Robotic Hand Mechanization

Engineering Technology

ET-494

Spring 2016

Students: David Cothell, Chad Newberry, Steven Walker

Advisor: Dr. Koutsougeras

Professor: Dr. Koutsougeras

1. Abstract

The goal of this project is to construct a mechanical limb that can have the functionality of a

human limb when controlled by an operator or user. The fabrication of this hand will take

place in SolidWorks (3D modeling software) and eventually be constructed or “printed”

using a 3D printer. This mechanical hand would be able to have close to (if not the same)

response, articulation, function, and operation through a series of cables, joints, motor,

gears and actuators that will emulate a human hand through the control of a user/operator

in a remote location using virtual reality (VR) headwear and hand controls. 3D printer

filaments such as PLA, ABS, and Vero White Plus are discussed in this paper, along with

snap joint calculations and gear designs.

2. Introduction

There is a remote location that has been put under a quarantine zone due to a

highly contagious epidemic. The people in this quarantine zone require a vaccine,

furthermore, someone to administer it. There is normally one option, and that is for

someone to risk their health and enter the quarantine zone to administer this vaccine. This

robotic hand mechanization or “RHM” project aims to alleviate that option. This project

is geared toward making a mechanized hand that will be fully mobile and remotely

controlled giving it the free range to enter hazardous conditions and carry out various

human duties. With advances in technology, robotics have taken the place of where

humans should not or cannot go.

Examples of robotic application:

UAVs (Unmanned Arial Vehicle)

Unmanned submersibles

EOD robots (Explosive Ordnance Disposal)

Robotic Surgical Systems

3. Purpose

The purpose of this project is to fabricate and mechanize a hand that can be

mobile, remotely controlled, and able to enter hazardous conditions. This task will be

achieved by designing and fabricating a mechanical hand that will be controlled by a

system of cables, actuators, or gears coupled with motors and flex shafts to mimic the

motion of a real hand all while being controlled by an operator/user in real time.

The final goal of the design aspect is to create a fully operational hand that will translate

human functionality in real time through remote control. The hand design will be created

using SolidWorks which is a 3D modeling software. While concurrently creating the

model in SolidWorks, component analysis and testing will be achieved using ComSol.

Once the simulations and analysis have been proven, the next step is to use a 3D printer

to produce the entire hand assembly for physical trials. The hand will be composed of

Vero White Plus Full 835, PLA (Poly Lactic Acid) 3D filament, ABS (Acrylonitrile

Butadiene Styrene) or a similar material.

4. Research

Previous research looked into many different types of robotic hands that are already

on the market. Some od these included free open source designs such as the InMoov robot

that is fully 3D printable, and other more expensive and sophisticated designs. These were

mainly the Shadow Hand and the Super Robust Hand.

The Shadow Hand is currently in use by NASA on their Robonaut. This hand has 20

degrees of freedom, and can close or open its fingers in about 0.5 seconds [8]. This hand

comes in pneumatic, hydraulic, or cable driven designs.

The Super Robust Hand was designed in Germany for an estimated cost of 70,000 to

100,000 Euros. This hand had 19 degrees of freedom and uses a system of stepper motors,

pulleys, springs and tendons. This allows the Super Robust Hand to be able to audibly snap

its fingers with 500 to 2000 degrees per second of joint rotation [7].

5. Final Design

The Bowden Cable (also known as push-pull cables) design uses a cable and

sleeve that fit tightly around a cable [11]. The cable is able to slide forwards or

backwards in this sleeve, but the cable is not able to bend or “kink”. As a length of cable

gets infinitely smaller, it will act like a solid rod and not bend like a long segment of

cable would when pushed. Therefore, this design allows the fingers to curl or fully extend

with the pull or push of the cable. This design allows for one cable to curl and extend the

finger drastically reducing the amount of controls needed for each digit.

The Finger

The finger design involves a cable running along the bottom of each finger

segment. When the cable reaches the tip of the finger it curves upwards, see figure 4

above. By using the end of the cable itself and pushing it towards the curved radius, the

fingers extend and lift outward and slightly up, as a hand would when its fingers are

outstretched. When the cable is pulled, the fingertip curls toward the palm.

