EMG

In this part of the  project we implemented a surface electromyograph that displays the signal using a TracerDAQ as an oscilloscope.

Electromyography detects the electrical signals that the human body generates to contract muscles. Detecting very low voltages in the milliVolt range on the skin surface is not a trivial task. As shown in figure 1, EMG signals are inherently low frequency signals.  The transmitted EMG data was displayed on a
tracerDAQ we setup to be a virtual oscilloscope.

This circuit will take the muscle potentials via electrodes, amplify them with a gain of 100, and then run the signal through a high pass and a low pass filter to so we get the correct EMG signal frequencies in the pass-band between approximately 10 and 500Hz. Next the signal will be run into computer substation which calculates a Pulse Width Modulation (PWM) duty ratio. The computer sends that information to a Phillips Function Generator, which outputs a PWM signal with the specified duty ratio to the motor controller. The PWM signal then is received by the gate of a MOSFET which adjusts the torque of a 12VDC motor that applies force the user will be pulling against.

Figure 1: Example EMG waveforms taken from different parts of a muscle

Circuit Description

Figure 2. Block diagram of the proposed system for EMG signal acquisition and processing.

The block diagram of the proposed system is shown in Figure 2.The quality of the EMG signal, in part, depends on the characteristics of the amplification process Figure 3.. While there may be several stages of amplification, the most important stage is often described as pre-amplification. Pre-amplification implies the first stage of amplification, close to the signal source.

Figure 3. EMG Test Circuit a). Pre-Amp. b).Differential Amp. c).Low - High pass. d)Rectifire

To implement this design, a LF353H amplifier for the circuit components was used the cutoff frequency for the low-pass and high-pass were 8 Hz, and 440 Hz respectively

 

Figure 4. Output EMG signal of the circuit

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