Download Robotic Finger

Robotic Finger
Yun Gong ’15, Patrick Norton ’15, Lisa Yamada ’15
Faculty Advisor: Dr. Taikang Ning
Trinity College Department of Engineering
Abstract
In the human body, fingers move due to electrical impulses sent from the brain. These impulses manipulate the muscle fibers of the finger, resulting in movement. This project contains a similar blend of electrical and mechanical components, except instead of a brain sending impulses, the
microcontroller outputs pulse width modulation signals to servo motors which then control movement of 3D printed fingers. These servo motors initiate a series of timing belts and pulleys which subsequently move the joints. The finger can flex/extend, move side-to-side, and curl. Each
movement can be achieved independently, similar to a human finger. The 3D printed finger has three joints, representing the three joints of a human finger. For the curling and flexion/extension movements, each joint can rotate 90° while the side-to-side motion has a limit of 50°. Additionally,
gripping pads are attached to the face of each individual section to add friction when holding objects. The overall finger model is quick to assemble, with each section broken down into halves that can be connected with a nut and bolt. An operating stand was also built to facilitate the
demonstrations and keep all electronics out of sight. In couple with the operating stand, the fingers are capable of various demonstrations to exemplify its functionality. These demonstrations include playing notes on a keyboard, holding a small object while in a curled position, and holding but
not crushing an egg. These demonstrations illustrate that the fingers are capable of not only strong grips but delicate grips as well.
Problem Statement
Theory: Mechanical Design
Demonstrations
The cost of current robotic prosthetics are exorbitantly high, ranging from
$11,000 to $120,000. The mission of this senior design project is to
alleviate this problem by designing and fabricating low-cost artificial
fingers with three degrees of freedom.
To generate the same movement that a human finger exhibits, the design needed to have two systems of movement. The first was a
method of moving the finger from an extended position to a curled position. The second system needed to be able to restore the
finger to an extended position. Three separate alternatives were developed: the cable alternative, the chain and sprocket
alternative, and the belt and pulley alternative. For each of these alternatives, the cable, chain, or belt would be attached to a servo
either inside the finger or below it to provide a force. The belt and pulley design concept was used for the final robotic finger.
The final design of the robotic finger should complete the following tasks:
• Play piano keyboard
• Hold an egg without crushing it
• Press the “easy” button
• Grip a wooden block
• Finger Gestures
For the demonstration, two robotic fingers will be 3D printed, and in order
to facilitate the demonstrations, an operating stand was constructed. The
operating stand was designed so that the orientation of the fingers can be
quickly changed from side-by-side to face-to-face, and vice versa.
Figure 1: Desired range of motions of designed finger [1]
Theory: Electrical Design
The joints of robotic fingers are driven by servo motors, which are
controlled by the pulse width modulation (PWM) signals sent from the
Arduino Mega microprocessor. The duty cycle of the PWM, defined as the
ratio of pulse width to pulse period, determines the position of the servo
motor. By writing an Arduino program to manipulate the duty cycles of
multiple PWM signals, the positions of the joints can be successfully
controlled.
For each finger, three HiTec mini servo motors are used (one servo for
each of the three desired motion shown in Fig.1), and each one requires a
power source of 5V. Since the Arduino Mega microprocessor only has one
5V output pin, a voltage regulator circuit was constructed to adjust the
output voltage of two 9.6V batteries in parallel to 5V. Connecting the two
batteries in parallel allows the output current to increase without changing
the voltage. The schematic of the voltage regulator circuit (combination of
the voltage regulator chip L7805CV and capacitors) is shown in Fig.6.
Although servos can be easily powered by an external power supply, the
use of batteries was preferred so that the design can be portable.
At the finger pad of each finger, a force sensing resistor (FSR) is placed to
sense the approximate amount of pressure applied to the finger. The FSR
varies its resistance depending on the amount of pressure applied at the
finger tip: the larger the force, the lower the resistance. By using a FSR, the
finger will be able to exert the appropriate amount of pressure for
particular tasks, such as gripping a delicate object.
