Affordable Prosthetics for Seamless Home Integration and Independence
Jun 2023
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Through a combination of innovative design, analog signal processing methods. A working hand prosthetic can be developed using 3D-printing technology.
Creating an EMG sensor from stretch is relatively straightforward process... though most of the time it never really is. It entails using Analog Signal Processing and Amplification methods to transform a noisy raw EMG signal from the forearm into a usable binary signal that can control a servo motor for a prosthetic hand.
When a muscle contracts, it generates electrical signals called action potentials. These action potentials propagate along the muscle fibers and cause changes in the electrical potential of the surrounding tissue. Electrodes are placed along the selected muscle fiber to detect these action potentials, which are very low voltage signals also prone to noise.
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The schematic details the basic connections of the EMG circuit, using an AD623 Instrumentation amplifier (Differential Amp) to measure the voltage drop between the Red (+ve) and Black (-ve) electrode along the forearm. Therefore, we chose to feed approximately 2.5v into the reference pin of the IC as this would create a 2.5v offset in the output since we cannot supply negative voltage in the battery.
Using an oscilloscope, we observed that the raw EMG signal was in the 100uV range, which required further amplification in later parts of the circuit.
EMG signal exists in a frequency range of 10Hz - 10kHz, however only 10 - 150Hz of the signal actually usable and important. Signal noise generated by the lights is about 60Hz. Therefore a bandpass filter was employed in a range of 60Hz - 160Hz to avoid noise and still capture the integrity of the signal.
A half-wave rectifier is a simple circuit that converts an alternating current (AC) signal into a pulsating direct current (DC) signal. It uses a diode as the key component to exclude negative portions of the signal.
If the input signal exceeds the reference voltage (2.5v), the comparator will output the inputted maximum voltage (5v), otherwise go to the negative rail (0V).
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In the initial stages of development, stages of the circuit were design and checked using falstad, the above simulation circuit is not an exact replication of the final circuit design, but it confirms our initial tests of expected behaviours from our circuit.
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