Spring 2017 Prosthetic Arm: Final Document

The Robot Company | CEO Professor Gary Hill

Blog Post Created by Project Manager | Bianca Esquivel

Mission, Systems, and Test Engineer | Phuong Tran Electronics and Control Engineer | Mikael Movsisyan

Design and Manufacturing | Cedric Yannick Mbetga

Executive Summary

By Project Manager | Bianca Esquivel

Project Objective

There are 3 main project objectives that address the problem of the customer being born without a right arm and cater to his preferred recreational activities:

Being able to operate a computer mouse by supporting and rotating the wrist that joins the prosthetic hand to the prosthetic arm.
Being able to pick up and drink a cup of water by allowing for proper wrist rotation and supporting the weight of both the prosthetic hand and the cup of water.
1. Ability to hold more than 8 oz. of water is a plus.
Being able to pick up and eat a chips ahoy cookie by allowing for proper wrist rotation and supporting the weight of both the prosthetic hand and the cookie.

Mission Profile

Mission Profile from Start to Finish:

Customer will sit down at a desk in front of a computer with a cup of water and a plate of cookies within the reach of his prosthetic arm
The computer will have a game of minesweeper ready to play
With the prosthetic arm he will be able to rotate his wrist to properly operate the mouse to play a game of minesweeper and be able to reset the game should he lose or restart the game should he win
He will be able to rotate his wrist in order to properly take hold of the cup of water within his reach to drink from at his leisure
He will be able to rotate his wrist in order to properly take hold of a chips ahoy cookie within his reach to eat one at a time at his leisure

The Design

Switch used to power the arm and hand on/off
Comfortable cuff fit for the customer
Room for PCB, Battery, and MCU in the limited space of the forearm
Room for the servo to rotate the wrist

Prosthetic Arm Design Image

Project Features

Wrist Rotation of 90 degrees
Personally designed to match the customer’s arm measurements
Comfortable arm attachment, unlike the over the shoulder strap that was often used before
PLA Material due to project budget
EMG (Electromyography) Sensor used to control the movement of the wrist
Most of our parts are off the shelf products to keep the budget low, while still keeping the form factor of the prosthetic arm

System Design

By Mission, Systems, and Test Engineer | Phuong Tran

System Design: Mechanical Perspective

Gray: Humeral Suspension Cuff

Red: Upper Forearm

Yellow: Lower Forearm

Green: Wrist

System Design: Electrical Perspective

System Interface

Link to Interface Definitions:

Prosthetic Arm Interface Definitions

Link to Interface Control Document:

Prosthetic Arm Interface Control Document

Cable Tree:

Link to Mission Command & Control:

Prosthetic Arm Mission Command & Control

Electronics Design

By Electronics and Control Engineer | Mikael Movsisyan

Buck Converter

LTSpice Simulation Buck Converter

Electronics Custom Parts

LT3971-5 Buck Converter from Linear Technology

Purpose:

To satisfy L2.2.1 – Wrist Motion
To step down 11.1V to 5V

Rationale:

Running current MG996R: 500-900mA (from datasheet)
Arduino Micro 5V pin rated 200mA
VCC fluctuations (Appendix 1.)
Voltage regulators (LM7805) tend to overheat

Reason for specified IC:

Relatively cheap
>1V logic (Enable Pin)
LTSpice simulation showed constant 5V output up to 1.2A

TI’s LM34 Temperature Sensor

Purpose:

To satisfy L2.4.1 Safety – Temperature
Measure analog temperature proportional to Fahrenheit

Rationale:

Above threshold analog input from sensor will cause MCU to power-down

Reason for specified IC:

Can measure required temperature 50oC i.e. 122F (Appendix 4)
Used through-hole component in 370L
Ease of use

GRS-4011 Switch

Purpose:

To satisfy L2.4.2 Safety – Kill Switch
Switch off supply to MCU (Appendix??)

Rationale:

User can manually shut-down prosthetic arm

Reason for specified component:

Available at hand
Current/Voltage rating 16A/125V

Sparkfun’s 2 DOF IMU with ADXL203 Accelerometer

Purpose:

To satisfy requirement from Con Ops
To measure orientation

Rationale:

Wrist can only turn in specified orientation (Appendix 3)

Reason for component:

Available in inventory
Analog outputEase of use

TowerPro MG996R Servo

Purpose:

To satisfy L2.2.2. Motion

Reason for component:

Trade-off-Study
Load Test

Firmware: Main Program

Firmware Flow Chart

FSM States Overview

Sleep Interrupt Firmware

Sleep Interrupt Flow Chart

PCB Schematic

Original Design Components

Arduino Micro
LM34 Temperature Sensor
MyoWare EMG Sensor

Unspecified components

Servo
Switch
LiPo Battery

Added Components

Buck Converter
IMU

Prosthetic Arm Fritzing Diagram

Breadboard Model

Rapid prototyped 3D design
MyoWare EMG sensor & IMU analog readings are used to control servo
Light-dependent voltage divider simulates analog reading from a temperature sensor. Used with sleep ISR to power-down MCU.

Rapid Prototype Breadboard Layout

Eagle CAD Schematic

PCB Layout

Hardware Design

Attachment Module

In order to meet the following requirement

L1-1 – Attachment
L2-1-Mass Management
L2-1-2- over shoulder strap

The module shown in figure below was created and modeled after the patented humeral cuff displayed below. Most of these parts were designed in solidworks and printed out. However the ¼ metal rods for the humeral cuff were purchased.

Solidworks Annotated 3D Exploded View

Solidworks Annotated Attachment Module

Humeral Suspension Cuff Module

Humeral Suspension Cuff Mounted on Socket

Cuff Sitting on Humerus Bone

Wrist Rotation Mechanical Design

The rotation of the wrist is controlled by the servo MG996R which was located in the lower arm. The 90 degree turn (L2-2-1 requirement) of the wrist was met by applying the appropriate gear ration of 17/11= 1.55.

Two gears are designed:

Driven gear with 17 teeth
Driver gear with 11 teeth

To avoid the prosthetic hand wire bundle hanging out of the arm, a hole was drawn through the driven gear such that the bundle could travel through it.

Driven Gear and Driver Gear

Servo Used

Cable Tree Hand & Arm Integration

Mechanical Connection

The prosthetic arm was assembled after final printing. This will be done using M3 screw and hex nuts. Hence hex houses were modeled in solidworks to house these nuts. Some of the advantages of these houses are that they: