WO2008004204A1

WO2008004204A1 - An electrical stimulation device for nerves or muscles

Info

Publication number: WO2008004204A1
Application number: PCT/IE2007/000065
Authority: WO
Inventors: Padraig Fogarty; Michael John Mcnulty
Original assignee: University Of Limerick
Priority date: 2006-07-06
Filing date: 2007-07-06
Publication date: 2008-01-10
Also published as: IE20070491A1

Abstract

An electrical stimulation device is for a biological load for applications such as transcutaneous nerve stimulation, neuromuscular electrical stimulation, or electrical muscle stimulation. The device comprises a controller, a power generation circuit for output of power to electrodes. A feedback compensation circuit has a sensor for sensing delivered current in the load and for providing feedback to the power delivery circuit. The feedback compensation components are integrated into the power generation circuit so that only the required power is generated and there is no need to dissipate excess power. Hence there is no need for a current limiting circuit between the power generation circuit and the load. The sensor comprises a sense resistor (Rsense,) in the load and a voltage tap (Vfb) across the sense resistor. The voltage tap is provided as a signal input to an error amplifier (EA) of the feedback compensation circuit, the other input being a reference voltage (Vref) representing a desired load current.

Description

"An electrical stimulation device for nerves or muscles"

INTRODUCTION

Field of the Invention

The invention relates to a stimulation device for nerves or muscles.

Prior Art Discussion

It is known for a long time that the effect of applying an electrical signal to a muscle is to cause an involuntary response. Electrical stimulation of both muscle and nerves is well established for both medical (e.g. pain relief, wound healing, rehabilitation) and cosmetic (e.g. muscle toning) purposes. Some of the more common applications of external stimulation devices are briefly described below:

Transcutaneous Electrical Nerve Stimulation (TENS)

This uses the delivery of electrical pulses to stimulate the nerves in the body. These signals are usually of low amplitude and when applied to the local area where pain relief is required cause a sensation that is similar to a gentle massage. The two most widely claimed effects are: the blocking of pain signals, and the stimulation of endorphin release.

Neuromuscular Electrical Stimulation (NMES) or Electrical Muscle Stimulation (EMSV

La this application the intention is to cause muscle contractions. The main market driver behind this technology is body toning, where the devices are used to 'exercise' targeted muscles on the body to improve strength / appearance. Typically these devices are sold directly to consumers, or as medical devices for specific conditions.

There is a significant overlap between FES and NMES, both in terms of technology and applications. Functional Electrical Stimulation (FES)

In a similar manner to NMES, FES is intended to cause muscle contractions, however in this situation the intention is to recreate the normal movement of a muscle and thereby cause it to 'function' in a controlled manner. There are conditions that result in patients losing or being unable to control their muscle movements (such as spinal cord injury or stroke). The objective in this situation is to cause the muscle to act as desired. FES often includes sensing technology to detect the trigger conditions, thereby giving the patient a better quality of life and more independence. Aside from the direct impact of improving mobility or control, there are additional benefits to this form of stimulation including improved blood circulation to prevent muscle wastage and prevent pressure sores.

The main components of a stimulation device are: • Power Source.

• User Interface.

• Controller. To configure the device to provide the desired stimulation pulses. This will usually include a CPU or equivalent circuit to time the pulse duration and repetition rate, adjust the pulse intensity according to the desired level and monitor the device to ensure correct operation, providing feedback and error handling in the event of a fault.

• Monitors. For safety and regulatory reasons it is generally necessary to monitor the supply voltage to ensure that the device is operating within its specified parameters and the output signal is being delivered as intended. Additional monitoring elements which may be linked to the output circuit and/or feed into the microcontroller could include: an over-current voltage protection circuit; an under-voltage lockout circuit, and an over-temperature protection circuit.

