GB2098489A

GB2098489A - Signal processing apparatus

Info

Publication number: GB2098489A
Application number: GB8214335A
Authority: GB
Original assignee: HODGSON DR JOHN ASHWORTH
Current assignee: HODGSON DR JOHN ASHWORTH
Priority date: 1981-05-15
Filing date: 1982-05-17
Publication date: 1982-11-24
Also published as: GB2098489B

Abstract

Signal processing apparatus comprises an amplifier 1 receiving a signal from a mammalian body on input terminals 10. The amplified signal is passed via a rectifier 2 to an integrator 3. The output of the integrator 3 is fed to a trigger stage 4, which responds to reset the integrator 3 each time the integrator output reaches a predetermined value. Each time the integrator 3 is reset, the trigger stage 4 emits a pulse which is amplified by an output stage 5, which thus provides an output pulse train, the amplitude of the pulses being adjustable up to 100 volts. The repetition frequency of the output pulse train is thus proportional to the average amplitude of the original signal at the input 10, and the output signal can be fed directly to a body to control a bodily response, or alternatively may be fed to a prosthesis, to simulate a bodily response. The output of the trigger stage may alternatively pass to a frequency-to-voltage converter to give an output with an amplitude related to the average amplitude of the input at terminal 10. The invention has more general application, in processing an input signal to provide a pulse train wherein the interval between each two successive pulses is proportional to the amplitude of the input signal since the first of the two pulses. In a modified stimulator, rectifier 2 is omitted, integrator 3 is arranged to give an output determined largely by the predominating polarity of the input, and trigger 4 may have positive and negative level triggers giving pulse trains with frequencies respectively dependent on positive and negative input signals. In a further variant, the circuitry in Fig. 1 following integrator 3 is omitted, and reset pulses are applied to the integrator 3 at a constant rate fast enough to prevent saturation therein, so that the integrator outputs immediately before a reset pulse is proportional to the average level of the input signal between reset pulses, and this output is applied to a sample and hold circuit or an analog to digital converter. <IMAGE>

Description

SPECIFICATION Signal processing apparatus This invention relates to signal processing apparatus, and is particularly although not exclusively concerned with apparatus for use in controlling a response of a mammalian body.

To date, experiments have been undertaken in several laboratories to record electrical signals from a mammalian body, and to process such signals such that they are suitable for use in driving prostheses. However, the processing of such signals tends to be of a fairly primitive nature, in that the signals are used merely as triggers. That is, function of a prostheses is controlled in dependence upon presence or absence of a signal, and the signals are not used as a source of information in any longer term sense.

Experiments have also been done to assess the benefits of stimulation of various areas of the body for the treatment of various disorders. Such experiments use electronically generated signals which are completely synthesised.

Preferred embodiments of the invention aim to provide apparatus which may be used to detect a signal from an area of a body, and to apply a stimulating current which is related to the detected signal, to the same or a different part of the body.

More generally, according to a first aspect of the present invention, there is provided signal processing apparatus including first means for processing a first signal from a body to produce a control signal which has a characteristic proportional to the amplitude of said first signal, and second means for feeding said control signal to said body thereby to control a bodily response.

Preferably, said first means is adapted to process said first signal when detected from a mammalian body.

The apparatus may optionally include detecting means for detecting said first signal.

Preferably, said characteristic is a frequency characteristic of said control signal. In such a case, the control signal may advantageously comprise a pulse train the repetition frequency of which is proportional to said amplitude of said first signal.

In an advantageous embodiment, said first means comprises an integrator arranged to receive said first signal or a signal proportional thereto, and reset means arranged to reset the integrator when the output thereof attains a predetermined limit value.

Said reset means may comprise trigger means arranged to produce an output pulse each time the output of said integrator reaches said limit value, which output pulse is arranged to reset the integrator.

The output pulse that is used to reset the integrator may also be used to drive an output stage of the apparatus. Alternatively, the trigger means may be arranged to provide a further output pulse each time the output of the integrator reaches said limit value, which further output pulse is used to drive such an output stage.

Alternatively the output pulse may be transmitted to a frequency to voltage converter, thus providing an analogue output related to average amplitude of said first signal.

