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