View Options - Click to expand
Print Options - Click to expand
E-Mail Options - Click to expand

Voluntary-Opening Prehensors with Adjustable Grip force

Daniel D. Frey, MS
Lawrence E. Carlson, DENG
Vidya Ramaswamy, MS

ABSTRACT

A novel concept for adjusting grip force in voluntary-opening prehensors is introduced. The vector mechanism allows the user to rotate a stiff elastic band within the prehensor to change grip force substantially with very low energy input. Two prototype vector prehensors are described: one modeled after a split hook, the other modeled after a TRS Grip III The performances of the prototypes are evaluated and compared with those of commercially available prehensors primarily through laboratory testing.

The vector prehensors allow grip force to be adjusted quickly and easily among 13 settings ranging from 1/2 to 20 lbs. This added capability was achieved without adding significant weight or bulk to the device.

Preliminary field tests demonstrated the practical feasibility of the device, but a clinical evaluation has not been performed.

Introduction

Although 85,000-90,000 people in the United States have lost an upper limb (1), only about 34,000 (40 percent) use prosthetic arms or hands (2). The low percentage of upper-limb amputees using prostheses may be a result of many factors, including financial limitations and social reasons, but the authors submit that a major factor is the limited function and flexibility of the devices currently available. Therefore, there is strong motivation for engineering research and development to improve upper-limb prostheses. About 90 percent of upper-limb amputees in the United States use body-powered prostheses, which are more reliable, lighter, quieter and less expensive than externally powered prostheses (3). Still, the authors believe there is considerable room for improvement in the technology, and advances in body-powered prostheses can have a significant effect on the lives of many amputees.

Among body-powered prehensors, voluntary-opening (VO) control is used more widely than voluntary-closing (VC) control. In a VO prehensor, springs or elastic bands provide grip force. This is advantageous because the amputee may relax cable tension while maintaining grip. However, a typical VO prehensor provides only one level of grip force while the appropriate grip force for common daily activities (from holding a paper cup to swinging a hammer) varies widely. In most cases, the amputee will select a grip force high enough to allow confident grasp during the heaviest tasks he/she frequently performs. However, light grasps are used much more frequently than heavy grasps (4) meaning the grip force of any given prosthesis is usually greater than necessary. This causes two major difficulties:

  1. The amputee must exert more energy and apply more force than is required for most tasks. This causes fatigue and discomfort since greater forces in the cable mean higher pressure applied to the skin through the harness. Higher cable forces also reduce the service life of prosthetic components such as cables, cable housing and associated hardware.
  2. It is very difficult for the amputee to hold an object gently. The only way to reduce grip force is to maintain cable tension, which defeats the primary advantage of VO prehension.

A VO prehensor that allows easy adjustment of grip force over a wide range would reduce these problems. A number of attempts have been made to produce such a terminal device. One example is the Sierra 2-load voluntary opening hook (see Figure 1 ). This design employs two torsion springs: One is engaged at all times, and the other is engaged only in the high grip-force setting when a switch is activated. Thus, the design provides two grip-force settings: 3 1/2 and 7 lbs.

The Sierra 2-load hook is comparable in size and durability to the Hosmer Dorrance 88X (medium adult) hook and is easy to adjust between high and low grip-force settings. (Measured from the wrist to the hook ends, the Sierra 2-load hook is more than 3/4-inch longer than the 88X hook, but the actual hooks on the Sierra 2-load hook are less than 1/8 -inch longer than those of the 88X.) Still, the design has not gained wide acceptance, perhaps due to its limited range of adjustability, limited number of settings and relatively high weight (over twice the weight of the 88X).

Another design that provides variable grip force is the Stanford Children's Hospital adjustable prehension device (see Figure 2 ). It fits onto any Hosmer Dorrance VO split hook and allows the amputee to add and remove one or two modules containing elastic bands prestretched to the hook. Each module adds a grip force of 1 1/2 lbs to the base grip force of about 1 lb. The amputee must carry modules separately and thus could potentially lose the modules. Also, a fair amount of effort is required to adjust the grip force. For these reasons, this design also failed to gain wide acceptance and is no longer commercially available.

