the french electric hand: some observations and …

THE FRENCH ELECTRIC HAND: SOME OBSERVATIONS AND CONCLUSIONSa

Luigi F. Lucaccini, M.A., Research Assistant Peter K. Kaiser, Ph. D., Research Assistantb

John Lyman, Ph. D., Director

Biotechnology Laboratory Department of Engineering

University of California Los Angeles, California 90024

I. INTRODUCTION

A. Purpose

Although the French Electric Hand has been in use in Europe on a lim- ited basis for about 20 years, little is kno'vn about the device in the United States. The purposes of this study were to provide a description of the components of the prosthesis, to outline its control and operation, and to assess the degree of functional regain that below-elbow amputees could achieve with it.

B. Background

The increase in prosthetic development activity that resulted from World War I1 included both conventional and radical approaches to the provision of functional regain to the amputee. Among the latter activities was n renewal of interest in the application of external power to prosthetics. Well- known results of this endeavor were the Alderson-IBM Electric Arm and the Heidelberg Pneumatic Prosthesis. Possibly the earliest and the least- known externally powered device to result from the war was the Vaduz Hand, now known as the French Electric Hand. Development work on that device was actually begun in Berlin during the war under Dr. Edmund Wilms (1).

The hand was developed with two purposes in mind. One was to pro- <.

vide an electrically powered terminal device that could be operated from an inexpensive, portable power source. The second was to employ a control

'Based on work performed under VA Contract V1005P-9779. Also catalogued as .I

Biotechnology Laboratory Technical Report No. 37, and UCLA Department of Engineering Report 66-44.

b N o ~ a Postgraduate Research Fellow in the Department of Psychology, University of Rochester, Rochester, New York.

Lucaccini et al.: French Electric Hand

system that was as natural as possible, that is, one which could be controlled by tension of those muscles naturally associated with prehension.

Shortly after the war a group was established under Dr. Wilms at Vaduz, Lichtenstein, to continue development of the hand. Approximately 70 pro- totype hands were constructed and used locally before the project was terminated in 1953 for lack of funds. Subsequently, French rights to the hand were purchased by the orthopedic firm of F. Guillot Institute Chir- urgie Orthopedic of Paris (2, 3) and Mr. Werner Kegel, technician with the Vaduz group and present holder of the German patents (4, 5 ) , joined the French firm. He continues to the present date with the firm as the chief developer of the device. He is the only member of the original group still active in the project (1, 6 ) .

The hand has not yet reached its final stage of development. Minor modifications are frequently made to increase reliability of operation and to decrease size and weight. This "gadgeteering" approach has been justi- fied on the) grounds that the project receives no government support. The hand is expensive (approximately $300.00). Amputees must purchase it with personal funds. Nevertheless it enjoys a growing popularity in France. Although the hand is intricate an&requires a number of adjustments to maintain optimal operation, it is reported to be reliable enough for normal daily use (6) .

Th'e particular model studied at UCLA was constructed in 1960. I t was purchased directly from the French orthopedic firm.

11. DESCRIPTION

A. General Description

The French Electric Hand is a prosthetic terminal device designed for use by below-elbow amputees. It provides one function - prehension - which the amputee controls by the tension of his forearm muscles against a pneumatic transducer located within the forearm socket. Full prehension span is 2.6 in. at the fingertips. The hand weighs about 0.8 lb. Other phys- ical dimensions of the hand are given in Table 1. A more recent version (1962) of the hand is shown in Figure 1. Its construction and operation are essentially the same as the model studied at UCLA.

TABLE 1. -Dimensions of the French Electric Hand

Dimension

Weight Weight with glove Length Breadth Circumference

Value

0.8 lb. 0.9 Ib. 6.0 in.

Dimension

Dorsal-volar diameter Maximum prehension span Index finger length

3.3 in. 10.0 in. '

I

Value

3.3 in. 2.6 in. 3.5 in.

Middle finger length Thumb length

3.8 in. 2.8 in.

Bulletin of Prosthetics Research - Fall 1966

FIGURE I . -French Electric Hand (1962 model) with shell cover removed. Power pack, pneumatic pump, and outer transducer are all visible. Thumb and first two fingers are covered by a cut-down prosthetic'glove.

