Circuit Arrangement for Checking the Position of Electrodes

The present invention relates to a circuit arrangement for checking the position of electrodes of an object, which is more particularly to be treated with electric energy supplied to the electrodes by a generator. In such an arrangement, the electrodes can slip out of a precise position and can e.g. come into contact with one another (short-circuit), drop (open), or have a too large or too small distance from one another or from the object. In certain circumstances, this can undesirably influence the supply of electric energy and in the simplest case this can lead to an inadequate action, but dangerous overloading is also possible. If e.g. the electrodes are used in a muscle stimulator, skin burns or scalds can occur.

The problem of the invention is to check the position of the electrodes, particularly in a continuous manner during the treatment process and in the case of an inadmissible variation in the electrode position a signal is emitted, so that intervention can take place either automatically or by the operating personnel.

According to the invention this problem is solved in that an electrical test pulse is periodically applied to the electrodes and the current or voltage caused by the test pulse on or in the transfer or transition impedence between the electrodes is checked by means of an evaluating device and on exceeding or dropping below respective predetermined limit values emits a warning signal.

If the transfer impedance only has resistive or an ohmic component, the current or voltage drop occurring during the test pulse can be evaluated, the transfer resistance determined and compared with a desired value, optionally set as a function of the particular treatment case.

Another solution of the invention is obtained in that the electrodes are at least approximately insulated relative to one another and consequently in the normal case the transfer impedance has an appreciable capacitive component, the load or voltage changes on the capacitive component resulting from the test pulse are determined in the evaluating means or device and on exceeding or dropping below respective predetermined limit values a warning signal is emitted.

On determining the ohmic and/or capacitive component of the transfer impedance by means of test pulses, the treatment energy can be fundamentally simultaneously supplied. If e.g. short wave energy with a frequency of approximately 27 MHz is supplied and the test pulses have a low frequency spectrum, e.g. a repetition frequency of 50 Hz, separation is possible through frequency separating means, such as frequency-dependent filters. The decoupling between the test pulses and the treatment energy is particularly simple if the generator has at least temporarily, e.g. by being inactivated or switched off, a high internal impedance and the test pulse is applied in these intervals of high generator internal impedance.

The decoupling is particularly simple if the test pulse is supplied when the treatment energy is not supplied, e.g. before switching on or in the periodic intervals between applications of treatment energy so that the supply of the test pulse energy is independent of the supply of the treatment energy.

A particularly preferred embodiment of the invention is characterized in that the test pulse is supplied to the electrodes as a current pulse and the voltage appearing at the electrodes during its duration is supplied to the evaluating device, which supplies a disconnection signal to the generator if the electrode voltage exceeds an upper limit value or drops below a lower limit value and/or if the capacitance of the capacitive component between the electrodes exceeds a predetermined maximum value and/or drops below a predetermined minimum value.

The warning signal is directly used for switching off the generator and upper and lower limit values can be fixed both for the ohmic and for the capacitive component of the transfer impedance.

Appropriately, the test pulse alternately has positive and negative polarity. This avoids a d.c. component flow through the treatment object, which could otherwise lead to electrolytic dissociation or other damage.

According to another embodiment, the capacitance can be determined by evaluating means evaluating the voltage change during the test pulse or immediately after its disconnection. As the test pulse leads to a charging or discharging of the capacitance, for the duration of the test pulse there is an approximately exponential voltage change at the transfer impedance. When the test pulse ceases, there is a load or charge change on the capacitance and a corresponding voltage change can be determined which, in accordance with the given time constant, also goes back again approximately exponentially. It is thus possible during the test pulse or after it has ceased to determine the course of the voltage curve by means of several test points and establish the capacitance therefrom. During the test pulse, the voltage then also contains the component resulting from the ohmic resistance whilst, after disconnecting the test pulse, it is only possible to determine the voltage at the capacitance. Thus, according to a simplified embodiment, the capacitance can be determined by evaluating the voltage immediately after the disconnection of the test pulse, it then being unnecessary to take account of the exponential potential gradient.

