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Myoelectric Controlled Prosthetic Hand

Dejan Duvnjak, Dan Hebb, Alexandra Nadeau, School of Information Technology and Engineering

Veronica Wajda, Glen Torontow, Thomas March, Department of Mechanical Engineering

University of Ottawa

 

 

  1. EXECUTIVE SUMMARY

This report describes a myoelectric-controlled partial-hand prosthesis design.  The proposed design senses an electromyogram (EMG) signal from the bicep when contracted.  This EMG signal is amplified and filtered and then sent to a microcontroller.  The microcontroller converts this signal to a pulse width modulated signal to drive the servomotor; which opens the prosthetic hand.  Each stage is analyzed in detail and the report presents simulation and experimental results of these stages. 

  1. INTRODUCT ION/OBJECTIVE

In today’s world there is great need for prosthetics.   Prosthetics is part of the field of biomechatronics, which aims to integrate mechanical, electrical and biological systems.  They are typically used to replace appendages lost to injury, missing from birth, or to replace a defective body part.  Prosthetics are important to improve amputees’ lifestyles and in order to do so there have been many studies aimed at allowing control over the prosthetic appendage.  In this report we will look at a myoelectric-controlled prosthetic hand that opens when the bicep is flexed.    

The proposed design can be split into three parts:

1) receiving/processing EMG signal from the bicep,

2) The microcontroller interpreting the EMG signal,

3) The prosthetic hand. 

The report explains the stages required for reading the EMG signal. These stages are filtering, amplifying, rectifying and the microcontroller interpreting the signal in order to the control the prosthetic hand. 

  1. ELECTRONIC STAGES/CIRCUITS

A.  The Electrodes

There are three electrodes used to control the operation of the prosthetic hand. Two are applied to the bicep and the other is connected to a neutral part of the body to act as a ground. When the subject flexes their bicep their brain sends a signal, also known as an action potential through a series of nerves and excites the muscle fibers to contract. This action potential has a fixed magnitude and speed it can travel through the nerve fibers known as axons. When this action potential travels through the synapse, from axon to axon, there is a chemical reaction that creates a changing electric field as it travels and this field can be detected by a sensor attached to the subject’s skin. Since the action potential is fixed in magnitude and propagation speed it is the frequency of impulses that controls the magnitude of the force your muscles apply. Therefore the electrodes must receive the impulses but filter out noise from frequencies that are outside of the possible rates of signaling of human nerve cells. Even when muscles are inactive they still have a resting potential which is what the ground electrode is going to detect and allow us to remove from the signal.

Fig. 1. Electrodes

B.  The Instrumentation Amplifier

The first stage of the system is an instrumentation amplifier which receives inputs from the two electrodes attached to the subject’s skin. The two electrodes are connected to different parts of the bicep and will receive impulses of 13-15ms in duration and of voltages between 20-20000uv. The instrumentation amp has a very high input impedance and doesn’t require impedance matching which makes the design simple and efficient. The instrumentation amplifier is essentially a difference amplifier which means that it only amplifies the difference between the two electrodes attached to the bicep which should cancel out noise which would be equally affecting both inputs and therefore will not be amplified. This implies that the placement of the electrodes on the bicep must be far enough apart to have dissimilar signals in order to get a coherent output from the instrumentation amplifier. The output of the instrumentation amplifier will be a signal consisting of the signal we are interested in between 50-500Hz and noise which is spread over the entire spectrum of frequencies.

Fig. 2. Instrumentation Amplifier

C.  The Low Pass Filter

The low pass filter is connected to the output of the instrumentation amplifier and is designed to remove frequencies that are above 500Hz. The low pass filter removes the frequencies above 500Hz because that is above the maximum signaling rate of the nerves in human muscles. Therefore any energy in frequencies above 500Hz is noise and will degrade the overall performance of the system if it is not removed.

