negative interactions with the internal motor magnet, as well as the various metallic pieces used in the

kit. This causes the shaft to have a tendency to ‘stick’ in certain rotational positions. This causes some

non-idealities in PID performance, causing some unpredictable overshoot as well as inability to settle

and constant fluctuations. However, these impacts are minimal and PID performance is still

acceptable. For the most part, experiments using the three gain terms are matched with the theoretical

trends. Therefore, it can be concluded that PID control works properly.

37

Durability of the Kit

Introduction:

For the lab kit to function properly, durability of the kit has to be good enough to stay in

good condition during whole semester. To check durability of the kit, dropping test, fragile test, and

fatigue test were performed.

Method:

The dropping test was performed for several heights. The standard height of a table (30”) is

going to be set as the minimum and benchmark height in the test. The kit should be fine when falling

from this height and 3 more tests will be performed at different heights by an increment of 30”. The

fragility test was examined by jumping in place with the lab kit in a backpack. The kit was placed

with 3 different loads (a notebook, 3 notebooks, and 5 notebooks) in the backpack, and determined

how long the kit remains in its normal condition. The kit was checked at every 100 times of jumping.

The fatigue is completed by calculating the estimated cycle life when the kit undergoes normal

operation forces. It was determined to not be feasible to perform a physical fatigue test, so an

estimation based on known physical parameters can provide a ballpark approximation.

Result:

1. Dropping test

Assumption:

a. There is no air resistance of air

b. The kit is sphere and gravity acts on center of the mass.

c. The kit acts like perfect inelastic collision, (coefficient of restitution, e=0)

d. Mass of system: 119.37g

Velocity of the kit before hit ground, V = √2gh

Impulse, I = ∆(mV) = m√2gh ∵ 𝑒 = 0

Height (inch) Endured Impulse(N∙s) Result

30 0.461 Fine

40 0.532 Fine

50 0.595 Fine

60 0.652 Fine

Table 3-1: Results of the Dropping Test

Discussion:

For dropping test, kit remained fine for every situation. According to Table 3-1, it can be shown

that the kit can resist minimum impulse of 0.652Ns.

38

2. Fragility Test

Carrying with Number of jumping in place Result

1 notebook 1500 Fine

3 notebooks 800~900 One of the hall effect

sensors was bent

5 notebooks 600~700 Both hall effect sensors

were broken and pulley

was disassembled

Table 3-2: Results of the Fragility Test

Discussion:

With one carried notebook, the kit remained fine even over 1500 times of jumping. However,

with three carried notebooks, hall effect sensors started to be bent. The pulley and both hall effect

sensors were broken at condition of five carried notebooks. Therefore, if the kit needs to be brought to

the class, we recommend that use separated container.

3. Fatigue Test

Assumption:

a. PCB is made of a layer that is Epoxy.

b. Shaft is made of ABS Polymer type

c. Force due to spring is only cyclic load.

d. 99.9 % of reliability.

e. Cyclic load is applied in bending situation.

i) Stress analysis for PCB

𝑆𝑛 = 𝑆′𝑛𝐶𝐿𝐶𝐺𝐶𝑆𝐶𝑇𝐶𝑅 = ((0.5)(15000))(1)(1)(1)(1)(0.753) = 5647.5 psi

𝑆𝑓 = 0.9𝑆𝑢𝐶𝑇 = 0.9(15000) = 135000 psi

𝜎𝑚𝑎𝑥 =k∆𝑥𝑚𝑎𝑥

𝐴=

(0.17)(0.625)𝜋2

(2.5)(0.04)= 1.668 psi

39

Figure 3-13: S-N Curve for PCB

ii) Stress analysis for the shaft

𝑆𝑛 = 𝑆′𝑛𝐶𝐿𝐶𝐺𝐶𝑆𝐶𝑇𝐶𝑅 = ((0.5)(5801))(1)(1)(1)(1)(0.753) = 2184.07 psi

𝑆𝑓 = 0.9𝑆𝑢𝐶𝑇 = 0.9(5801) = 5220.9 psi

𝜎𝑚𝑎𝑥 =𝑀𝑦

𝐼=

(𝐹𝑑)(𝑦)

(𝜋𝑅4

4)

=(0.625)

𝜋2

(0.17 × 2)(0.55)(0.25)

𝜋(0.254)4

= 14.96 𝑝𝑠𝑖

Figure 3-14: S-N Curve for shaft

0

1

2

3

4

5

1.00E+00 1.00E+02 1.00E+04 1.00E+06

log(P

eak a

ltern

ating s

tress

), S

log(Life), N

S-N Life Curve

Stress.Max

0

1

2

3

4

1.00E+00 1.00E+02 1.00E+04 1.00E+06log(p

eak a

ltern

ating s

tress

), S

log(Life), N

S-N Life Curve

Stress Max

40

Discussion:

For loading and unloading spring process, 1.669 psi is assumed to be a bending stress at the

PCB and 14.96 psi is assumed to be a bending stress at the shaft. According to Figure 3-13 and 3-14,

both maximum stress lines always lie below S-N curve, so it can be theoretically concluded that

numerous repeating processes of assembly and disassembly can’t affect too much on PCB and shaft.