Said first means preferably includes an amplifier arranged to receive said first signal Preferably, the amplifier is followed by rectifier means connected to receive the output of the amplifier and provide a rectified signal to the input of the integrator.

Said first means is preferably arranged to receive an input signal in the range 0 to 2 m.V., and the apparatus is preferably arranged to process signals in the frequency range 50500Hz.

Preferably, the apparatus includes noise rejection means for rejecting unwanted noise.

The apparatus preferably includes means for blanking processing of said first signal during emission of said control signal.

In a variant of the invention, said control signal may be used to control operation of a prosthesis, rather than for controlling a bodily response directly.

Therefore, in certain embodiments of the invention, said second means may be omitted.

For better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: Figure 1 is a circuit diagram of a preferred embodiment of the invention; and Figure 2 shows examples of wave forms occurring at certain points in the circuit of Figure 1, in use.

The illustrated apparatus is adapted for use in receiving a first signal from a human body (for example), and processing that signal to produce a control signal which can be used to control a bodily response. One example of where the apparatus may be used is with a stroke patient suffering from flaccid paralysis. Some slight activity is detected in an affected muscle as an electrical signal, which signal serves as the first signal which is input to the illustrated apparatus.

The signal is then processed in the illustrated apparatus, to provide a control signal which is fed directly back to the same muscle to produce a significant force. Such operation will become more readily apparent form the following description.

The apparatus comprises five stages arranged in series-namely an amplifier stage 1, a rectifier stage 2, an integrator stage 3, a trigger stage 4, and a stimulator stage 5, which latter stage is the output stage of the device.

The amplifier stage is arranged to receive a first input signal on terminals 10, which input signal is fed to a differential amplifier 11. The amplifier 11 is arranged to affor a high input impedance, and in the illustrated example, it has a gain of approximately 1,000. Capacitors 12 serve to AC.

couple the input terminals 10 to the amplifier 11.

The output of the amplifier 11 is fed to two parallel half-stages 20 of the rectifier stage 2. The circuit configurations of the two half-stages 20 are almost identical, and each half-stage is arranged to provide substantially unity gain.

However, one of the half-stages 20 is arranged to pass only positive signal portions, whilst the other half-stage is arranged to pass only negative signal portions. Also, one of the half-stages is arranged to provide inversion of the input signal, while the other half-stage'is arranged in a non-inverting manner.

The rectified signal appearing at the output of the rectifier stage 2 is fed to a variable resistance 30 at the input of the integrator stage 3. The variable resistance 30, together with a capacitance 31 , define an R.C. time constant of an operational amplifier 32, which is arranged as an integrator. A transistor 33, arranged as a switch, is provided to discharge the capacitance 31, and thereby reset the integrator stage 3.

The output of the integrating amplifier 32 is fed as an input to a trigger 40 of the trigger stage 4.

The trigger 40 has offset switching thresholds, such that it changes state when the output of the integrating amplifier 32 reaches a first predetermined limit value, and changes state again when the output of the integrating amplifier 32 falls below a second predetermined limit value, which is greater in magnitude that the first limit value.

The output of the trigger 4 is connected to inputs of respective monostable devices 41 and 42.

The first monostable device 41 is arranged to respond to a change of state of the trigger 40 corresponding to the output of the integrating amplifier 32 reaching said first limit value, whereupon the monostable device 41 emits a 2mS (for example) pulse, which is operative to switch on the switching transistor 33, and thereby reset the integrator stage 3.

The second monostable device 42 responds in a similar manner to the monostable device 41, but emits a shorter pulse in 1 mS duration, which pulse is fed to an input transistor 50 of the stimulator stage 5. All the circuitry up to and including the input transistor 50 is powered from low voltage-in this example, +volts.

The input transistor 50 in turn drives a further transistor 51, which is powered by a relatively high potential source-in this example, +100 volts. A variable resistance 52 is connected in the emitter path of the transistor 51, and an input to an output transistor 53 is tapped from the variable resistance 52 such that, upon a 1 mS pulse appearing at the base of the input transistor 50, the transistors 51 and 53 are switched on accordingly, such that a corresponding 1 mS pulse appears at output terminals 54 in the collector path of the output transistor 53. The value of the output pulse on the terminals 54 depends upon the selective setting of the variable resistance 52, and may be in the range substantially 0--100 - .

volts.

