Changing the World of Prosthetics

Aerospace materials, advanced motor designs, and special safety circuitry eliminate problems in motion-controlled hands.

—edited by Richard Mandel

Substituting a missing limb with a motion-controlled replacement has helped many people live better, more full lives. Nonetheless, technology continues to evolve, reducing the problems associated with under-designed solutions that lack the strength, durability and finesse of our own original flesh and blood.

A prosthesis is put together onto a socket molded from the end of the patient’s remnant limb. Controlling signals for the prosthesis, referred to as myo-electric control, is provided by a series of quarter-sized sensors placed over muscle sites on the remnant limb to which the prosthesis is attached. The muscle signals, which offer the strongest EMG output, are in micro-volt values which are amplified to a useable level.

 Motion Control Inc, Salt Lake City, UT, designs and manufactures upper extremity prosthetic devices and controllers. Their primary product line includes a myo-electric controlled electric elbow, the Utah Arm 2, the Pro Control below-the-elbow hand and wrist microprocessor controller, and the Motion Control Hand. The company also manufactures a variety of other equipment for use in prosthetics. 

Eliminating problems

When Motion Control went into the design phase for their next generation wrist and hand, they focused on user-specified needs first. This method of research and design allowed the company to eliminate some of the nagging problems that made life irritating to prosthetic device users.

For example, one problem described by users was that occasionally the hand would produce a lock-down effect. After grabbing a doorknob or shopping cart handle, if the hand lost power for any reason, it would clamp down, making it nearly impossible to release easily. Obviously, this effect posed a safety hazard, as well as embarrassing or awkward moments. 

Motion Control responded with a mechanical safety release mechanism that, when pushed, would release the drive mechanism and allow the hand to be passively opened. This patented design feature not only creates a safety mechanism, but also provides emotional freedom to the users.

After safety, another common concern that users voiced was the need to get more use out of a single battery charge. Earlier products used circuitry that continued to supply current to the motor, even though the hand could not pinch hard enough to perform its assigned function. This caused an unnecessary drain on battery power. To eliminate this problem, the company incorporated a current limit circuit into their design. Once the current rises above a specified motor stall limit, voltage is cut-off from the device until the user applies the control signal for movement in the opposite direction. 

This circuitry solved only part of the problem relevant to conserving battery power. A second useful solution was to use a mechanical back-lock on the device so that as the power was removed, the pinch force would not weaken. Since constant pinch-force takes up a lot of current, eliminating the need for it added valuable life to the battery.

The motor and transmission

A key advantage of the Motion Control Hand is the capability to be used based on a variety of input voltages ranging from 6V to 18V. This spread is due to the variety of battery packs and types used in such devices, as well as the various speed and torque requirements of the particular end product. This situation means that the motor designed into the device has to easily accommodate the variety of voltages without sacrificing torque, response speed, or accuracy. 

After careful consideration, Motion Control chose to use graphite brushed motors from Maxon Precision Motors, Burlingame, CA. These motors offer the ability to work within the specified voltage range, while also providing the added benefits of compact size and lightweight for ease of movement, and highly efficient operation for increased battery life. Use of graphite brushes allowed the motor life to exceed that of alternative designs. Precious metal-brushed motors will often burn out if a variety of drive voltages are used. 
Motion Control also wanted the hand to respond more quickly than previous models, while providing high pinch forces. When using typical electric motor gear trains, designs often need to sacrifice speed for torque or vice versa. To avoid this drawback, Utah Arm uses an automatic, mechanical two-speed transmission that, as the torque increases when the hand encounters an object, senses the change in torque and automatically shifts gear ratios for high pinch force. When the object is released, it will automatically up-shift to the faster gear ratio. 

The motor is mounted in the hand in a position perpendicular to the axis of the forearm. Through the combination of the two-speed transmission, special gearing, and a belt drive, the hand easily opens and closes when muscle signals are engaged. Clamping articulation is performed though two joints — one connected to the thumb and one connected to the combination of index and middle finger. The other fingers are passive. 

Building for normal use

Advancements in electronics, motors, materials, and other aspects of design have allowed the company to save space in prosthetic limbs. Motion Control has produced a version of the hand that incorporates an in-hand microprocessor control, eliminating the need for an external controller to be worn elsewhere on the patient. 

It’s amazing how often we use our hands and how seemingly durable they are. When Motion Control created their mechanical, motion-controlled hand, they wanted to ensure the prosthesis would have comparable strength. Aircraft grade 7075 aluminum provided the device with a strong and durable material that would rival that of a real hand. 

The covers for the internal components are made of Lexan plastic for high durability and impact resistance. This design allows the company to over-mold the gaskets directly onto the part for a strong, secure bond. To combat harsh dirt- and moisture-ridden environments, the company used o-ring seals and sealed covers. Research is continuing in this area to better protect the hand and its circuitry from the outside environment. Other modifications include fitting the hand with a hook for tasks that don’t require great dexterity. For this, the hook does not require a covering, since it often is subject to even more abuse than the standard hand. 

One of the company’s latest devices, the Flexion Wrist, features 30 degrees of flexion and extension. Additional features of the Motion Control Hand include a short hand that is available for wrist disarticulation, a quick-disconnect wrist, and stronger, longer life motors. Also included in the newer versions are fingers that are reinforced at the base, higher-performance and faster speeds driven by two battery options, a battery-save feature, a wide grip, and a patented safety release feature. 

Motion Control Inc,
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Maxon,
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