GB2169710A

GB2169710A - Process and apparatus for measuring the distance between a target and a sensor

Info

Publication number: GB2169710A
Application number: GB08531590A
Authority: GB
Inventors: Pierre Azpiroz
Original assignee: E C E L EQUIPEMENT ET LA CONST
Current assignee: E C E L EQUIPEMENT ET LA CONST
Priority date: 1985-01-10
Filing date: 1985-12-23
Publication date: 1986-07-16
Also published as: GB8531590D0; GB2169710B; DE3600055A1; FR2575820B1; FR2575820A1

Abstract

According to this process, a perturbation of the potential in a triangular wave signal supplied directly to the sensor is detected. The apparatus comprises a resonant series circuit supplied by an oscillator, a comparator and a source of stabilised supply, characterised in that the oscillator (0) supplying the sensor (1) is a triangular wave oscillator connected to a power amplifier (10) directly supplying the sensor (1) and arranged to produce an output potential signal having a perturbation when the impedance of the sensor varies and comprising a three-input logic circuit (9) and a resettable monostable circuit (12). By the use of a complementary pair transistor amplifier (10) having a strapped-emitter output, the triangular waveform has flat portions where it passes through a potential equal to one-half the peak value. If the approach or recession of a magnetisable target (C) detunes the sensor (1), the flat portions are advanced or retarded relative to the triangular wave. <IMAGE>

Description

SPECIFICATION Process and apparatus for measuring the distance between a target and a sensor The present invention relates to an apparatus for detecting the proximity of a movable target having ferro-magnetic properties, with respect to a fixed point where a detector is located, over a range of predetermined distances.

Measurement of the proximity of an object or target with respect to a fixed predetermined point finds numerous applications, among which can be mentioned differentiation between open and closed positions of a metal door, detection of the position of the undercarriage struts of an aircraft and others.

The measuring capability of such an apparatus is of particular interest when the movable target is surrounded by relatively large metal structures, which must not alter the displacement values to be measured.

In certain applications, the places where the point of detection and the electronic means for processing the information supplied by the detector device are located are occasionally separated by a distance which can range from several centimetres to several tens of metres.

In the particular case of aircraft, in which the installation weight of such an apparatus, which may include several proximity detectors, is a critical factor, it is customary to separate the sensitive part or sensor from the electronic measuring circuits, which are assembled into a block or a board installed at a distance.

Apparatus comprising an inductance disposed in the sensor has already been made, the information processing unit comprising a standard inductance measuring bridge (Maxwell bridge). Such a system requires a connecting line having three conductors between the sensor or sensors and the processing unit.

To reduce the weight further, it is preferable for the sensor or sensors and the processing unit to be connected only by two conductors.

Various forms of apparatus made for this purpose are already known and comprise a resonant series circuit disposed in the sensor.

French Patent Application No. 82/03694 in particular describes an inductive sensor device having a resonant series circuit supplied at low frequency with a sinusoidal signal, in which dephasing is measured by means of a phase comparator.

While operation of such a device is always satisfactory from the standpoint of the accuracy of measurement of the proximity of the target, it does not as such satisfy an important need, in the case of its use on a modern aircraft, namely simulation of its function in the area of the particularly severe security controls required on modern aircraft.

The invention has the purpose in particular of dealing with this disadvantage by providing a proximity detection device of the type described above, which can itself allow such a simulation, namely without the addition of a supplementary material.

This purpose is attained according to the invention by means of a process by which measurement of the proximity of the target is effected by detection of, a perturbation of the potential in a triangular wave signal.

For this purpose, the invention provides a proximity detection device for a movable target having magnetic properties, surrounded by relatively large metal structures, of the inductive sensor type, comprising a resonant series circuit supplied by an oscillator, a comparator and a source of stabilized the sensor is a triangular wave oscillator connected to a power amplifier arranged to produce an output potential signal having a perturbation when the impedance of the sensor varies and comprising a three-input logic circuit and a resettable monostable circuit.

The description which follows, in conjunction with the drawings by way of non-limitative examples, will show how the invention can be carried out.