• Pulse Output Delivery Circuit. This circuit is required to switch the necessary voltage/current sources to the output pads such that the desired signals are delivered. As the controller is typically a logic level device and the output voltage is much higher, this circuit must contain the necessary voltage conversion elements to safely interface between these two levels. Various implementations exist such as: optical coupling, relay switching and isolating/level shifting transistors.

• Stimulation Power Generation Circuit. Since the device power source is typically a low voltage source an additional circuit block is often required to produce the higher voltage necessary for effective stimulation.

• Current/Voltage Control. Depending upon the device design, the pulse delivered may be constant-current, constant-voltage or a combination of both. In any case is desirable to make the pulse amplitude adjustable. One approach is to generate a fixed voltage using the stimulation power circuit, which is set slightly above the maximum required, and then dissipate any excess power using an active circuit stage when the pulse is being delivered. This approach has the advantage of simplifying the power circuit design and is capable of adjusting to a certain degree of load variability. However, the main disadvantage of this approach is inefficiency, as any power dissipated in this way is wasted (usually in the form of heat). For implanted devices the additional heat generated may not be desirable.

The purpose of the stimulation device is to generate, control and deliver pulses to the electrodes placed on the user's/patient's skin. The pulse characteristics (such as amplitude, duration, and frequency) determine the evoked response, but these are generally based upon the application and the operational requirements (such as electrode type, body region, power circuit) for any given device. It should be noted that various pulse characteristics are proposed by the many manufacturers of these devices, and no general set of parameters can be defined as being optimum.

WO03/006106 describes an apparatus having a fixed high voltage power supply. The amplitude of the applied stimulation signal is adjusted by means of sensing the load current and feeding this back to a constant current circuit which limits the voltage/current to the required level. The excess energy available from the power supply is dissipated by the constant current circuit. As the output current is controlled (to keep it at a constant level) by dissipating excess output power it appears that efficiency of the arrangement would suffer.

US4917093 describes a stimulation device having an adjustable high voltage power supply. The amplitude of the applied voltage is adjusted according to feedback indicating the amplitude of the voltage drop across the electrodes. The purpose of the adjustment is to minimize the difference between the applied voltage and that across the load, to improve energy efficiency.

Fig 1 illustrates the typical output voltage, stimulation waveforms, and loss for a complex body load for a stimulation device of the prior art in which voltage across the electrodes is monitored to provide feedback. Although for such a circuit a single fixed high-voltage rail would be sufficient to deliver the required stimulation signals, it is clearly of benefit to minimise the output voltage (V_OUT) such that the loss and power dissipation are also minimised. Since the parameters of the complex body load are not well defined and may vary over time, a conservative design approach is usually adopted to ensure a sufficiently high voltage is available for the entire pulse duration as dictated by the peak voltage of the waveform. However, it is clear from the waveforms as shown that even this peak voltage level still represents a significant loss of power during the rising period of the first pulse and that the loss is even greater during the second (balancing) pulse since the initial load voltage levels are not equal in both phases.

The invention is directed towards providing an improved stimulation device. More specifically, the invention is directed towards providing improved power efficiency, and/or reduced voltage clipping and attendant charge imbalance, and/or improved user/patient safety and comfort, and/or reduced heat dissipation, component count, cost, and size.

SUMMARY OF THE INVENTION

According to the invention, there is provided an electrical stimulation device for a biological load, the device comprising: a controller, a power generation circuit for output of power to electrodes, a feedback compensation circuit connected to the power generation circuit and comprising a sensor for sensing delivered current in the load, and means in the power generation circuit for operating in response to the feedback compensation circuit to generate only the power required for the load.

In one embodiment, the sensor comprises a current sensing device in the load current path and a voltage tap across the current sensing device to provide a voltage representative of load current.

In one embodiment, the current sensing device is a resistor of known value.

In one embodiment, the voltage tap is provided as a signal input to an error amplifier of the feedback compensation circuit, the other input being a reference voltage.

In another embodiment, the reference voltage input to the error amplifier represents a desired load current.

In one embodiment, the power generation circuit comprises a boost power converter,

hi one embodiment, the boost power converter is of the current mode control type.

