GB2079959A - Evaluation circuit for short- circuit ring sensors - Google Patents

Abstract

The measuring and reference coils (L1, L2) of a magnetic short- circuit ring type position or angle sensor are connected in series with fixed resistor (R7) and transistor (T). Junction (D) between the coils is connected to the non-inverting input (11) of a comparator (K1) whose inverting input (10) is connected to a voltage divider (R1, R2) and, via a feedback resistor (R3) to the junction of reference coil (L2) and the collector of transistor (T). When transistor (T) is off, the decay current flows through diode (D1). The output of comparator (K1) is connected to the base of transistor (T), whereby an oscillatory circuit is completed. While the transistor is off, the decaying current through the coils flows through diode (D1) and fixed resistor (R7) and the coils from a voltage divider for the correspondingly decaying voltage across the fixed resistor (R7). The potential at E between resistor (R7) and measuring coil (L1) reaches a peak value Us alpha L1/L2 which is applied to a peak voltmeter (comparator K2) and a corresponding output voltage (U8) appears across capacitor (C). <IMAGE>

Description

SPECIFICATION Evaluation circuit for short-circuit ring sensors The invention relates to an evaluation circuit for a magnetic short-circuit ring type sensor having a measuring coil whose inductance is variable in dependence upon the position of the short-circuit ring, and a reference coil having an independent (reference) inductance. In such type of sensor, the magnetic short-circuit ring is displaceable along a limb of a yoke on which the measuring coil is disposed and completes a magnetic flux path through the measuring coil. The length of this flux path and thus its reluctance depends on the position of the short-circuit ring.

An object of the invention is to provide the simplest possible electrical circuit by means of which the inductance of a position sensor of the above-mentioned type can be evaluated, at the same eliminating, as far as possible, troublesome properties of the magnet core such as temperature-dependence of the permeability. The evaluation is to lead to an output voltage which is proportional, over a wide range, to the ratio of the measuring inductance to the reference inductance.

The present invention provides an evaluation circuit for a magnetic short-circuit ring type sensor having a measuring coil whose inductance is variable in dependence upon the position of the short-circuit ring, and a reference coil having an independent (reference) inductance, in which the reference coil is connected in series with the measuring coil and with a fixed resistor and is connected to the collector of a transistor whose base is connected to the output of a comparison device, one input of the comparison device being connected to a voltage divider comprising two resistors and the other input of the comparison device being connected to the junction between the coils, and in which a peak volt-measuring device is connected to the junction between the measuring coil and the fixed resistor.

The invention is further described hereinafter by way of example, with reference to the drawings in which Figs. 1 and 2 are circuit diagrams of two embodiments of evaluation circuit according to the invention.

Referring to the circuit diagram, illustrated in Figure 1, of the first embodiment of an evaluation circuit in accordance with the invention, the position-dependent inductance of a coil forming part of a position or angle sensor is designated L1, and the reference coil set to a constant inductance value is designated L2. The reference coil L2 is connected in series with the measuring coil L1 and with a fixed resistor R7 and is connected to the collector of a transistor T. The transistor T is a n-p-n transistor and its base is connected to the output of a reference device, that is to say, a comparator K1.One of the two inputs, that is to say, the inverting input 10, is connected to a voltage divider comprising two resistors R1 and R2, while the other input, that is to say, the noninverting input ii of the comparator K1, is connected to the junction D between the two coils L1 and L2.A damping resistor R8 is connected between the junction D and the common negative lead 1 2. The non-inverting input 1 4 of a second comparator K2, operating as a peak voltmeter, is connected to the junction E between the fixed resistor R7 directly connected to the common positive lead 13 and the measuring coil Ll,the output of the comparator K2 being connected by way of a diode D2 to a storage capacitor C which, together with a parallel-connected discharge resistor R10, is connected to the inverting input 1 5 of the comparator K2.

A feedback branch, comprising a resistor R3, leads to the inverting input 10 of the first comparator K1 from the collector of the transistor T. Moreover, a reverse-biassed diode D1 is connected between the collector of the transistor T and the positive lead 13 and, together with the aforementioned feedback branch, enables selfexcited oscillations to be formed in the manner described hereinafter, the frequency ratio and duty ratio of the oscillations being dependent upon the magnitude of the prevailing inductance value of the coil L1.

