WO1992009328A1

WO1992009328A1 - Stimulator for surface stimulation in paralyzed patients

Info

Publication number: WO1992009328A1
Application number: PCT/US1990/006877
Authority: WO
Inventors: Daniel Graupe; Amihadar Arber; Silvano Romeo
Original assignee: Sigmedics, Inc.
Priority date: 1989-04-07
Filing date: 1990-11-26
Publication date: 1992-06-11

Abstract

The system of the present invention is a stimulation system (100) for providing upper-motor-neuron paralyzed patients with capabilities of unbraced standing and unbraced walking, which consists of a single pulse power and amplifier (103) that outputs various trains of stimulation pulses and which is totally under microprocessor (104) generation and control, where the microprocessor's microcomputer selectively performs multiplexing of the pulse trains and their selective alternate distribution to the various surface electrodes attached to the skin of the patient at the appropriate stimulation locations. It also controls pulse-width, pulse-duration, pulse-amplitude and pulse-shape. Furthermore, the system can compute and activate (audible and/or visual) warning signals to warn the patient when the system is close to its maximal output. This would happen when muscle fatigue is such that no further increase in stimuli level can keep the patient upright. The system computes and outputs indications of the various stimuli levels to a visual display, and it provides automatic fail-safe and override capabilities to help ensure that the patient does not fall in case of major circuit or system failure or misactivation. This is a critical capability since this system will be used by paraplegics who lack sensation in their lower extremities.

Description

STIMULATOR FOR SURFACE STIMULATION IN PARALYZED PATIENTS

Technical Field

This invention relates to functional electrical stimulation (FES) of paraplegics and more particularly to an improved microcomputer controlled apparatus and methodology.

Background Art

Work on functional electrical stimulation of paraplegics is based on the discovery of the Italian physiologist Luigi Galvani in the late 18th century that a muscle will contract when in contact with an electrical charge. This has been first applied systematically to paralyzed patients by W. Liberson in 1960 (in W. Liberson et al., Arch. Phys. Med. Rehab., Vol. 42, p. 101, 1961). Since then considerable work has been devoted to that topic, as reviewed by Graupe et al. (J. Biomed Eng. Vol. 5, pp. 220-226, July 1983) , by Graupe et al. (Critical Reviews in Biomedical Engineering, CRC Press, Vol. 15, pp. 187- 210, 1988) and in a recent text by A. Kralj and T. Bajd ("Functional Electrical Stimulation: Standing and Walking after Spinal Cord Injury", CRC Press, Boca Raton, FL, 1989) .

It was thus shown that the application of trains of pulses of adequate amplitude, pulse-width and pulse-repetition-frequency at appropriate locations above the region of certain key muscle, enables paraplegic patients with complete or near-complete upper-motor-neuron lesions to stand up and to take steps with the support of parallel bars or a walker. The merits of such standing and of the primitive walking accomplished by these steps is both psychological and physiological in the exercise provided to the patient which enhances circulation, cardiac output and movement of joints to prevent or slow down ossification and contractures (Kralj and Bajd, "Functional Electrical Stimulation: Standing and Walking after Spinal Cord Injury," CRC Press, Boca Raton, FL, 1989, pp. 33, 68, 130-131). Furthermore, there are indications (not yet fully proven due to the limited application of FES which is presently available only in research labs and research clinics) , that FES is beneficial in prevention or reduction of incidence of pressure sores and of osteoporosis (Krajl and Bajd, same as above, pp. 8, 33, 49, 69, 131 and 135) and in the reduction of severity of spasticity (Krajl and Bajd, same as above, pp. 3, 8, 37-47) . FES is limited to upper-motor neuron lesions since in that case the peripheral nerves (at the lower extremities, in our case) are intact though they cannot communicate with the central nervous system due to the spinal- cord lesion. However, since the peripheral nerves are healthy and intact, they respond to FES even after many years of paralysis without stimulation. In two patients, after 35 years of paralysis and with no stimulation over that whole time, the peripheral nerves responded to FES fully satisfactorily. Disclosure Of Invention In accordance with one aspect of the present invention, the present system provides non-invasive electrical stimulation for paralyzed patients with upper-motor-neuron lesions so as to provide capabilities for unbraced standing and walking. In a preferred embodiment, only a single pulse power and amplitude amplifier of stimulation pulses is utilized, in contrast to the multiple pulse amplifiers that are presently used (usually one per stimulation channel) , noting that systems using multiple pulse-amplifiers require relatively heavy and cumbersome hardware, since, for patient walking purposes, a multitude of output-stimulus pulse channels (four at least) are required.

The use of a single pulse-amplifier for multiple channel stimulation is achieved in the present invention by the use of a microprocessor

(microcomputer) controller system that provides output channel multiplexing and which also generates stimuli pulses and controls pulse-width, pulse-amplitude, pulse-repetition-frequency and which provides warning

(preferably audible) to the patient when the system saturates. As an example, the system provides warning when muscle fatigue is such that no further increase in pulse-amplitude is possible to combat the fatigue

(i.e. to recruit further muscle fibers not reached by the present electrical fields produced by the stimuli) , due to reaching maximal predetermined pulse levels.

In accordance with another aspect of the present invention, the system provides the combination of multiplexing and of complete microprocessor (microcomputer) control including microprocessor controlled warning and microprocessor-controlled provisions of fail-save features. The warning aspect is of major importance since the patient, being paralyzed, cannot feel muscle fatigue. The warning is determined responsive to computerized sensing of the appropriate control input levels, which the patient sends, such as through activating manual finger- switches attached to the walker, as control inputs to the microprocessor controller, to increase the level of the stimuli when he senses, via pressure in his arms (which hold the support of the walker,) that he needs higher such levels. Alternatively, the control inputs can be automatically generated from feedback signals derived from the patient (such as via sensing electrodes) .

The significance of the provision of fail-save features for situations where certain major system failures occur is important, since the system is designated for use by paralyzed persons who have no sensation at their lower extremities. These persons depend on these stimulator's safety features, and the above failures without fail-safe features, may cause them to fall, noting that it is of utmost importance to prevent falls.

In accordance with another aspect of the present invention, directed to the safety feature of preventing falls, the system provides an increased level of stimulus to the standing remaining quad for the duration of a step by the other leg during each step, by a predetermined increase level value. This provides a safety mechanism to prevent possible falls of a patient while taking a step. When taking a step, all of the patient's weight is on the leg not stepping, the stepping leg being in the air (being moved to produce a step) . The computing portion of the system determines both when to take the step, and at the appropriate time of taking the step, increases the stimulus to the remaining quad by between ten and twenty percent above the ten steady state level.

