Current
upper extremity prosthetic technology can be classified into three categories:
cosmetic, internally powered and externally powered. Cosmetic prosthetics are
used solely to enhance appearance. Internally powered prosthetics (IPP) use
motion of the amputee to function. These systems usually consist of gyros,
cables and other basic mechanical systems. Externally powered prosthetics offer
a wider range of motion as well as more internal functions such as pulley or
servo motor mechanics.
Principal
differences between human limb functionality and mechanical prosthetics are
functionality, strength, range of motion and reciprocity. Though most of these
issues are being advanced within prosthetics using various material
breakthroughs, the reciprocity of the human hand is often overlooked.
The
power of the human hand does not lie solely in the fact that it can manipulate
its surroundings, but that its surroundings can also manipulate it. A human
limb is more than just a mechanized tool; it is a sensory powerhouse that
allows a person to understand his or her environment. The sensory nerves in the
hand allow a person to sense stimuli such as temperature, humidity, chemical
stimulation, acceleration and pressure.
To
mimic these capabilities, a communication system to support a mechatronic
prosthetic is critical. After all, a nervous system feedback system is
essentially a biological mechatronics system complete with wires (nerves),
signals and actuators (muscles and tendons). The purpose of this project was to
investigate the concept of designing a prosthetic hand that could mimic this
reciprocal ability of the human hand, how one would be designed, what
components could be used to create it, and how functional the resulting design
could be.
Basic Hand Design
The
"Sensory Input Bio Prosthetic Hand" created for this project is a concept
design of a sensor feedback system in a prosthetic system. The hand has two
modes: automatic and manual. The modes can be toggled between by flipping the
mode switch on the underside of the wrist. When the hand is in automatic mode,
it is simply responding to its environment. Any time it detects touch on any of
its five fingertips; it moves that finger into a grabbing position. After a
pause, it moves the finger back to its original position.
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Grabbing force can be controlled to protect an item being grabbed and to protect the hand itself. When set in automatic mode, the force dial on the underside of the wrist can be turned to adjust the maximum force the fingers can impart. If the dial is turned all the way counter-clockwise, then the hand will move its fingers as long as any level of touch is sensed. If the dial is turned all the way clockwise, it will only move until it feels a slight force resisting its movement. For example, if the operator wants to grab something solid, like a block, he would turn the dial all the way counter-clockwise. If the operator wants to grab something light and fragile, like a flower, he would turn the dial all the way in the clock-wise direction. Many different settings can be attained for the hand's gripping strength in between these two settings by adjusting the dial position between the two extremes.
When the hand is in manual mode, it only responds to direct input from the user and not to any input from the environment. A joystick located below the OLED screen on the base of the hand can be manipulated to move different fingers. Each of the four directions that the joystick can be moved triggers a certain finger to move. The finger will continue moving until the joystick is released or until it reaches the fully closed position. When a finger is moved via the joystick, a map of the hand appears on the OLED screen highlighting the finger currently being moved. Each of the fingers can be manipulated separately to create a large variety of finger arrangements. To return the fingers to the fully open position, the joystick is pushed in and held, triggering the fingers to return.
Any time the hand is in manual mode, touching the fingertips does nothing; when the hand is in automatic mode, moving the joystick in any of the four primary directions does nothing. The joystick can be pushed whenever the fingers are in the fully open position, regardless of mode, to display the battery life of the hand on the OLED screen.
Simple Mechatronics
The
hand works as a simple mechatronics system. In automatic mode, force sensors in
the fingertips detect touch and convert the force of this touch into a signal.
The force dial on the underside of the wrist is connected to a potentiometer,
which turns its setting into a signal. A PIC chip, located in the forearm,
compares the signal coming from the fingers and the signal coming from the
force dial. If the signal from the fingers is greater, the PIC sends out a
signal to the servo control board, which sends a signal to the servo motors located on the back of the hand. The servo
motors each have two wires that are pulled when the servo motors turn. These
wires are attached to the fingertips, one along the front and one along the
back. When the wires are pulled one way the finger closes, and when the wires
are pulled the other way the finger opens.
When
the mode switch on the underside of the wrist is switched, the servo control
board no longer focuses on the PIC for its signals. Instead, it focuses on the
OLED joystick. When the joystick is pushed in a direction, it sends a signal to
the servo control board telling it which finger to move. The joystick also
sends a signal to the OLED screen to display a corresponding picture that is
saved on the OLED.
The
device is powered with four battery packs. Upon start up, an intro movie plays
on the OLED screen introducing the hand. It then moves all the fingers in a set
pattern to check that they are all functional.
Reacting to Touch
When the hand is in automatic mode, the only input for
finger movement comes from the force sensors on the fingertips. Force sensing
resistors are a polymer thick film device which exhibits a decrease in
resistance when met with an increase in applied force. The resistor consists of
three layers: a flexible substrate with a printed semiconductor, a spaced
adhesive and a substrate with printed interlinking electrodes. As force is
applied to the flat surface, the semiconductor squeezes together with the
electrodes, thus causing a decrease in resistance.
This sensing and reacting process is simple, but the
result is substantial and accurate to more than 10,000 Gs. The resistance of
each of these resistors is monitored using a PIC's pot command, which measures
the resistance of a circuit based off a resistor-capacitor circuit. The PIC's
pot command can only store up to 255 resistance values, so the scale of the
PIC's pot command is set so that the highest value of the force sensors, 80 K
ohms, reads as 255 on the PIC. During development it was found that, for the
force sensors, the potentiometer scale value is 215.
The force dial is a 10 K ohm potentiometer. When it is
turned all the way in the counterclockwise direction, it has a resistance of
0.001 K ohms. When turned all the way in the clockwise direction, it has a
resistance of 88 K ohm. The PIC reads the resistance of the potentiometer in
the same way that it reads the resistance of the force sensors. The command
scale value for the potentiometer was found to be 40.
The PIC compares the resistances of the force sensors and
the resistance of the potentiometer. If the resistance of one of the force
sensors is higher than the resistance of the potentiometer, the PIC sends out a
serial code at a baud rate of 9600. This is received by the servo control board
serial input line which is waiting to receive information at this baud rate.
The first character turns the control board into receive mode, the next character
tells it what command it's waiting for and the following bits direct such
functions as which servo to move, how fast and how far.
In manual mode, the OLED is programmed to send an incremental
serial signal to the servo control board to allow for accurate and incremented
control of each individual servo. This was accomplished by having only the last
two packets of information sent by the serial signal to direct for position.
These packets of information can be set in a loop to iterate increasingly with
a small pause. When choosing a finger to move, the OLED displays which finger
is to be moved.
Though the hand project was a
success, we were not able to get the potentiometer to work exactly as intended.
The program works as designed, but since the servo motors are programmed to run
for a period of time once triggered, it is impossible to push the force sensors
with enough force to trigger the potentiometer cut-off function before the
servo motors are triggered.
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