Prosthetic limbs

Abstract: A prosthetic limb system comprises a prosthetic limb and activation means therefor attachable to an amputee stump, in which the activation means comprises means for sensing myoelectric signals generated at the stump surface by specified muscles, means for filtering the sensed signals, means for inputting the filtered signals to an artificial neural network and means within said neural network for classifying input signals with muscle contractions representative of desired digit movements; and the prosthetic limb comprises servo-drive means to enable digits thereof to perform specified mechanical movements, said classified signals being operatively connected to said servo-drive means to enable one or more of said digits to perform an actual movement on receipt of an appropriate desired movement from said artificial neural network.

Description: This invention relates to prosthetic limbs and especially to prosthetic limbs having distal free digitate appendages such as prosthetic hands. Prosthetic hands for arm amputees have been available for several decades but until the 1970's were generally cable-operated. More recently, a prosthetic hand has been developed in which the thumb and coupled index and middle fingers cooperate in a single degree of freedom pincer-like action, where the action is controlled by sensing the myoeletric signals (MES) generated at the skin surface of the stump when the brain sends a "muscle contract" command. For example, two MES sensors may be applied to the skin surface below the elbow and, when the sensed MES exceed a specified threshold, the thumb and coupled finger open or close. A prosthetic limb responding to MES is known from EP-A-421 780. Further work on MES has established that the amplitude and frequency thereof vary for different hand movements for a given muscle site, and that the MES signature for a particular movement is reasonably consistent and repeatable. It is an object of the present invention to provide a prosthetic limb, especially an arm, where the prosthetic hand attached thereto can perform more realistic digit movements than have hitherto been possible. According to one aspect of the present invention, there is provided a prosthetic limb system according to the subject-matter of claim 1. According to another aspect, the invention provides a method for controlling a prosthetic limb, according to the subject-matter of claim 6. Preferably, the prosthetic limb comprises a prosthetic hand and the elements thereof are an artificial thumb and at least one artificial finger. The sensing means are preferably disposed at selected zones of the skin of the stump, according to the limb movement which it is intended to provide, since it is known that myoelectric signals reflecting or appertaining to particular limb movements and tissue filter functions occur at discrete sites on the upper limb. The sensed signals are amplified and normalised and are preferably filtered and classified according to their frequency content in a perception subsystem. The signals are converted from the time to the frequency domain by the use of bandpass filters operating within specified frequency ranges between 0 and 1000 Hz, preferably 0 - 400 Hz, and processed to calculate the RMS value of each filter output, preferably with performance standards for speed of recognition (e.g. <0.4 seconds), percentage of non-recognitions (e.g. <5%) and percentage of mis-recognitions (e.g: <0.5%). The artificial neural network may comprise interconnected input, hidden and output layers and can be trained to recognise and react to specific patterns; the outputs are associated with discrete limb positions. Preferably, a back-propagation learning algorithm is utilised to train the logic base of the neural network, preferably modified with added momentum and learning coefficients. A neural network with three binary output signals would provide up to eight desired hand positions. The decoded myoelectric signals from the neural network are passed to a fuzzy logic classifier in order to provide a definitive desired hand position. This is then passed to a cognition system which will compare the actual finger positions with a suite of stored desired finger positions, any difference existing being used to actuate the fingers until the difference is zero. Significant performance criteria would include stability over all operating regimes and a fast response time with no overshoot; desirably the settling time should be short, (e.g. <0.8 seconds). All processing calculations including digital filtering by the use of bandpass filters, neural network pattern recognition and post-processing can be undertaken within one sampling period, typically one-thousandth of a second. The servo-drive means preferably, for a prosthetic hand, will operate five active fingers in as many as eleven degrees of freedom of movement within performance criteria e.g. size of actuator, range of movement, resolution and repeatability, linearity, force to signal ratio, quietness of operation and power consumption. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings of which: Fig. 1 is a block diagram showing the parts of an entire prosthetic hand system according to the invention; Fig. 2 shows myoelectric signal spectra from four specific muscle contractions; and Fig. 3 shows three useful positions of a virtual robot hand which may be achieved from the application of the system of Fig. 1 in sensing myoelectric signals from an upper limb. In Figure 1, the parts shown enclosed in the box defined by a dashed line correspond with the activation means of the present invention. In Figure 2, the four specific positions of the hand are as follows:- S1: hand relaxed with fingers and thumbs extending straight; S2: ring finger bent so that tip touches the palm; S3: hand relaxed with wrist bent inwards towards the palm; and S4: little finger bent inwards to touch the palm. The myoelectric signals were obtained from an individual but it has been found that, although different individuals produce their own unique spectral identities, nevertheless the overall relative pattern for a given movement or position remains reasonably consistent. In particular, the frequency components of the myoelectric signal from a single sensor carry sufficient information to discriminate between several hand positions. This information is amplitude-independent. However, the amplitude of the signals is a function of the force applied and may be used to provide a force-sensitive component to the hand movements. With reference to Figure 3, three useful positions of a prosthetic hand are illustrated; such positions can be provided from the control system acting on the myoelectric signal spectrum of Figure 2 and are not intended necessarily to replicate the natural movements for a given myolelectric signal but rather to provide useful functionally-equivalent positions which can be reliably controlled using simple muscle movements of the upper limb. The positions shown are a) hand relaxed; b) parallel grasp and c) chuck grasp.

