Creating a Sensory Input Bio-Prosthetic Prototype

DN Staff

October 8, 2010

8 Min Read
Creating a Sensory Input Bio-Prosthetic Prototype

Currentupper extremity prosthetic technology can be classified into three categories:cosmetic, internally powered and externally powered. Cosmetic prosthetics areused solely to enhance appearance. Internally powered prosthetics (IPP) usemotion of the amputee to function. These systems usually consist of gyros,cables and other basic mechanical systems. Externally powered prosthetics offera wider range of motion as well as more internal functions such as pulley orservo motor mechanics.

Principaldifferences between human limb functionality and mechanical prosthetics arefunctionality, strength, range of motion and reciprocity. Though most of theseissues are being advanced within prosthetics using various materialbreakthroughs, the reciprocity of the human hand is often overlooked.

Thepower of the human hand does not lie solely in the fact that it can manipulateits surroundings, but that its surroundings can also manipulate it. A humanlimb is more than just a mechanized tool; it is a sensory powerhouse thatallows a person to understand his or her environment. The sensory nerves in thehand allow a person to sense stimuli such as temperature, humidity, chemicalstimulation, acceleration and pressure.

Tomimic these capabilities, a communication system to support a mechatronicprosthetic is critical. After all, a nervous system feedback system isessentially a biological mechatronics system complete with wires (nerves),signals and actuators (muscles and tendons). The purpose of this project was toinvestigate the concept of designing a prosthetic hand that could mimic thisreciprocal ability of the human hand, how one would be designed, whatcomponents could be used to create it, and how functional the resulting designcould be.

BasicHand Design

The"Sensory Input Bio Prosthetic Hand" created for this project is a conceptdesign of a sensor feedback system in a prosthetic system. The hand has twomodes: automatic and manual. The modes can be toggled between by flipping themode 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 ofits five fingertips; it moves that finger into a grabbing position. After apause, it moves the finger back to its original position.

Grabbingforce can be controlled to protect an item being grabbed and to protect thehand itself. When set in automatic mode, the force dial on the underside of thewrist can be turned to adjust the maximum force the fingers can impart. If thedial is turned all the way counter-clockwise, then the hand will move itsfingers as long as any level of touch is sensed. If the dial is turned all theway clockwise, it will only move until it feels a slight force resisting itsmovement. For example, if the operatorwants to grab something solid, like a block, he would turn the dial all the waycounter-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 inbetween these two settings by adjusting the dial position between the twoextremes.

Whenthe hand is in manual mode, it only responds to direct input from the user andnot to any input from the environment. A joystick located below the OLED screenon the base of the hand can be manipulated to move different fingers. Each ofthe four directions that the joystick can be moved triggers a certain finger tomove. The finger will continue moving until the joystick is released or untilit 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 currentlybeing moved. Each of the fingers can be manipulated separately to create alarge variety of finger arrangements. To return the fingers to the fully openposition, the joystick is pushed in and held, triggering the fingers to return.

Anytime the hand is in manual mode, touching the fingertips does nothing; when thehand is in automatic mode, moving the joystick in any of the four primarydirections does nothing. The joystick can be pushed whenever the fingers are inthe fully open position, regardless of mode, to display the battery life of thehand on the OLED screen.


Thehand works as a simple mechatronics system. In automatic mode, force sensors inthe 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 theforce dial. If the signal from the fingers is greater, the PIC sends out asignal to the servo control board, which sends a signal to the servo motors located on the back of the hand. The servomotors each have two wires that are pulled when the servo motors turn. Thesewires are attached to the fingertips, one along the front and one along theback. When the wires are pulled one way the finger closes, and when the wiresare pulled the other way the finger opens.

Whenthe mode switch on the underside of the wrist is switched, the servo controlboard no longer focuses on the PIC for its signals. Instead, it focuses on theOLED joystick. When the joystick is pushed in a direction, it sends a signal tothe servo control board telling it which finger to move. The joystick alsosends a signal to the OLED screen to display a corresponding picture that issaved on the OLED.

Thedevice is powered with four battery packs. Upon start up, an intro movie playson the OLED screen introducing the hand. It then moves all the fingers in a setpattern to check that they are all functional.

Reactingto Touch

When the hand is in automatic mode, the only input forfinger movement comes from the force sensors on the fingertips. Force sensingresistors are a polymer thick film device which exhibits a decrease inresistance when met with an increase in applied force. The resistor consists ofthree layers: a flexible substrate with a printed semiconductor, a spacedadhesive and a substrate with printed interlinking electrodes. As force isapplied to the flat surface, the semiconductor squeezes together with theelectrodes, thus causing a decrease in resistance.

This sensing and reacting process is simple, but theresult is substantial and accurate to more than 10,000 Gs. The resistance ofeach of these resistors is monitored using a PIC's pot command, which measuresthe resistance of a circuit based off a resistor-capacitor circuit. The PIC'spot command can only store up to 255 resistance values, so the scale of thePIC's pot command is set so that the highest value of the force sensors, 80 Kohms, reads as 255 on the PIC. During development it was found that, for theforce sensors, the potentiometer scale value is 215.

The force dial is a 10 K ohm potentiometer. When it isturned all the way in the counterclockwise direction, it has a resistance of0.001 K ohms. When turned all the way in the clockwise direction, it has aresistance of 88 K ohm. The PIC reads the resistance of the potentiometer inthe same way that it reads the resistance of the force sensors. The commandscale value for the potentiometer was found to be 40.

The PIC compares the resistances of the force sensors andthe resistance of the potentiometer. If the resistance of one of the forcesensors is higher than the resistance of the potentiometer, the PIC sends out aserial code at a baud rate of 9600. This is received by the servo control boardserial input line which is waiting to receive information at this baud rate.The first character turns the control board into receive mode, the next charactertells it what command it's waiting for and the following bits direct suchfunctions as which servo to move, how fast and how far.

In manual mode, the OLED is programmed to send an incrementalserial signal to the servo control board to allow for accurate and incrementedcontrol of each individual servo. This was accomplished by having only the lasttwo 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 witha small pause. When choosing a finger to move, the OLED displays which fingeris to be moved.

Though the hand project was asuccess, 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 runfor a period of time once triggered, it is impossible to push the force sensorswith enough force to trigger the potentiometer cut-off function before theservo motors are triggered.

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