Shoulder disarticulation prosthetic system

Abstract

The invention is an externally powered prosthesis for shoulder disarticulation amputees. Proportional control of the prosthesis is provided by a displacement sensor preferably comprised of a movable magnet and a stationary semiconductor element which responds to changes in magnetic field strength. The movable magnet is displaced by the transverse motion of the amputee's skin (caused by controlled flexure of a suitably located muscle), thereby producing a signal in the semiconductor element which is proportional to the skin motion. Control of the prosthesis is then accomplished by amplification and utilization of the signal to drive an electric motor powered by an external battery.

Description

STATEMENT OF GOVERNMENT INTEREST

The invention herein described was made in the course of or under a contract, or subcontract thereunder, with the Department of the Navy.

CROSS-REFERENCE TO RELATED APPLICATIONS

The subject matter of the invention relates to a copending patent application, entitled "Myoelectrically Controlled Prosthesis," Ser. No. 114,262, filed Feb. 10, 1971 and now U.S. Pat. No. 3,735,425.

BACKGROUND OF THE INVENTION

The invention relates to prosthetic and orthotic systems wherein a terminal device is opened in direct proportion to voluntary movement by a user. More particularly, the invention is a prosthesis especially suited for shoulder disarticulation amputees, the prosthesis being controlled by the signal produced by a motion transducer in response to voluntary skin movement. The high-level, essentially DC signal thus produced may then be conditioned and fed into known servo electronics in order to eventually drive a geared-down DC motor which, in turn, drives a mechanical linkage for operating the moving parts of the prosthesis.

SUMMARY OF THE INVENTION

The present invention is particularly useful for prosthetic applications where suitable electromyographic signal sites are not available for "emg" control of a prosthesis. Such situations require that a signal be produced by other means in order to control an externally powered prosthesis. Skin motion on the remaining portion of an amputee's stump is used in the present invention to proportionately control the prosthesis. The motion of the skin is sensed by a motion transducer, the output of which transducer is a signal which may then be conditioned and used to operate a battery-powered servo electronics and motor system. A terminal device on the prosthesis is opened in direct proportion to the signal amplitude as controlled by the user's movement of a particular skin location. Essentially, skin motion produces a control signal which causes a servo control unit to drive a DC motor, consequently opening the prosthesis. The prosthesis begins to open until a feedback voltage proportional to the opened position is equal to the control signal. In this manner, the terminal device of the prosthesis is servo controlled for all positions between fully closed and fully opened. In a similar fashion, elbow motion of the prosthesis may be controlled.

A primary object of the present invention is to provide a prosthetic system suitable for those amputees who do not possess "emg" sites of sufficient strength to control a myoelectrically operated prosthesis.

A further object of the invention is to provide a motion sensor system for a prosthesis whereby voluntary movement of an amputee's skin produces a proportional signal which may be utilized to operate the prosthesis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating the major components of the invention;

FIG. 2 is a basic block diagram of the invention;

FIG. 3 is a schematic of the control unit;

FIG. 4 is a chart illustrating the input and output waveforms of the pulse width modulator;

FIG. 5 is a schematic of the power unit;

FIG. 6 is a cross-sectional view of the power unit; and

FIG. 7 is an idealized perspective of a prosthesis configured according to the invention, certain parts being shown in phantom.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a generalized system diagram illustrating the basic elements of the present invention is provided. Voluntary skin movement at 10 is detected by a motion transducer 12, the signal 11 from the transducer 12 being conditioned at 14 before being fed into a control unit 16. The control unit 16 comprises servoelectronic mechanisms well-known in the art and will be described in detail hereinafter. External power is provided to the control unit 16 by battery 18, the control unit controllably supplying power to power unit 20 according to the signal 11 and also according to a position feedback signal 13 generated by power unit 20. The power unit 20 is comprised of a DC torque motor (not shown) which drives a cable 22 for operation of the prosthesis 24 in a well-known manner.