In one embodiment, the power generation circuit dynamically varies output voltage in response to the sensed load current without an intermediary circuit between the power generation circuit and the load.

In another embodiment, the power generation circuit comprises a high frequency switching power converter.

hi one embodiment, the transient response of the power generation circuit can be set to provide the required output for a desired body load, while providing a reduced output for alternative loads. In one embodiment, the switching frequency (F_Sw) is sufficiently high to enable the power converter to achieve the desired stimulation transient response without the need for a large output storage capacitor.

DETAILED DESCRIPTION OF THE INVENTION

Brief Description of the Drawings The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:-

Fig. 1 is a plot showing typical stimulation waveforms and power loss from a prior art device;

Fig. 2 is a block diagram of a stimulation device of the invention;

Fig. 3 is a set of plots showing stimulation waveforms and power loss of the device of the invention;

Fig. 4 is a plot illustrating the response for a resistive model with modified load compensation; and

Fig. 5 is a simplified schematic of a feedback compensation circuit arrangement for constant current applications.

Description of the Embodiments

An electrical stimulation device of the invention enables generation of the precise voltage required during the entire stimulation pulse period. This is achieved by the power generation circuit being capable of dynamically generating the output voltage required across the body load, and responding directly to variations in this impedance. This dynamic response may be achieved using an entirely hardware feedback circuit arrangement, thereby providing a faster, safer and more reliable control method.

Referring to Fig. 2 the stimulation device comprises: a microcontroller, a power generation circuit comprising a comparator (COMP), a transistor driver (DRIVER), a power sense resistor (Res), a power transistor (NMOS), an inductor (L), a diode (D), and an output capacitor (C_OUT) for output of power to a body load (Z_LOAD) (comprised of the electrodes and biological tissue); a feedback compensation sensor (R_SENSE), compensation network (Z; & Z_f), and error amplifier (EA) for directly sensing delivered current in the load and feeding it back to a comparator (COMP) of the power generation circuit, which operates in response to provide a desired current through the load. An important aspect of this circuit is that only the required power is generated, and hence there is no need to dissipate excess power.

The sensor Rsense (providing voltage feedback signal (Vfb),) is in the load current path. The current-sensing resistor R_SENSE has a known value so that the voltage tap Vfb is a direct representation of load current in real time. The voltage tap Vfb is provided as a signal input to the error amplifier EA, the other input being a reference voltage V_REF representing a desired load current.

The power delivery circuit comprises a boost power converter. L, D, C_OUT and NMOS. The boost power converter is of the current mode control type, with the current being sensed at Res-

As is clear from Fig. 2, the power generation circuit dynamically varies output voltage in response to the sensed load current without an intermediary circuit between the power generation circuit and the load. In effect, the load and the feedback compensation circuit are integrated into the power generation circuit. Intermediary circuits such as constant current circuit of the prior art are avoided. The power generation circuit comprises a high frequency switching power converter, with the switching frequency being determined by the signal applied to Fsw- The switching frequency (Fsw) is sufficiently high to enable the power converter to achieve the desired stimulation transient response without the need for a large output storage capacitor (C_OUT)-

A major benefit of the circuit is elimination of the need for a power dissipating current control circuit stage. As shown, this is achieved by including the output stage and load as part of the feedback compensation circuit. The load current (Vfb = R_SEN_SE X I_LOA_D) is sensed and fed back to control the stimulation power generation circuit from which the pulse is delivered, hi this way the voltage generated is only that which is required to maintain the required load current, thus providing better efficiency.

Fig 3 illustrates the typical output and stimulation waveforms for the device. The output voltage level (V_OUT) is seen to track but is slightly above that required by the load (the stimulation voltage,) leading to a small power loss. This loss is due to the unavoidable conduction losses in the pulse output delivery circuit components

(switches and resistive circuit elements). While this loss can be minimised by careful component selection and design, it can never be completely eliminated as it is inherent for all such circuit arrangements.

