Development, Testing, and Application of DANTE: A Prototype
Lego Prosthetic Arm
by
C. Starrett
B. White
Jordan Zink
Gahanna Lincoln High School
140 S Hamilton Road
Gahanna, Ohio 43230
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DEVELOPMENT, TESTING, AND APPLICATION OF DANTE: A
PROTOTYPE LEGO PROSTHETIC ARM
C. Starrett, B. White, and Jordan Zink
Gahanna Lincoln High School, 140 S Hamilton Road, Gahanna, Ohio 43230
The purpose of the project was to develop a robotic arm out of Legos capable of accomplishing
several tasks. The arm was based in design of a human arm with some modifications. The final
arm contained five joints: a shoulder joint, a elbow joint, two wrist joints, and a hand grip joint.
Each joint was powered by a motor controlled by two NXT, a programmable Lego brick. To
control the arm, a new control system using an ultrasonic sensor and touch sensors was
developed and proved to be successful.
Several tests were run on the arm. It was found that it took 10 seconds for the shoulder joint to
rotate 90 degrees, meaning it takes a somewhat long amount of time for the arm to rotate. Also, it
was found that the arm could lift a maximum of 300 grams, which is more than adequate for the
tasks. Finally, it was found the hand gripped on average 3.9 N compared to a light human grip of
5.7 N.
The tasks required to be completed included picking up a graduated cylinder filled with water
and pouring the contents, threading a thin rope through a small opening and threading a piece of
string through the same small opening. Testing found that DANTE was able to perform all three
of the tasks.
DANTE proved to be an effective robotic arm. While speed was lacking in certain joints, the
intuitive control system and strength provided major advantages for the arm.
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Table of Contents
I. Introduction…………………………………………………………………………….……….4
II. Review of Literature…………………………………………………………….…...…...…….4
III. Methods…………………………………………………………………..……………...…….7
IV. Results……………………………………………………………………………….….……12
V. Conclusion………………………………………………………………………………...….14
VII. Works Cited……………………………………………………………………...………….15
VIII. Appendix………………………………………………………………………………...…16
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Introduction
The modern world is increasingly becoming a more robotic one. Robotic technologies are
becoming more advance every day and are being implemented in many aspects of life, including
military robots and assembly line robots. One field of robotics closer to human is one that seeks
to combine humans to robots: prosthetics. People who have lost limbs to car accidents, diseases,
and any other reason will soon be able to replace the lost arms and legs with fully functioning
robotic prosthetics allowing the individual to live out a much more normal life.
The goal of this project was to make a robotic arm that closely represented the same ideas
of a normal human arm. The arm would need to be portable, accurate, have a wide range of
motion, and be able to accomplish several tasks, such as picking up a graduated cylinder of water
and pour the contents. This arm would represent a simple, cheap robotic prosthetics most likely
not suited for use on a person. However, the arm should share many of the same qualities that a
million dollar advanced robotic arm also has.
Review of Literature
Muscles
A major component of this project is an understanding of the human arm and hand and
how they work. The human arm is made up of several muscles which include the deltoid,
pectoralis major, biceps brachii, coracobrachialis, subscapularis, teres major, and the latissimus
dorsi. Other muscles include the deltoid, supraspinatus, infraspinatus, and the triceps brachii.
These muscles are responsible for all the movement the human arm does Along with the arm
muscles we also had to study the muscles of the hand which include the extensor carpi radialis
longus, extensor carpiradialis brevis, extensor carpi ulnaris, flexor carpi radialis, flexor carpi
ulnaris, and the palmaris longus. Along with the other muscles that include the extensor
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digitorum, the extensor indicis, extensor digiti minimi, flexor digitorum superficialis, and the
flexor digitorum profundus. These muscles all help control the wrist and the hand and nee to be
re created in our bionic arm. With a combination of the use of both arm and hand muscles we are
able to do many things, write, open cans, twist bottle caps open, so our goal for our robot is to
have it be able to recreate all the range of motions a human hand and arm have (“Muscles of the
wrist and hand”, 2000).