FIG. 2 provides a more detailed diagrammatical illustration of the present prosthesis system. The system is essentially a closed-loop position servo arrangement with position feedback that follows the signal 11 generated by the motion transducer 12 in response to voluntary skin movement at 10. The motion transducer 12 preferably comprises a magnet 30 attached to a string 32, the string 32 being joined to a user's skin by an adhesive tab 34. Movement of the user's skin causes the string 32 to displace the magnet 30 in a lateral direction, this displacement being sensed by a magnetic-field sensitive semiconductor unit 36. The semiconductor unit 36 is comprised of semiconductor elements (not shown) which sense changes in the magnetic field experienced by the unit 36. The unit 36 is known in the electronic arts and, on exposure to a changing magnetic field such as is produced by the movement of the magnet 30, produces a signal proportional to the change in said field. Therefore, the signal 11 is proportional to the movement of the user's skin at 10. Electronic excitation is applied to the transducer 12 by signal conditioning circuit 14 which circuit 14 also provides gain control before the signal is applied to the control unit 16. The signal applied to the control unit 16 is a DC control signal that is approximately proportional in amplitude to the amplitude of the skin movement detected by the motion transducer 12.

Signal utilization is now accomplished in a known fasion that will be described for convenience. In order to minimize electrical power consumption a pulse-width modulation system is used to control the output torque of a DC torque motor 38 which actuates the prosthetic device 24. More specifically, a pulse-width modulator 40 utilizes the output of a triangular wave generator 42 and the output 43 of a summing amplifier 44 to provide a pulse-width modulated signal which controls motor current. The DC torque motor 38 is driven in one direction only. The output 43 of the summing amplifier 44 corresponds to the difference between the amplitude of the signal 11 and the position of control cable 46 as represented by a shaped position signal 47 of a lead-lag circuit 48. Since output 43 of the summing amplifier 44 reflects the above mentioned difference, it will hereinafter be referred to as error signal 43. If the amplitude of error signal 43 is small, current in the motor 38 flows for a small part of the duration of the output of the triangular wave generator 42. Accordingly, the "on" time of the motor 38 is a function of the magnitude of the error signal 43. By operating a power switching transistor 50 in a power switching mode and causing the motor 38 to operate only when the output of the lead-lag circuit 48 is less than the signal 11, relatively little power is dissipated. When the motion transducer 21 is not sensing skin displacement, standby power consumption in the electronic component is quite low, i.e., less than 300 milliwatts No mechanical switches or special power cutoff relays or circuits are required to switch from "standby" to "operate" condition.

A potentiometer 52 supplies the shaped position signal 47 via the lead-lag circuit 48 to the summing amplifier 44. This feedback arrangement provides high gain at low frequencies and less gain as frequency is increased. Such signal processing makes opening of a terminal device on the prosthetic device 24 relatively easy to control at all elbow flexion positions with or without an object in the terminal device. This control technique facilitates a simple interface between the amputee and his prosthesis. The amputee need generate only one signal when he desires to open the terminal device on the prosthesis, the terminal device being automatically closed by spring or elastic action when the user's skin is relaxed and returned to normal position. Thus, an amputee maintains a grasp force without additional effort or attention on his part. When the amputee wishes to disengage the terminal device, he again moves his skin, which motion is detected by the transducer 12. The signal 11 thus produced provides the voltage needed for opening of the terminal device, thereby freeing the object being grasped.

Referring now to FIG. 3, a schematic diagram is shown of the control unit 16. The control unit 16 is known in the art, having first being described in the copending patent application referenced hereinabove. Due to the differing inputs into the control unit 16 of the present invention when compared to the aforementioned patent application, a brief description of said unit 16 follows. The signal 11 generated by the motion transducer 12 in response to movement of a user's skin is passed to the summing amplifier 44 and then to the pulse width modulator 40. The output of the pulse width modulator 40 is subsequently applied to the power unit (not shown) which ultimately actuates the prosthetic device (e.g., a hand or an elbow). In order to minimize electrical power consumption within the power unit, the pulse width modulation system is employed. The pulse width modulator 40 utilizes the outputs 60 and 62, respectively, of the triangular wave generator 42 and the output of a servo amplifier 64 to provide a pulse-width modulated signal 66 which regulates motor current.