Finally, according to a further embodiment, the warning signal, e.g. its amplitude, can be dependent on the magnitude of the determined ohmic or capacitive component and can be supplied in regulating manner to a device for setting the electrode position, e.g. the distance from the object or the bearing pressure on the object, or a device for setting the electric energy supplied by the generator. Thus, within a given range of variations, it is possible to automatically make the necessary corrections without involving the operating personnel.

The invention is described in greater detai hereinafter relative to a non-limitative embodiment and the attached drawings, wherein show:

FIG. 1 a circuit arrangement according to the invention.

FIG. 2 the test pulse and in dotted line form the potential gradient occurring at the transfer impedance between the electrodes.

FIG. 3 a more detailed circuit diagram compared with FIG. 1 of an evaluating device and the stages associated therewith.

FIG. 1 shows a circuit arrangement according to the invention in a muscle stimulator. A first electrode 1 and a second electrode 2 are arranged on a treatment object 3, e.g. an upper arm muscle represented in cross-section through its contour line. Electric energy is supplied by a treatment generator 4 in its normally operative state to electrode 1 from its first output 5 via a first terminal 6 and to electrode 2 by its second output 7 via a second terminal 8. This energy can consist of pulses, whose width is adjustable between 10 and 990 .mu.s and whose amplitude can be 160 V.sub.ss with alternating polarity. During the pulse, a constant current is supplied, which can be set between 1 and 99 mA. The frequency of these pulses can also be selected between 1 and 99 Hz. Thus, between the pulses is a gap in which the generator 4 is switched off i.e. by activatable statechanging means the state of the treatment generator is switched from its normally operative state to a non-operative state, so that it, supplies no energy and has a high internal resistance compared with the remaining switching part, so that in the intervals the generator branch does not have to be taken into consideration.

The distance between electrodes 1 and 2 can according to the equivalent circuit diagram shown in dotted line form, be represented as a series connection of a capacitor 10 of 0.47 .mu.F and a resistor 11 of 1500 ohm, with a 220 ohm resistor 12 being parallel to capacitor 10. These are naturally specific values of a particular embodiment and in practice can vary considerably upwards and downwards in the individual case.

Test impulse supply means, such as a test signal generator 14 supplies test pulses to terminals 6 and 8 and consequently to electrodes 1 and 2 and corresponds to the 5 V power source with an internal resistance of 15 kOhm. Thus, with the given values, the test signal generator 14 supplies transfer impedance 10, 11, 12 with a virtually constant current, the test pulse being supplied for a duration of 220 .mu.s in the intervals of the treatment pulses, in which generator 4 is switched off.

Such a test pulse is diagrammatically shown in the continuous line and with a current amplitude a. Particularly due to resistor 11, at the start of the pulse interval there is a voltage amplitude b1. As capacitor 10 is charged during the pulse, the voltage between points 6 and 8 increases and at the end of the pulse interval reaches the somewhat higher value c, so that the difference between c and b1 is due to the charging of the capacitor. If test pulse (a) ceases, the voltage represented by dotted lines in FIG. 2 instantaneously decreases by an amount b2 between terminals 6 and 8 where b2 equals about b1, because instantaneously the current ceases through resistor 11. The voltage (c-b2) from capacitor 10 is maintained and this then decays exponentially, roughly corresponding to the time constant due to capacitor 10 and resistor 12. The size of the capacitance can be determined from the exponential course of the dotted line curve during test pulses 15 or subsequent thereto, e.g. by amplitude measurements at several points. It may be adequate for the intended monitoring and warning signal means to evaluate only the residual voltage left after disconnecting the test pulse 15 for comparison with a desired value and optionally for forming the warning signal.

