GB2053487A

GB2053487A - Inductive differential position sensor

Info

Publication number: GB2053487A
Application number: GB8019176A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1979-06-15
Filing date: 1980-06-12
Publication date: 1981-02-04
Also published as: IT1131327B; DE2924093C2; GB2053487B; IT8022784A0; DE2924093A1

Abstract

Two inductors (L1, L2) are arranged on respective limbs of a common iron core along which a short-circuit ring is displaceable (linearly or arcuately) in accordance with the displacement value to be measured. The outputs from the two coils are fed to an integrator (25) whose output is in turn connected to a Schmitt trigger (29). The output of the Schmitt trigger is fed back alternately to the two coils via a change-over switch(s) to form an oscillator whose frequency depends upon the inductance (L1 or L2) of the respective coil and thereby upon the displacement value to be measured. A flip-flop (32) produces an output voltage (UA) whose duty ratio depends upon the oscillation periods obtained by the two inductances (L1 and L2). The change-over switch(s) is controlled by the flip-flop output (33). By applying the output voltage (UA) to a low-pass filter an analog function of the measured displacement value is obtained. Alternatively a digital function can be obtained by applying the output voltage (UA) to a counter. <IMAGE>

Description

SPECIFICATION Inductive differential position sensor The invention relates to an inductive position sensor systems.

German Patent Specification Auslegeschrift No.

23 52 851 describes a position sensor having two coils mounted on a common iron core, each of which coils is mounted on a respective one of the yokes formed by the two shorter limbs of a rectangular iron core, and the coils being influenced by a short-circuit ring which encircles each of the two longitudinal limbs and can be adjusted along these limbs in a contactless manner in dependence upon the travel to be measured. During this adjusting movement, the inductance of one of the two coils is increased and the inductance of the other coil is at the same time decreased. The above-mentioned Auslegeschrift also describes a rotary angle sensor which is constructed on the differential principle and in which two axially extending yokes are interconnected by annular limbs which are surrounded by the short-circuit ring adjustable in dependence upon the angle of rotation.

In the evaluation circuit proposed in the said Auslegeschrift, the same current flows through the two coils, and the voltages appearing in the coils are amplified in amplifiers and are rectified, so that the difference between the rectified voltage can be formed in a differential amplifier. The said Auslegeschrift also recommends the use of an active rectifier with an operational amplifier instead of amplifiers having rectifiers connected to the outputs thereof.

An object of the invention is to provide an evaluation circuit for a differential positional sensor in which, with simple means, the high measuring accuracies obtainable with these position sensor can also be used primarily for controlling electronic diesel injection systems or devices for regulating travelling velocity, such that the output signals of the evaluation circuits can be further processed both directly digitally and analogously.

The present invention provides an inductive position sensor system having two coils arranged on a common iron core, the inductance of at least one of which coils is variable by means of a shortcircuit ring which surrounds one limb of the iron core and is adjustable along the said limb in a contactless manner in dependence upon the displacement value to be measured, the two coils being connected to the inverting input of an integrator and being connected to a change-over switch which is adapted alternately to connect each of the coils to the output of a Schmitt trigger which is connected to the output of the integrator.

The invention is further described, by any of example, with reference to the drawings, in which: Fig. 1 is a perspective view of an angle sensor, Fiy. 1 a is a circuit diagram of the coils of the angle sensor of Fig. 1, Fig. 2 is a perspective view of a linear travel sensor, Fig. 2a is a circuit diagram of the coils of the travel sensor of Fig. 2.

Fig. 3 is a circuit diagram showing one of the coils of the sensor of Fig. 1 or 2, Fig. 4 is a graph showing the output of the circuit of Fig. 3, Fig. 5 is a circuit diagram, similar to Fig. 3, but showing the other sensor coil, Fig. 6 is a graph showing the output of the circuit of Fig. 5, Fig. 7 is a block circuit diagram of one embodiment of position sensor system according to the invention, Fig. 8 is a graph to explain the operation of the circuit of Fig. 7, and Fig. 9 is a block circuit diagram of another embodiment of position sensor system.