In order to more clearly describe the operation of the control unit 16, discussion of the lead-lag circuit 48 and the associated feedback loops will be temporarily omitted. The output signal 62 of summing amplifier 44 represents a DC signal that is proportional to skin movement. This DC signal is applied to input 68 of the pulse width modulator 40. Also, the output 60 of the triangular wave generator 42 is received at input 68. The operation of the pulse width modulator 40 can best be explained by additional reference to FIG. 4. The DC output 43 of the servo amplifier 64 is shown by waveform (a) of FIG. 4. In this condition, there is no sensed signal being received from the motion transducer. The DC output signal as shown in waveform (a) is obtained from a potentiometer 70 and applied to input 72 of operational amplifier 74. The output, E.DELTA., from output terminal 76 of the triangle wave generator 42, shown by waveform b) of FIG. 4, is applied to the negative input 68 of the operational amplifier 74. Waveforms (a) and (b) are combined within the amplifier 74 to produce the summed waveform (c) of FIG. 4. As long as the summed waveform is less than 0 volts, the output of the pulse width modulator 40 is as illustrated by waveform (d) which represents the control unit 16 in the "off" condition. Thus, the prosthesis does not respond since there is no effective enabling signal. When the motion transducer senses movement of the skin, there is applied to input 78 of the amplifier 64 a positive DC signal. When combined with the threshold level signal as produced by the potentiometer 70, the signal as represented by waveform (e) is thus applied to input 68 of amplifier 74. When waveform (e) is combined with the E.DELTA.-waveform (f), the resultant signal as shown by the upper half of waveform (g) is produced. Whenever the upper peak of waveform (g) exceeds 0 volts, the pulse width modulator 40 shifts from the non-enabling Ep.sub.1 voltage level to the enabling Ep.sub.2 level. In this manner does the prosthetic device actuate only when the sensed skin motion exceeds a predesignated (and variable) threshold. As the prosthesis is being actuated its physical position is indicated by the wiper arm of a position potentiometer (not shown). The feedback position signal 80 is applied to the lead-lag circuit 48 at terminals 82 and 84. The function of the lead-lag circuit 48 is to prevent actuation of the prosthesis by a short term, high gain signal, e.g., noise. Via resistors 86, 88, 90 and 92 and capacitor 94, the prosthesis will only respond to a long term signal, thereby preventing the prosthetic device from continually opening and closing upon every sensed signal.

Referring to FIG. 5, there is shown a schematic diagram of the power unit. The power unit is also known in the art but is briefly described hereinafter to provide a more complete description of the system used to operate a prosthesis according to the present invention. Emergent from the pulse width modulator 40 is a series of enabling pulses 103, as shown by the lower portion of waveform (g) of FIG. 4. These pulses are applied to the motor 38 after being amplified by power transistor 100. Transistors 102 and 104 serve as driver transistors for the transistor 100. Upon energization, the rotation of the armature of the motor 38 causes like rotation in gear reduction means 106. After the necessary gear reduction is accomplished the control cable 46 is connected to a pulley that is attached to the last gear element (not shown). Attached to the end of the control cable 46 is the prosthetic device 24. In this manner, rotation of the motor shaft causes activation of the prosthetic device 24. Also connected to the gear means 106 is the wiper arm 108 of the potentiometer 52. In this manner, position feedback signal 80 is provided for the lead-lag circuit 48. A transient suppression circuit is provided for the power unit for inhibiting undesired interference with power transistor 100 and driver transistors 102 and 104 when the motor 38 is switched on and off. The transient suppression circuit consists of resistor 110, zener diodes 112 and 114, and diode 116.

The gear box reduction means 106 is shown in the cross-sectional drawing of FIG. 6. The output signal of power transistor 100 is applied to brush assembly 118 causing rotation of the rotor 120, and thus rotor pinion 122. Rotation of rotor pinion 122 induces rotation in spur gear 124 and pinion gear 126. Final gear reduction is accomplished by spur gear 128 to which is attached at its upper end pulley 130 and at its lower end potentiometer 52. Trained around pulley 130 is the control cable 46 which actuates the prosthetic device. The position feedback signal is provided by the potentiometer 52 and wiper arm 108, as previously described.

