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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
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.
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
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 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 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 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 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 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. This motor simulates the motions achieved by the pronators and supinator which allow for rotation at the rodioulnar joints.
Hand