The differential position sensor shown in Figure 1 is an angle sensor and operates on the initially mentioned short-circuit ring principle. It has two concentric, arcuate limbs 1 and 2 which are interconnected at their ends by two radially extending bridge portions 3 and 4 (yokes) which, like the limbs, are made from thin sheet metal laminations.

A respective one of two coils 5 and 6 is disposed in the vicinity of the bridge portions 3 and 4 respectively, and their ends are designated 51, 52 and 61, 62 respectively. The coils 5 and 6 surround the outer arcuate limb 1. A shaft 7, whose variable prevailing angular position is to be measured by the angle sensor and converted to an electrical signal, is arranged concentrically of the arcuate limbs 1 and 2.

For this purpose, the shaft 7 carries a radially projecting arm 8 having a short-circuit ring 9 which is in the form of a single, closed turn and surrounds the outer limb 1. The inductance of the coil 5 is designated L1 in Figure 1, and the inductance of the coil 6 is designated L2. As will be seen in Figure 1 a, the two coils are connected in series. When the inductance L1 decreases, the inductance L2 of the coil 6 increases to the same extent.

Figure 2 shows a half-differential translatory or linear travel sensor which operates on the shortcircuit ring principle and has a common iron core 11 incorporating two upwardly directed limbs 12 and 13 and three downwardly directed limbs 14, 1 5 and 1 4a. A coil 6 having an unchanging inductance L2 is mounted in the central limb 15. A coil 17 on the two upwardly directed limbs 12 and 13 of the iron core 11 is split up such that half of the turns of the coil surrounds the limb 12 and the other half surrounds the limb 1 3. The inductance L1 of the coil 1 7 is variable in dependence upon the adjusting travel S of a short-circuit ring 1 8 which constitutes a short-circuit turn surrounding each of the two limbs 12 and 13.The two coils 16 and 1 7 are connected in series in the manner shown in Figure 2a, the winding ends being designated 51,52 and 61,62 in Figure 2a, analogously to Figure 1 and Figure 1 a.

In order to simplify the description, only the inductance L1 and the inductance L2 will be discussed hereinafter in the same manner as with reference to Figures 3 to 11, it being assumed that the inductance L1 varies in dependence upon linear travel or in dependence upon angle of rotation, it being immaterial whether the industance L2 is variable as in the embodiment of Figure 1 or constant as in the embodiment of Figure 2. Advantageously, an oscillator 20 can be used to determine the prevailing value of the inductance L1 or L2. The oscillator 20 includes an operational amplifier as an active component and, as is shown in Figure 3, has a feedback path which leads from its output to its negative input and which includes the inductance L1 to be measured.

The square-wave output voltage U 1 shown in Figure 4 is obtained, the period of the output voltage U 1 being designated T1. If the second inductance L2 is connected into the feedback path of an operational amplifier 21 in an analogous manner, a square-wave output voltage U2 is produced whose period T2 is proportional to the inductance L2. The ratio of the two inductances L1 and L2 can be obtained from the circuits of Figures 4 and 6 by simple comparison of the periods T1 and T2, and a high degree of accuracy can be obtained, since the two periods are independent of external conditions such as the supply voltage of the oscillators 20, 21 or the temperature thereof etc.

In detail, the circuit, shown diagrammatically in Figure 7, is provided for period-analogous evaluation and operates on the principle of the relaxation oscillators indicated in Figures 3 and 5.

The two inductors L1 and L2 are commonly connected to the negative input 24 of an integrator 25 whose output 26 is back-coupled to the negative input 24 by way of resistor R and is connected by way of a resistor R1 to the positive input 28 of an operational amplifier 29 operating as a Schmitt trigger, to which operating voltages Ue and UB are fed in a conventional manner. The output 30 is connected by way of a lead 31 to a switch S which alternately connects the inductors L1 and L2 to the output of the Schmitt trigger 29, so that the inductances L1 and L2 alternately lie in the oscillatory circuit completed by the feedback 31. The change-over at each cycle of oscillation is effected by means of a flip-flop 32 from the output 33 of which is taken a duty-ratio-analogous voltage UA.This voltage is fed by way of a control lead 34 to a logic circuit 35 which actuates the change-over switch S.