In accordance with another aspect of the present invention, solid state cascaded voltage-doublers are used, instead of the heavier pulse-transformers, for the purpose of stimuli pulse-generation. Another innovative aspect of the present invention is the employment of a telemetry link between the walker- mounted patient-operated switches and the stimulator itself, to avoid the employment of wires between walker and the usually patient-borne stimulator.

In accordance with yet another aspect of this invention, in order to avoid high voltage at channel outputs under no-load conditions, the output of the pulse-amplifier circuit may be connected to a voltage- sensitive load, such as a fast ZNR transient surge resistor (e.g. a zinc-oxide nonlinear resistor device) in series with an appropriate load resistor, such that the output channels to the stimulation electrodes, that are attached to the patient, will be loaded by a resistance of no more than a few thousand ohms if the output of the pulse-generator circuit (e.g. pulse- transformer) exceeds some predetermined voltage (such as in the range of 50 to 130 volts) .

In accordance with another aspect of this invention, opto-isolators are coupled from the output of the channel outputs and to the electrodes, so as to provide isolation of the various output channels, so as to avoid back current from feeding back into the microprocessor and other circuitry, and so as to isolate the various output channels to prevent cross¬ talk.

In accordance with another aspect of this invention, any command to activate a "sit-down" mode (in order to stop stimulation when the patient wishes to sit-down) , may be overridden by pressing of any command switch, to avoid that the patient may fall if inadvertently activating the "sit-down" command. Also, when the "sit-down" command switch is pressed, the sit-down function which under the present invention implies gradual cessation of stimulation to the patient's quadriceps muscles, will be delayed under an aspect of this invention in its execution of this gradual reduction of stimuli to zero by a predetermined number of seconds, (e.g. 5 to 12 seconds) . Furthermore, under again another aspect of this invention, the activation of the sit-down command will immediately give the patient a warning signal which may be a flashing light on the walker to indicate to the patient that the "sit-down" function has been initiated.

Additional safety features are also associated with the "sit-down" mode. First, since the computer is determining when to take steps, it can time events precisely. As one additional safety feature, when going to a sit-down mode, the system increases stimulus level to both the right and left quadriceps muscles via the electrodes for attachment thereto, thus giving extra support to the quads for sitting, since the person may already be fatigued at this point where they are ready to sit-down. The increase in level is in the range of ten to 20%.

In accordance with another aspect of the present invention, the system measures its own output current between the multiplexer and the electrodes (or alternatively it can measure the voltage drop over a fixed impedance at that location) and couples the measurement value back to its input for determination of whether the output current (or voltage) . drop is below a predefined threshold. The predefined threshold, in a preferred embodiment, is approximately 50% of the lowest output level to any electrode. Alternatively, by trial and error, other thresholds can be chosen. If there is a drop below this predefined threshold, this implies that enough current is not being output to some channel (or channels) , i.e. such as an electrode is loose or disconnected. The computing unit of the apparatus then provides an audible sound alarm and/or visual alarm for the user/patient, and disallows any step, but maintains the patient in a standing mode (at least on one leg since during a step one leg is in the air) . If the other leg happens to be the one disconnected (the one being stood on is the one where the electrode disconnects) , the patient will fall, except as self- supported by his/her arms on a walker unit.

In describing use of this system for upper- motor-neuron paraplegics, noting that this invention is concerned with standing and walking by such paraplegics implies limitation of the system to patients whose spinal cord lesion is virtually at the thoracic T12 level or higher (approximately above the belt level) . The functions and features as discussed above and their combination in the manners discussed above are essential and unique for applications to standing and walking by paralyzed patients. . This constitutes a major and essential difference from any stimulator for pain relief or for exercise of muscle where no walking is intended to be stimulated and controlled.

Brief Description Of Drawings

The present invention may be better understood by reference to the written specification in conjunction with the drawings, wherein:

Fig. 1 is an electronic schematic block diagram of one embodiment of an FES Stimulation System in accordance with the present invention;

Fig. 2A shows the complete system with walker, patient and FES stimulator box;

Fig. 2B shows a side view of Fig. 2A;

Fig. 2C shows a top view of the walker of Figs. 2A-B;

Fig. 2D shows a perspective view of the walker of Figs. 2A-C; Fig. 3 illustrates one embodiment of the walker mounted switch control unit;

Fig. 4 illustrates the voltage pulse waveform for a typical FES system pulse output;

Figs. 5A-D provide voltage vs. time waveforms illustrating the envelope of pulses as distributed to four channels during FES activated walking;

Fig. 6 illustrates an electrical block diagram for a specific alternate embodiment of a FES stimulator box;

Fig. 7A shows the stimulus level plotted vertically versus time plotted horizontally for the right quadriceps muscle;

Fig. 7B illustrates the stimulus level versus time for the left quadriceps;

Fig. 8A and 8B illustrate stimulus level versus time waveforms for the right quadriceps (Fig. 8) and left quadriceps (Fig. 8B) ;

Figs. 8C and 8D illustrate step occurrence versus time waveforms for step right (Fig. 8C) and step left (Fig. 8D) ; and.

Fig. 9 is an electrical schematic block diagram illustrating the output level detect safety subsystem of the present invention.

Mode For Carrying Out The Invention

The FES stimulation device under this invention is a system as in Fig. 1, that comprises of an FES stimulation box 100 that is battery-operated 111, using AA or AAA 1.5 volt batteries or similar batteries, and which includes a stimuli pulse- generator 103, a microprocessor-control circuit 104 and related interface and which has a control panel 107 of pressure switches on its cover.

The same stimulator box 100 also houses the telemetry (ultrasound or radio frequency, or infra- red) receiver in realizations of the invention where telemetry is employed. Alternatively, connector interface circuitry can be provided for non-telemetry applications. It may also house the audible warning element 108, and may house a low-battery voltage indicator 109 and/or a low-battery visual 110 or audible warning 108 and a display of stimuli levels that is activated to show the level of a given channel when the appropriate control switch is activated by the user. The stimulation box 100 is connected on its input side 121 to walker-mounted hand (finger) switches (switching unit) 101 and on its output side 124 to the stimulation surface electrodes 102 attached to the patient. The complete system with a patient is shown in Fig. 2A.

