Electronic Design

On The Other Hand, It's Myoelectric

Healthcare solutions need to be affordable as well as effective. That's what a team of engineering students from ITESO graduate school at the Universidad Jesuita de Guadalajara in Guadalajara, Mexico, decided when they entered their latest project in Freescale Semiconductor's Black Widow $10,000 Design Challenge. The team's myoelectric prosthetic hand not only maintains user comfort and functionality, it also minimizes costs (see the figure).

The contest required teams to use a member of Freescale's S08 family in the design, and the team's hand uses the MC9S08QG4 microcontroller. Yet the project itself has its own requirements. In many countries, government subsidies for the development of such technologies aren't available, requiring designers to choose their components carefully. Also, the technology must be functional, aesthetically pleasing, and affordable.

Myoelectric prostheses measure the small (microvolt) electrical emissions produced by muscles during contraction and relaxation. After a signal is detected, it is amplified, filtered, and rectified to determine what action (if any) should be applied to the prosthesis.

According to ITESO project lead Alan Collins, the hand has four movements: open, close, and right and left rotation. "It was made thinking \[of\] hand amputees that can move their forearm muscles. The forearm muscles directly control the four movements of the mechanical hand," he says.

The movements are determined by four pairs of electrodes that detect the myoelectric impulses from the forearm. The electrodes use a method that's similar to electroencephalograms (EEGs) that measure brain activity. There are four main signal-conditioning stages: amplification, filtering, analog-to-digital conversion, and the motor drivers.

"The brain of the project is an 8-bit \[S08 family\] MCU from Freescale, which receives the converted digital signals and controls the two dc motors in the mechanical hand," Collins says.

Two Freescale MC33887 H-Bridge devices use low-power modes to extend the life-span of the battery that powers the circuitry. They also drive the hand's actuators. A potentiometer regulates the maximum pressure applied to objects being grasped, letting users grab anything from a glass to a tool with the proper force.

Freescale selected the ITESO project as the winner of the Black Widow Design Challenge from a field of more than 775 participants. The entries were judged on creativity, design efficiency, technical complexity, number of Freescale devices used, and overall application innovation and usefulness.

The total cost of the prototype materials was around $1600, including a quick turn on the printed-circuit board. The team plans to use the prize money to find and test lighter and tougher materials and conduct research to design an affordable prosthetic glove.

ITESO: Universidad Jesuita de Guadalajara
Freescale Semiconductor

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