The parts of the hand include four fingers, a thumb, the hand frame and shell, internal components, and two external units (a battery and a transducer). The hand frame and shell are constructed of cast aluminum. The frame includes two bases - one at the wrist and one supporting the fingers and thumb. The bases are connected by a thin bar and by the hand shell. One part of the shell is attached to the frame with screws and can be easily removed to allow access to the inner mechanisms of the hand. The remainder of the shell is welded to the upper and lower bases of the frame. The outer surfaces of the shell and frame are covered with a flesh-colored plastic coating. The thumb, index finger, and middle finger are aluminum They are padded with foam rubber and covered with plastic. The ring and little fingers are flexible coiled springs, also covered with plastic. A hollow, threaded attaching stud at the wrist permits attachment to standard prosthetic units and allows wiring and pneumatic tubing to pass into the hand. The entire hand can be covered with a cosmetic glove. Detailed views of the inner mechanisms of the hand are provided in Figures 2, 3, 4,5 and 6. The hand shell has been cut away in these figures.

Prehension occurs through the closure of three digits of the hand (the thumb, index, and middle fingers) in a three-jaw chuck configuration. The - fingers are located on a common axis of rotation. The thumb is located on a separate axis. Rotation of the motor is transformed through a gear train and scissor links into simultaneous rotation of thumb and fingers around their axes. Although the ring and little fingers rotate together with


FIGURE 2. -Side view of hand, fully FIGURE 3. -Side view of hand, fully closed. Principal mechanical compon- opened. Feedback linkages are labeled. ents are labeled.

I

FIGURE 4. - Side view of hand.

the first two fingers, they are not opposed and thus serve only a cosmetic purpose.

The hand system contains four units: 1. Pneumatic, to initiate movement; 2. Electrical, to provide power; 3. Mechanical, to effect prehension;


and 4. Feedback, to modify or halt operation of the hand depending on its degree of opening and the forces encountered. These units interact func- tionally in the control of movement and cannot be considered to be completely distinct. They will be discussed in the following sections.

F I ~ U R E 5. -Top view of hand. Prin- FIGURE 6. - Bottom view of hand. cipal electrical components are labeled.

B. Functional Units: Components and Operation

1 . Pneumatic Unit. The pneumatic unit provides a means to transform the amputee's control movement (muscle bulge) into closure of the appropriate electrical control circuit within the hand. The pneumatic unit con- sists of two interconnected rubber bladders: an outer transducer and an inner transducer. The outer transducer is located within the forearm shell at a point where the amputee can provide maximum muscle bulge against it. The inner transducer is located within the hand. The transducers are connected by rubber tubing which passes into the hand through the center of the threaded attaching stud. The amputee can adjust the pressure within the pneumatic unit by means of a manual plunger-type pump located in the line connected to the transducers (see Fig. 1 ) .

Pressure against the outer transducer, resulting from a contraction of the appropriate musculature by the amputee, causes an expansion of the inner transducer. Small circular aluminum plates are cemented to each side of the inner transducer. The proximal plate is constrained, so expansion of the inner transducer causes upward (distal) movement of the distal plate


and of an attached spindlec against a spindle spring. Thus the reversing switch activates the motor to close the hand. With relaxed muscle (Fig. 7 ) , the spindle spring pushes the spindle downward (proximally), returning air from inner to outer transducer and moving the reversing switch to open the hand until the open limit switch breaks the circuit.

LIMIT SWITCH

SPINDLE BODY

OPEN INNER TRANSDUCER

HAND n OUTER TRANSDUCER c

FIGURE 7. - Electrical circuit and parts of mechanism; hand fully open; spindle moves insulated ends of 2nd and 4th leaves of reversing switch.

2. Electrical Unit. The electrical unit is composed of two control circuits, one for closing and one for opening the hand. Both circuits include the same elements: a 3-volt reversible, permanent-magnet motor, a 5-volt

'The directions "up" and "down" will be used to refer to movement within the hand towards or away from the fingertips. Directions agree with Figure 5.


nickel-cadmium battery, two leaf spring switches, and a spindle. These components with the exception of the battery are located within the hand. Leads pass from the battery to the motor through the center of the threaded attaching stud. The motor draws 300 to 500 ma. The battery is small enough to be carried in a pocket, hooked to the belt, or located within the forearm socket. I t has a rating of 500 ma.-hr. and is sufficient for 8 hours of normal use. The motor, one switch, and the spindle are visible in Figure 5, and the other switch in Figure 3.

One leaf spring switch (the reversing switch) works together with the spindle to govern the direction of current flow through the motor. The position of the spindle in relation to the switch determines which of the two control circuits is activated. The reversing switch, which is double pole, double throw, has five leaves; two of these (the second and fourth from the top) are anchored to the spindle at the other end. Upward movement of the spindle, resulting from expansion of the inner transducer, causes contact to occur between the first and second leaves and between, the third and fourth leaves of the reversing switch. The switch is wired so that these contacts cause current to flow across the motor in the direction necessary to cause closing of the hand. Downward dovement of the spindle, resulting from deflation of the inner transducer when the amputee relaxes his contraction, causes contact between the second and third leaves and between the fourth and fifth leaves of the reversing switch. The switch is wired so that for these contacts current flow across the motor is in the direction opposite to that for closing. These two control circuits are illustrated sche- matically in Figure 7.

The second leaf spring switch (the limit switch) acts to halt movement either when the limit of the rang? of motion is reached or when a mechanical resistance is encountered during grasp or opening of the hand. The limit switch is single pole, double throw. Two of its three leaves are anchored to the spindle at their far end. Both the opening and closing control circuits pass through the limit switch. During movement, both circuits remain un- broken at the limit switch, although only one of these circuits is closed at the reversing switch. When mechanical resistance to finger movement is encountered or when the end of the range of motion is encountered, the motor begins to stall. Through a mechanical feedback linkage a bending moment is applied to the limit switch. This moment displaces the unan- chored leaf relative to the anchored leaves such that one of the two contacts is broken. The limit switch is wired so that the control circuit that is broken is the one that was responsible for movement.

Current flow at the switches occurs at contact points on each leaf adja- cent, but external, to the spindle body. No current flows through the spindle itself. The spindle serves only as an attachment point for the leaves of the two switches. The leaf ends are electrically isolated by insulators


mechanically interlocked with them. These leaf ends and their insulators make up the spindle body.

3. Mechanical Unit. The mechanical unit is composed of the motor, a gear train, a sector gear, two scissor links, four fingers and a thumb mounted on separate axes, and supporting structures. The principal components of the mechanical unit are visible in Figures 2 and 6. Rotation of the motor shaft is transformed into motion of thumb and fingers in the following manner: a gear attached to the motor shaft drives the gear train. The gear train in turn drives the sector gear, causing it to move up or down. The radius of the sector gear serves as an anchor point for the ends of two scissor links. One link is attached at its other end to the finger axis, the other to the thumb axis. Motion of the sector gear results in downward or upward displacement of the ends of the scissor links attached to it, and in simultaneous inward or outward movement, respectively, of thumb and fingers.

The mechanical unit is driven through either a high or low gear reduction ratio. In high gear, a reduction ratio of 6: 1 exists between the motor shaft gear and the gear that drives the sector gear. This ratio is in effect at all times during opening of the* hand. I t is normally in effect during closing as well. However, the transmission ratio will shift to low gear (reduction ratio of 125 : 1 ) during closing if two conditions are met:

1. The fingers meet mechanical resistance as the result of contact with an object.

2. The motor has run long enough for the crutput gear of the gear train to reach full speed.

The shift to low gear has the advantage of permitting grasp to occur with a much higher ~rehension force than is possible in high gear. The shift to low gear is due to a back torque that develops when finger or thumb motion is arrested during closing. Blocking of the fingers locks the position of the sector gear, with two results. One is that a back torque is reflected onto an intermediate part of the gear train, called the gear change unit. If the gear train has reached full speed before the fingers are blocked, then the reflected back torque will be sufficient to overcome the restraining spring that holds the gear change unit in its normal position (high gear). The gear change unit will then shift 12 deg. placing two new gears in the transmission system to yield the larger reduction ratio. The second result of blocking of the fingers is that the output gear of the gear train will now climb the sector gear, ultimately breaking the active control circuit at the limit switch. The hand will remain locked at whatever level of prehension force was developed until the amputee relaxes his contraction.

4. Feedback Units. The hand contains two feedback units: a position feedback linkage and a force feedback linkage. In order to understand their action, the mounting of motor, gear train, and sector gear should first be described.


a. Mounting. The motor and gear train are attached to an H-frame by screws. The H-frame and the sector gear are both mounted at one end on a common axis of rotation. The axis is a short cylindrical rod that is supported by two triangular plates attached to the supporting bar of the hand frame (see Fig. 2 ) .

The H-frame is held in place by its mounting on the rod and by a feedback link to the limit switch. The H-frame rotates around the rod only when the fingers are blocked or when the end of the range of motion has been reached; its motion results in breaking the current to the motor.

The sector gear is held in place by its mounting on the rod, by contact with the output gear of the gear train, and by connection to the scissor links to thumb and finger axes. The sector gear rotates around the rod when driven by the gear train, and, as described previously, its rotation is transformed into movement of thumb and fingers.

b. Position Feedback Linkage. The position feedback linkage acts as a volume compensator. It has two effects: 1. it preserves the resiliency of the leaf springs in the reversing and limiting switches by counteracting the dis- torting force placed on them by the inner transducer during closing of the hand, 2. it permits the hand to close in a graduated or step-wise fashion when the amputee applies a moderate control bulge against the outer transducer.

Two parts connect the sector gear to the bottoin plate of the inner transducer. These are the position feedback link and the position feedback lever (see Fig. 3 ) . The position feedback link is a thin rod attached at one end to the sector gear and at the other end to the position feedback lever. The lever is attached at its other end by a ball and socket arrangement to a supporting bar attached to the lower base of the hand. A second ball and socket in the middle of the lever connects it to the bottom transducer plate. The net effect of this mechanical linkage is that when the sector gear is driven down during closing of the hand, the bottom plate of the transducer is correspondingly pulled down. As a result, the inner transducer expands downward, reducing the force against the upper plate, the spindle, and the switch leaves. Conversely, as the hand opens, the position feedback link pushes the bottom plate back up, tending to increase the upward force of the transducer against the spindle.

If the amputee applies a strong force against his transducer, the position feedback linkage will not interfere with closing of the hand. However, if a moderate level of force has been applied, at some point along the range of closing the position feedback linkage will reduce the force on the spindle body enough to allow the spindle pressure spring to move the spindle down slightly, breaking the closing circuit. The hand will then be in an electrically neutral state with neither circuit closed. A subsequent increase in force by the amputee will again initiate closing, while a decrease will initiate open-


ing. If successive changes in control force are slight, the hand will close or open by small amounts. Hence, precise control of prehension span is possible.

c. Force Feedback Linkage. The force feedback unit acts to break the active control circuit whenever movement of the hand is blocked. I t serves a protective function by eliminating drain on the battery when the motor is stalled and by preventing the motor from overheating.

The force feedback unit has one part, a thin rod (force feedback link), which ties the H-frame to the limit switch body (see Fig. 3 ) . As has been noted already, the H-frame pivots around its axis when motion of the hand is halted. Depending on the direction of hand movement before blocking occurs (and the direction of rotation of the motor), the force feedback link pulls or pushes on the switch body. As a result, the center leaf of the switch is bent relative to the two anchored leaves, and one of its two contacts is broken, halting current flow to the motor. The hand remains locked in position until the amputee reverses direction or the obstruction to movement is removed.

FORCE ON OUTER TRANSDUCER (LB)

FIGURE 8. -Prehension span as a function of force on outer transducer.


C. Engineering Tests

Bench tests with the electric hand revealed that the most sensitive control capabilities were provided when the pneumatic unit was inflated to 5 p.s.i. At this setting, required input forces and displacements were min- imal. A force against the outer transducer of about 1.0 lb. was just sufficient to close the hand a slight amount at this setting. Forces in the range from 1.0 to 2.2 Ib. permitted operation of the hand in the step-wise closing modes. Forces beyond 2.2 Ib. resulted in complete closure of the hand, with a shift to low gear during grasp. Figure 8 presents the amount of closure obtained for a given input force. Figure 9 shows the amount of closure obtained as a function of the displacement of the outer transducer.

Prehension forces in high and low gear were measured. Maximum steady- state prehension force never exceeded 0.7 Ib. in high gear. In low gear, maximum force ranged from 2.5 to 7.0 Ib., depending on the adjustments made to the hand.

D. Adjustments

The major adjustments provided by the developer are listed in Table 2. Only the first of these, regulation of pressure in the pneumatic unit, can

DISPLACEMENT OF OUTER TRANSDUCER ( 0 . 0 1 IN

FIGURE 9. - Prehension span as a function of outer transducer displacement.

40


be made by the amputee. This adjustment should be made while the hand is worn in order to test the effectiveness of the adjustment. All other adjustments necessitate removal of the cosmetic glove and shell cover and the use of special tools. A good knowledge of the functioning of the units of the hand is necessary to determine and to make needed adjustments, particularly since any adjustment is likely to result in the need to make minor compensating adjustments to other units.

E. Modes of Operation

The hand may be classified as a voluntary-closing terminal device. That is, to close the hand or to main~ain it closed the amputee must apply pressure against his control transducer. Otherwise the hand will open to the limit. No special locking mechanism is employed; the fingers and thumb are locked automatically by the gear train whenever motion is halted. Five modes of operation of the hand are possible. These modes are summarized in Table 3.

The first mode of operation, normal opening, occurs when the amputee has relaxed his contraction against the outer transducer. -

The second and third modes, steywise opening and closing, occur when a slight to moderate range of force is applied to the control transducer. Precise control of opening and closing is possible, but only very light objects may be grasped. The position feedback linkage is responsible for these modes of operation, as described in Section IIB4 of this report.

The fourth mode, normal closing, occurs when a strong control force is applied by the amputee. The hand closes quickly and grasps with a higher range of force than in step-wise closing.

The fifth mode, normal closing - strong grasp, is identical to normal closing except that grasp force is greater than usual. A very large muscle bulge by the amputee is necessary. I t acts to displace the spindle body to a higher than usual position relative to the limit switch. The result is that a greater than usual bending moment must be applied to the limit switch through the force feedback linkage to halt the motor. Consequently, the motor runs longer than usual and develops a higher torque in grasp.

F. Mechanical Difficulties

Although the hand that was evaluated at UCLA was a production model, a number of difficulties were encountered early in its use. Immediately upon receipt of the hand, its operation was found to be erratic. A number of improvements were made, none of which altered the design of the device. Flexible wires were substituted for rigid ones in the electrical circuits. The insulation between switch leaves was improved. The gear train was taken apart and burrs and rough spots were removed from the gears. Leaks within the pneumatic system were repaired several times, until it finally

TABLE 2. - Major Adjustments

Part

Pneumatic unit

Spindle pressure spring

Position feedback linkage

Adjustment

Increase pressure within pneumatic unit by manual pump.

Decrease pressure.

Increase force of spring, by screw adjustment.

D~~~~~~ force.

Increase effective length of feedback link - by adjustment screw at point of connection to position feedback lever.

Decrease length.

Function

Transmits force to activate closing circuit.

Acts on spindle body counter to force of inner transducer; causes spindle to move down and engage opening circuit when amputee relaxes his contraction; inter- acts with force feedback on limit switch during opening by altering position of anchored leaves of limit switch.

Provides volume compensation at inner transducer proportional to amount of closing of hand.

Result

Reduces level of amputee contraction necessary to close hand.

Increases level of amputee contraction necessary to close hand.

Increases level of amputee contraction necessary to close hand; increases moment necessary to break opening circuit at limit switch.

Decreases level of amputee contraction needed to close hand; decreases moment necessary to break opening circuit at limit switch.

Increases range of motion over which step-wise hand motion is possible; decreases minimum amputee contraction needed to achieve step-wise operation of hand.

Decreases range of motion over which step-wise hand motion is possible; increases minimum amputee contraction needed to achieve step-wise operation of hand.

TABLE 2. - Major Adjustments-Continued

Part

Force feedback linkage

Limit switch

Adjustment

Change point of attachment of link to limit switch body - three attachment points provided.

Increase contact pressure of upper and lower leaves of switch with center leaf by separate screw adjustments for each leaf.

Decrease contact pressure.

Function

Acts on limit switch to break active circuit upon grasp of an object or upon reaching end of range of motion.

Acts together with force feedback linkage to halt motor upon grasp of an object or upon reaching end of range of motion.

Result

Alters bending moment applied to limit switch by changing lever arm of moment; amplified moment results in greater bending of switch, earlier break in control circuit, and lower level of prehension force upon grasp.

Increases bending moment needed to break circuit at limit switch, therefore, motor runs longer after movement is blocked and higher level of prehension force develops during grasp.

Decreases bending moment needed to break circuit a t limit switch, therefore, motor runs for shorter time after movement is blocked and lower level of prehension force develops during grasp.

TABLE 3. - Modes of Operation of the Hand

circuit operation

Normal Completely Opening opening relaxed. circuit.

Step-wise Moderate to Opening opening 1 slight, in 1 circuit.

decreasing steps. I

Normal closing

Step-wise closing

Normal closing, strong grasp

Strong, applied quickly.

Slight to moderate, in increas- ing steps.

Very strong, applied quickly.

Closing circuit.

Closing circuit.

Closing circuit.

Action of position feedback

Action of force feedback

No effect on opening.

position. I

Breaks opening circuit.

Causes break in opening circuit when volume compensation com- presses inner transducer sufficiently to move spindle up from opening

Breaks opening circuit.

No effect on closing.

Causes break in closing circuit when volume compensation overcomes amputee's contraction force at inner transducer.

Breaks closing circuit when finger movement blocked or hand fully closed.

Breaks closing circuit &hen finger movement blocked or hand fully closed.

Action of hand

No effect on closing.

Hand opens to limit unless fingers blocked.

Breaks closing circuit when finger movement blocked or hand fully closed.

Hand opens in small increments, one for each small decrease in amputee contraction.

Hand closes in small increments, one for each small increase in amputee contraction; gear change to low ratio does not occur upon grasp.

Hand closes at maximum speed; gear change to low ratio occurs upon grasp, ~ielding higher prehension force.

Hand closes a t maximum speed; gear change to low ratio occurs upon grasp ; highest range of prehension force develops.


became necessary to replace both transducers with a neoprene system fabri- cated in the laboratory. The deterioration of the original rubber system was probably the result of atmospheric smog.

After 25 hours of use, the motor frequently could not be started unless the shaft was spun by hand. Inspection revealed that the commutator was badly scorched. The commutator was sanded, cleaned, and the position of

.+ the brushes was adjusted. No further problems occurred with the motor. Frequent adjustments were necessary to the feedback systems and the

electrical switches. The hand simply would not maintain a given state of r: adjustment for more than a few hours of use. Readjustment was compli-

cated by the fact that large thread screws were used at adjustment points and the switch leaves were too easily bent out of position.

Suggestions for improving the hand, in addition to raising the general quality of workmanship and materials, would include substitution of switch leaves of higher resiliency and finely-threaded adjusting screws. A tighter coupling between the gears and fingers would increase the power transfer- ence from motor to fingers.

Il l . AMPUTEE PERFORMANCE

A. Subjects

Two below-elbow amputees were selected as pilot-wearers of the hand. Both were young, in good health, and regular wearers of conventional below-elbow prostheses (APRL hand, Dorrance hooks) . These subjects are described in Table 4.

TABLE 4. -Description of Subjects

1tem 1 Subject 1 I Subject 2

Sex Age Occupation Amputation Cause of

amputation Present

prosthesis

Total experience with present

P prosthesis

Male 2 1 Student Right BE, 62% stump

Traumatic BE socket with flexible hinges,

APRL hand or one of four Dorrance hooks

4 years

Male 20 Student Right BE, 32% stump

Traumatic BE split socket, APRL hand

or Dorrance 5X hook

7 months

Subject 1 had a long below-elbow stump (62 percent). I t was possible to attach the electric hand to the socket of his regular prosthesis. The control


transducer was located within the socket at the point of maximum bulge of the superficial volar flexors. A tight-fitting harness was used to insure that the control transducer remained in contact with the area of maximum muscle bulge. Subject 2 had a very short below-elbow stump (32 percent). The electric hand was attached to the socket of his conventional prosthesis, but two modifications were made. A cable assist for flexion was added, and the control transducer was located within the socket against the end of the stump. The amputee activated the hand by pushing his stump into the socket about in. to contact the transducer. This arrangement was not ideal, but it permitted control at all times without fear of accidental activation of the hand during flexion.

Both amputees were able to operate the electric hand at all recommended locations ( 7 ) .

B. Performance Tests

1. Introduction. After being fitted with the electric hand, each amputee received a few hours of instruction and practice in its use under the super- vision of laboratory personnel. Adjustments to the harnessing and to the hand were made where necessary. A series of seven performance measures were then taken on each amputee. Each amputee performed each test three times in the following order: 1. with his left hand, 2. with his regular prosthesis, and 3. with the electric hand. The locking mechanism of the conventional prosthesis (APRL hand) was removed for these tests to make its operation more closely comparable to the electric hand.

2. Thickness Discrimination. In this test the amputee was asked to report whether he could detect differences in width between pairs of aluminum blocks placed in his hand. The amputee was blindfolded during these tests. The smallest differential used, 0.1 in., was reliably detected (75 percent of the time or better) by both amputees with the left hand and with the APRL hand. Neither amputee was able to detect differences as large as 0.5 in. with the electric hand.

3. Minimum Motion. Each amputee's ability to exercise precise control over his prosthesis was determined by measuring the smallest closing move- ments he could make with it. The amputee opened his hand to 1.5 in. and then made the smallest closing motion he could. Ten trials were performed by each amputee with each hand.

Table 5 summarizes the results of this test. Precision of control, measured as the smallest closing motion possible, was found to be quite good for both prostheses compared to the intact hand. Both amputees were more variable in their response with the electric hand. The median response for Subject 1 was larger with the electric hand than with the other hands. Nevertheless the differences obtained were slight and of no practical significance.


TABLE 5. -Minimum Closing Motion (In.)

.I 4. Reaction Time. Reaction time was measured as the time elapsed from the onset of a closing command signal (light) to the beginning of the closing movement by the fingers. Each amputee completed 110 trials with each hand.

Table 6 summarizes the results of this test. For both amputees, perform- ances were best with the left hand and worst with the electric hand. The differences were not statistically significant.