Referring to Fig. 2A, paraplegic user 99 is shown supporting himself with his arms, and by use of the present FES system, with a walker 98 having finger switches control unit 101 mounted thereto, and the FES stimulator box 100 affixed to his belt.

Fig. 2B illustrates a side view of Fig. 2A.

Fig. 2C illustrates a top view of the walker 98, while Fig. 2D provides a perspective view of the walker 98 and finger switches control unit 101. The walker 98 is preferably constructed of aluminum pipe, of from 1/2" to 1-1/2" diameter, ergonomically determined, but typically 1/2" or 3/4".

The switching unit 101 that is mounted on the handles of the walker 98 as illustrated in Fig. 2A, is divided into two sub-units, interconnected by wire, one on the right hand side hand-bar and one on the left-hand side hand-bar. The walker should preferably be a reciprocating walker, using aluminum tubing of approximately 1 inch diameter. As illustrated in Fig. 3, the right hand-side walker-mounted switching sub-unit consists of an "on/off" switch 131 for stand-up and sit-down, and of a switch 132 for activating a right step and of a switch 133 for increasing the stimulation level for both the stimulus to the right region of the quadriceps muscles (for strengthening right quadriceps muscle contractions in standing) and for the stimulus to the right peroneal nerve (for the right step) , and where a short duration pressing of that switch increases the stimuli level to the quadriceps and a substantially longer such duration increases it for the right step. The left hand-side switching sub- unit includes a "sit-down" switch 134, such that the sit-down function is activated only when both the left "on/off" switch 131 and the right "off", (namely "sit down") switch 134 are simultaneously pressed, whereas standing requires pressing the right "on/off" switch 131 alone, and where one must start pressing the left "off" switch 134 before pressing the right "on/off" switch 131 and continue pressing it until after releasing the right switch 131. The left hand-side sub-unit also includes a left step switch 135 and a level increase switch 136 that again serves to both increase level of left quadriceps and of left step stimuli. Thus, there are 3 switches on each sub unit as illustrated in Fig. 3.

As illustrated in Figs. 1-3, the switching unit 101 is connected for use with an ultrasound transmitter which via a coding circuit, both mounted on the walker, transmit the appropriate codes to the stimulator box. The codes as illustrated herein are:

(i) stand-up : press switch 131.

(ii) right step : press 132.

(iii) left step : press 135. (iv) sit-down : press 131 and 134 simultaneously, (v) increase stimulation to right quads

(quadriceps) : press 133 (short duration) . (vi) increase stimulation to left quads

(quadriceps) : press 136 (short duration) . (vii) increase stimulation for right step : press

133 (long duration) . (viii) increase stimulation for left step : press 136 (long duration) . This ultra-sound communication link coding circuit and transmitter, together with the appropriate receiver 140 mounted in the stimulation box 100 constitute the communications link of 121 of Fig. 1. The receiver 140 couples the received coded signal for input to the microprocessor circuit 104 where this input is decoded to determine which command is being sent from the input commands (i) to (viii) above. In an alternate embodiment, a wire link substitutes this coded ultrasound link as link 121 above. Switch 131 can be omitted in some realizations or could serve only for stand-up purposes. In these cases, sit-down is activated by a long duration activation of switch 134 alone, which could be located at the right or left sub-unit with the "stand-up" switch, if employed, being located at the opposite sub-unit.

In addition, in either realization, inputs (i) to (viii) can alternatively be inputted from the stimulator-mounted switching control panel 107 where, additionally, four further inputs can be generated. These inputs are as follows:

(ix) decrease stimulation to right quads (x) decrease stimulation to left quads (xi) decrease stimulation to right step (xii) decrease stimulation to left step. However, on this panel 107 each function (i) to (xii) may have its own switch. In a realization where the region of the gluteus maximus muscles are stimulated, then any of the above functions relating to a step should be considered as functions relating to the gluteus maximus of the same side (right or left) , since it is considered that persons with a relatively unstable trunk who require stimulation of the gluteus maximus are not supposed to walk with the present system. In cases of minor trunk instability, a corsette may be worn by the patient, and walking may be executed without stimulating the gluteus maximus if approved by a medical practitioner.

The microprocessor control circuit of 104 has further inputs 123 via internal adjustment circuit 112 which includes a set of pins where applied voltages are input as (4) commands to adjust pulse rate, pulse duration (width) , maximal stimuli levels, and can additionally be used to adjust duration of ramp-like envelopes of stimuli amplitudes that are employed at the initialization of stand-up and/or at the end of sit-down, these ramps being a gradual initial- increase/final-decrease of stimuli amplitudes, to avoid a too sudden start/cessation of contraction in stand-up/sit-down. For stand-up, a certain overshoot at the end of the ramps is possible, to provide contraction force for standing-up that is above the force required otherwise for standing, noting the energy required to stand-up from a sitting position.

The microprocessor 104 generates pulse trains and controls at its output 124 the pulse characteristics according to the inputs from 112 and from the switch- inputted functions concerned with stimuli level which may either modify pulse duration or pulse amplitude. All these are outputted through 125 to the pulse and amplitude amplifier 103 whose output is multiplexed in response to the control as determined by microprocessor 104, and which serves to provide stimuli for all functions (stand, right step, left step) . The pulse amplifier 103 is controlled responsive to the microprocessor via link 126 to output these pulses through the distribution interface circuit 105 as determined by the processor 104 according to the input switches of 101 or of 107. The circuit 105 thus couples the stimuli to the various skin surface electrodes (2) that are attached with tape to the skin at the appropriate stimulation locations as discussed above. Note that components 103, 104, 105, 107 and 117 are all mounted on the stimulator box 100.

Opto-isolators as available from many of the commercial manufacturers of opto-isolators can be utilized, including those from Texas Instruments, Hewlett-Packard, and many, many others.

Opto-isolators 205 are illustrated in both Figs. 1 and 6. The optoisolators 205 are shown in dashed form in both Figs. 1 and 6, since they may optionally be included or not included depending upon design criteria. In the preferred embodiment, the opto- isolators 205 are included, having inputs coupled from the channel distribution and multiplexing circuitry 105, and optically isolating, amplifying and coupling to the surface electrode array 102, being adapted for coupling to connectors which are coupled to cables coupled to the electrodes. Similarly, for Fig. 6, the opto-isolators 205 are shown as coupling between the electrode multiple interface unit 105C outputs and providing opto-isolation to and from the skin electrodes on the patient.