Referring to Fig. 2 again, it is clear that the high- voltage stimulation power generation circuit (power converter) does not monitor its own output voltage, but instead the current flowing though the load. This is compared with the desired current level, which is given by the adjustable voltage input Vref and the hardware circuit is designed to maintain the output voltage at the optimum level to sustain the desired current with no additional adjustment of the output voltage being necessary.

In power converter applications, it is desired to keep an output voltage/current stable, in spite of variations/disturbances in input voltage, converter elements, and load current/voltage, which in this instance may result from variations in the load impedance itself. The circuit automatically adjusts to such disturbances, regulating the output current/voltage at the desired output with the desired accuracy. Closed loop control is used to maintain a stable, regulated output.

The feedback compensation involves modifying the open loop transfer function of the system, to ensure adequate phase margin at the cross-over frequency (frequency at which the open loop gain is 1 (Odb)). The error amplifier (EA) and its associated compensation network (Zi and Zf) give adequate gain and phase margins to the open loop transfer function to ensure that closed loop stability and transient response specifications are met.

By means of compensation network (Zj and Z_f) the transient response of the controller can be set to provide the required output for a desired body load, while providing a reduced output for alternative loads. Fig. 4 illustrates how the response could be further tailored to provide a slowly rising output voltage when the load is purely (or substantially) resistive, while the response for complex loads containing a large capacitive component would remain as previously shown. By adopting this approach it is possible to greatly improve safety and comfort for the patient.

The device of the invention eliminates the resistive network required in the prior voltage-sensing devices and instead places the load in the sensing path. This provides a number of benefits including the ability to eliminate the separate current control circuitry. In the invention, the current flowing in the load is sensed by a resistor R_sense, and this is fed directly to the power converter circuit and compared against a reference. However, using this scheme the power converter will generate only the voltage required to maintain the desired current through the load, thereby saving considerable power. A significant advantage is that variations in the load impedance are automatically adjusted for.

The power converter allows voltages of 100V or higher to be delivered even though the power source is merely a set of batteries. The power converter circuit allows for generation of larger output voltages from lower input voltages. This power converter is designed to deliver the maximum current/voltage under the worst-case load conditions. Although the topology shown in Fig. 2 may be described as a 'boost' power converter, alternative topologies such as 'flyback' or 'sepic' may also be used, and the same basic principles as described can be applied to these other topologies.

The boost power converter as shown can be further described as a Current Mode Control (CMC) type, operating in a Discontinuous Current Mode (DCM). This CMC converter is described as a one-design solution, which offers many benefits including:

• Inherently provides period-by-period peak switch current limiting.

• Removes one pole from the control to output transfer function. This leads to greater loop bandwidth (faster transient response) and also simpler loop compensation.

• Input voltage feed-forward provided for by the fact that the inductor current rises proportional to the line voltage.

Operating this converter in Discontinuous Current Mode (DCM) simplifies the design as it eliminates the control loop instability at duty cycles > 50% seen with continuous current mode (CCM) operation. However, continuous current mode operation is also possible with the addition of suitable slope compensation to minimise this instability.

A Voltage Mode Control (VMC) boost power converter scheme could alternatively be used, and although this offers a simpler single feedback loop and good noise margin, the slower transient response and more complex compensation required (due to the double pole in the plant characteristic and loop gain variations) make this less advantageous.

The output capacitor C_OUT is chosen to give the specified output voltage/current ripple. Using higher switching frequencies (Fsw) enables the use of a smaller capacitor and has the additional benefit of moving the dominant pole in the small signal converter model to a higher frequency, further simplifying the feedback compensation circuit.

This feedback compensation circuit can be shown to be equivalent to a simplified form as shown in Fig. 5. Since the circuit node monitored by the voltage generation circuit is within the current path of actual load, the feedback control loop (compensation network Fig. 5 Zi and Zf) the characteristics of the load contribute to the system response.

