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Electronics

The Essentials

 

Since our prosthetic is robotic, it requires electrically powered components in order to function. These components include physical components (i.e. motors and motor drivers) as well as computational/logical components (i.e. the Arduino microprocessor). The main groups of electronics are:

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  1. Motors and motor drivers

  2. Arduino microprocessor

  3. Electro-Myographic (EMG) sensors

  4. User Interface (LED and button)

  5. Battery

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The main goal of the electronics system is to function as the brain of the prosthetic. The Arduino stores and runs the algorithm responsible for proper functioning of the prosthetic. It takes inputs and based on the algorithm decides on the necessary outputs. (See Algorithm for a more detailed explanation on how this works.) The EMG sensors read voltage levels in different muscle groups, dependent on muscle activation, and transmit that information to the Arduino. Based on this input and the algorithm, the Arduino decides if a motion must be made and if so, which motion. Commands are then sent to the motors through the motor drivers to perform specific motions. In addition to this there is an LED and button that serve as a user interface, indicating certain functions of the prosthetic and allowing the user to provide input, including positive reinforcement feedback to better improve the prosthetic's function.

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The majority of the electronics are wired together on a custom design Printable Circuit Board (PCB). This is a board that serves as a base on which the components must be soldered. To order your own PCB before or after modifications, you can use any PCB manufacturer. Usually this can be done over the internet. We ordered our boards from PCBway. You can also send us an email (see Contact page) requesting that we ship you a PCB (not including any of the other electronics) as they are far cheaper in both manufacturing and shipping costs when ordered in bulk. We plan to stock a number of extras for this purpose, and will ask for compensation only for our costs (as we are not doing this for profit).

Essentials
board.jpeg

First Circtuitboard

circuitboard v1 PCB.PNG

Circuitboard Layout

circuitboard v1 schematic.PNG

Circuitboard Schemati

Details

The Details (for Developers)

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Our PCB was designed using standard and easily available components to make it accessible and easily put together. Theoretically it could be shrunk much more to use surface mounted (SMD) electronics. We soldered our lab versions with female header pin rows for our tests and to allow us to reuse our chips, you could save on height (requiring a different dimension for the housing of the circuitboard) by soldering chips directly to the PCB, although this would make replacement more difficult. The PCB is printed on 1 oz copper as this makes ordering it cheaper ($5 for 10 pieces through PCBway).

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Solder the circuitboard according to the schematics provided.

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Arduino nano was chosen for its size. Polulu dual motor drivers were chosen for size and for their ability to feedback current sensing data. This capability has to be enabled by making cuts and soldering surface mount resistors into the correct locations. See the data sheet for Polulu DRV8833 here.

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Motors are cheap micro-gearmotors. Motors are run until the current sensed through them is above a certain threshold (based on desired grip strength and limited by our current measurement resolution). This eliminates the need for more expensive servo motors and allows flexibility when gripping. As a result, the fingers will close until they feel resistance, naturally forming around whatever object happens to be in the way. 

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The EMG sensors are currently the most complicated and most expensive part of the system. We are using the Myoware muscle sensors (see here). The sensors read muscle movements by measuring voltage differences across the skin and after some built in signal processing (through hardware electronics) that outputs an amplified, rectified, and integrated signal that can be used directly with microcontrollers. The gain of the signal can be adjusted. We are working on better and more comfortable ways to attach them. They require a decent amount of calibration, and are the first thing we recommend checking (using the GUI or a standalone Arduino code, we describe the process in our Build/Use guide. See the datasheet for more information.

processing of EMG signals.PNG

EMG sensor signal processing

Files
Working On

A multiplexer (TI4051) is used to reduce the number of pins required to read the various inputs required for the algorithm to run properly. 

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Files

 

To view and edit the files used in designing the circuitboard you will need:

  1. NI MultiSim

  2. NI UltiBoard

 

Full parts list excel sheets will be added later

Link to datasheets

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What We're Working On

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We are currently working on our next version of the circuitboard, which will allow proper interfacing with the next version of the design of the prosthetic. 

  1. Eliminate the need for an encoder in the PCB

  2. Allow control for six motors (probably a different version of the board)

  3. Choose and implement a battery

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