The stimuli levels are computed by counting the number of times the appropriate switches are pressed, so that each time an appropriate switch is pressed, the stimuli level at the corresponding channel is increased by a predetermined increment.

The pulse amplifier 103 can consist of a single pulse transformer or, alternatively, of a single solid state cascade of voltage doublers to output a train of pulses which may all be positive or all negative, or may have a positive component followed immediately by a negative component (or vice versa) as illustrated in Fig. 4. When quads and steps are stimulated then the pulse-rate is set by microprocessor 104 at approximately 48 pulses per second which is multiplexed at the pulse amplifier's output responsive to microprocessor 104 into 2 channels, each having a rate of 24 pulses per second. If the gluteus maximus and the quads are stimulated, then no step is activated, and the pulse rate is 96 pulses per second divided into 4 channels each having a rate of 24 pulses per second responsive to the microprocessor 104.

The distribution of the stimuli as determined by microprocessor 104 is shown in Figs. 5A-D, showing the envelopes of stimuli pulse trains for stand and sit- down functions as applied to Right Quads (Fig. 5A) and Left Quads (Fig. 5C) , and to the right and left peroneal nerves for activating. Right Step (Fig. 5B) and Left Step (Fig. 5D) .

In addition, microprocessor circuit 104 computes the actual stimulus level per a given function and outputs via 128 to display circuit 106. Display circuit 106 can be an LED light-bar display, or alternatively a numerical read-out display. The display can be mounted on the stimulator box 100.

The microprocessor 104 also determines when the quads' stimulus level at any one of the two quads reaches a level close to the maximal level, as determined by 104 responsive to the inputs via 112, and then activates an audible alarm 108 to warn the patient that he cannot increase the level any further. This aspect is important since the patient has no sensation at the quads and cannot determine the degree of fatigue at the stimulated muscle. When walking with the FES system of the present invention, the patients' arms carry only 2% to 5% of his body weight according to measurements, whereas his stimulated leg muscles carry 95% to 98%. Whenever the patient feels that his arms which support him on the walker carry increased weight, he will tend to increase quads stimuli levels at the appropriate arm side. However, when he reaches the maximum level he must be given time to sit down (possibly, to go to a chair) . Hence, the alarm of 108 is to be activated in sufficient time before that complete fatigue level is reached, this level being also adjustable by processor 104 responsive to inputs at 112. The audible alarm can be mounted on the stimulator box 100.

The microprocessor circuit 104 also computes fail-safe provisions to: (i) guarantee that a step can be taken only at one leg at a time; (ii) guarantees that, if no input is received from the walker mounted switches 101 or from the panel switches 107 on the stimulator box 100, concerning commands to one leg or to both, then the patient will always be or remain in the "stand" (both quads "on") mode, to guarantee that with a failure in link 121 or 122, be it an acoustic, IR, radio frequency or wire link or transmitter or receiver failure, that the patient will remain standing and will not fall (and, of course, he still has the walker support) ; (iii) guarantees that after every step the system automatically returns to the "stand" mode (of both quads being stimulated) , and in certain realizations the microprocessor 104 also computes a fail-safe provision that a step is limited in time so that even if a step switch is stuck in the "step" mode, then after a predetermined reasonable time (of the order of a second) the concerned leg is returned to "stand" (quads) mode by an appropriate microprocessor-controlled decision and channel switching. In certain realizations the latter situation also provides for an appropriate warning signal, audible or visible or both, to alert the patient to that faulty situation.

Additional safety features are also associated with the "sit-down" mode. First, since the computer is determining when to take steps, it can time events precisely. As one additional safety feature, when going to a sit-down mode, the system increases stimulus level to both the right and left quadriceps muscles via the electrodes for attachment thereto, thus giving extra support to the quads for sitting, since the person may already be fatigued at this point where they are ready to sit-down. The increase in level is in the range of ten to twenty percent (10% - 20%) . Referring to Figs. 7A-B, Fig. 7A shows the stimulus level plotted vertically versus time plotted horizontally for the right quadriceps muscle, and Fig. 7B illustrates the stimulus level versus time for the left quadriceps muscle. Note that at the time indicated, TSD indicating the time of the sit-down command from the standing position as received and recognized by the computer, that the stimulus levels to both the right and left quadriceps muscles are increased, by between ten to twenty percent preferably, for a short time duration (on the order of seconds) prior to decreasing the stimulus level by ramping the level of the stimulus signals to the right and left quadriceps to the zero level of full sitting.

In accordance with another aspect of the present invention, directed to the safety feature of preventing falls, the system provides an increased level of stimulus to the standing remaining quad for the duration of a step by the other leg during each step, by a predetermined increase level value. This provides a safety mechanism to prevent possible falls of a patient while taking a step. When taking a step, all of the patient's weight is on the leg not stepping, the stepping leg being in the air (being moved to produce a step) . The computing portion of the system determines both when to take the step, and at the appropriate time of taking the step, increases the stimulus to the remaining quad by between ten and twenty percent above the then steady state level.

Referring to Figs. 8A-D, Figs. 8A and 8B illustrate stimulus level versus time waveforms for the right quadriceps (Fig. 8A) and left quadriceps (Fig. 8B) . Figs. 8C and 8D illustrate step occurrence versus time waveforms for step right (Fig. 8C) and step left (Fig. 8D) . Each of Figs. 8A-8D is plotted against the same time axis in the horizontal direction. As illustrated in Figs. 8A-C, when step right occurs as indicated by the positive pulse on Fig. 8C, the stimulus level to the right quadriceps (Fig. 8A) is turned off during leg movement to produce the step, while at that same time, the stimulus level to the left quadriceps is incremented as illustrated on Fig. 8B, approximately 15% over the steady state level. (An increase within the range of approximately 10 to 20% is acceptable) . Thus, the stimulus level to the left quadriceps is increased temporarily during the step occurrence of the right step, corresponding to the turning off of the stimulus to the right quadriceps. Similarly, referring to Figs. 8A, 8B and 8D, at the step occurrence of step left, the positive pulse in Fig. 8D, the stimulus to the left quadriceps. Fig. 8B, is turned off, while the stimulus level to the right quadriceps, Fig. 8A, is increased by the predefined value to increase the level of stimulus to the standing leg, the right leg, during the left step. To provide extra smooth landing of a given leg after a step has been taken with that leg, it may be advisable not to stimulate the quadriceps of that leg with a sharp instantaneous voltage rise, but to provide a voltage ramp rise-time of the stimuli to these quadriceps after each step.