In stimulation applications pulse widths may range from as little as 50μs upward. For these short pulses, transient response times much less than the pulse duration are required. This indicates that power converters with high switching frequencies are desirable. For example, for a critically damped system (fastest response without any overshoot) at time t=l/f_c, the output voltage/current has reached 98.6% of its final value where fc is the loop crossover frequency. In general, fc__MAx ⁼ 10% Fsw, therefore maximizing Fsw is necessary for fast transient response time. Using faster switching speeds also, reduces the size of the magnetics in the power converter

'plant'. Thus a reduction in size also results.

The stimulation device has an efficient method of generating and delivering a biphasic constant current stimulation pulse via skin electrodes, typically used in TENS and NMES applications. However, the techniques described may be applicable to other stimulation applications including those using implanted electrodes, applications using monophasic pulses or constant voltage pulses.

The following summarises some of the benefits of the invention:

• More power efficiency

• The output current to be maintained at the optimum output level irrespective of load variations.

• Programmable/adjustable amplitude is provided, without issues associated with voltage clipping, (with exception of maximum operating voltage)

• Better matching of pulse positive and negative phases is achieved.

• Power stage generates the required voltage to maintain the output current at the desired level (voltage rail does not dip)

• Reduced number of components, leading to size and cost savings

The device may include a high frequency switching power converter, which would bring the following additional benefits. • Longer constant amplitude pulses can be delivered, as the power required is generated as required, and not sourced from a large reservoir capacitor.

• Smaller magnetic components and output capacitors can be used (& cheaper).

As the load forms part of a feedback compensation network, and so therefore stability and transient response of the device can be optimised for the application.

• Circuit is more stable over a wide range of loads.

• Safer delivery of output pulse when bad contact (resistive load)

Improved safety (regulator compliance)

• Complete hardware control scheme; software control and error detection is not required; although these may still be included.

• High voltage need only be generated during pulse delivery.

• It is not necessary to maintain a high voltage on the device when stimulation is not required.

• The high voltage rail may be discharged after each pulse reducing the risks this can pose.

The invention is not limited to the embodiments described but may be varied in construction and detail.

Claims

1. An electrical stimulation device for a biological load, the device comprising: a controller, a power generation circuit for output of power to electrodes, a feedback compensation circuit connected to the power generation circuit and comprising a sensor for sensing delivered current in the load, and means in the power generation circuit for operating in response to the feedback compensation circuit to generate only the power required for the load.

2. A stimulation device as claimed in claim 1, wherein the sensor comprises a current sensing device in the load current path and a voltage tap across the current sensing device to provide a voltage representative of load current.

3. A stimulation device as claimed in claim 2, wherein the current sensing device is a resistor of known value.

4. A stimulation device as claimed in claims 2 or 3, wherein the voltage tap is provided as a signal input to an error amplifier of the feedback compensation circuit, the other input being a reference voltage.

5. A stimulation device as claimed in claim 4, wherein the reference voltage input to the error amplifier represents a desired load current.

6. A stimulation device as claimed in any preceding claim, wherein the power generation circuit comprises a boost power converter.

7. A stimulation device as claimed in claim 6, wherein the boost power converter is of the current mode control type.

8. A stimulation device as claimed in any preceding claim, wherein the power generation circuit dynamically varies output voltage in response to the sensed load current without an intermediary circuit between the power generation circuit and the load.

9. A stimulation device as claimed in any preceding claim, wherein the power generation circuit comprises a high frequency switching power converter.

10. A stimulation device as claimed in claim 9, wherein the transient response of the power generation circuit can be set to provide the required output for a desired body load, while providing a reduced output for alternative loads.

11 A stimulation device as claimed in any of claims 6 to 10, wherein the switching frequency (Fsw) is sufficiently high to enable the power converter to achieve the desired stimulation transient response without the need for a large output storage capacitor.

12. A stimulation device substantially as described with reference to the accompanying drawings.