Past variable grip VO designs have failed in the marketplace not from a lack of need for them, but because they do not fulfill all of the demands of upper-limb amputees. The goal of this research has been to develop VO prehensors that have a much wider range of grip force than previously available yet are quick and easy to adjust and comparable in size and weight to the most popular prehensors on the market.

The purpose of this article is to present the results of the authors' engineering research and development effort. The concepts behind the devices are explained, the prototypes are described, and the results of laboratory testing are discussed. To help guide the development process, the authors solicited comments from a few individuals who field-tested the prototypes. The results of the preliminary field tests provide some anecdotal evidence of the strengths and weaknesses of the prehensors developed. However, this article is not intended to provide a clinical evaluation of the prototypes.

The Vector Prehensor Concept

Figure 3 depicts a split-hook prehensor and a simplified schematic showing the forces that make it work. The grip force applied by the prehensor is proportional to the torque applied by the bands about the pivot. One obvious way to change the torque is to vary the band force. This is the strategy employed by the designers of the Sierra 2-load hook and the Stanford Children's Hospital adjustable prehension device. With this approach, an additional band or spring is added for each grip-force setting. The number of settings must be limited to avoid excessive bulk or complexity.

Another way to change the band force is to increase the tension in the bands. To do so, energy must be added to the bands, making the procedure physically strenuous or time-consuming.

To avoid these difficulties, the grip force can be adjusted by varying the band angle instead. A vector, such as band force, is defined by its direction as well as its magnitude. The concept behind the vector prehensors is to be able to change the direction of the band force while leaving its magnitude constant and to be able to swing the band on an arc without changing its length.

When the band angle is 90 degrees (as shown in Figure 3 ), grip force is maximized. As the band angle is reduced, the torque applied by the band about the pivot decreases. Because the band is not stretched or relaxed, no energy change is required, and the adjustment can be made with minimal effort. Since the band angle can approach zero, the grip force can be adjusted as low as desired. Because band angle can vary continuously, many different gripforce settings can be provided without added complexity. Since these advantages are afforded by exploiting the vector nature of the band force, the prototypes have been named "vector prehensors."

To demonstrate the utility of the vector prehensor concept, several prototypes were constructed in two basic configurations: a split hook and a TRS grip-type prehensor. A VO vector hand also is possible but has not yet been designed. The next section describes the two most advanced prototypes developed to date.

Description of Prototypes

The vector hook prototype is shown in Figure 1 . The body is made of black anodized aluminum. Neoprene-lined aluminum fingers from a Hosmer Dorrance 88X split hook mate with the case and are screwed in place. The prototype therefore can be configured as a left- or right-handed device by replacing the fingers. The shape, size (5 1/4 inches long) and weight (4 1/4 oz) of the device are very close to those of an 88X hook (4 5/8 inches long and 4 oz). Unlike the 88X, however, the vector hook prototype does not have a protrusion near the cable attachment.

The vector grip prototype is shown in Figure 4 . The body is made of a titanium alloy. The molded polyurethane fingers were adapted from a TRS Grip III prehensor. This device is symmetrical and therefore can be worn on the right or left side. It is similar in shape, size (5.64 inches long) and weight (9.34 oz) to the TRS Grip III (5.70 inches long and 9.80 oz). This is one of the first prototypes to employ grip-type prehensile shapes in a VO prehensor.

In the vector prehensors, adjustment of grip force is accomplished by depressing and then sliding a rubber-coated button (see Figure 5 ).The deliberate actions of pressing then sliding are designed to minimize the possibility of accidental adjustment. The lowest gripforce setting is nearest the wrist, and the highest grip-force setting is at the most distal position. There are a total of 13 settings. The button is wide and accessible so adjustment can be accomplished employing any available surface (e.g., against a countertop or one's leg). Adjustment can be made without use of the opposite hand and without visual cues. The grip force can be adjusted only when the prehensor is fully closed; when the prehensor is open, the button cannot be depressed.