Figure 2 shows examples of signal wave forms' at various parts of the circuitry, in use.

There are four rows and three columns in Figure 2. In each of the rows numbered 1 to 4, there are shown wave forms for a particular type of input signal. Row 1 corresponds to the case wherein there is low activity of a patient. Rows 2 and 3 correspond to cases wherein there is medium activity of a patient. Row 4 corresponds to the case wherein there is high activity of a patient.

The first column shows wave forms at the output of the amplifier stage 1. The second column shows wave forms at the output of the integrator stage 3. The third column shows a wave form at the output of the trigger stage 4.

Looking first at row 1, it will be seen that there is a small ripple voltage at the output of the amplifier stage 1. This ripple voltage corresponds to 50 H.z. noise. This noise is rectified in the rectifier stage 2, and drives the integrator stage 3 to provide an output as shown in the second column.

It is in fact desired to filter out the 50 H.z. noise shown in row 1, and this can be done at any convenient stage in the apparatus. For example, the noise may be filtered out by any suitable frequency filter up to the integrator stage 3.

Alternatively, the amplifier 11 may be provided with an offset so that any noise up to a predetermined level falls below a cut-off level of the rectifier stage 2. As a modification on the same theme, the rectifier stage 2 may itself be provided with an offset level, below which input noise is ignored. Alternatively, the integrator stage may be so arranged that it ignores noise below a predetermined level or frequency.

As yet another alternative to noise elimination, means may be provided for resetting the integrator stage a predetermined time after the last trigger pulse from the trigger stage 4. Thus, with noise below predetermined level, the output of the integrator stage cannolt reach a level sufficient to trigger the trigger stage 4. A further alternative is to provide means which does not pass to the stimulator stage 5 any two pulses having a time interval between them which is greater than a predetermined minimum interval.

Considering now row 2 in Figure 2, there is shown in the first column the output of the amplifier stage in the case of medium activity of a patient. In this case, stainless-steel electrodes, of 1 1 centimetre diameter and 50k. Q impedance, are applied to the skin in the vicinity of an affected muscle. When a brain stimulus signals the muscle to function, the electrode detects a signal of the form shown in the first column of row 2. This signal is rectified by the rectifier stage 2, and drives the integrator stage 3 to provide an output of the form shown in the second column of row 2.

The third row 3 shows an output at the amplifier stage 1, which is generally similar to the amplifier output shown in row 2. However, in row 3, it is the wave form at the output of the trigger stage 4 that is shown, rather than the wave form at the output of the integrator stage 3. Thus, in the third column of row 3, there can be seen a pulse train, of which the repetition frequency is proportional to the average of the amplitude of the detected signal fed to the input terminals 10.

As indicated above, the output of the trigger stage 4 drives the stimulator stage 5, whereby a wave form similar to that in the third column of row 3 is obtained, the pulses of the train now having an amplitude between 0 volts and 100 volts, as determined by setting of the potentiometer 52.

In row 4, the first column shows an amplifier output which is representive of a high activity signal at the respective muscle. As can be seen, the amplitude of the signal is much greater than in the previous case. Thus, as may be seen in the second column of row 4, the output of the integrator stage 3 has a considerably higher frequency, leading to a considerably higher repetition frequency of the pulse train at the output of trigger stage 4 (not actually shown in this case).

In this instance the intervals between output pulses, here represented as integrator resets, would be shorter than intervals between successive peaks in the input signal.