Referring to Fig. 9, in accordance with another aspect of the present invention, the system measures its own output current such as between the opto- isolator 205 (or multiplexer means 105) and the electrodes 102 (or alternatively it can measure the voltage drop over a fixed impedance R at that location) , and couples the measurement value back to its input for determination of whether the output current (or voltage) drop is below a predefined threshold. As illustrated in Fig. 9, the output current to one electrode is determined by measuring the voltage drop (V_D) across a fixed impedance R. The voltage (V_D) is compared to a predefined threshold level (V_τ) (illustrated as being variable for different preset predefined levels) by a comparator 300. The output of the comparator 300 is coupled to an input of the system's microprocessor 104. The predefined threshold, in a preferred embodiment, is approximately 50% of the lowest output level to any electrode. Alternatively, by trial and error, other thresholds can be chosen. If there is a drop below this predefined threshold, this implies that enough current is not being output to some channel (or channels) , i.e. such as an electrode is loose or disconnected. The computing unit 104 of the apparatus then provides for an audible sound alarm and/or visual alarm for the user/patient, and disallows any step, but maintains the patient in a standing mode (at least on one leg since during a step one leg is in the air, preferably increasing the stimulation level to the remaining electrode) . This stimulation level can be increased by increasing either the peak voltage, pulse frequency, pulse width, or a combination of these. In addition, it has been shown in the literature (of Kralj and Bajd book, page 127, Fig. 3) that by increasing pulse width and/or pulse frequency during a "TAKE STEP" mode, one achieves a higher step (a larger angle of hip joint flexion) . Whereas the figure of p. 127 related to the CUTANEOUS FEMORIS nerve, our experience over many tests with patients indicates similar increases of step height (namely, of hip flexion angle) also when a peroneal nerve is being stimulated. If the other leg happens to be the one disconnected (the one being stood on is the one where the electrode disconnects) , then the patient could fall, except as self-supported by his/her arms on a walker unit.

To avoid excessively high voltages at the output of the pulse-amplifier circuit 103, that output may also be connected, in parallel, to a ZNR type voltage sensitive load. This load presents to that output a load of no more than a few thousand ohms, if the voltage at that output reaches a certain predetermined value of between 50 and 130V, and which otherwise presents an additional open circuit.

A battery supply 111, as illustrated, consists of 8 AAA 1.5 VDC batteries, which supplies power to all the circuits of the stimulator box 100 via 130. The battery supply is also coupled to activate a low- battery voltage warning via circuit 109 to provide the audible warning 108 and also a visual flashing warning light 110. The "low-battery" warning sound is programmed by the processor 104 to differ from the sound of the stimulus-level-saturation alarm discussed above.

Fig. 6 illustrates a specific embodiment of the stimulator box 100 of Fig. l, for a wired link 121, with corresponding numerals indicating like elements. Specific electronic device designations are illustrated in Fig. 6, such as the Hitachi HD63701 processor, 104.

While there have been described herein various illustrated embodiments, it will be understood by those skilled in the art that various other embodiments emanate from those disclosed. This description is illustrative and not limiting, and the true scope of this invention is as set forth in the appended claims.

Claims

1. An electrical stimulation system, for coupling to a plurality of electrodes coupled to locations at the right and left side of the lower part of the body of a patient, said system characterized by: pulse generator means for selectively outputting trains of pulses having selectively variable duration and pulse repetition frequency responsive to a control signal; memory for storing operational instructions and data; control means for selectively outputting said control signal responsive to said memory and to input commands; pulse amplifier means for selectively amplifying pulse amplitude and power responsive to said control means; multiplexer means for selectively multiplexing outputs of said pulse amplifier means from said pulse generator and alternatively between selected ones of said electrodes responsive to said control means; and a battery power supply; wherein said electrodes are wired in pairs as channel, wherein there are at least 4 channels, one to the right quadriceps muscle, one to the left quadriceps muscle, one to the right common peroneal nerve and one to the left common peroneal nerve.

2. The system according to claim 1 further characterized by two additional channels, one to the right gluteus maximus muscle and one to the left gluteus maximus muscle.

3. The system according to claim 1, further characterized by said electrodes wired in pairs as channels, wherein there are two additional channels, one to the right gluteus maximus muscle and one to the left gluteus maximus muscle.

4. The system according to claim 2 further characterized by the system having an off-state and on-state, wherein at each initial switching from said off-state to said on-state, the pulses output to the quadriceps muscles and gluteus maximus muscles are gradually increased in amplitude having a slope of ramplike form, for an initial predefined time interval commencing with said initial switching.

5. The system according to claim 4, further characterized by said initial time interval is in the range of approximately one to two seconds.

6. The system according to claim 4, further characterized by at the end of said initial time interval, the envelope of the amplitude of the output pulses slightly overshoots and then reaches a steady state level, wherein overshoot takes place for a short time duration of the order of a few seconds or less.

7. The system according to claim 6, further characterized by the duration of said initial interval, the initial amplitude of said pulse outputs, the overshoot level, and the steady-state level are determined responsive to said control means.

8. The system according to claim 1 further characterized by input command means for providing mode input command signals, wherein said controller means selectively provides one of a plurality of modes responsive to said input command means.

9. The system according to claim 2 further characterized by input command means for providing mode input command signals, wherein said controller means selectively provides one of a plurality of modes responsive to said input command means.

10. The system according to claim 8, further characterized by said input command means are switches.

11. The system according to claim 9, further characterized by said control means switches modes of operation responsive to said input command means to provide a stand mode, a take left step mode, a take right step mode, and a sit down mode.

12. The system according to claim 11, further characterized by during said stand mode the pulse outputs are selectively multiplexed to the right and left quadriceps channels and to the right and left gluteus maximus or medius channels.

13. The system according to claim 11, further characterized by channels coupled to the area of right and left common peroneal nerves.

14. The system according to claim 13, further characterized by during the take left step mode the pulse outputs are selectively output to the right quadriceps channel and left common peroneal channel, while at the same time no stimulus is output to the left quadriceps muscle and to the right common peroneal nerve, and wherein during the take right step mode the pulse outputs are selectively outputted to the left quadriceps channel and the right peroneal channel, while at the same time no stimulus is outputted to the right quadriceps muscle and to the left peroneal nerve.

