CIRCUIT FOR MEASURING VARIABLE INDUCTANCE

Abstract: An oscillator (10) supplies signals to the fixed (Lo) and variable (L) inductances of an inductive displacement transducer, the signal at the common terminal (RW1) being used to provide an output signal (Uact) and a voltage (V) for controlling the amplification of an amplifier (B3) in the path from the oscillator (10) to the inductance (L). The signal from the common terminal (RW1) is processed in a differential amplifier (30) and an amplification control circuit (40) comprising a rectifier (B4, S), a delay element (VZ1), an integrator (I) and a sample-and-hold circuit (S and H) controlled by a synchronisation ouput of the oscillator stage (20). The controllable amplifier (B3) is part of a multiplier (50), which may comprise analogue or digital components. The controllable amplifier may be replaced by an amplitude-controllable oscillator connected to a timing signal generator which is also connected to the first oscillator (10).

Description: Circuit for Measuring variable inductance The present invention relates to a circuit for measuring a variable inductance and in particular the variable inductance of an inductive displacement transducer. Circuits associated with such transducers are disclosed in, for example, DE-A-2924093 and DE-A-3119162. In existing circuits a number of problems arise. For example low pass output filters are required which introduce unwanted phase shifts. In addition, there are often stability problems. The present invention seeks to overcome or reduce at least one of the above problems. According to the • present invention there is provided an inductance evaluating circuit comprising an oscillator supplying a fixed inductance and a variable inductance having a common connection point and signal-processing means connected to the common connection point to provide an output signal representative of the value of the variable inductance, characterised in that the oscillator signal is supplied by respective paths to the ends of the inductances remote from the common connection point, one of said paths incorporating a controllable signal-generating device and an output of said signal-processing means is connected to control the signal-generating device. The above circuit has a number of advantages. It provides an output signal with a low residual ripple so that a low frequency filter is not required. There are no problems with stability, the circuit is not sensitive to phase shifts and there are only a few component sub-assemblies which affect the accuracy of the circuit. When used in connection with an inductive displacement transducer, high- resolution measurement may be achieved. The displacement may be converted by the transducer into an " inductance ratio / o which is proportional to displacement and is then converted by the signal-processing circuit into an electric output signal which may take any desired form, e.g. an analogue signal, a binary value or a mark/space ratio. In a preferred embodiment the signal-processing means comprises an amplification control circuit comprising means for rectifying the signal from the common connection point, and means for integrating the output of the rectifying means. The rectifying means may comprise a switch which is switchable by a synchronising output desired from the oscillator to receive an input signal directly or via inverting means. " The amplification control circuit may further comprise a delay element between the rectifying means and the integrating means, and the output of the integrating means may be supplied to a sample-and-hold circuit the operation of which is controlled by a synchronising output derived from the oscillator or from timing signal generating means which also control the oscillator. Such a control circuit provides both the output signal U act of the whole circuit and a control signal V for ensuring that changes in the value of inductance L are quickly and accurately followed and that any errors are quickly corrected. In a preferred embodiment,between the common connection point and the amplification control circuit there is connected a circuit for matching the signal at the common connection point to a reference d.c. voltage level. The matching circuit is preferably constituted by a differential amplifier circuit. This has the advantage of compensating any offset voltages present. .A voltage divider may be connected between the oscillator and the input of the differential amplifier circuit. This has the advantage of detecting any breaks in a cable or a short-circuiting to ground of the common connection point. A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which: " Fig.l is a circuit diagram of a prior art arrangement; Fig.2 is a circuit diagram of an arrangement in accordance with the present invention; and Fig.3 is a signal diagram indicating the signals occurring at various locations within the circuit of Fig.2. Referring now to Fig.l, there is shown a prior art circuit comprising an oscillator 10, a half-wave control device 11, which supplies a synchronising input to a half-wave rectifier 13, and an output drive stage 14. The common connecting point of a fixed inductance Lo and a variable inductance L is connected to an inverting amplifier 12, the output of which is supplied to the half-wave rectifier 13 to produce an output U RW which represents the value of the variable inductance. Referring now to Fig.2 there is shown an arrangement in accordance with the present invention comprising an oscillator stage 20 which comprises a sine-wave oscillator 10 and a synchronising output stage. The output of the oscillator is shown at Al in Fig.3. This signal is supplied directly to a first impedance transformer Bl and then to a terminal RWO connected to a first end of fixed inductance Lo and indirectly to a second impedance transformer B2 and thence to a terminal RW2 connected to a first end of the variable impedance L. The impedance transformers Bl and B2 supply the sinusoidal oscillations to the inductances with low resistance and without retroactive effect. The oscillator output Al is also supplied to the synchronisation stage which comprises a comparator K and a dead-time member T. The "sync" signal is delayed by the dead-time member T, to compensate the phase shift of signals RWO and RW2 relative to signal "RWl. The synchronising output signals of this stage are shown at A4 in Fig.3. The connection between the oscillator output Al and the impedance transformer B2 is via a controllable, inverting amplifier B3, which forms part of a multiplier circuit 50. The output of amplifier B3 is shown at A2 in Fig.3. wnen the ratio of the amplitude of the signal at terminal RW2 to the amplitude of the signal at RWO is exactly equal to - ~ /-< 0 t there is balance, and the signal amplitude at terminal RWl becomes zero. This corresponds to the signals shown in the centre column of Fig.3. If, however π RW0 j ~^~ U RW2 a corresponding amplitude appears at RWl (see left or right columns of Fig.3). By means of a differential input- amplifier 30 connected to terminal RWl, the average value of this oscillation is adjusted so that the average value at measuring point A3 corresponds to the internal reference voltage U R . Amplifier 30 comprises an adder circuit 32, a plus input of which receives the signal from terminal RWl, and a minus input of which receives an output signal from a delay member VZ2 the input of which is also connected to receive the signal RWl. The output of adder 32 is supplied via an inverting amplifier B5 to measuring point A3. The A.C. signal at measuring point A3 is rectified with reference to U R by an analogue inverter B4 and an analogue switch S. The signal "Sync" from point A4 serves for synchronisation and changes over at each zero-crossing point. The output of switch S is indicated at A5 in Fig.3. The rectified signal at A5 is then filtered by delay element VZl and integrated • at I with reference to the reference voltage ϋ R . The deviation from U R of the average value of the signal at A6 is proportional to the difference of the actual amplitude at RW2 relative to the stabilized amplitude at RW2; by means of the integration of this deviation there is obtained a tracking control value. This is quantised in time by a sample-and-hold circuit S & H which is controlled D a onostable trigger M connected to the "sync" signal. The control voltage V thus obtained, see line A7 in Fig.3, controls the amplification of the controllable amplifier B3 via a converter 51 within multiplier 50. The output signal of the circuit U act is also obtained from converter 51. Finally, to detect a break in a cable or short circuiting to ground of the signal at RWl, -there is provided a voltage divider X which is connected to the output of oscillator 10 to provide a divided-down sine-wave which is added in adder circuit 31 to the RWl-signal. Various modifications may be made to the above described arrangement. For example, the implementation of the converter 51 and the amplifier B3 is dependent upon the desired form of the output signal u act and upon the required accuracy. As described an analogue signal is produced by an analogue multiplier. The multiplier may comprise an operational transconductance amplifier. Alternatively a 10-Bit word output could be obtained from an A/D converter 51 and an amplifier B3 with a digitally determined amplification factor. The output signal could also be in the form of a signal with a variable mark/space ratio. Where desired, a timing input may be provided to the converter 51 from the output monostable trigger M, as shown in a broken line in Fig.2. Figure 2 shows a preferred embodiment of the present invention and various integers can be omitted if desired, for example the differential amplifier 30, the voltage divider X, the delay element VZl and/or the sample and hold circuit S & H and/or the monostable trigger M of the amplification controller 40, and the converter 51. If desired the control amplifier B3 could be alternatively connected in the path from oscillator 10 to the fixed inductance Lo. Instead of the synchronising output being derived from the oscillator 10, it may be derived from a common timing signal generating means which also controls oscillator 10. In a further modification, the controllable amplifier B3 is replaced by an amplitude-controllable oscillator which is also connected to the common timing signal generating means.