Along with knowing the muscles we had to understand how muscles work. The basic
action of any muscle is contraction. The brain triggers the contraction by sending a signal down a
nerve cell to the biceps muscle telling it to contract. The actual contraction works A muscle fiber
contains many myofibrils, which are cylinders of muscle proteins. These proteins allow a muscle
cell to contract. Myofibrils contain two types of filaments that run along the long axis of the
fiber, and these filaments are arranged in hexagonal patterns. There are thick and thin filaments.
Each thick filament is surrounded by six thin filaments. Thick and thin filaments are attached to
another structure called the Z-disk or Z-line, which runs perpendicular to the long axis of the
fiber. Running vertically down the Z-line is a small tube called the transverse or T-tubule, which
is actually part of the cell membrane that extends deep inside the fiber. Inside the fiber,
stretching along the long axis between T-tubules, is a membrane system called the sarcoplasmic
reticulum, which stores and releases the calcium ions that trigger muscle contraction
(Freudenrich, 2010).
Joints
Another aspect of the body we had to understand was how joints and tendons work.
Joints are found where two bones meet. They make skeletons flexible and without them
movement would be impossible. There are several different types of joints, such as hinge joints
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like our knee and elbow joints which allow us to bend and unbend parts of our bodies. Along
with hinge joints there are also ball and socket joints which allow for radial movements in almost
any direction. They are found in the hips and shoulders. Another type of joint is the saddle joint,
these allow movement back and forth and up and down, but does not allow for rotation like a ball
and socket joint. Another important joint is the pivot joint which allows rotation around an axis.
The neck and forearms have pivot joints. In the neck the occipital bone spins over the top of the
axis. In the forearms the radius and ulna twist around each other. The last type of join is a gliding
or plane joint which allows bones to slide past each other. Metacarpal and metatarsal joints are
gliding joints. Joints are major part of our musculoskeletal system and enable us to do everyday
physical activities (“The Joints”, 2009).
Gear Ratios
We had to recreate the motions that the human arm and hand can achieve and easy way to
do this was by the use of gear ratios. A common way to transfer power from one axel to another
is the use of gears. Gears are teethed machine parts that mesh with other teethed parts to transmit
motion or to change speed or direction (Brian, n.d). In this project gears are used to increase the
amount of force from a motor, as well increase the speed from a motor. Gears are commonly
used to trade torque for angular velocity. Angular velocity is the rotational speed (revolutions per
minute), and torque is the twisting force on a gear. The relationship is expressed between the
number of teeth, n, and angular velocity, w, at one gear to the number of teeth and angular
velocity of a second gear. The equation is as follows:
n1w1=n2w2
When a big gear is driving a little gear the little gear will spin faster than the big gear, this
is called “gearing up”. Conversely when a little gear is driving a big gear the big gear will rotate
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slower then the smaller gear, but have a increase in torque, this is called “gearing down”. This
relationship is important because if an increase in torque, T, is needed then a decrease in angular
velocity, w, is needed. Likewise if an increase in angular velocity is needed, then there needs to
be a decrease in torque. The equation is as follows:
T1w1=T2w2
After comparing the two formulas, the torque can be related to the number of teeth on the
gear. This can be expressed as:
T1n2=T2n1
(Wang, Catt, 2007).
Sonar Sensor
Another aspect of this project was the use of a sonar sensor in our control system. We
used the sonar system to determine the distance a sliding wall was away from it, this would
trigger a change in motors. The sonar sensors works by sending out a sound wave, when the
sound wave hits an object the sound bounces back to the sensor. This allows the sensor to
calculate how far away the object is. This is how the control system works by determining the
distance of the sliding wall which allows the NXT to switch motors it is powering. Sonar can
also be a sound that isn’t audible to the human ear with out extremely sensitive equipment
(“Underwater Conflict”, 2005).
Methods
Early on in the project, it was decided that the robotic arm (also referred to as DANTE)
would be constructed out of Lego pieces exclusively. Other options included wooden pieces
powered by hydraulics, however this option was abandon due to lack of flexibility and strength.
The arm consisted of five joints/axes: shoulder, elbow, two wrist joints, and the hand grip. A
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basic diagram of the arm can be seen in Figure 1.1, and a picture of the final arm can be seen in
Figure 1.2.
