GB2059604A - Evaluation circuit for a transducer - Google Patents

Abstract

A variable inductance coil L1 and a reference inductance coil L2 are connected in series with one another and with a resistor R7 in the collector circuit of transistor T whose base is connected via a voltage divider R5, R6 to the output of a comparator K. The inverting input A of the comparator is connected to a fixed voltage divider R1, R2 and, via a diode D1, to the collector of the transistor T. When transistor T conducts, A is held at a low potential and the current in resistor R increases until the voltage at point B falls below the voltage at A, whereupon the transistor is blocked and the decay current flows through diode D2. The resulting potential at point D between L1 and L2 depends only on the ratio L1/L2 and is integrated on a capacitor C1. Temperature and other adverse influences on L1 and L2 cancel out. <IMAGE>

Description

SPECIFICATION Evaluation circuit for a transducer The invention relates to an evaluation circuit for a transducer having a coil whose inductance is variable in dependence upon a displacement to be measured.

An object of the invention is to provide as simple as possible a circuit by means of which evaluation of the signal of an inductive displacement transducer may be effected with substantial elimination of extrinsic properties of the magnetic core, such as the dependence of permeability on temperature.

It is known that these influences can be detected by means of a reference coil and processed together with the signal of the displacement-dependent coil.

The present invention provides an evaluation circuit for a transducer, comprising a transducer coil whose inductance is variable in dependence upon the position of a magnetic flux shortcircuiting ring, a reference coil having an independent (reference) inductance, the two coils being connected in series with each other and with a fixed resistor in the collector circuit of a transistor, a comparator one of whose inputs is connected to the fixed resistor and whose other input is operatively connected to the collector of the transistor and to one end of the winding of one of the two coils, the output of the comparator being connected to the base of the transistor and a peak value measuring device connected to the junction between the two coils.

The purpose of the transduction is to provide a position-dependent d.c. voltage.

The invention is further described, by way of example with reference to the drawing, which is a circuit diagram of an evaluation circuit in accordance with the invention.

In the evaluation circuit according to the present invention, the position-dependent inductance of a coil pertaining to a half-differential displacement or angle transducer is designated as Ll,and the reference coil, which is adjusted to a constant inductance value, is designated as L2.

The coil L1 is wound on a core along which a magnetic flux short-circuit ring is displaceable in accordance with the displacement linear or angular to be measured, thereby altering the inductance correspondingly. The two coils are serially connected and are arranged in series with a resistor R7 in the collector circuit of an NPN transistor T, whose base is connected to a voltage divider, formed by two resistors R5 and R6. The voltage divider R5/R6 is arranged between the output of a comparator K, formed by an operational amplifier, and a common negative supply lead 10, and is itself connected by means of a resistor R4 to a positive supply lead 11 which is connected to a positive operating voltage UR. A "free-running" diode D2 is connected across the series circuit L1, L2, R7.The inverting input A of the comparator is connected to a voltage divider formed by resistors-Ri and R2, and is connected by way of a resistor R3 and a diode D1 to the collector C of the transistor. The non-inverting input of the comparator K is connected to the junction B between the reference coil L2 and the series resistor R7. The positive input of an operational amplifier 0 is connected to the junction D between the two inductarice coils L1 and L2, by means of a resistor R8. The operational amplifier 0 operates as a peak-value rectifier, and its output is connected via a diode D3 to a storage capacitor C1, in parallel with which there is connected a high-ohmic discharge resistor R9.

Let it be assumed that the current through the resistor R7 is only very small. The non-inverting input of the comparator is therefore at practically the potential of the full supply voltage UR, while the inverting input is at a lower potential. The transistor T is therefore switched on, and an increasing current starts to flow via the resistor R7 and the two inductance coils. The feedback path D1, D3 ensures that the potential at the inverting input is additionally reduced. This state continues until the current through R7, L1, L2 and the conductive transistor T has reached such a value that, because of the voltage drop at R7, the potential at the non-inverting input of the comparator has fallen below that at the inverting input.The transistor T is now blocked, and a path for the current flowing through the inductance coils is provided by the free-running circuit, that is, the circuit R7, L2, L1, D2, during which it decays in dependence on time. Since the transistor T is blocked, the potential at the inverting Input A jumps to a higher value which is set by the resistors R1 and R2. This state continues until the current in the free-running circuit, and hence also the voltage drop at the resistor R7, have decreased to such a value that the potential at the non-inverting input of the comparator exceeds that at the inverting input. The transistor T then again conducts, and the process is repeated.

During the switch-off period of the transistor, the voltage drop at tie resistor R7 decreases owing to the decaying coil current. The inductance coils L1 and L2 form an inductive voltage divider for this voltage. While, therefore, the potential at the point B reaches the value a.UB given by the voltage divider Ri, R2, the potential at the junction D reaches a peak value Us given thereby and by the quotient of the inductances. Neglecting the voltage drop at the diode D2, we have

where Ua designates the supply voltage and a the ratio R2:(R1 + R2). This peak voltage is taken to the capacitor C1 in the serially-connected peak rectifier and appears as a d.c. voltage at the terminal E. A matching amplifier, which permits matching to less high-ohmic evaluation circuits, may also be connected to this point. Since the voltage Us includes only the quotient of the inductances, all tolerance influences which similarly effect the two inductances -- such as the properties of the magnetic ore - are eliminated.

An advantage of this circuit (which operates according to the principle of the blocking oscillator) compared with other known methods which, for example, use an inductive voltage divider in combination with a sine wave oscillator, lies in the reduced need of switching elements.

Claims (4)

1. An evaluation circuit for a transducer, comprising a transducer coil whose inductance is variable in dependence upon the position of a magnetic flux short-circuiting ring, a reference coil having an independent (reference) inductance, the two coils being connected in series with each other and with a fixed resistor in the collector circuit of a transistor a comparator one of whose inputs is connected to the fixed resistor and whose other input is operatively connected to the collector of the transistor and to one end of the winding of one of the two coils, the output of the comparator being connected to the base of the transistor and a peak value measuring device connected to the junction between the two coils.

2. An evaluation circuit according to claim 1, in which the comparator comprises a voltage divider whose division ratio is alterable by the transistor.

3. An evaluation circuit according to claim 2, in which the alteration to the division ratio is effected via a diode.

4. An evaluation circuit constructed and adapted to operate substantially as herein described with reference to and as illustrated in the drawing.