In order to clarify the mode of operation, it will first be assumed that the current flowing through the fixed resistor R7 has onty a very low value. The non-inverting input ii (positive input) of the comparator Ski is then almost at the potential of the full supply voltage UB fed by way of the positive lead 13, while the inverting input 10 (negative input) is at a lower potential determined by the resistors R1, R2 and R3. The transistorT is thus triggered. An increasing current commences to flow through the resistor R7 and the two coils L1 and L2. The potential at the inverting input 10 of the comparator Ski is additionally decreased by way of the positive feedback path R3.This state continues until the current flowing through the resistor R7 and the two coils L1, L2 and the conductive transistor T has reached a value such that, as a result of the voltage drop across the resistor R7, the potential at the non-inverting input ii of the comparator K1 has dropped below that of the inverting input 10. The transistor T is then blocked, and the current hitherto flowing through the coils L1, L2 can continue to flow in the decay circuit by way of the diode D1 and can decay in dependence upon time. Since the transistor T is blocked, the potential of the inverting input 10 jumps to a higher value determined by the resistors R1 and R2.The non-conductive state of the transistor T continues until the current in the decay circuit R7, L1, L2. D1, has dropped to a value such that the potential of the non-inverting input ii of the comparator K1 exceeds that of the inverting input 1 0. The transistor T then becomes conductive again and the oscillation operation which has just been described is repeated.

During the non-conductive period of the transistor T, the voltage drop across the resistor R7 also falls as a result of the decaying current in the coils. The coils L1 and L2 form an inductive voltage divider for this voltage, including the forward voltage of diode Dl. Thus, whilst the potential of the junction D is running to the value a UB determined by the voltage divider R1, R2, the potential of the junction E reaches a peak value Us determined by the quotient of the inductances of the coils L1 and L2. Thus, an output Ua, proportional to the quotient L1/L2, appears across the capacitor C.

In the circuit diagram of the second embodiment of Figure 2, the following components are provided in addition to the components which have been described with reference to Figure 1 and which are provided with the same reference numerals in Figure 2.

The variation in frequency which is caused by the variation in the inductance of the coil L1 can be substantially reduced by introducing an additional, timing circuit R1 1, C1 which is connected to the non-inverting input (positive input) ii by way of a diode D4, and a charging device R12, D5 associated therewith, and a coupling device R13 in conjunction with the diode D4. This advantage requires only a small expenditure on electrical components and permits a large variation of the inductance L1.

Claims (7)

1. An evaluation circuit for a magnetic shortcircuit ring type sensor having a measuring coil whose inductance is variable in dependence upon the position of the short-circuit ring, and a reference coil having an independent (reference) inductance, in which the reference coil is connected in series with the measuring coil and with a fixed resistor, and is connected to the collector of a transistor whose base is connected to the output of a comparison device, one input of the comparison device being connected to a voltage divider comprising two resistors and the other input of the comparison device being connected to the junction between the coils, and in which a peak volt-measuring device is connected to the junction between the measuring coil and the fixed resistor.

2. An evaluation circuit as claimed in claim 1, in which a first diode is connected across the series combination comprising two coils and the fixed resistor and is poled so that it does not conduct the collector current of the transistor.

3. An evaluation circuit as claimed in claim 1 or 2, in which a damping resistor is connected in parallel with the series combination comprising the transistor and the reference coil.

4. An evaluation circuit as claimed in claim 12 or 3, in which a timing circuit, comprising a resistor and a capacitor connected in parallel therewith, is connected by way of a diode to that input of the comparison device which is connected to the junction between the two coils.

5. An evaluation circuit as claimed in claim 4, in which the timing circuit and the diode associated therewith are connected to the collector of the transistor by way of a resistor and a further diode connected in series therewith.

6. An evaluation circuit as claimed in any preceding claim, in which the peak voltage measuring device is formed by an operational amplifier operating as a comparator.

7. An evaluation circuit for a magnetic shortcircuit ring type sensor constructed and adapted to operate substantially as herein described with reference to and as illustrated in the drawings.