Shoulder
The shoulder joint connected the arm assembly to the base. The joint is similar to a pivot
joint in the human body; that is it allows for rotation along one axis. To build the joint, a
turntable was used to provide good support. The turntable was powered by a worm gear directly
connected to a motor. The worm gear was used due to its superior torque. In a human shoulder,
many muscles contribute to the rotation of the shoulder joint including the latissimus dorsi,
pectoralis major, infaspinatus, subscapularis, teres minor, and teres major. Rather than using
many linear acting muscles though, DANTE uses one circular motor and a turntable.
It was found that, due to the heavy weight of the rest of the arm, the upper arm had a hard
time staying perpendicular to the ground. To counteract this, extra supports between the base and
the upper arm were added. This included a wheel on one side and a slider on the other side. Both
of these provided vertical support without hindering the rotation of the joint.
Arm and Elbow (Hinge)
The arm section consisted of two main parts: the upper and lower arm. The upper arm
connected the base (shoulder) to the lower arm and can be associated with the humerous in a
human arm. At the top of the upper arm is the elbow joint. The elbow joint is a hinge joint that
allows the lower arm to move up and down like a lever. To control the joint, a worm gear was
used again to handle the heavy weights of the rest of the arm. In a human arm, the bicep and
triceps would be used to move the elbow, and while the motor provides circular motion rather
than linear motion, the same result can be achieved. The elbow joint proved to be rather tricky
and required many modifications and reinforcements to be made effective. However, a minor
problem still exists in that the lower arm will wobble for a short period of time after any
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movement. Reconstructing the elbow joint could have fixed the problem, but time constraints
prevented this form being accomplished.
The lower arm corresponds to the radius and ulna in a human body, but has two major
deviations. Rather than using two “bones’ to achieve rotational movement for the wrist, a single
bone was use (rotation would be achieved by a separate assembly). Also, rather than extending in
only one direction from the elbow, the lower arm extends both ways like a crane. The advantage
of this is that, by adding weight to the side opposite the hand, the other side of the arm serves as
a counterweight, which relieves large amounts of stress from the elbow joint. The main weight
used was two NXTs, but later washers were also used to counterweight. Originally, the lower
arm was very short, but as the weight of the hand became heavier, the other side of the lower arm
was extended to properly balance the arm.
A problem arose when the lower arm, which was originally very thin, would bow very
easily. This was solved by adding a sort of truss system with Lego pieces running across the top
of the arm. This created a sort of box support system that prevented the arm from bowing.
Wrist (Hinge, Piviot)
The wrist, which was attached to the lower arm on the end opposite the NXTs, consisted
of two joints. The first joint, which directly connected to the lower arm, allowed the hand to
move up and down. To power the joint, the motor was geared down to increase torque. The
gearing down involved going from a 8 spoke gear to a 40 spoke gear twice. This motor simulates
the actions created by the flexor capri radialis, flexor capri ulnaris, extensor capri radialis, and
extensor capri ulnaris in an actual human which cause the wrist to flex and extend.
The second joint allowed the wrist to rotate. Using the same turntable used in the
shoulder, the joint was powered by a worm gear. In addition to the advantage of torque, the
worm gear also allowed for more fine movement, which is critical for the task of pouring liquids.
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This motor simulates the motions achieved by the pronators and supinator which allow for
rotation at the rodioulnar joints.
Hand
The hand presented an interesting challenge due to the fact that it had to be compact and
light while still accomplishing the goals required. The first design pursued proved to have major
flaws. The fingers were rather small and lack an effective finger tip. Also, a design feature that
allowed the motor powering the hand to be located in the wrist proved to be more hassle than
advantage.
A second hand was developed after learning from the failures of the first design. The
second design was much simpler for the connection between the motor and the fingers. The
motor simulated the motions created by the extensor and flexor digitorum in a human hand
which allow the fingers to open and close. The fingers themselves were longer and possessed
fingertips made of rubber tires that were found to be very effective in picking up objects. The
original fingertips were later redesigned to better grip a cylindrical object such as a graduated
cylinder.
Once construction was completed, a ProE drawing of the entire arm was created. A
screen shot of the ProE model can be found in Figure 1.3. Also, a TinyCAD drawling of the
wiring of the arm was completed and can be found in Figure 1.4.
Control System 1: Front Panel
A proper control system for DANTE required the ability to easily control five motors.