Claims: Claims 1. An inductance-evaluating circuit comprising an oscillator (10) supplying a fixed inductance (L Q ) and a variable inductance (L) having a common connection point (RWl), and signal-processing means (30,40) connected to the common connection point to provide an output signal (U act ) representative of the value of the variable inductance, characterised in that the oscillator signal is supplied by respective paths (Bl,B2) to the ends of the inductances remote from the common connection point, one of said paths (B2) incorporating a controllable signal-generating device (B3), and an output (A7) of said signal-processing means is connected to control the signal-generating device. 2. A circuit according to claim 1, wherein the signal- generating device is a controllable amplifier (B3). 3. A circuit according to claim 1, wherein the signal- generating device is an amplitude-controllable oscillator connected to timing signal generating means which also control said first mentioned oscillator (10). 4. A circuit according to any preceding claim wherein the signal-processing means comprises a control circuit (40) comprising means (B4,S) for rectifying the signal from the common connection point, and means (I) for integrating the output of the rectifying means. - 5. A circuit according to claim 4, wherein the rectifying means comprises a switch (S) which is switchable by a synchronising output derived from the oscillator (10) to receive an input signal directly or via inverting means (B4). 6. A circuit according to claim 2 and to claim 4 or 5, wherein the amplification control circuit (40) further comprises a delay element (VZl)- between the rectifying means and the integrating means, and wherein the output of the integrating means is supplied to a sample-and-hold circuit (S & H), the operation of which is controlled by a synchronising output derived from the oscillator (10) or from timing signal generating means which also control the oscillator. 7. A circuit according to claim 3 and to claim 4 or 5, wherein the control circuit (40) further comprises a delay element (VZl) between the rectifying means and the integrating means, and wherein the output of the integrating means is supplied to a sample-and-hold circuit (S & H), the operation of which is controlled by a synchronising output derived from the oscillator (10) or from said timing signal generating means. 8. A circuit according to any of claims 4 to 7, wherein oetween the common connection point (RWl) and the control circuit (40) there is connected a circuit (30) for matching the signal at the common connection point to a reference d.c. voltage level (U R ). 9. A circuit according to claim 8, wherein the matching circuit is a differential amplifier circuit (30) and a voltage divider (X) is connected between the output of the oscillator (10) and the input of the differential amplifier circuit. 10. A circuit according to any preceding claim, wherein the output of the signal-processing means is connected to a multiplier circuit (50) compris-ing a converter (51) which supplies output signals (U act , V) to an output terminal of the circuit and to the control input of the controllable signal-generating device (B3). 11. A circuit according to any preceding claim wherein the iinndduuccttaannccees ( ,L 0 ) are part of an inductive displacement transducer

Inventors: FISCHER WERNER, FISCHER, WERNER
Applicants: BOSCH GMBH ROBERT, ROBERT BOSCH GMBH
Application year: 1988
Main classification: G01R27
IPC codes: G01R27