Control signals are supplied from a control stage 16 via a line 17 to a test pulse generator 14 and via a line 18 to the treatment generator 4 ensuring that the test pulses occur in the intervals of the treatment pulses. The voltage occurring during test pulses 15 at terminal 6 relative to the earth terminal 8 and which is shown in dotted line form in FIG. 2, is supplied by connecting line 19 between terminals 6 and test pulse generator 14 to an input 20 of evaluating means, such as an evaluating device 21. In the latter the amplitude values b1, c of the dotted line voltage curve in FIG. 2 are measured and prepared at least to preferably reach an adjustable desired value. This setting can take place as a function of the object to be treated, in order to ensure an optimum energy supply. If there is an inadmissible divergence from at least one desired value, a warning signal occurs at output 22 of evaluating device 21, which is supplied to the treatment generator 4 at input 23, switches off the generator and possibly releases a signal, e.g. a flashing lamp or bell. Control stage 16 can also supply the necessary sensing signals to an input 24, so that in evaluating device 21, the amplitude checking of the voltage between terminals 6 and 8 takes place at the correct time, which are to be selected for an appropriate evaluation.

Appropriately, means are provided so the test pulses have alternating polarity, so that they cannot cause a direct current through the treatment object.

Particularly if treatment energy is very high in the frequency range and low frequency test pulses are used, it can be admissible for the treatment energy to act continuously. Corresponding separating filters 25, 26 should then be connected in line 27 between terminal 6 and treatment generator 4 or in line 19 between terminal 6 and evaluating device 21 or test signal generator 14.

FIG. 3 again shows the object 3 between electrodes 1 and 2, which are connected to terminal 6 with lines 19, 27 and to terminal 8. The treatment generator 4 is omitted. The circuit according to FIG. 3 is controlled by evaluating means, such as a central unit 30, which in particular contains a sensing generator and supplies the necessary control and test signals, whilst also carrying out the evaluation of the voltage occurring at terminal 6 relative to terminal 8.

From output 31 of central unit 30, the test pulses are supplied to an amplifier 32 to whose output is connected line 19. The voltage occurring at terminal 6 is supplied to the input of a comparison amplifier 34 across amplifier 33. Via a line 34a, central unit 30 supplies control signals for a staircase generator 35, to whose outputs 36, 37 are connected the inputs of a differential amplifier 38. Amplifier 38 is controlled in such a way that a stair case voltage occurs at its output during test pulse 15 and is indicated by curve 39 in FIG. 3. This staircase voltage is supplied to a terminal 40 and can be used from there for other functions in the particular apparatus. Stair case voltage 39 is also supplied to the second input of comparison amplifier 34, which is e.g. set in such a way that it supplies an output pulse when the test voltage received from the output of amplifier 33 and supplied to the first input of amplifier 34 is higher than the staircase voltage 39 supplied to the second input. Thus, in accordance with the time position, at the output of comparison amplifier 34 a signal is received, which is representative of the particular amplitude stage. This signal is supplied to the input 41 of evaluation stage 30. Signal 39 preferably appears in the starting part of test pulse 15 and again immediately after the end thereof. Thus, two values are supplied to input 41, whereof the first substantially corresponds to the amplitude jump b in FIG. 2, whilst the second substantially corresponds to the amplitude jump (c-b2) in FIG. 2 immediately after the end of test pulse 15. The first amplitude jump gives information on the size of resistor 11 and central unit 30 supplies a warning signal at output 42 if this resistance value is above or below a set desired range.

The second amplitude jump represents the size of capacitor 10. This amplitude jump is also corresponding evaluated compared with predetermined desired values and if one of these limits is exceeded, a corresponding warning signal is supplied via output 42. As in FIG. 1, the warning signal can be used for switching off treatment generator 4 and for switching on a warning lamp or a warning bell.

As the amplitude (c-b2) of the exponential voltage range between terminals 6 and 8 caused by capacitor 10 can be much smaller than the voltage jump at the beginning or end of the test pulse caused by ohmic component 11, it can be appropriate to switch the gain of amplifier 33 to a higher, e.g. triple value for determining the exponential drop after the end of the test pulse. This can be brought about by central unit 30 via connection 43 to amplifier 33.