Electromyography

Project Overview

Measuring muscle activation via electric potential, referred to as electromyography (EMG), has traditionally been used for medical research and diagnosis of neuromuscular disorders. However, with the advent of ever shrinking yet more powerful microcontrollers and integrated circuits, EMG circuits and sensors have found their way into prosthetics, robotics and other control systems.

This part of the tutorial provides instructions to understanding and working with EMG from the basic theory to teaching you the knowledge to build practical projects interfacing microcontrollers with EMG sensors. The tutorial contains theory explanations, hardware, and software procedures/experiments to provide a foundational level of understanding while you will apply and demonstrate your knowledge through a project based lab and tasks.

By the end of this section, the projects you make will show your proficiency in many aspects of EMG such as hardware (wiring, circuitry, and troubleshooting) and software (signal analysis, processing, and information visualization).

In this project, we will use the following materials and tools:

• 1 Breadboard

• 1 STEPico microcontroller

• 1 AD8221 sensor module with electrodes attached to it

• 1 MEGO Dual

• 1 1.54” 240x240 pixel LCD display

• Multiple DuPont wires

• 2 USB-C cables

• (optional) 1 MEGO

• (optional) 1 Zoolark

Circuiting Instruction

The AD8221 is a high performance instrumentation amplifier. It is designed to accurately measure weak signals. AD8221’s design allows it to reject wideband interference and line harmonics, greatly simplifying filter requirements. Possible applications include precision data acquisition, biomedical analysis, and aerospace instrumentation.

Unlike the previous projects, the AD8221 module is not dedicatedly designed for electromyography. Before we can begin wiring and working with the sensor, we need to understand what the 5 pins on the module do.

The three on the left are for powering, while the two on the right are for signal transferring. This module needs a +Vs and a -Vs to work properly, which can be seen both in the image above(Figure 3.3.2). So, there must be a potential difference of 9V between +Vs and GND as well as between GND and -Vs. To achieve that, you can either use two separate MEGO power sources, each responsible for two pins; or you can use a MEGO dual, which acts as an extension of MEGO, to achieve the same goal.

+Vs	Provide a 9V potential difference	Anodal of one MEGO	VCC+ on MEGO dual
GND	Grounding	Cathode of one MEGO Anodal of another	GND on MEGO dual
-Vs	Provide a -9V potential difference	Cathode of another MEGO	VCC- on MEGO dual
SIG	Send out the amplified signal of the EMG	Any GPIO pin with ADC capability on pico	Same
GND	Grounding	Any GND pin on pico	Same

To finish wiring, we also need to attach 3 electrodes to our body and connect them to the AD8221: red and yellow electrodes at the proximal end of a limb or a muscle, while green one at the distal end.

The last step is to wire the st7789 display. Following the figure to complete the circuit.

After uploading program into STEPico, you can see obvious changes in the indicator when you do actions.

The bar will also show a certain height even if the electrode pad is not connected. This is because the signal read by the ADC will not be a perfect zero. Even if the electrode pad is not connected, there will still be some noise, power supply coupling or floating input effects that cause the ADC reading to be non-zero. In the above figure, using the Zoolark oscilloscope could visualize the noise signal.