On a New Electromechanical Hand for Young Children

AL-TEMEN, W. LITEROWICH, M. MIFSUD AND M. MILNER*Toronto, Ontario


Introduction

The available range of externally powered prosthetic devices for children, particularly in the 2-to-6-year old bracket, has been limited, very likely for economic reasons. At the Ontario Crippled Children's Centre (OCCC) a large population of upper-extremity amputees is seen. In recent years more attention has been focused upon the needs of the younger age group. While the existing Systemteknik hand provides a fine solution for the needs of many of the children, added advantages would be lighter weight and readily and economically replaceable components. Earlier fittings might well be possible in some instances. A newly designed electromechanical hand for preschool children is in the final stages of testing prior to manufacture and marketing by the Variety Village Electrolimb Production Centre which is managed by OCCC. Features of the hand include a glass-fibre-reinforced plastic body, readily replaceable fingers and thumb and a new braking system which enables locking on hand closure. The brake can be bypassed manually to allow for opening and closing of the hand using a screwdriver after the cosmetic glove has been rolled down. The drive system is comprised of an integral, readily removable, repairable and replaceable motor-gearbox assembly. A highly efficient bridge circuit and an optional energy-saver circuit for myoelectric control are included.

Design Considerations

Design suggestions were solicited from physicians, prosthetists, therapists, parents, users and others. Special emphasis was placed upon the issues of reliability, weight, ease of manufacture, cost of assembly and serviceability.

It was decided to fabricate most hand parts using a plastic material in short-run injection-molding tooling. By encapsulating the motor and related drive mechanisms in a single metal sleeve, the needs for precision in these parts and their fit in the hand were addressed. The size of the drive was minimized to reduce weight. All parts of the drive are accessible and repairable.

A one-way anti-rollback mechanism was incorporated to insure locking upon closure at any position. Provision was made for the fingers to be manipulated manually without degrading the anti-rollback feature. This is accomplished by rolling the glove down far enough to expose a hole on the hand body and inserting a small screwdriver to engage with the main drive spindle which could then be turned to open or close the fingers.

The wrist unit is an integral part of the hand assembly and incorporates an adjustable friction collar.

The hand body houses the power bridge; the wrist unit houses the energy-saver circuit. Both modules are accessible for trouble- shooting and repair. The power bridge controls the direction of current flowing through the motor windings. To achieve high electrical efficiency and low voltage losses the bridge system utilizes miniature relays developed for military purposes which for typical use would have a projected life of about four years for 1000 daily cycles.

The energy-saver circuit responds to the time taken for the hand to move its fingers together or apart over the whole range of movement. Command-signal levels from the electro-myographic processor indicate the start of finger movements and provide power for the energy-saver circuit which draws no current in the absence of a command signal. Essentially, motor power is discontinued after the maximum opening or closure time is reached and can be restored once the command signal is removed, that is, myoelectric control signal is absent. After early tests on several juvenile users, battery life has been extended by 50-100 per cent leading to a day and a half of use.

The hand assembly is supplied complete and ready for attachment to the relevant prosthetic element.

Specifications Attained

The motor operates on 6 volts and has built-in 15:1 gear reduction. The first stage of gearing uses spur gear reduction.

An epicyclic gear-reduction unit is used for compactness.

The braking system design emphasizes reliability.

The hand body is a single-piece plastic injection-molded part. The material used is Acetal with 25 per cent glass.

The hand cover is an injection-molded part made out of Acetal. It serves to house the power bridge.

First and second fingers are common for left and right hands. They and the thumbs are made of Zytel. A cosmetic glove needs to have the remaining fingers packed with a suitable filler.

The maximum (stall) current is 34OmA, the pinch force with the glove fitted is 1800 gm, opening and closing times are each 1.0 second and the weight is less than 185 gm. The length without the wrist units is 9 cm and the maximum width 5.4 cm.

Intensive bench testing has been done with the hand. The first prototype ran 250,000 cycles before signs of pinch-force degradation were noticed. Pertinent corrective action was taken at this and subsequent stages to refine the material choices and mechanical finishing. It is anticipated that annual servicing will be required for production models.

User Experience

A 6-year-old boy aggressively used developmental prototypes over a period of two years. In several months of use with the prototype production model only minor problems, such as wire breakages and glove breakdowns, have been reported. He indicated that he likes the grip and can tie his shoelaces well with it. Steps have been taken to correct the identified deficiencies.

Conclusions

The new hand is appropriate for wide distribution.

The technology has been transferred to the Variety Village Electrolimb, Production Centre for manufacture and marketing.

Acknowledgment

We gratefully acknowledge the generous support of the Variety Club of Ontario (Tent 28) for this project.

*Rehabilitation Engineering Department, Ontario Crippled Children's Centre, 350 Rumsey Road, Toronto, Ontario M4G 1R8 Canada