As theoretical expectation, the kit was fine until 100 times of assembly and disassembly process.

Ease of Setup

Introduction:

One more requirement of this lab kit is that it not be difficult for the students to build. The

purpose of ME 3281 is not to teach students how to build or wire, so the emphasis on the kit is not,

either. The design of the kit has been created in order to minimize the amount of wiring and reduce

the complexity of putting the components together, with the idea, in mind, that all students will have

varying levels of skill in these areas. With that said, it is also assumed that the students in this course,

having made it so far in their education, have some baseline knowledge and skill. To ensure that our

kit conformed to the requirements set forth, students were asked to participate in testing. Through this

testing, we sought to gauge student reaction to the setup of the kit. Students were asked to build the

kits and comment on their experience, elucidating on the frustration, confusion and difficulty they

faced while doing so. The students were also timed, to ensure that the time taken to construct the kit

was not considerably long. An ideal value for the time to construct was set at twenty minutes and,

marginally, less than one hour was deemed acceptable. Due to a lack of materials available for the

purpose of this testing, the students were asked not to make any permanent solders, during the

construction.

Methods:

Four student volunteers took part in the testing. The students were given all of the

components necessary to construct a full kit. They were also given written instructions along with a

picture of a completed kit. The students were given no outside help and were left on their own,

simulating the experience of students working by themselves at home. Entering the testing,

background information was taken. The students were asked about prior experience and their

confidence in their ability to construct the kit. The students were then timed while constructing the kit.

At the end of testing, they were asked, generally, about their experience while building the kit,

pressing further for opinions on their level of frustration and the complexity of the kit, as well as

whether or not they faced any particularly great challenges.

Results:

The results of this experimentation were not surprising. Entering testing, all of the students

expressed a great deal of confidence in their abilities. All of the students were engineers and in fact

were all of differing sects, being electrical, chemical, mechanical and aerospace. All but the electrical

engineer expressed a lack of practical experience in projects like this, but remained confident. The

time taken by each of the students is shown below, in Table 3-3. Having made no mistakes in

constructing the kit, the confidence of each of them was well justified. Interviews, after testing, were

equally unsurprising. One student shared that he had experienced some trouble with inserting the Hall

Effect sensors, having bent a lead, he said it was difficult to get it straight enough, again, for it to slide

in easily. All of the students expressed their concern for the connection that would be created by the

glue, but provided no viable alternatives. They all agreed that the construction of the kit provided

them with no great challenge.

41

Student # Time (min)

1 25

2 31

3 28

4 30

Table 3-3: Student Time to Construct

From the results obtained, many positive justifications can be made. The students expressed

no particular trepidation when faced with what the final kit looked like. It is hoped that this is an

indicator of the reaction of ME 3281 students, showing that they will not be intimidated by the task

before they even attempt it. Furthermore, as seen in the above table, the time necessary to construct

the kit was well within the acceptable limit, and was surprisingly consistent. Some students tend to

have a small attention span, and only having to take thirty minutes out of their day to construct this

kit, they may even enjoy themselves. Furthermore, testing indicated no particularly great obstacle in

the construction, meaning that students will not find themselves stuck at any particular point. Time

was the main metric, being tested, but the most interesting and enlightening conclusion that can be

drawn, from the interviews, is the ease of mind, in all of the students. As students, ourselves, we know

how frustrating some homework assignments and labs can be, and therefore feel that it is of great

importance to mitigate the amount of negativity students will find, and are proud to see that, during

testing, the construction was seen as no big deal.

3.2 Cost Analysis

Most of the components used in the lab kit were ordered from McMaster-Carr or DigiKey.

Since they are all stock components and are ordered in bulk, the per-unit cost is low. The custom shaft

was designed to minimize manufacturing operations, and only requires one cut and low tolerance hole

drilled. This part could be ordered from the QuickParts internet service, or from a local supplier. The

PCB was ordered from ExpressPCB, using the economical “Standard” option. By meeting these

design requirements for this option, the PCB can be purchased for $3.00 in quantities of 250. The total

cost of all components is included in the BOM.

The kit was designed to simplify assembly, allowing the students to assemble the kits

themselves. Thus, there is no cost for assembly. Besides the shaft and PCB, all parts are standard and

unmodified, so no machining operations will need to be performed by the student.

The total cost of the components from the BOM is estimated to be $30.05 in lots of 250.

Compared to the cost of the previous model, $41.60, this is kit offers improved functionality at a

significantly lower price. The major cost savings come from simplification of assembly, allowing the

students to assemble the kit themselves. Also, the use of an Arduino for the microcontroller, which

most students already have, removes the cost of the PIC and supporting electronics.