The vector prehensors are powered by two custom-molded elastomer bands. The best design to date will last for more than 70,000 cycles at the highest grip-force setting. The bands are contained within the casing where they are well-shielded from abrasions, nicks and sunlight. The casings are interwoven so the bands are covered even when the hooks are fully open. The de sign intent is such that the bands should last the life of the prehensor, but more thorough field-testing will be required to establish if the prototype elastomer bands meet this goal.

With the current design, the elastomer bands can be replaced by a single individual with Phillips' head and flathead screwdrivers in about 10 minutes. The prehensor itself acts to pretension the bands so no specialized tools are required.

Test Methodology

Laboratory testing of the vector prehensors was performed to determine their grip-force performances at various angles of opening and at all available grip-force settings. The button on the side of the prehensor was used to set the grip force to its lowest setting. Grip force was recorded at opening angles from 50 to nearly 0 degrees. For every angle examined, the grip-force setting was increased by one increment, and the testing was repeated until the performance at each of 13 settings had been evaluated.

Grip force was measured by a curved beam grip-force transducer. The outside of the curved portion of the beam was fitted with strain gauges for computerized data acquisition (see Figure 6 ). As the grip transducer was compressed, the strain gauges were placed in tension causing their resistance to increase. This change in resistance was measured and converted to a force measurement then stored in a computer. Bolts of various lengths were fitted to the grip transducer to set the angle of opening of the prehensors to values from 50 to 10 degrees in 10-degree increments. The opening angle was measured with a protractor.

The technique described did not permit measurement of grip force at opening angles less than 10 degrees due to the thickness of the grip transducer. Therefore, to measure the grip force at a nearly 0-degree opening angle, the prehensors were pulled open a small amount with a string (see Figure 7 ).The tension in the string was measured with a load cell (an electronic device for measuring forces).

For comparison, commercially available split hooks also were tested. The grip-force performance of a Hosmer Dorrance 88X (medium-sized adult aluminum split hook) was measured with one, three, eight and 10 standard prosthetic bands installed. These numbers of bands were chosen to represent the wide range of grip forces typically used by upper-limb amputees. A Sierra 2-load hook also was tested to permit comparison with a currently available adjustable prehensor. Its grip-force performance in both the low and high settings was measured.

Limited field-testing was conducted by subjects with varying levels of amputation. A transradial, a transhumeral and a wrist disarticulation amputee (all unilateral) each wore the vector hook in place of his/her usual terminal device. Following a test period of several days, each tester completed a questionnaire eliciting his/her observations on the performance of the prototype.

The primary purpose of the field-testing was to provide rapid feedback on the prototypes for continued research and development and to gauge the economic value of the design concept. The field-testing was not intended as a scientific clinical evaluation of the prototype. The appropriate methods for defining a sample population, selecting subjects, etc., were not employed. Therefore, the authors do not claim the field-test results scientifically demonstrate the advantages of the vector prehensor prototypes. The results of the field tests are offered only as anecdotal evidence of the feasibility of the vector concept and its current embodiment and to point out areas in need of further development.

Results

The results of laboratory testing of the vector hook are shown in Figure 8 . The grip force as measured at the tips of the hook fingers is plotted against the opening angle of the hooks for each available grip-force setting. In the lightest setting, the grip force remains nearly constant at 1/2-lb throughout the range of opening. In the highest setting, grip force begins at about 11 lbs and gradually rises to nearly 20 lbs as the hooks are opened. The 11 intermediate settings provide a choice of grip-force levels evenly spaced between these two extremes.

Figure 9 compares the vector hook's performance with those of similar prehensors. The vector hook at its highest setting performs roughly the same as an 88X hook with eight to 10 bands and provides nearly twice the grip force of the Sierra 2-load hook at its high setting. The vector hook's lowest setting was designed as the lightest possible grip that still allows the hook to close while pulling 1/16-inch steel prosthetic cable through unlined steel cable housing. In the lowest setting, the vector grip can hold a Styrofoam coffee cup without crushing it. This is not possible with the 2-load hook or with an 88X hook that has only one standard prosthetic band.