Fig. 3. Low Pass Filter

D.  The High Pass filter

The high pass filter is connected to the output of the low pass filter and filters out frequencies that are below 50Hz. For the same reasons as with the low pass filter we filter out any frequencies that are below 50Hz because we know that they are noise. Putting a low pass filter is series with the high pass filter effectively creates a band pass filter that only allows frequencies between 50-500Hz to pass through without attenuation; which is the range of frequencies human nerves can transmit signals.

Fig. 4. High Pass Filter

E.  The Precision Rectifier

The precision rectifier is a half wave rectifier that is configured so that the op-amp never goes into saturation due to the diode D1 in parallel with the resister R11. The output of the filter must be rectified so that it can be an input to a comparator. The output of the precision rectifier is a signal that is always positive and consists of a series of impulses when the bicep is flexed and relatively close to zero when the muscle is relaxed.

Fig. 5. Rectifier 

G.  The microcontroller

The microcontroller receives the output of the rectifier and interprets the impulses and converts them to a pulse width modulated signal to control the servomotor.  The microcontroller used was the Arduino Duemilanove which is a cheap and relatively simple to use microcontroller. The code implemented is provided below.

 

  1. MECHANICAL CONFIGURATION

 The prosthesis was designed using a 4-bar linkage to produce clamping motion. The pressure of the clamp is derived from a Hitech HS-311 servo motor. The servomotor produces up to a ±90° rotation based on the programming of the microcontroller.

Inside the servo motor is a simple potentiometer. The potentiometer, or variable resistor, will allow the motor to rotate until the intended position is reached. Once the position is reached the motor will stop rotating. It will only be prompted to move again if a new signal is provided by the microcontroller.

The hand, designed in SolidWorks, was printed to full scale as a template to ease manufacturing. The length of the follower link is based on the open and closed position of the hand. A length too long and the hand would not close; a length too short and the hand would not open.

 

Figure 6. Hand as designed in Solidworks

 

  1. TESTING

 SIMULATION RESULTS

The input voltage is shown in a 5 volts/div scale.  Below is the result after each stage to insure that each is performing as expected.  The scale changes from 5 volts/div to 2 volts/div after the instrumentation amplifier and so we can clearly see that the input voltage has been rectified and amplified for valid inputs to our microcontroller.  

A.  The input voltage

B. The signal after the instrumentation amplifier

D.  The signal after the band pass filter

E.  The signal after the precision rectifier

EXPERIMENTAL RESULTS

The following tests were done with a function generator for systematic testing. The output being tested is the signal going into the microcontroller.

 

From the above results, we see that the band-pass filter is successfully filtering the correct frequencies. 

  1. PROBLEMS

 The first attempt to build the circuit using 741 op-amps did not work. There were very high levels of noise in the output that were big enough to render our EMG signal undetectable. The other sources of significant error were the components used to build the circuit which receives the EMG signal and amplifies it. This circuit was made out of standard 741 op-amps which produced a significant DC offset. The resistors and capacitors were accurate to +- 20% which also added to the DC offset. This offset was compensated for by adjusting the supply voltage to the op-amps which has eliminated much of the offset but lead to unpredictable outputs. The circuit was rebuilt with a high quality instrumentation amplifier and high quality op-amps which reduced the DC offset to zero. The high quality components produced an output with little or no noise and allowed for accurate control the hand.  

  1. CONCLUSION

Overall this project was a success and all of the components performed as expected.  

PICTURES

 

 

 

 REFERENCES

[1]  T. Pan, P. Fan, H. Chiang, R. Chang, and J. Jiang, “A Myoelectric Controlled Partial-Hand Prosthesis Project,” in proc. IEEE Transactions on Education, 2004.

[2]  E. Widmaier, H. Raff, and K. T. Strang, Human Physiology; The Mechanisms of Body Function, 9th e.  New York: McGraw-Hill, 2004.

[3]  H. Takeda, N. Tsujiuchi, T. Koizumi, H. Kan, M. Hirano, Y. Nakamura, “Development of Prosthetic Arm with Pneumatic Prosthetic Hand and Tendon-Driven Wrist”, IEEE, 2009.

[4]  Campbell and Reece, Biology, 6thed.  San Francisco: Benjamin Cummings, 2002.

 

PRESENTATION

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