Claims: A prosthetic limb system comprising a prosthetic limb and activation means therefor attachable to an amputee stump, in which the activation means comprises means for sensing myoelectric signals generated at the stump surface by specified muscles, means for filtering the sensed signals, means for inputting the filtered signals to an artificial neural network and means within said neural network for classifying input signals with muscle contractions representative of desired digit movements; and the prosthetic limb comprises servo-drive means to enable digits thereof to perform specified mechanical movements, said classified signals being operatively connected to said servo-drive means to enable one or more of said digits to perform an actual movement on receipt of an appropriate desired output signal from said artificial neural network, characterised in that the filtering means comprises bandpass filters operating within specified frequency ranges between 0 and 1000 Hz. A system according to claim 1, in which the prosthetic limb comprises a prosthetic hand and the digits thereof are an artificial thumb and at least one artificial finger. A system according to claim 1 or claim 2, in which the artificial neural network comprises interconnected input, hidden and output layers and is trained to recognise and react to specific patterns. A system to any preceding claim, in which the frequency components of the myoelectric signal from a single sensing means carry information sufficient to enable the activation means to discriminate between several hand positions. A system according to any preceding claim, in which the classified signals are passed to a cognition system comprising a fuzzy logic classifier including a rule-based system using fuzzy inference techniques for decision making. A method for controlling a prosthetic limb, the method comprising the use of sensing means for measuring myoelectric signals generated by a specified muscle at the surface of an amputee stump, digitally filtering said signals, applying said digitally filtered signals to an artificial neural network programmed to produce an output signal for a prosthetic limb representing a desired limb movement for a given input signal, and causing elements of said limb to move in response to said output signal, characterised in that the signals are converted from the time to the frequency domain by the use of bandpass filters operating within specified frequency ranges between 0 and 1000 Hz. A method according to claim 6, in which the sensing means are disposed at selected zones of the skin of the stump, according to the muscle activity which is available within the stump and the tissue filter functions which occur within said stump. A method according to claim 7, in which the frequency components of the myoelectric signal from a single sensing means carry information sufficient to enable the output signal to be representative of several limb positions. A method according to any of claims 6 to 8, in which the sensed signals are amplified and normalised and are preferably filtered and classified according to their frequency content in a perception subsystem. A method according to any of claims 6 to 9, in which the frequency-domain signal are processed to calculate the RMS value of each filter output, preferably with performance standards for speed of recognition, percentage of non-recognitions and percentage of mis-recognitions. A method according to any of claims 6 to 10 in which the decoded myoelectric signals from the neural network are passed to a fuzzy logic classifier in order to provide a definitive desired hand position. A method according to any of claims 6 to 11 in which the definitive desired hand position is passed to a cognition system which will compare the actual finger positions with a suite of stored desired finger positions, any difference existing being used to actuate the fingers until the difference is zero.

Inventors: BURNS ROLAND STEPHEN, NURSE PETER, BURNS, ROLAND STEPHEN, NURSE, PETER
Applicants: ADVANCED CONTROL RES LTD, ADVANCED CONTROL RESEARCH LTD.
Application year: 2000
Main classification: A61F2
IPC codes: A61F2