Recent, major improvements that eliminate numerous problems in the design of motion controlled limbs have come from a better understanding of users’ needs. Add to this knowledge base the application of aerospace materials, advanced motor designs, and special safety circuitry, and you have the latest generation of prosthetic devices. Motion-controlled wrists and hands have helped many people with missing limbs live better, fuller lives.
Designing and building prosthetic devices is a highly specialized business. First, prosthetic devices are assembled from a socket that is molded from the user’s remnant limb. This lets specialists attach the prosthetic device and electrodes to a myo-electric controller. Myo-electric controls connect to a series of quarter-sized sensors placed over muscle sites, which generate µV signals for operating the controller.
Control Inc., Salt Lake City, Utah, designs and manufactures upper
extremity prosthetic devices and controllers. Their primary product
lines include 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.
One problem that occasionally cropped up in early designs was that a hand would experience a lock-down effect. After grasping a doorknob or shopping cart handle and the hand lost power, it would clamp down and be nearly impossible to release. This effect posed a safety hazard as well. A solution to the problem is a mechanical safety over-ride feature that releases the drive mechanism and lets the hand open passively. This feature is not only a safety mechanism, but frees the user emotionally as well.
Another serious concern was the need to have a single battery charge last longer. Earlier
devices used circuitry that continued to supply motor current 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, engineers at Motion Control incorporated a current limit circuit.
After 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.
However, this circuitry solved only part of the problem of conserving battery power. A second solution uses a mechanical back-lock on the device so when the power was removed, the pinch force would not weaken. Since constant pinch-force draws substantial 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 its capability to be used based on a variety of input voltages ranging from 6 to 18V. This wide operating voltage range is based on the assortment of battery packs used in such devices, as well as the various speed and torque requirements of the particular product. This means that the motor designed into the device has to accommodate a range of voltages without sacrificing torque, speed, or accuracy.
After carefully considering the alternatives, engineers selected Maxon’s RE 16 motors with graphite brushes. These motors offer no-load speeds to over 14,000 rpm, stall torques up to 31mNm, and continuous torques to over 5mNm. They operate within the specified voltage range, while providing the added benefits of compact size and little weight. This makes motion control easier with higher efficiency for increased battery life. Graphite brush life exceeds that of precious metal brushes that often burn out when operated with a variety of drive voltages.
The range of motion for the prosthetic hand closely follows that of the human hand.
In addition, the hand had to respond quickly with high pinch forces. With typical electric-motor gear trains, the device would need to sacrifice speed for torque or vice versa. Motion Control’s Utah Arm uses an automatic, mechanical two-speed transmission that senses the increase in torque when the hand contacts an object and automatically shifts gear ratios for high pinch force. When the object releases, the transmission automatically shifts up to the next higher gear ratio. The motor mounts in the prosthetic hand in a position perpendicular to the axis of the forearm. The two-speed transmission, special gearing, and a belt drive, let the hand open and close when muscle signals are engaged.
It’s amazing how often we use our hands and how utterly durable they are. When Motion Control created their mechanical, motion-controlled hand, they had to provide a device with equal strength to that of a human. They chose to use an aircraft grade 7075 aluminum, which rivals that of a real hand.
Further concerns about hand operation included the harsh dirt and moisture-ridden environment that exists in our everyday world. To combat this environment, the company used o-ring seals and sealed covers to create a watertight environment for the hand. Additional research is continuing in this area to protect the hand better and its circuitry from the outside environment. Further modifications to the hand will make it waterproof.
clamps use two joints, one connected to the thumb and the other
connected to the combination of index and middle finger. The other
fingers are passive.
Advancements in electronics, motors, materials, and other aspects of design have allowed the company to save space in the prosthesis as well. Motion Control has produced a version of the hand that incorporates an in-hand microprocessor control. This breakthrough in technology eliminates the need for an external hand controller in many arms.
Other changes include fitting the hand with a hook that does not require a covering. This means that it will be subject to even more abuse than the standard hand. The covers for these new devices are made of Lexan plastic for high durability and impact resistance. Furthermore, the gaskets can be over-molded directly onto this material for a strong, secure bond.
A new device, the Flexion Wrist, features 30°of flexion and extension. The Pro Hand version, which can be used with a competitor’s component as well, can upgrade a patient’s system without requiring an entirely new prosthesis. Additional features of the Motion Control Hand include a short-hand that is available for wrist disarticulation, a quick-disconnect wrist, and stronger, long-life motors. Also included in the newer versions are fingers reinforced at the base, faster speeds driven by two battery options, a battery-save feature, a wide grip, and a patented safety release feature.
Information for this article was provided by Motion Control, Inc., Salt Lake City, Utah. www.utaharm.com.
For more information on the Maxon Precision Motors see: www.maxonmotorusa.com.
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Filed Under: Medical, Materials • advanced, Mechanical, Motion control • motor controls, Mechatronics, Motors • servo