15. The system according to claim 13, further characterized by between each of said take right step mode and said take left step mode said system automatically returns to said stand-up mode, where none of the common peroneal nerves receive stimuli.

16. The system according to claim 11, further characterized by the system is only permitted to switch to said sit-down mode from said stand-up mode.

17. The system according to claim 11, further characterized by during said sit-down mode the amplitude of the pulse outputs to the quadriceps channels and gluteus maximus channels are gradually reduced to zero, over a predetermined time period on the order of seconds.

18. The system according to claim 8, further characterized by said input command means includes a manual command switch circuit.

19. The system according to claim 18, further characterized by said manual switch command circuit is mounted on a patient support device.

20. The system according to claim 18, further characterized by said manual command circuit is mounted on a panel of a casing having the electronics of the system and on a patient support device.

21. The system according to claim 19, further characterized by said patient support device is a walker.

22. The system according to claim 19, further characterized by said patient support device is a reciprocal walker.

23. The system according to claim 18, further characterized by said manual command switch circuit communicates with said control means to provide input commands to determine the mode of the system.

24. The system according to claim 23, further characterized by said manual command switch circuit further communicates additional input commands to said control means to determine the level of pulse output to multiple channels.

25. The system according to claim 24, further characterized by said multiple channels are the right and left quadriceps channels, the right and left gluteus maximus channels, and the right and left peroneal channels.

26. The system according to claim 18, further characterized by said manual command switch circuit is further comprised of mode switches for right-step, left-step and sit-down.

27. The system according to claim 18 further characterized by said manual command switch circuit is further comprised of level adjustment switches for communicating input commands to said control means to selectively increase and decrease the level of stimulus to said right quadriceps channel, said left quadriceps channel, said right peroneal channel and said left peroneal channel.

28. The system according to claim 19, further characterized by the manual switch command switch circuit is mounted on the patient support device permits only level adjustment.

29. The system according to claim 35, further characterized by interconnection means for coupling said input commands from said manual switch command circuit to said control means.

30. The system according to claim 1, further characterized by said pulse outputs are selectively and alternatively multiplexed between said electrodes in a sequence and order and having selectively variable amplitude responsive to said control means to provide for multiple modes of stimulation.

31. The system according to claim 23, further characterized by said input means is further comprised of switches to provide command inputs for increase and decrease of stimulation levels of said pulse outputs to each of said channels.

32. The system according to claim 32, further characterized by said switches are mounted to a housing encasing said stimulation system.

33. The system according to claim 13, further characterized by four output channels, wherein two of said channels provide for stimulating the vicinity of the quadriceps muscles, and wherein a two position switch is incorporated into the system to switch the other two of said four output channels so that in one switch position the output of said other two channels is coupled for output to the vicinity of the common peroneal nerve and in the other switch position the output of said other two channels is coupled for output to the region of the gluteus maximus.

34. The system according to claim 2, further characterized by the amplitudes of the pulse outputs coupled for output to the quadriceps and gluteus maximus muscles, are gradually increased over an initial time period after switching the system "on" for initial stand up, to a first level that exceeds a subsequent steady state level of these pulses, and wherein that amplitude is dropped to said steady state level at the end of said initial time period.

35. The system according to claim 34, further characterized by said drop in amplitude of said pulse outputs to their respective steady state level at the end of said initial period is gradual.

36. The system according to claim 34, further characterized by said initial time period is in the range of approximately one to two seconds.

37. The system according to claim 34, further characterized by said first level exceeds said steady state level in the range of zero to one hundred percent of said steady state level.

38. The system according to claim 29, further characterized by said patient support device is physically separate from the remainder of the system which is encased in a housing, wherein said interconnection means is further comprised of a set of cables having two ends each with appropriate fast connect polarized plugs and sockets for connection to said housing on one end and to the manual switch control circuit, which is mounted on said patient support device, on the other end.

39. The system according to claim 20, further characterized by said patient support device is physically separate from the remainder of the system which is encased in a housing, wherein said interconnection means is further comprised of a wireless telemetry device having a transmitter mounted on the patient support device and having a receiver mounted on said housing.

40. The system according to claim 39, further characterized by said telemetry device is one of a radio frequency telemetry device, an ultra-sound acoustic telemetry device, and an infrared telemetry device.

41. The system according to claim 20, further characterized by said level adjustment is one of adjusting pulse width, and adjusting pulse amplitude.

42. The system according to claim 28, further characterized by each pressing of a switch of said level adjustment switches the control means selectively provides for a change in output pulse stimuli level by a predefined increment.

43. The system according to claim 41, further characterized by the degree of change in the pulse output is determined responsive to the duration of the pressing of said switch.

44. The system according to claim 28, further characterized by means for displaying the pulse output stimulus level for the channel having its level manually modified responsive to switch activation at the time of modification and for a short time thereafter.

45. The system according to claim 44, further characterized by if no level adjustment is made at a given time, then the level displayed is that of a quadriceps channel having the highest stimulus level.

46. The system according to claim 1, further characterized by said pulse output stimuli are in the form of trains of pulses in pairs, wherein a pulse of one polarity is followed in close succession by a pulse of the opposite polarity, and wherein the time distance between the pulse of the first polarity and the successive pulse of opposite polarity in a pair is very much shorter than the time interval between the one pair of pulses and the next such pair of pulses.

47. The system according to claim 20, further characterized by said control means automatically switches said system to said stand-up mode in the absence of command inputs to provide a fail-safe feature for the system.

48. The system according to claim 47, further characterized by said fail-safe feature is operative in case of circuit failure, and in case of communication link failure.

49 The system according to claim 11, further characterized by if a step is activated and remains activated for a time duration that exceeds a predetermined duration then the control means automatically returns the system to said stand-up mode after a predetermined time.

50. The system according to claim 49, further characterized by following an activation of a step for a time duration beyond said predetermined duration, then the system returns to said stand mode and remains in that mode until sit-down mode is activated, wherein no further step can be taken.

51. The system according to claim 49, further characterized by following an activation of a step for a time duration beyond said predetermined duration, the system returns to said stand-up mode, whereafter further steps can be activated to enable the patient to take further steps.

52. The system according to claim 49, further characterized by following an activation of a step for a time duration that exceeds said predetermined duration, a warning signal that is distinguishable from other warning signals of the system is activated responsive to said control means.