It may thus be appreciated that, where the patient has, for example, flaccid paralysis of the respective muscle, the illustrated apparatus is operative to detect an electrical signal representing a small degree of activity in the muscle, and subsequently amplify the process the signal to provide a control signal at the output of the stimulator stage 5, which control signal is fed back to the affected muscle, to provide a considerably amplified response thereof. The response of the muscle will depend upon the repetition frequency of the pulses from the stimulator stage 5, and also the setting of the potentiometer 5, which in turn determines the output voltage at the terminals 54.In such a condition as described, it is important to understand that an affected muscle can in itself be quite capable of providing a desired force, and that it is the neural circuitry controlling the muscle that has become affected, such that ordinarily the muscle is of little or no use. There are a number of medical conditions in which these conditions can occur, and apparatus as illustrated can be of immense value in restoring to a patient full (or improved) response of a muscle which is otherwise of at least some degree of uselessness.

In operation of the illustrated apparatus, it will be appreciated that the 2 m.S. pulse from the first monostable device 41 not only resets the integrating amplifier 32, but also blanks operation thereof whilst the 1 m.S. pulse is being applied to the respective muscle from the terminals 54.

Thus, the apparatus is so arranged as to ignore at the input terminals 10 any effect of control pulses appearing at the output terminals 54. The width of pulses at the outputs of the monostable devices 41 and 42 can be adjusted as circumstances require, in order to meet this criterion.

Although the illustrated apparatus has been described with reference to use with a patient suffering from flaccid paralysis of a muscle, it will nevertheless be appreciated that it may be used in any circumstance where it is desired to process a signal from a body, in order to produce a control signal for controlling a bodily response.

As outlined above, the apparatus may be adapted for use in driving or controlling a prosthesis, rather than controlling a bodily response directly.

It may be appreciated that, in the wave form shown in the first column of Figure 2, there are many frequency components present (other than in row 1), and this is typical of an electrical signal generated by the activity of a muscle as it develops force. However, for the purposes of the illustrated apparatus, the highly non-uniform fluctuations which are experienced in such signals are largely irrelevant to the desired control of the muscle, as the amplitude of the signal is roughly proportional to force, and is sufficient to form the basis of a subsequent control. Thus, the illustrated apparatus forms an average based upon the detected signal, and effectively rejects irrelevant information contained in the original signal, for the purposes of producing the control signal at the output terminals 54.

In an alternative arrangement to that illustrated, the rectifier stage is omitted, and the integrator stage is so arranged to provide an output signal which is determined largely by the predominating polarity of the input signal. Then, the trigger stage 4 may comprise both positive and negative level triggers, which provide two respective frequency coded pulse trains, one providing information on positive signals, and the other providing information on negative signals.

The illustrated apparatus may be powered by any suitable means. However, in a preferred case where the apparatus is manufactured as a small unit, preferably using micro-chip technology, the apparatus may be battery driven, with a circuit ground which is floating with respect to the wearer. Thus, a patient may carry apparatus as illustrated as a permanent aid to facilitating otherwise useless bodily response.

The processing of alternating signals to provide amplitude information often requires a processing delay which may be unacceptably long for use in some control systems. This is the case with low pass filtering of rectified signals and computer techniques. At present there appears to be no method whereby amplitude information may be extracted from rapidly fluctuating signals (say 50-500 Hz.) at a speed compatible with control systems having a response time within one or two cycles of the signal (say 5ms). The apparatus described herein will accept signals alternating at any frequency and convert them to a pulse train where the interval between pulses depends upon the average amplitude of the input signal since the last pulse and upon the values set for components within the apparatus.Thus, by selecting the appropriate components the processing time can be reduced to whatever level is acceptable.

More generally, according to a second aspect of the present invention, there is provided signal processing apparatus comprising means for receiving an input signal and means for processing the input signal to provide a pulse train wherein the interval between each two successive pulses is proportional to the amplitude of the input signal since the first of the two pulses.

According to a third aspect of the present invention, there is provided a method of signal processing, wherein an input signal is processed to provide a pulse train wherein the interval between each two successive is proportional to the amplitude of the input signal since the first of the two pulses.

Preferably, the repetition frequency of the pulse train is within an order of magnitude of the frequencies occurring in the input signal.

In a variant of the illustrated arrangement, a useful analogue voltage, accurate in time, may be produced by dispensing with the circuitry following the integrator 3 and applying reset pulses at a constant interval, short enough to prevent saturation of the integrator. The integrator output immediately before the reset pulse is then proportional to the average signal between reset pulses. This voltage may be applied to a sample and hold circuit or an analogue to digital converter triggered before the reset pulse is applied. The signal may thus be "remembered" until the next reset pulse.