Subject 1

TABLE 6. - Reqction Times (msec.) - - -- - - -

Subject 2

5. Prehension Span Reproduction. Each amputee was asked to adjust the degree of opening of his hand or prosthesis to match the width of a small metal block. Four block widths, ranging from 0.2 to 2.2 in., were used. Forty-eight trials were performed with each hand at each width. Half of the trials were performed with the comparison stimulus present and half after it had been presented and removed.

The results are presented in Table 7. Average deviations were quite small for each subject, regardless of the prosthesis used. Accuracy of reproduction was the same with or without the stimulus present.

6. Speed of Closing. Average closing speed was measured with each hand for each amputee. Eleven trials were performed. Closing speed was calculated as the ratio of distance traveled by the fingers in closing to the total closing time. The results of these measurements are presented in Table 8. Subject 1 was faster with the left hand than with the electric hand, but the difference only approached a significant level ( t = 1.94, df = 20, p < . lo ) . Subject 2 was faster with the left hand than with the

Unit

Left hand APRL hand Electric hand

Unit


0.01-0.03 0.01-0.04 0.01-0.04 0.01-0.04

0.04 0.02-0.06 0.0 1-0.45

Subject 1

Mean .

154 181 240

Subject 2

SD

24 28 68

Mean

181 222 252

SD

20 43 51


APRL hand (t = 1.41, d f= 20, p < .20), and with the electric hand than with the APRL hand (t = 1.38, df = 20, p. < .20), but again the differences only approached a significant level.

TABLE 7. -Average Deviations for Prehension Span Reproduction

a

TABLE 8. - Closing Speed (In./msec.)

Unit

Subject 1 : Left hand APRL hand Electric hand

Subject 2: Left hand APRL hand Electric hand

7. Grasp and Transport. This test required the amputee to grasp a small object, transport it 3 ft., and place it in a predetermined location on a table. Twelve geometric shapes were used, weighing from 1 to 5 oz., and ranging from 0.75 to 1.50 in. at the point of maximum width. Each arn- putee performed 10 trials with each object for a total of 120 trials. The direction of transport was to the right for half of the trials and to the left for the other half.

The results of this test are given in Tables 9 and 10. Both amputees were fastest with the left hand and slowest with the electric hand. The differences in task performance time were not significant.

Chi-square tests of the number of errors (regrasps, dropped objects, etc.) revealed that significantly fewer were made with the left hand than with either prosthesis (subject 1 : x2 = 5.92, df = 1, p < .02 ; subject 2 : x2 =

21.88, df = 1, p < .01). Differences between the two prostheses were not significant.

Prehension span to be reproduced, in.

Unit


0.2

0.04 0.04 0.07

0.04 0.04 0.03

0.8

0.04 0.07 0.10

0.06 0.05 0.04

Subject 1

Mean

32.7 25.3 20.5

Subject 2

1.4

0.07 0.09 0.13

0.06 0.11 0.08

SD

5.8 4.2 2.3

Mean

33.3 20.9 27.5

2.2

0.08 0.11 0.10

0.05 0.09 0.08

SD

7.7 4.3 2.1


TABLE 9. - Transport Task: Average Completion Times (See.)

TABLE 10. - Transport Task: Tota l Errors

Unit


8 . Table Setting Task. The amputee was instructed to set a standard place setting for one person. Four Cishes and four utensils were used. Ten trials were performed with each hand.

Tables 1 1 and 12 summarize the results of this test. Subject 1 was significantly faster with his left hand than with the electric hand ( t = 2.35, df = 18, p < .05) or with the APRL hand ( t = 268, df = 18, p < . 0 2 ) . Subject 2 was also faster with his left hand than with the electric hand ( t = 3 . 2 3 , d f = 1 8 , p < .01) or the APRL hand ( t = 6 . 1 7 , d f = 1 8 , p < . O l ) . No other differences were significant.

Subject 2

TABLE 1 1 . - Table Setting Task: Average Completion Times (See.)

Mean

1 .O 2.2 2.9

Subject 1

Subject 2

3 22 2 7

Unit


SD

0.2 1.3 1.4

Mean

1.1 2.0 3.0

Subject 1

14 2 1 30

TABLE 12. - Table Setting Task: Total Errors

SD

0.4 1.3 1.5

Unit


Subject 2

Unit


Mean

10.9 29.4 29.3

Subject 1

SD

1.0 2.8 5.6

Mean

11.3 20.4 35.5

49

Subject 1

2 3

26

SD

1.2 3.2

10.2

Subject 2

1 7

13


In terms of the number of errors made (regrasps, dropped objects, etc.), both amputees performed the task best with the left hand (Subject 1: x2 = 32.83, df = 1, p < .005; Subject 2: x2 = 8.64, df = 1, p < .005). Subject 1 made fewer errors with the APRL hand than with the electric hand (x" 16.69, df = 1, p < .005). No other differences were significant.