53. The system according to claim 1, further characterized by the pulse amplifier's output is connected to a non-linear voltage-sensitive load such that this output is selectively loaded by a resistance of appropriate value when any channel output to the stimulation electrodes exceeds a predetermined value.

54. The system according to claim 11, further characterized by said input command means is further comprised of a manual switch command circuit mounted on a patient support device, wherein said "sit-down" function can also be activated from the manual switch command circuit, and wherein this activation of the "sit-down" function from said manual switch command circuit is responsive to a predetermined long time duration pressing of an appropriate command switch, of an order of seconds, wherein the execution of the "sit-down" function by the control means is delayed from the time of the switching of said appropriate switch by another predetermined time delay of several seconds in the range of 5 to 12 seconds, so as to allow the patient to position himself properly to sit- down, and wherein during said predetermined time delay, the patient can override the activation of the above "sit-down" function by pressing of any other switch wherein the system is returned to the "stand mode."

55. The system according to claim 54, further characterized by the "sit-down" activation on the manual switch command circuit is performed responsive to the pressing of either of said "take left-step" and take right-step" switch to command the execution of a take step command, responsive to pressing either of said take step switches for a time duration that is longer than a predefined time of approximately one to five seconds, wherein the execution of either of said take step commands is responsive to a shorter duration pressing of said switch to distinguish between said commands to take a step and said command to sit-down, wherein said system prevents inadvertently activating said "sit-down" function.

56. The system according to claim 54, further characterized by the system activates a visual signal to inform the patient that said "sit-down" command has been activated.

57. The system according to claim 55, further characterized by the system activates a visual signal to inform that patient that said "sit-down" command has been activated.

58. The system according to claim 56, further characterized by said warning signal is a flashing light signal.

59. The system according to claim 54, further characterized by said override is executed responsive to pressing any switch.

60. The system according to claim 18, further characterized by said manual switch command circuit is mounted on the portable casing housing the electronics.

61. An electrical stimulation system, for coupling to a plurality of electrodes coupled to locations at the right and left side of the lower part of the body of a patient, said system characterized by: pulse generator means for selectively outputting trains of pulses having selectively variable duration and pulse repetition frequency responsive to a control signal; memory for storing operational instructions and data; control means for selectively outputting said control signal responsive to said memory and to input commands; pulse amplifier means for selectively amplifying pulse amplitude and power responsive to said control means; multiplexer means for selectively multiplexing outputs of said pulse amplifier means from said pulse generator and alternatively between selected ones of said electrodes responsive to said control means; and a battery power supply and a portable casing within which said system is mounted, wherein said pulse generator means and said control means are further characterized as comprising a single microcomputer, and an Analog to Digital Converter (ADC) , wherein said microcomputer selectively outputs through said ADC a single pulse train to said pulse amplitude and power amplifier, wherein said pulse power and amplitude amplifier is a single pulse amplifier, wherein the output of the pulse amplitude and power amplifier is multiplexed to distribute its output to at least 4 channels, wherein the multiplexer means is further comprised of solid state switch devices, wherein said control means switches modes of operation responsive to said input command means to provide a "stand" mode, a "take left step" mode, a "take right step" mode, and a "sit down" mode, wherein the levels of said pulse trains are ramped at an initial "stand" mode stand-up from an initial "sit- down" mode sit position, wherein at a subsequent "sit- down" mode the levels of said pulse trains decreases to zero in a ramp manner to avoid sudden cessation of power, wherein said microcomputer receives said input commands from both an external switching array mounted on a patient support device that is coupled by a coupling to said casing enclosing said microcomputer, pulse amplifier, ADC and multiplexer, and, from a switching array mounted on said casing, wherein switching input commands are coupled from said external array including two sets of switches, one mounted on the right side of patient's support device and one mounted on the left side of same, wherein said right side switches include one step switch to selected command of "take right step" and one level switch to select command of both increasing pulse level for right quadriceps and for increasing pulse level for producing a right step, wherein increasing of pulse level for right quadriceps is commanded by a single short pressing of said level switch, wherein each such pressing produces a fixed increment increase in pulse level, and wherein a long duration pressing of said level switch beyond a predetermined duration selects a command to increase level of pulse to the common peroneal nerve to produce a step, wherein said left side switches do the same respectively for the left quadriceps and left step, wherein switching to command sit down is performed by long duration pressing for a duration longer than some predetermined duration, of a selected switch, and wherein once said sit down has been commanded, said control means delays execution of sit down command for a different predetermined time duration.

62. The system according to claim 61, further characterized by said pulse trains have levels which are ramped at initial stand-up and are further characterized in that the initial ramp overshoots slightly above a subsequent steady state pulse level.

63. The system according to claim 61, further characterized by said patient support device is a reciprocal walker.

64. The system according to claim 61, further characterized by said external switching array is coupled to said casing by one of cabling, and by telemetry.

65. The system according to claim 61, further characterized by said two sets of switches are all automatic release switches that automatically return to their non-switched position upon releasing the switch pressure.

66. The system according to claim 61, further characterized by said pulse level adjustments are in terms of adjusting pulse amplitude.

67. The system according to claim 61, further characterized by said pulse level adjustments are in terms of adjusting pulse width.

68. The system according to claim 61 further characterized by said selected switch for sit-down is further characterized as having a separate sit-down switch.

69. The system according to claim 61, further characterized by said switches on said casing are further comprised of separate switches for increasing level of pulses to right and left quadriceps muscles and level of pulses to common peroneal nerve to execute each step, wherein there is also a separate switch for sit down and one for stand, wherein there are also separate switches to command decrease of each of the above pulse levels and where there is also a system on-off switch, wherein said casing also includes the battery power supply pack and a pulse level display that displays the level of the pulses to the channel whose level is manipulated at the time of a short predetermined time duration afterwards, and wherein the casing also includes a visual display of "low battery" and a warning display of maximal level of pulses to that quadriceps channel whose level is the highest, wherein the level at which said warning is activated is selectively determined responsive to the microprocessor, wherein there is a first audible alarm for low battery and another audible alarm, distinguishable from said first audible alarm, for maximal level of pulses that parallels the respective said visual alarm displays, the audible alarms being placed within said casing, wherein there are output sockets from the casing for output of said stimuli to the skin surface electrodes attached to the patient at the appropriate locations, wherein said microcomputer provides means for providing safety features, comprising means for returning the system to stand mode after sit down mode override, means for returning to stand mode after each step mode, and of returning to stand mode responsive to said step mode being activated in excess of a predefined time, and which disallows right and left step from being activated simultaneously, wherein said safety means delays execution of sit down mode for a predetermined time duration and allows override of said sit down mode by pressing any switch during said time duration, said safety means providing means for returning the system to said stand mode whenever otherwise no take step or sit down command is received and allowed by said microcomputer, including when no command inputs at all are received by said microcomputer, wherein all interpretation of switch pressure duration and all other interpretations and setting of the various predetermined time durations are determined by the microcomputer, wherein all safety features are provided responsive to said same microcomputer and all outputs to and determination of warnings except for low battery warning, are set responsive to said microcomputer, wherein all overrides are set by said microcomputer, and wherein said microcomputer sets pulse repetition frequency, pulse width and pulse amplitude, and degree of overshoot responsive to input commands.