Claims

1. Signal processing apparatus including first means for processing a first signal from a body to produce a control signal which has a characteristic proportional to the amplitude of said first signal, and second means either for feeding said control signal to said body, thereby to control a bodily response, or for feeding said control signal to a prosthesis, to simulate a bodily response.

2. Signal processing apparatus according to Claim 1, wherein said first means is adapted to process said first signal when detected from a mammalian body.

3. Signal processing apparatus according to Claim 1 or 2, including detecting means for detecting said first signal.

4. Signal processing apparatus according to Claim 1,2 or 3, wherein said characteristic is a frequency characteristic of said control signal.

5. Signal processing apparatus according to Claim 4, wherein the control signal comprises a pulse train the repetition frequency of which is proportional to said amplitude of said first signal.

6. Signal processing apparatus according to Claim 4, wherein said first means comprises an integrator arranged to receive said first signal or a signal proportional thereto, and reset means arranged to reset the integrator when the output thereof attains a predetermined limit value.

7. Signal processing apparatus according to Claims 5 and 6, wherein said reset means comprises trigger means arranged to produce an output pulse each time the output of said integrator reaches said limit value, which output pulse is arranged to reset the integrator.

8. Signal processing apparatus according to Claim 7, wherein the output pulse that is used to reset the integrator is also used to drive an output stage of the apparatus.

9. Signal processing apparatus according to Claim 7, wherein the trigger means is arranged to provide a further output pulse each time the output of the integrator reaches said limit value, which further output pulse is used to drive an output stage of the apparatus.

10. Signal processing apparatus according to Claim 7, wherein the output pulse is transmitted to a frequency to voltage converter, to provide an analogue output related to average amplitude of said first signal.

11. Signal processing apparatus according to any preceding claim, wherein said first means includes an amplifier arranged to receive said first signal.

12. Signal processing apparatus according to Claim 11, as appendant to any one of Claims 6 to 10, wherein the amplifier is followed by rectifier means connected to receive the output of the amplifier and provide a rectified signal to the input of the integrator.

13. Signal processing apparatus according to any preceding claim, wherein said first means is arranged to receive an input signal in the range 0 to 2 mV.

14. Signal processing apparatus according to any preceding claim, wherein the apparatus is arranged to process signals in the frequency range 50-500 Hz.

15. Signal processing apparatus according to any preceding claim, including noise rejection means for rejecting unwanted noise.

16. Signal processing apparatus according to any preceding claim, including means for blanking processing of said first signal during emission of said control signal.

17. Signal processing apparatus comprising means for receiving an input signal and means for processing the input signal to provide a pulse train wherein the interval between each two successive pulses is proportional to the amplitude of the input signal since the first of the two pulses.

1 8. A method of signal processing, wherein an input signal is processed to provide a pulse train wherein the interval between each two successive pulses is proportional to the amplitude of the input signal since the first of the two pulses.

19. A method of signal processing, according to Claim 18, wherein the repetition frequency of the pulse train is within an order of magnitude of the frequencies occurring in the input signal.

20. Signal processing apparatus substantially as hereinbefore described with reference to the accompanying drawings.

21. A method of signal processing, the method being substantially as hereinbefore described with reference to the accompanying drawings.

22. Signal processing means comprising an integrator adapted to receive an input signal and means for resetting the integrator at constant intervals, sufficiently short to prevent saturation of the integrator, such that the output voltage of the integrator at the end of each said interval is proportional to the average amplitude of the input signal during that interval.

23. Signal processing apparatus according to Claim 22, including a sample and hold circuit adapted to receive the output of the integrator.

24. Signal processing apparatus according to Claim 22, including an analogue digital converter arranged to receive the output of the integrator.

25. Signal processing apparatus according to Claim 22, 23 or 24, and also being in accordance with Claim 1,2 or 3, or in accordance with any one of Claims 11 to 16 as appendant to Claim 1, 2 or 3.