IV. SUMMARY

A series of seven performance tests was completed by each amputee. As would be expected, performance was faster and more accurate with the normal left hand than with either prosthesis. Comparisons between the two prostheses alone failed to reveal significant differences in performance, with a few exceptions. The most notable of these was the thickness discrimination task. The electric hand was found to be seriously deficient in supplying (nonvisual) feedback about the amount of opening of the hand.

Overall, both amputees tended to perform slightly better with the conventional prosthesis than with the electric hand. The difference was larger for the experienced wearer, Subject 1, and can probably be attributed to his long experience with the conventional prosthesis. Again, it should be noted that the differences were not significant.

The maximum level of prehension force possible with the electric hand is sufficient only for light-duty tasks. Informal observations revealed that the amputee wearers were not able to turn handles or doorknobs easily with the hand, or to lift heavy objects. A maximum grasp force double that provided by the hand has been recommended for terminal devices (8), although most activities require forces of 3 1b. or less (9) .

Another drawback of the hand is the fact that the amputee has only two prehension forces available which he selects by choosing to effect a gear change or not.

The hand was acceptable to both pilot wearers. They preferred its method of control to that of the cable-controlled APRL hand, even though one amputee used an unorthodox control movement. Subject 1 reported the control movement to be compatible with the normal neuromuscular control pattern. He stated that this was the first time since his amputation that a prosthesis had responded when he attempted to close his "phantom hand." Appearance, speed of motion, and the fact that the control motion

- -

was invisible were all given as acceptable features of the device. Neither amputee objected to the noise of operation, which was loud enough to attract attention to the wearer, particularly when changing gears.

The reliability of the French Electric Hand was found to be quite low, contradicting reported experiences of European wearers (6). Operating difficulties were traced to poor workmanship and quality of materials in


the model tested, the difficulty of making adjustments to the hand, and the seeming inability of the hand to maintain a given level of adjustment.

No safety problems were encountered during operation of the hand. The hand was purchased from its developer directly at a cost of about

$300. A local orthopedic supply firm estimated that it would be possible to sell the hand in this country for about $600, including servicing.

In conclusion, the performance inadequacies of the electric hand (low grasp force, inability to vary grasp force, lack of feedback to amputee, and noisy operation) coupled with its high cost and low reliability argue against adoption of the device by American amputees. The unique feature of the hand - its control system - was well received by both pilot wearers; they found it easy to operate, without fear of accidental activation. Implementa- tion of improvements in the areas noted should make the French Electric Hand a valuable prosthetic device.

ACKNOWLEDGMENTS

Richard N. DeCallies, MS., and Ranjit K. Roy, B.S., contributed to the evaluation while they were members of the Biotechnology Laboratory Staff. Mr. Carl Sumida of the Child Amputee Prosthetics Project, UCLA, provided technical advice and assisted with prosthetic fitting. Their assist- ance is gratefully acknowledged.

REFERENCES

1. TAYLOR, C. L.: Investigation of Upper Extremity Prosthetics in Europe and the Near East. Department of Engineering, University of California, Los Angeles, Biotechnology Laboratory Special Technical Report No. 22, September 1954.

2. WILMS, E.: French Patent 1,018,865, April 6, 1950, Prosthetic Apparatus. 3. WILMS, E.: French Patent 1,018,866, April 6, 1950, A Switching Apparatus

Designed to Run An Electrically Powered Work Tool.

4. WILMS, E.: German Patent 895,044, October 29, 1953, Prosthesis.

5. WILMS, E.: German Patent 904,793, February 22, 1954, Arrangement for the Actuation of An Electrically Controlled Work Tool.

6. GROTH, HILDE: A Survey of Research Activities in Western Europe for Se- lected Areas of Biotechnology. Department of Engineering, University of California, Los Angeles, Biotechnology Laboratory Technical Report No. 9, April 1961.

7. SANTSCHI, W.: Manual of Upper Extremity Prosthetics, 2nd Edition. Depart- ment of Engineering, University of California, Los Angeles, 1958.

8. KELLER, A. D., TAYLOR, C. L., AND ZAHM, V.: Studies to Determine the Functional Requirements for Hand and Arm Prosthesis. Department of Engi- neering, University of California, Los Angeles, July 1947.

9. FLETCHER, M. J. AND LEONARD, F.: The Principles of Artificial Hand Design. Artificial Limbs, Vol. 2 ( 2 ) : 78-94, May 1955.

the french electric hand: some observations and …

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