70. The system according to claim 69, further characterized by a mode switch to change between stimulation of common peroneal nerve and of gluteus maximus such that at the gluteus maximus mode no steps may be taken but otherwise all commands to level change of pulses to common peroneal nerve to execute a step become commands to level change of pulses to gluteus maximus.

71. The system according to claim 69, further characterized by said visual warning includes a visual warning when sit down function has been activated and for a short time thereafter.

72. The system according to claim 71, further characterized by said visual warning is placed on said patient's support device.

73. The system according to claim 61, further characterized by said system returns to stand mode responsive to the pressing of any switch during said predetermined time duration after sit down has been commanded.

74. The system according to claim 19, wherein said patient support device is crutches.

75. The system according to claim 1 further characterized by: opto-isolator means, for coupling between said multiplexer means outputs and said electrodes, for buffering and optically and electrically isolating said outputs coupling to said electrodes and said electrical stimulation system.

76. The system according to claim 1 further characterized by: input command means for providing mode input command signals, wherein said control means selectively provides one of a plurality of modes responsive to said input command means; wherein said control means switches modes of operation responsive to said input command means to provide a stand mode, a take left step mode, a take right step mode, and a sit down mode; wherein said electrodes are wired in pairs as channels, one to the right quadriceps muscle, one to the left quadriceps muscle, and one each to the area of right and left peroneal nerves; wherein during the "take left step" mode the pulse outputs are selectively outputted to the right quadriceps channel and left peroneal channel, while at the same time no stimulus is outputted to the left quadriceps muscle and to the right peroneal nerve; said output to the right quadriceps channel being increased by a predefined amount during the time that no stimulus is being output to the left quadriceps channel.

77. The system according to claim 1, further characterized by: input command means for providing mode input command signals, wherein said control means selectively provides one of a plurality of modes responsive to said input command means; wherein said control means switches modes of operations responsive to said input command means to provide a stand mode, a take left step mode, and take right step mode, and a sit down mode; wherein said electrodes are wired in pairs as channels, one to the right quadriceps muscle, one to the left quadriceps muscle, and one each to the area of right and left peroneal nerves; wherein during the "take right step" mode the pulse outputs are selectively outputted to the left quadriceps channel and the right peroneal channel, wile at the same time no stimulus is outputted to the right quadriceps muscle and to the left peroneal nerve, said outputted to the left quadriceps channel being increased by a predefined amount during the time that no stimulus is being output to the right quadriceps channel.

78. The system according to claim 1 further characterized by: input command means for providing mode input command signals, wherein said control means selectively provides one of a plurality of modes responsive to said input command means; wherein said control means switches modes of operation responsive to said input command means to provide a stand mode, a take left step mode, a take right step mode, and a sit down mode; wherein said electrodes are wired in pairs as channels, one to the right quadriceps muscle, one to the left quadriceps muscle, one to the right gluteus maximus muscle and one to the left gluteus maximus muscle; wherein said sit down mode the amplitude of the pulses to the right and left quadriceps channels is initially increased by a predefined amount, for a predetermined time duration, and wherein thereafter the amplitude of the pulse outputs to the right and left quadriceps channels and to the right and left gluteus maximus channels are gradually reduced to zero, over a predetermined time period on the order of seconds.

79. The system according to claim 78 further characterized by said predetermined time duration is on the order of seconds.

80. The electrical stimulation system according to claim 1 further characterized by: input command means for providing mode input command signals, wherein said control means selectively provides one of a plurality of modes responsive to said input commands means; wherein said control means switches modes of operations responsive to said input command means to provide a stand mode, a take left step mode, and take right step mode, and a sit down mode; wherein said electrodes are wired in pairs as channels, wherein there are at least two channels one to the right leg quadriceps muscle and one to the left leg quadriceps muscle; means for detecting a drop below a predefined threshold level in output current level of said output of said multiplexer means, and responsive to detecting said drop, to automatically activate an audible alarm, and automatically disallowing left and right step modes.

81. The system according to claim 80, further characterized by said drop in output current level is measured as a change in an electrical voltage drop over an impedance in parallel to the output to the stimulation electrode.

82. The system according to claim 80, further characterized by said drop in output current level is measured as a change in an electrical voltage drop over an impedance in series with the output to the stimulation electrodes.

83. The system according to claim 78 further characterized by said amplitude level increase is an increasing peak voltage of the pulse.

84. The system according to claim 76, further characterized by the pulse width of the pulse output is increased by a predetermined amount only during said take left step that mode.

85. The system according to claim 77, further characterized by during the take right step mode, the pulse width of the pulse output is increased by a predefined amount only during that mode.

86. The system according to claim 76, further characterized by during the left take step mode, the pulse frequency of the pulse output is increased by a predetermined amount.

87. The system according to claim 77, further characterized by during the take right step mode the pulse frequency of the pulse output is increased by a predefined amount only during that mode.

88. The system according to claim 1 further characterized by: input command means for providing mode input command signals, wherein said control means selectively provides one of a plurality of modes responsive to said input commands means; wherein said control means switches modes of operation responsive to said input command means to provide a stand mode, a take left step mode, a take right step mode, and a sit down mode; wherein said electrodes are wired in pairs as channels, one to the right quadriceps muscle, one to the left quadriceps muscle, and one each to the area of right and left peroneal nerves; wherein when the stimulus to the peroneal nerves at the end of a given step is ceased, then the stimulus voltage to the quadriceps of the leg taking that given step is increased in a ramp-like graduated rate versus time.

89. The system according to claim 88, further characterized by said ramp voltage lasts less that one second.