For students that don’t already have an Arduino or breadboard, this kit design is more

expensive than the previous model. The Arduino adds an extra $27.83 and the breadboard $3.49,

resulting in a total kit cost of $61.37. However, both of these components are reusable; many students

will find uses for these components after the class ends. Further, our survey found that 87% of

students already possess an Arduino, so most students will not need to purchase one.

The demand for the lab kit is large, with 74% of students stating that ME 3281 could benefit

42

from a take home lab kit. However, most students also indicated that they would be unwilling to pay

over $50 for such a kit. With the reduced cost of approximately $30, this kit is more affordable than previous

models while improving features and usability.

3.3 Environmental Impact

The impact on the environment, from this ME 3281 lab kit, is incredibly minimal. There are

however some considerations that should be made with any product that contains electronic

equipment. The main base of the kit is constructed from a printed circuit board (PCB). In general, the

process by which these boards are made and the materials from which they are made, do have some

negative impacts on the environment. More specifically, a large amount of water is used for rinsing

boards, during production, and at which time becomes contaminated with potentially dangerous

substances like heavy metals and solvents. Many available vendors, including ExpressPCB,

manufacture lead-free boards which reduce environmental impact. Environmental conscious vendors

also ensure that any chemicals are recycled whenever possible, or neutralized and dealt with in a safe

manner. It was initially suggested that a custom piece of plastic could be made, for the purpose of

mounting the pieces of the kit. This, however, would make the use of the Hall Effect sensors much

more difficult and frustrating for students. In addition, that solution would have represented a

considerable cost increase for the kit, which was a main design concern.

Continuing, another possible impact of this lab kit on the environment is simply waste.

Around 150 students will be purchasing this kit each year, and will presumably never again have a

need for it, when their semester in ME 3281 comes to an end. Unfortunately, this means that many of

the students will simply throw the kit away. This acts in two ways. First, it is never good, for the

environment, to increase the amount of waste produced or to fill the landfills. Secondly, as said

before, PCBs are produced using heavy metals which, if the kit is thrown in the trash, will be in

contact with the environment, free to cause pollution. This concern can, at least, be mitigated in two

ways. First, students, upon buying the kit, should be informed of the proper methods of disposal for

PCBs, and the means by which they can go about recycling them, when they are finished. Next, the

University can consider a buyback program, and take what is salvageable, recycle the boards

themselves or simply resell the kits as used. If the kits were reused, a great deal of impact would be

taken off of the environment.

Transportation costs associated with the lab kit can be minimized. Many of the parts come

from Digi-Key and McMaster-Carr, both of which have local warehouses so waste associated with

shipping is minimized. Digi-Key also uses Geami, a 100% recycled packaging product. The kit

components are picked up by the students themselves while on campus, so no further shipping is

required.

Finally, the use of the kit requires electricity, the production of which is responsible for a

majority of air pollution. The power usage, however, is very low, especially if students do not leave

the kit powered on when not in use. Also, there is really no feasible alternative, as the kit must

interface with a computer.

All sources of environmental impact, stated above, are present in nearly every product

available today. In the grand scheme of things, they are quite minimal. This is especially true when

one considers the potential impact, due to this lab kit, on students’ ability to learn and understand the

material presented to them in ME 3281.

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3.4 Regulatory and Safety Considerations

The ME3281 lab kit is the lab kit for the student. This lab kit used in the ME3281 class is to

help student understand the concepts. Since most of students will use this, safety should be considered

to prevent accident. The lab kit does not have any hazardous substance. According to Restriction of

Hazardous Substance Directive (RoHS), it defines following six substances as hazardous substance

and restricts using those; Lead, Mercury, Cadmium, Hexavalent chromium, polybrominated biphenyls,

Polybrominated diphenyl ether. To assembly the lab kit, student should solder wires to PCB. The

solder contains lead that is hazardous substance. When the solder is heated, it forms lead oxide fumes.

Excessive exposure to this fume can cause lead poisoning. Therefore, students should be informed to

conduct soldering in a well-ventilated location.

Another safety consideration is current flow through the circuit. To supply power to the

motor, 12V is used and this voltage is going through the DC power jack. DC power jack is attached to

the breadboard directly. This can be detached easily by force. It is important to keep the power jack fit

into breadboard so that current cannot flow to else dangerous.

Sharp corners of the PCB are a concern for safety and can cause injury. This can be

prevented by using the kit carefully.

Magnet used to track position of the pulley is really strong. It is really important to keep the

distance from electrical hardware such as hard drive. Allowing the magnet to be close to electrical

components, particularly hard drives, can cause damage and should be avoided.

List of safety concerns

1) Voltage from DC power jack

2) Sharp edges of PCB board

3) Lead in solder

4) Hot temperature of solder

Recommendations

1) Do not hold the corners of PCB too tight

2) Lock DC power jack so it cannot be detached from PCB

3) Do not solder in closed environment.

4) Keep it away from children.

5) Keep distance when you solder.