At the base of the finger, a ball and socket joint allows the finger to be able to

move up and down and left and right. The ball and the socket that the ball fits into have

matching flats designed into them that prevent the finger from rotating axially beyond a

few degrees. Figure 5 below illustrates this:

Figure 4. Bowden Cable Finger Design

The Palm and Thumb

The palm is a solid design, and the back is hollowed out to reduce weight and

material. There are cables running through the base of the palm all the way to the ball and

socket joints that the fingers mate into. The top of the palm has slots designed into it to so

that the ball and socket joints can be pressed into the palm as separate pieces. These slots

are offset from the front of the palm to give room for the driving cables. Due to this offset

the fingers themselves are 10mm longer than average fingers to make up for the increase

in distance from the front of the palm. Also to help with this issue, indents are designed

into the top of the palm to allow the fingers to curl approximately 15 degrees below

Figure 5. Ball and Matching Socket

horizontal. This lets the finger tips contact the palm closer to its center. The palm has a

slot where the thumb is mounted to it. Figure 6 below depicts the palm:

The thumb is designed so that it can be rotated towards the center of the palm and

still be curled and extended like the other four fingers. This was accomplished by having

a small platform that holds the ball and socket joint, and connects to the palm via a hinge

joint. The Bowden cable that actuates this part of the thumb is attached to the far back of

the platform to increase torque when it is rotated towards the center of the palm. The rest

of the thumb is the same as the fingers minus the middle section. The thumb assembly is

shown is figure 7:

Figure 6. Front (left) and Back (right)

of the Palm.

Figure 8. Final Design, Complete Hand

Front and Side Views

Figure 7. Thumb

6. Control

The hand is controlled by a Raspberry Pi (RPi) microcontroller. On the RPi,

Python 2 programs are used to control the DC and servo motors that are used in the

mechanisms that drive the hand. L298N DC motor driver boards are used to drive the

motors and shield the controlling RPi from back emf and current.

Currently, the hand is controlled via reading keystrokes from a keyboard attached

to the RPi. As long a certain key is held down the corresponding DC motor will be turned

on. Once the key is released, the DC motor will be shut off. This is done via the pygame

library using even detection. The servo motors are simply attached to certain keys that

when pressed the servo turns to either 0, 90 (Neutral), or 180 degrees to open or close the

finger horizontally.

A glove that can be worn by a user has been built and programmed to control the

hand. This glove uses an Arduino Lillypad that is a slave to the RPi. This was

accomplished via the Nanpy module available for python. The glove has five flex

sensing resistors (FSRs) sown on each finger that as the resistor flexes the resistance

changes. These changes in resistance are then read by the RPi through the slaved

Arduino. Once these resistances are read the DC motors will be turned on or off based

on the values read. This FSR glove is not a new or novel idea and many instructions to

build one can be easily found on the internet [13]. However using the Arduino on the

FSR glove as a wireless, Bluetooth slave to the RPi is not easily found if available. For

more info on Nanpy or the design of the glove see the links in the references: [13][15].

7. Mechanisms of Actuation

To push and pull the cables 12 volt 20.4:1 DC motors are being used. These

motors spin at 500rpm with a torque of about 0.6Nm. The motors are used to turn a

power screw that pushes and pulls a sheath that the cables are connected too. Six of these

DC motors are used. One for each finger, and one to move the thumb hinge into the palm.

Captive linear screw actuators were the first choice to push and pull the cables.

However, the prices per one that met the required needs of the project, (speed, torque)

were too high (cheapest was $122) for this stage in the design. Instead the above

mentioned DC motors and a designed and 3D printed power screw [9] assembly is being

used. The power screw is about 4.5 inches long and has a pitch of .25 inches. This allows

approximately 2 inches of linear travel for the sheath per second. See figure 9 below:

Figure 9. Power screw and Sheath in

the mounting bracket for DC Motor.

This mount keeps all parts in line with

the key shaft of the DC motor.

While the DC motors and power screws are what cause the fingers to curl and

extend, the servo motors are used to move the fingers horizontally left and right via

fishing line attached to a pulley on top of the servo. The servos spec out at 60deg/.12sec

with about 0.43Nm of torque at 6 volts. Five servos are used, one for each finger.

8. Data and Properties

The following tables are the data collected from testing, and the physical

properties of the hand assembly. The physical properties were determined by using the

evaluation features present in the SolidWorks modeling software.

Finger Open and Close Times

Opening Times (seconds) Closing Times (seconds)

1 1.32 1 1.62

2 1.49 2 1.59

3 1.52 3 1.72

4 1.71 4 1.71

5 1.46 5 1.65

Physical Properties

Part Surface Area Volume

Hand Assembly 110487.17 square millimeters 277157.86 cubic millimeters

Power Screw Assembly 37651 square millimeters 57435 cubic millimeters

6 1.52 6 1.52

7 1.57 7 1.57

8 1.62 8 1.63

9 1.43 9 1.59

10 1.55 10 1.75

Average 1.52 +/- 0.1 Average 1.64+/- 0.07

Finger Force

Resistance Measured from Applied Force

(kOhms)

1 23.4

2 31.2

3 19.5

4 28.6

5 22.4

6 32.1

7 18.9

8 22.3

9 24.8

10 20.7

Average 24.4

Force found via graph: ~ 25N (5.5lbs)

9. Testing and Evaluation

To measure the force that the finger can close into the palm with its fingertip, a

force sensitive resistor was used. An ohm meter was attached to the resistor and the

finger was curled into the palm and resistor via the DC motor and power screw setup.

