Low cost bio-robotic system using biometric signals : a thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering in Mechatronics at Massey University, Manawatu, New Zealand
The high cost of bio-controlled prosthetic devices is prohibitive to the general amputee population. This causes the majority of amputees to be forced to use mechanical or passive prosthetic devices which provide little to no extra functionality to the amputee’s residual limb. The mechanical prostheses can actually cause damage to other parts of the amputee’s body by the way they are mounted and actuated. By contrast, bio-controlled prostheses actually improve the functionality and quality of life to the amputee with little to no adverse side effects. The mounting device does not cause injury to other parts of the amputee’s body and the amputee is able to lead a near-normal life. The barrier to these devices however is the price. In most cases, amputations involve a third party to pay for medical care and rehabilitation through either government funding or medical insurance. These organisations don’t want to spend lots of money for every amputee and therefore have a maximum expenditure unless a special case is made. A passive or mechanical prosthesis is commonly able to be obtained for less than $3000, the price for a bio-controlled prostheses however is upwards of $5000. This maximum expenditure only qualifies most amputees for the mechanical type prostheses. This research funded by the Dick and Mary Earle Scholarship aims to break the cost barrier to the bio-controlled prosthesis by creating a bio-controlled device for a competitive price.
A proof of bio-controlled prosthesis design and a prototype testing platform was achieved using a series of low cost manufacturing and electronic techniques. The prototype was required to provide competitive functionality to the current bio-controlled prostheses on the market while retaining a similar cost of the mechanical prosthesis. To keep the cost of prototyping to a minimum a physical test platform was manufactured in-house at Massey University using the mechanical workshop and manufacturing technologies available. The mechanical prototype was first designed in a Computer Aided Design (CAD) package, and then transferred to a Computer Aided Manufacturing (CAM) software package. The resulting program was then loaded into the Computer Numerical Control (CNC) machining centre where the machine would follow the provided program and manufacture the required part out of aluminium. The CNC machine however was unable to create all of the mechanical parts used in the prototype prosthesis and some manual machining was required to bring the design to completion. The final mechanical system was functionally sound but lacked aesthetic appeal as it is a prototype testing platform and could be improved by changing manufacturing technologies. An alternative to the conventional manufacturing processes available to Massey was laser sintered 3D printing of titanium alloy. By changing from “material removing” technologies to “material adding” manufacturing the shapes of the prototype components would be able to be dramatically changed for both aesthetic qualities and for improved mechanical properties. Based on the prosthesis cost study, the titanium alloy would provide a lighter, stronger and more durable base for the prosthetic device for a similar price.
The prototype was to be controlled by Electromyography (EMG), a method of detecting electrical potential across a muscle when it is activated. When an amputation occurs the muscles controlling the lost limb are commonly left intact and the amputee is able to still control these muscles without any extra training. EMG produces a small differential voltage when the target muscle is activated. The ability to read the changes in potential provides the opportunity to use this signal as a control mechanism. The current market proprietary EMG sensors are part of the reason why bio-controlled prosthetic devices are
out of reach for so many amputees. These sensors cost above $500 each and include filtering and signal conditioning. This cost was dramatically reduced by creating an EMG sensor from scratch. The market EMG sensors output a signal that is suitable for pattern and feature recognition for high-level control. Eliminating the need for this high-level analysis in the control system means that the quality of signal and filtering was not required to be as stringent. Instead of outputting a high fidelity waveform, the conditioning circuit outputs a slow moving averaged waveform that is suitable for the input to a microcontroller. In such a way the overall sensor and conditioning circuit costs were reduced by almost half.
The control system was designed around an “accurate enough” mentality so the developed prosthesis would be able to provide a suitably accurate performance without spending excessive amounts of money. Because the majority of the processing was completed in hardware before the EMG signal reached the microcontroller, the specifications of the microcontroller were not onerous. This allowed the purchases of a relatively inexpensive microcontroller, a further cost reduction. The level of control required by the “accurate enough” control method was very limited, only requiring: two EMG inputs, five motors and five current sense modules. The two EMG inputs are used to activate the prosthesis in the forward and reverse directions. The action is undertaken by the five motors, one for each finger. To prevent the damage to the gripped objects and the motors, the current sensing modules are used to detect both the force and stalling of the motors. The only calculation that the microcontroller is required to perform is to compare the EMG signal and the current sensing inputs to the predefined threshold values.
The overall developed system was able to achieve the desired functionality for an overall price of $3,770. This price is not as expensive as other bio-controlled prostheses and is close to the price of the current hook prosthesis. At this price point a strong case could be made to the third party purchasing organisations to purchase a higher functionality prosthesis to greatly improve the quality of life for the amputee community. Taking into account that an initial prototype is inherently more expensive to produce than a commercial variant due to economy of scale, there is much promise for future revisions to become more competitive in the bio-controlled prostheses market.
The research reported in this thesis has published two articles in two international conference proceedings and also won a runner up best presentation award at one of these conferences.