Since there were no sensors on the arm, sensor input viewing or control was not needed. Because
there was five motors, two NXTs were required (each NXT contains three motor ports). To allow
for control of both NXTs, a master slave setup was used. In this setup, one NXT, designated the
master, runs the main portion of a program. The second NXT, designated as the slave, listens for
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instructions from the master. The two NXTs were connected via a wireless Bluetooth
connection, allowing for no direct connection between NXTs to be required.
The first control system developed utilized front panel; a part of Robolab that allows for
the user to input data easily. The front panel used can be found in Figure 1.5. Five number boxes
were used, with each number box corresponding to one of the motors. The number entered
would equal the degrees desired for the selected aperture to move. For instance, if “45” was
entered into the wrist rotation number box, it would result in the wrist rotating 45 degrees, not
just the motor rotation 45 degrees. The whole program can be found in Figure 1.6.
Due to problems with Robolab, real time control of the motors could not be achieved
easily. This presented a weakness of the system. Each time a new number was entered, the
operator would have to upload the program to the master NXT and start the program. This made
for slow reaction time and slow movements. While not ideal, the system still worked.
Control System 2: Ultrasonic Sensor
While the front panel method proved to be sufficient, it lacked a certain amount of
control. To achieve better control, a second system was developed. This system utilized an
ultrasonic sensor (which measures distance) and a small Lego wall built to slide on a track.
Based on the position of the wall, the ultrasonic sensor would give off a different distance. Based
on this distance, the program could determine which motor was being selected. Then, two touch
sensors determine movement: one button makes the motor move forward; the other button makes
the motor move backwards. The ultrasonic sensor setup can be seen in Figure 1.7. The master
slave construct was kept for this system. All sensors were connected to the master NXT. The
master NXT ran the main program and the slave followed instructions given by the master.
A problem discovered late on in testing of the system was the grip. When the power to
the hand grip motor was stopped, the grip strength reduced to the point where it could no longer
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pick up an object. To get around this problem, a third touch sensor was attached to the controller.
This touch sensor, when pressed, would tell the program to activate the grip motor, allowing for
constant pressure to hold an object securely. When pressed again, the program would release this
pressure. The final master and slave programs can be found in Figure 1.8.
There are many advantages to this system. Because the system is separate from a
computer, the NXT does not need to be connected via a wire to a USB port, giving the arm more
freedom. The system is more easy to use since the operator merely needs to move a slider and
push buttons. Also, the system allows for more real-time feedback, because the operator can run
a motor for exactly the right time that he or she desires. One disadvantage is that the arm must be
connected to this control system. This could be avoided by using a third NXT connected to the
control system. This NXT would be the master and the two NXTs would be slaves. This would
be ideal since the control system could be wirelessly connected to the arm, but lack of resources
prevented this modification from being pursued. Ultimately, the ultrasonic sensor system proved
to be a much easier to use and effective system than the front panel system.
Results
Speed of Shoulder
Several tests were preformed on DANTE. The first test preformed was a speed test. The
joint tested was the shoulder joint because it was the slowest and it represents how long it would
take to move an object from one side of a table to another. The test involved measuring the time
it required to rotate the arm 90 degrees. Time was measured via a stopwatch.
Results found that it took 10 seconds for the shoulder to turn the arm 90 degrees. This
means that it would take the arm 20 seconds to move 180 degrees, which would be the farthest it
could move an object from its original location. This speed was not ideal because it meant that it
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would take a while to move objects around, but it did show an advantage. With less speed comes
greater control, which for a robotic arm can be a great advantage.
Strength of Arm
To measure the amount of weight the arm could pick up, a test was run where the arm
lifted varying amounts of weight. A simple rig was setup that allowed the hand to lift a plastic
bag. 10 gram washers were used as weight. Beginning at 100 grams (10 washers), the arm was
made to lift the bag off a desk and then set the bag down again. After each successful run,
another 10 gram washer was added to the bag. This process was repeated until the arm could not
lift the bag.
The maximum amount of weight that could be lifted was 300 grams. 300 grams equates
to 300 ml of water, although including the weight of the graduated cylinder or other vessel used
to hold the water it is estimated that the arm could lift between 200 and 250 ml of water.
