IMPROVED CONTROL OF BODY-POWERED PREHENSION
Lawrence E. Carlson, D.Eng.
University of Colorado at Boulder
For a variety of reasons, body-powered upper limb prostheses are still widely used worldwide. One significant reason is certainly the feedback the amputee receives when operating such a system. Body-powered prehensors can be classified as either voluntary opening (V.O.), meaning the prehensor is normally closed and opens with tension in the control cable, or voluntary closing (V.C.), in which the prehensor fingers are open until pulled closed with cable tension.
This paper discusses the relative merits of body-powered prostheses, as compared to externally powered ones, and the relative merits of voluntary opening vs. voluntary closing prehensors. Specific projects which offer improved body-powered prosthetic prehension are described:
Most upper limb prostheses are either body-powered or electrically powered. Both systems have advantages and disadvantages. However, the low cost, high reliability, light weight and simplicity of body-powered systems, combined with the kinesthetic feedback provided by the harness system, argue that body-powered prostheses have a place for the foreseeable future. This in turn justifies continued efforts to improve these systems to make them lighter weight, more functional and more efficient.
Body-powered prehensors can be operated in either a voluntary opening or voluntary closing mode. V.O. prehensors are more widely used than V.C. devices, probably because of the convenience of being able to maintain a grip force without any effort. However, the inverse relationship between cable tension and grip force makes it difficult to adjust the grip force of most V.O. prehensors. V.C. prehensors are more natural and grip force can be easily modulated. However, the necessity of maintaining tension in the control harness to maintain grip is awkward and tiring. A reliable and efficient holding assist, which would solve the latter problem, has not yet been perfected. This paper describes several projects that are intended to improve both types of body-powered systems.
USE OF SPECTRON 12 CABLE FOR HIGH-EFFICIENCY FORCE TRANSMISSION
A body-powered prosthesis should be as efficient as possible. The typical stainless steel aircraft cable routed inside a wound cable housing can be less than 50% efficient, depending on the total angle of wrap1. SpectronTM 12 cable is manufactured from SpectraTM fiber, an ultra-high molecular weight (UHMW) extended chain polyethylene fiber. Spectra fibers exhibit high tensile strength, high toughness, high abrasion resistance and good ultra-violet radiation resistance. Using specially designed end fittings, Spectron 12 has been successfully used in clinical tests at 16 prosthetic clinics in the U.S.2 In a typical prosthetic system, the efficiency of the Spectron 12 cable in a Teflon liner is about 82%, vs. 61% for steel cable. This translates into smoother, easier prosthesis operation.
VARIABLE MECHANICAL ADVANTAGE V.C. PREHENSOR
Prehension has two distinct phases: sizing the fingers in order to grasp an object, and gripping the object by increasing the finger force to a level which will prevent slipping. Optimum sizing requires rapid finger movement, but little force. Gripping, on the other hand, requires large forces but, for most applications, little finger excursion. Since mechanical work is force exerted through a distance, neither phase of gripping should theoretically require much work to be done, which opens the possibility of highly efficient prosthetic prehensors. For example, the synergetic prehensors developed at Northwestern University3 are electrically powered prehensors with two opposable moving digits. One moves very rapidly to adjust the finger opening, but can exert minimal force; the other slowly builds high levels of grip force when an object is encountered. The resulting prehensors have speed and grip force comparable to the human hand, but are also light weight and efficient.
The variable mechanical advantage (VMA) prehensor is a body-powered gripper that can rapidly close the fingers with minimal cable excursion4. When an object is encountered, a simple mechanism shifts to a high mechanical advantage mode, allowing large grip forces to be developed. A prototype VMA prehensor has demonstrated increased mechanical advantage, a holding assist capability to help maintain grip with minimal cable force, improved amputee mobility while maintaining grip, and reduced cable excursion requirements in the sizing mode. It is not clear, however, that these features are worth the added complexity.
VECTOR PREHENSORS EASILY ADJUST GRIP FORCE OF V.O. PREHENSORS
A V.O. prehensor with enough grip force for heavy tasks is uncomfortable and tiring to open. A light grip force, on the other hand, may be inadequate for many tasks. Vector prehensors utilize simple mechanics to easily adjust the grip force over a range from as little as 2 N. to over 80 N. Two prototype versions have been designed and built5. One is patterned after a Hosmer split hook (Fig. 1) and the other is modeled after a TRS Grip III (Fig. 2). Adjustment among the 13 grip force settings available is quick and easy.
Figure 1. Vector Hook
Figure 2. Vector Grip