Pros
• Chain can rotate finger in both
directions; can serve as curling and
restoring force
• Sprocket can be attached directly to
servo motor
• Making sprockets within finger larger
than servo sprocket amplifies grabbing
force and gives more precise movement
Cons
• Sprockets difficult to 3D print accurately;
may not properly link with standard
chain
Curling force: Cable
Restoring force: Torsion springs
Pros
• Simple system of forces
• Does not require complex finger
model to implement
• Keeps size of finger to a minimum
Cons
• Cannot apply a grabbing force at
flexed position
• Cables stretched after long use
Figure 5: Cable Alternative
Figure 6: Chain and Sprocket Alternative
Parts Selection
Pros
• Same functionality as sprocket
design
• Can 3D print pulleys to interlock
with timing belt teeth
• Timing belts are non-stretchable and
maintains length at all positions
• Assembly involves fewer parts than
sprocket design
Cons
• To achieve all forms of movement,
servo housing unit needs to be big
enough to fit servos; resulting in an
increase in size
• Servo housing unit prevents multiple
fingers from being placed side by
side
Figure 7: Belt and Pulley Alternative
Figure 3: Schematic of Voltage
Regulator Circuit
Figure 2: Pulse Width Modulation
diagram [2]
Figure 4: Voltage Regulator Circuit
Table 1: Cost of Robotic Finger Design
Cost per Robotic Finger
Materials/Equipment
Quantity Unit Price
Cost
Arduino Mega 2560 Rev3
1
$37.95
$37.95
HS-225MG Hi-Tec Mini Servo
3
$35.99
$107.97
Timing Belts (14")
1
$4.50
$4.50
Timing Belts (9")
1
$3.75
$3.75
Timing Belts (7")
1
$3.45
$3.45
Force Sensing Resistor
1
$6.95
$6.95
3D Printing Expenses
13 Parts
--$193.64
TOTAL COST $358.21
Cost Per Hand
Cost
$37.95
$539.85
$22.50
$18.75
$17.25
$34.75
$968.20
$1639.25
Arduino Mega 2560 Rev3
• Arduino language
• Contains 15 PWM output pins
HS-225MG Mightly Mini Servo
• Dimensions: 1.74 x 1.22 x 0.66 in3
• Torque: 67 oz-in
• 180° rotation
• HiTec C1 Standard Spline (24 teeth)
Force Sensing Resistor (FSR)
• 0.5 inch diameter
• 0.5mm thickness
• Peel Sticker Backing
• Used frequently for robotic grippers
• Force range: 1N to 100N
• (0.225lb force to 22.5lb force)
3 Timing Belts
• 14” circumference
(allows top joint movement)
• 9” circumference
(allows middle joint movement
• 7” circumference
(allows bottom joint movement)
Figure 8: Arduino Mega
2560 Rev 3 [3]
Figure 9: HS-225MG
servo [4]
Figure 12: Operating Stand
(Face-to-face orientation)
Figure 13: Operating Stand
(Side-by-side orientation)
Conclusion
After multiple revisions, the final design of the robotic finger is able to
complete three degrees of motion: flexion/extension, side-to-side, and
curling. The functionality of the robotic finger was exhibited through a
series of demonstrations. Integrating a complicated mechanical and
electrical system together, the robotic finger was able to display fine
microcontroller control to complete fundamental actions required of a
human finger.
Future work of this project include replicating the current design in order
to create an entire hand. By doing this, the range of applications expands
enormously. Another useful feature to incorporate is wireless control so
the robotic finger or hand can be controlled remotely. To make the robotic
prosthetic even more advanced, one could make it controlled by
electromyography (EMG) signals. Finally, the robotic device can always be
enhanced by mastering smoother control of the joints.
Robotic prosthetics are now more accessible than ever due to the progress
in 3D printing. The cost of a finger using this design concept is $358.21,
and the estimated cost of the full hand is $1639.25. By continuing research
in this field, countless number of lives will be transformed daily.
Bibliography
[1] Potratz, Jason. "A Light Weight Compliant Hand Mechanism With High Degrees of
Freedom." Journal of Biomedical Engineering 127.6 (2005) ASME Digital Collection.
Web. 27 Apr. 2015.
[2] Touch Bionics. I-limb Ultra. Touchbionics.com. Web. 27 Oct. 2014.
[3]"Arduino Mega2560 Rev3." Arduino Mega2560 Rev3. Robot Mesh. Web. 11 Nov.
2014.
[4] HS-225MG Mighty Mini." Servo City. Web. 25 Mar. 2015.
[5] ".5 Inch Force Sensing Resistor (FSR)." Trossen Robotics. Web. 11 Feb. 2015.
[6] "Timing Belts." ServoCity. Robotzone. Web. 28 Apr. 2015.
Acknowledgements
Figure 10: Force
Sensing Resistor [5]
Figure 11: Timing Belts [6]
NASA CT Space Grant College Consortium
Trinity College Engineering Department
Engineering Department Technician: Andrew Musulin
Department Chair: Professor Mertens