Considering that the task required lifting only 100 ml of water, this means that the arm is easily
able to lift the amount of water required. Considering this is less than half of the maximum
quantity DANTE can lift, we can safely say that the arm can both lift the water and hold the
water without straining joints or pieces.
Grip Strength
The final tests run involved measuring the grip strength of the hand. To perform this test,
a Vernier hand grip sensor was used, which uses force pads on opposite sides of a square rod to
measure hand grip strength in Newtons. For testing, the sensor was held in the hand in a manor
that did not apply any force. Using Logger Pro, the data of the force applied was collected for 5
seconds. During the five seconds, the motor powering the hand was activated. This trial was run
three times. An additional three trials were run with a human gripping the sensor as if they were
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holding a graduated cylinder full of water. Two example graphs of data can be found in Figure
2.1.
On average, DANTE put out a peak force of 4.1 N and a leveled out force of 3.4 N. For
comparison, the human grip on average peaked at 9.3 N and leveled out at 5.7 N. However, this
was a light grip, and when the human gripped the sensor more realistically, the force became
approximately 12 N on average. All this data means that the hand can grip adequately, but not on
the level that a human hand can grip. Modifications to the hand in the future could fix this
problem.
Tasks
There were three main tasks required for DANTE to perform. The first involved picking
up a graduated cylinder filled with 100 ml of water, moving the cylinder, and pouring the content
into another vessel (such as a beaker). The arm was successfully able to pick up a beaker full of
water and pour the contents out. Due to the low speed of the wrist rotation joint, high levels of
control were achieved during the pouring process, providing a major advantage for the arm.
The second task involved grapping a piece of thin rope and feeding it through a relatively
small opening. DANTE was able to successfully accomplish this task with relative ease. The
third task was essentially the same, but instead of a thin rope, a piece of string was used. Due to
the smaller size, this task was harder to perform, but the arm was still able to complete the task.
Conclusion
Overall, DANTE proved to satisfy all the requirements made for it. The design had the
advantage of easy control and strength. However, certain joints of the arm were slower than
ideal. Also, grip strength was lacking compared to a humans. Further modifications could
eliminate many of these weaknesses and improve the arm further.
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One unique feature of the arm that had positive results was the ultrasonic sensor control
system. The system allowed for intuitive control of the arm that was much easier to handle than a
computer interface. Also, the system did not require a computer to run, allowing the arm to be
more mobile. This system could also be applied to any number of other robotic devices, such as
ROVs, to allow for greater control and easier human interaction.
Works Cited
Freudenrich, Craig. “How Muscles Work.” How Stuff Works. N.p., n.d. Web. 8 Apr. 2010.
<http://health.howstuffworks.com/muscle1.htm>.
“Muscles of the wrist and hand.” moon.ouhsc.edu. N.p., 18 July 2000. Web. 9 Apr. 2010.
<http://moon.ouhsc.edu/dthompso/namics/handmm.htm>.
“Muscles That Act on the Arm.” Get Body Smart. N.p., 4 May 2008. Web. 4 Apr. 2010.
<http://www.getbodysmart.com/ap/muscularsystem/armmuscles/menu/menu.html>.
“The Joints.” The Shock Family Web Page. N.p., 26 May 2009. Web. 11 Apr. 2010.
<http://www.shockfamily.net/skeleton/JOINTS.HTML>.
“Underwater Conflict .” Exploratorium.edu. N.p., 16 July 2005. Web. 10 Apr. 2010.
<http://www.exploratorium.edu/theworld/sonar/sonar.html>.
Wang, Eric, and Bernard Catt. Engineering with Lego Bricks And Robolab. Knoxville,
Tennessee: College House Enterprises, 2007. Print.
Woodford, Chris. “Gears.” Explain That Stuff. N.p., 18 Aug. 2009. Web. 24 Dec. 2009.
<http://www.explainthatstuff.com/gears.html>.
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Appendix
Figure 1.1
Elbow
NXT
Lower Arm
Upper Arm
Hand
Wrist
Shoulder
Base
Figure 1.2
DANTE
17
Figure 1.3
Figure 1.4
18
Figure 1.5
Figure 1.6
Master Program
19
Slave Program
Figure 1.7
20
Figure 1.8
Master
21
Slave
Figure 2.1
DANTE’s grip
Human grip (light)