The voltages appearing at the end 51 of the inductor L1 and the end 62 of the inductor L2 are shown in the two topmost curves of Figure 8. The input 24 of the integrator 25 remains connected to the coils having the inductances L1 and L2. The third curve in Fig. 8 shows the voltage U3 appearing at the output of the integrator 25. At the end of each time interval t1, the switch S switches the lead 31 from the inductor L1 to the inductor L2, that is to say from the position designated 01 in Figure 7 to the position 02, when the hitherto activated inductor L1 has become currentless and thus the period S1 has expired.

The period S2, whose duration is determined by the value of the inductance L2, then follows. The switching times t1 and t2 of the change-over signal U30 at the output of the Schmitt trigger 29 form the square-wave oscillation UA, modulated in the duty ratio of the inductance values L1 and L2, at the output 33 of the flip-flop 32. This flip-flop 32 ensures that the change-over is effected whenever one of the inductors L1 and L2 has become currentless, and prevents falsification of the duty-ratio-analogous evaluation which otherwise appears as a result of voltage peaks.

Figure 9 shows a tested circuit for use in motor vehicles. In this circuit, a flip-flop 32 (obtainable under trade designation 4013) acting as a storage component is provided as an essential component of the change-over device. In the illustrated embodiment, the function of the switch 35 realised by gates 41,42 having tristate outputs, and decoupling amplifiers 43 and 44.

The square-wave oscillation UA of Figure 8, of moldulable duty ratio, renders it possible to perform anglogous evaluation by averaging by means of a low-pass filter (not shown) connected to the output of the flip-flop 32. Furthermore, digital evaluation can be effected in a simple manner by evaluating the switching times t1 and t2 by means of a counter (not shown) connected to the output of the flip-flop 32. If, in the case of the counting-out frequency available, the sampling times t1 and t2 are too short for a counter of this type connected on the output side, the flip-flop 32 can be advantageously replaced by a frequency divider, whereby the times t1 and t2 can be prolonged by an optional factor. The circuit of Figure 9 renders it possible to perform adjustment in various parts and to select the frequency, particularly by adjusting the amplification factor of the decoupling amplifiers 43 and 44.

The circuits, in accordance with the invention, of Figures 7 and 9 are likewise suitable for analog and digital processing of measured values. Mutual interference or influencing does not take place, since only one of the two inductors L1 and L2 is activated at any given time. Furthermore, the wholly common use of the same parts of the circuit ensures a maximum of drift and error compensation.

Claims

1. An inductive differential position sensor system having two coils arranged on a common iron core, the inductances of at least one of which coils is variable by means of a short-circuit ring which surrounds one limb of the iron core and is adjustable along the said limb in a contactless manner in dependence upon the displacement value to be measured, the two coils being connected to the inverting input of an integrator and being connected to a change-over switch which is adapted alternately to connect each of the coils to the output of a Schmitt trigger which is connected to the output of the integrator.

2. A sensor system as claimed in claim 1, in which the change-over switch is connected to the output of the Schmitt trigger.

3. A sensor system as claimed in claim 2, in which the output of the Schmitt trigger is connected to a store, which actuates the changeover switch.

4. A sensor system as claimed in claim 3 in which the store comprises a flip-flop.

5. A sensor system as claimed in any of claims 1 to 4, in which the output of the Schmitt trigger is connected at least indirectly to a low-pass filter to obtain an analog function of the displacement value.

6. A sensor system as claimed in any of claims 1 to 4, in which the output of the Schmitt trigger is connected at least indirectly to a counter to obtain a digital function of the displacement value.

7. An inductive differential position sensor system, constructed and adapted to operate substantially as herein described with reference to and as illustrated in the drawings.