The change in resistance was recorded over 10 trials and the average was taken. This

average was then compared to a resistance versus force chart [14]. It was determined that

the fingers can close with about 5.5lbs or 25N of force at the fingertip. It should be noted

that this is merely a ballpark estimate as force sensing resistors are not always the best

measure of force output [14], and a load cell would be need for a more accurate force

reading.

The time that it takes to curl and extend the finger were also measured over 10

trials each. To extend the finger it took on average 1.5 seconds, and to curl the finger in

fully it took 1.64 seconds. This test was done with simply a stop watch and looking for

when the finger was fully extended or curled.

10. Conclusion

The hand is in its final design and not many tweaks, if any, are needed. Simple

control of the hand has been implemented, allowing the hand grasping power. . This is

done through keyboard keystrokes and a glove that utilizes flex sensing resistors. The DC

motors and power screws work as intended to push and pull the cables that drive the

fingers, and the servo motors work well to give the finger left and right horizontal play.

To improve on the hands response and gripping ability the hand itself needs to be

outfitted with feedback sensors of some sort. Flex sensing resistors or potentiometers on

the finger joints to relay positions of the figure should be the next step in this project. The

controls are, at this time, fairly simple and can be vastly expanded upon to give better

function and response. Considering the hand is in a prototyping or experimental phase it

functions fairly well, with room to enhance and better certain areas as bugs and

limitations are found.

Appendix:

A: DC Motor Specs

11. Deliverables

I. Final hand design

II. Raspberry Pi controls

III. Bill of materials

IV. Working hand analysis

V. Working 3D printed hand

Work Done:

David – SolidWorks Design of Bowden Cable Hand, ComSol analysis of new parts and 3D

printer filaments, fitting of printed parts, some research/implementation of controls, part

analysis/evaluation, Raspberry Pi coding,

Chad – fitting of printed parts, cable and motor and controls integration, programming the

Raspberry Pi/ controls, SolidWorks thumb design, Python programming debugging, DC motor

platform integration

Steven – 3D printer logistics, SolidWorks hand components design, fitting/testing of parts, part

analysis/evaluation, linear screw actuator implementation, component platform fabrication and

installation, Virtual Reality (VR) research/construction/implementation

References

1) Leon Maryasin. (2013). Robotic Hand[online]. Available FTP

https://grabcad.com/library/robotic-hand-4GrabCad.com/November 17, 2013.

2) Robert Dennis, Jon Edwards, “Considering Endoscopic Design,” Digital Object

Identifier 10.1109/MPul.2013.2262141, July/August 2013.

3) Langevin, Gael. "InMoov: InMoov Finger Prosthetic." InMoov: InMoov Finger

Prosthetic. N.p., 14 July 2013. Web. 18 Nov. 2015.

4) "Spring Durability and Spring Fatigue." Lesjoforsab Springs & Pressings. N.p., 2015.

Web. 18 Nov. 2015.

5) "MechGuru's Worm Gear Box Design Calculator." MechGuru's Worm Gear Box

Design Calculator. MechGuru, n.d. Web. 18 Nov. 2015.

6) Budynas, Richard G., J. Keith. Nisbett, and Joseph Edward. Shigley.Shigley's

Mechanical Engineering Design. 8th ed. New York: McGraw-Hill, 2008. Print.

7) Guizzo, Erico. "Building a Super Robust Robot Hand." IEEE Spectrum. IEEE, 25

Jan. 2011. Web. 18 Nov. 2015.

8) "Dexterous Hand." Shadow Robot Company. N.p., n.d. Web. 18 Nov. 2015.

9) Oberg, Erik, Franklin Day Jones, Holbrook Lynedon Horton, and Henry H. Ryffel.

Machinery's Handbook. 27th ed. New York: Industrial, 2004. Print.

10) Harris, M., Kyberd, P. J., & Harwin, W. S. (2005). Design and Development of a

Dexterous Manipulator. Transactions of the Institute of Measurement and

Control, 137(2), 137-152. Retrieved January 24, 2016.

11) Bowden Cable » Hindle Controls. (n.d.). Retrieved February 02, 2016, from

http://www.controlsandcables.com/products-to-sort/bowden-cable/

12) VeroWhite [PDF]. (2015). Stratasys.

13) How to make a remote controlled Robotic Hand with Arduino. Retrieved March 28,

2016, from http://www.instructables.com/id/Wireless-Controlled-Robotic-Hand/

14) Interlink Electronics. (n.d.). FSR Integration Guide [PDF]. Sparkfun.

15) Nanpy 0.9.6 : Python Package Index. (n.d.). Retrieved May 02, 2016, from

https://pypi.python.org/pypi/nanpy