FIG. 7 illustrates an embodiment of the present prosthetic system which is capable of elbow movement. The prosthesis 150 is particularly suited to individuals who have shoulder disarticulation amputations. In such situations, the signal to be controllably generated by the amputee for the purpose of driving the prosthesis is derived from voluntary movement of the skin surface 152 near the amputation site. As has been described previously, an adhesive tab 154 disposed on the skin surface 152 holds a string 156 which is connected to a magnet (not shown). The magnet comprises a portion of a motion transducer 158 which senses the movement of the skin surface 152. The output signal of the transducer 158 is directed to a control unit 160 via signal cable 162. Control signals from the control unit 160 to the power unit 164 as well as position feedback signals from the power unit 164 to the control unit 160 are transmitted via signal cable 166. Upon receiving energization commands from the control unit 160, a motor within the power unit 164 causes the control cable 46 to retract (as described previously), consequently opening the fingers 168 on a prosthetic hand 170 or the digits of a prosthetic hook. Since the operation of the prosthetic hand 170 is well-known in the art, description of this mechanical process has been deemed unnecessary. Retraction of the cable 46 causes forearm 172 to move upwardly if the elbow joint 174 is unlocked. When the forearm 172 is in the desired position, the amputee pulls on a locking cable 176 by means of a shoulder harness 178, thereby causing lever arm 180 to lock the forearm 172 into position. When the forearm 172 is locked into position via the lever arm 180, further retraction of the control cable 46 causes the fingers 168 of the hand 170 to open. The motor and power supply may be placed within the physical confines of the prosthesis 150 or may be disposed on a special belt to be worn about the amputee's waist to reduce prosthesis weight.

Claims

We claim:

1. A control system for actuating a prosthesis from voluntary skin movement of a user, comprising:

a prosthesis;

displacement sensing means for measuring the amplitude of movement of a point on the skin of a user of the prosthesis, the sensing means comprising

a magnet,

connecting means joined to the magnet and adapted to be connected to a point on such user's skin, and

magnetic field sensitive means for sensing a change in the magnetic field on displacement of the magnet by motion of that point on such user's skin to which the connecting means is joined;

means for actuating said prosthesis in response to said sensed skin movement; and

means for providing a position feedback signal to said actuating means for identifying the degree of actuation imparted to said prosthesis.

2. The control system of claim 1, and further comprising means for delaying in time said position feedback signal.

3. A control system for actuating a prosthesis from voluntary skin movement of a user, comprising:

a prosthesis;

means for sensing displacement of a point on the skin of a user of the prosthesis, said means comprising,

a magnet,

connecting means joined to the magnet and adapted to be connected to a point on such user's skin, and

magnetic field sensitive means for sensing a change in the magnetic field occurring on displacement of the magnet relative to said magnetic field sensitive means, the displacement of the magnet occurring due to voluntary motion of that point on such user's skin to which the connecting means is joined;

means for converting the sensed skin displacement into a DC level signal having an amplitude proportional to the magnitude of the skin displacement;

means for identifying the exact physical position of said prosthetic device and for producing therefrom a position feedback signal;

means for actuating said prosthesis in response to said sensed skin displacement, said actuation means providing an electrical signal sufficient to actuate said prosthesis and including a summing amplifier for producing said electrical actuation signal when the resultant threshold output amplitude of said means for converting the sensed skin displacement into a DC level signal is greater than the amplitude of said position feedback signal;

means for converting said electrical signal into mechanical energy; and,

means for producing a control signal effective to actuate said prosthetic device only when the sensed skin displacement is above a predetermined threshold level, said degree of prosthetic actuation being proportional to the amplitude of said sensed skin displacement, said means for producing said control signal comprising a standard waveform generator producing a predetermined amplitude signal, said electrical actuation signal being generated when said output amplitude of said summing amplifier is greater than said predetermined amplitude of the standard waveform signal.

4. The control system of claim 3 wherein the magnetic field sensitive means is a semiconductor.

5. The control system of claim 3 wherein said means for producing a position feedback signal comprises a potentiometer whose wiper arm is mechanically actuated in direct proportion to the degree of actuation imparted to said prosthesis, the voltage sensed by said wiper arm being received by said actuating means.

6. The control system of claim 3 and further comprising means for delaying in time said position feedback signal.

7. The control system of claim 3 wherein said means for converting said electrical signal into mechanical energy comprises a DC motor.