The results of the field-testing, although preliminary, are very encouraging. The vector hook was rated the same as or superior to standard VO hooks by each tester in every activity rated. Each tester responded that he/she would replace his/her standard VO hook with a vector hook if it were available. One tester noted that the highest position is useful for holding tools but requires too much force to open without excessive stress on the cable and other prosthetic components. A longer lever arm for cable attachment could be employed to alleviate this problem. One tester noted that a stainless steel version of the vector hook would be preferable for performing tasks that involve heavy work.

Future Work

Because the vector prototypes have proven promising, an application has been submitted for a patent on the vector prehensor mechanism. Once the patent is issued, commercial production of vector prehensors will commence.

However, before such an effort begins, some engineering challenges must be met. An elastic element that consistently will last the life of the device should be developed to eliminate band replacement. A high fatigue life elastomer with adequate stiffness and strain to failure must be identified. The shape of the band should be optimized to evenly distribute stress. A reliable process of bonding an elastomer to a bushing should be developed.

Manufacturing processes that will allow vector prehensors to be produced at a competitive cost must be identified. Some components of the prototypes may be redesigned for ease of manufacture and assembly. The prosthetics research group at the University of Colorado has already begun to address these issues.

Several questions remain to be answered through clinical evaluation. It remains to be established that use of vector prehensors in place of standard VO devices results in measurable improvement in performance of tasks. Also, since lighter settings may be used frequently and require less cable tension to actuate, one may test the hypothesis that vector prehensors increase the service life of prosthetic components (such as cable, housing and other hardware). It may be valuable for future engineering development to establish how the vector prehensors tend to be used (i.e., how often the lightest and heaviest settings are employed) and how durable they are in extended field service.

As vector prehensors are introduced in the marketplace, it will be important to determine what training can be provided to the users to allow them to make the most of their new capabilities. Over a longer term, the health benefits (e.g., reduced harness discomfort) resulting from use of vector prehensors should be investigated.

Conclusion

Laboratory tests demonstrate the prototype vector prehensors provide a range of grip forces far exceeding those of any currently available voluntary opening prehensor. They allow grip force to be varied with minimal effort among 13 settings ranging from 'A to 20 lbs. The improved function of the prehensors was made without incurring any substantial penalty in weight or bulk. Preliminary field tests suggest vector prehensors will make everyday tasks easier for upper-limb amputees, but this claim remains to be substantiated by clinical evaluation.

Acknowledgments

The contributions of Robert Radocy as a consultant and as a field evaluator are gratefully acknowledged. We thank Richard Stewart and Donald Delforge for field-testing the prototypes. This research was supported by the NCMRR under Grant 1-RO1-HD30131-O1.


DANIEL D. FREY MS, is a graduate student in mechanical engineering at the Massachusetts Institute of Technology in Cambridge, Mass.

LAWRENCE E. CARLSON, DENG, is a professor of mechanical engineering at the University of Colorado at Boulder in Boulder, Colo.

VIDYA RAMASWAMY MS. is a graduate student at Pennsylvania State University in University Park, Pa.

References:

  1. Digest of data of persons with disabilities. National Institute on Disability and Rehabilitation Research. Washington, D.C., 1992:29.
  2. LaPlante MP, Herdershot GE, Moss AJ. Assistive technology devices and home accessibility features: prevalence, payment, need and trends. Vital and Health Statistics of the Centers for Disease Control/ National Center for Health Statistics 1992; Advance Data No. 217:6.
  3. Childress DS (Director of the Northwestern Prosthetics Research Laboratory, Chicago, IL). Personal communication to M. LeBlanc.
  4. Fletcher JF, Leonard F The principles of artificial-hand design. Artificial Limbs; 2:2:78-94.