Figure 1 is a synoptic diagram of a proximity detector device according to the invention; Fig. 2 is a power amplifier circuit; Figures 3a to 3d show the deformation of the output potential of the amplifier when the current in the sensor changes direction, Fig.

3b showing deformation of the amplifier output when the target is located at the detection threshold with respect to the sensor, Fig. 3c showing the same potential when the target is nearer the sensor and Fig. 3d corresponding to the target being at a distance.

Referring to Figure 1, the apparatus comprises a proximity sensor designated by the reference 1 and an electronic circuit designated generally by the reference 2, which is connected to the sensor by a two-conductor cable 3.

As known per se, the sensor 1 comprises a resonant circuit forrned by an inductance 4 and a capacitor 5 in series in an enclosure 6.

The electronic device 2 according to the invention comprises an oscillator 0, of the VCO type constituted essentially by three voltage comparators la, 2a, 3a, resistances R1, R2, R3, R4 and R5 and a condensor C1, known per se.

The oscillator 0 is connected to a potential amplifier 7 and to an input to a comparator 8 and also to a logic circuit 9 (AND gate). The second input of the comparator 8 is connected to the junction between the potential amplifier 7 and a power amplifier 10 of a particular type, described below, and to one of the inputs of another comparator 11, the second input of which is connected to the output from the power amplifier 10. The output of the comparator 8 and that of the com parator 11 are likewise connected to the logic circuit 9.

The output of the logic circuit 9 is connected to a resettable monostable circuit 12, the outputs Q and Q of which are utilized for connection to an indicator device of any suitable type.

The condensor 5 of the sensor 1 is connected to the output of the power amplifier 10, while the inductance 4 is connected to earth. The assembly is energized from a stabilized supply source 13 as shown.

The power amplifier 10, connected to the output of the potential amplifier 7, comprises, as shown in Fig. 2, two complementary NPN and PNP transistors designated by the respective references 14 and 15.

The bases of the two transistors 14,15 are connected together and to the output of the potential amplifier 7. The junction of these two bases is also connected via a resistor 16 to the collector of the transistor 14.

The emitters of the transistors 14,15 are connected together and their junction is connected to the condensor 5 of the sensor 1.

The collectors of the two transistors are connected to the source of potential so that the utilization circuit of the power amplifier 10 is the sensor 1.

The particular feature of this amplifier is that it produces a perturbation in its output signal when there is a variation in the impedance of a circuit LC which it supplies directly.

Operation of the device is as follows: The circuit LC of the sensor is supplied from the oscillator through the intermediary of the power amplifier 10 at a frequency near the resonance of the circuit LC.

The condensor C1 of the oscillator is charged linearly through the resistance R1 and discharges through the resistance R2.

The potential at the point S1 is a wave of triangular form but of low level. This potential is amplified by the amplifier 7 to range over the whole interval of the supply potential, from 0 to +V, then amplified in power by the amplifier 10.

The NPN transistor of the amplifier 10 applies a charging current to the circuit LC of the sensor, which current increases linearly from 0 to +V.

When the current ceases to increase, the energy stored in the circuit LC is recycled, except for losses, through the PNP transistor.

This change of mode, charge and discharge, creates a deformation in the output potential of the power amplifier 10, due to the baseemitter potential threshold of each transistor necessary to ensure their conduction and to the fact that the amplifier 7 charge is reactive.

This deformation is manifest by a step in the potential.

As the LC circuit of the sensor is near resonance, it behaves as a pure resistance, supplied through a capacity of infinite value. In this case, the step in the potential is located at a value of +V/2.

When the target C which has magnetic properties (for example mild steel) approaches the sensor, the value of the self-inductance 4 increases as also does the impedance of the LC circuit. This variation causes a perturbation in the waveform of the output potential of the power amplifier. When the target C comes near, the current in the LC circuit decreases and potential-current dephasing is such that conduction in the pair of complementary transistors NPN and PNP ceases for a potential value between +V/2 and +V. The potential step "rises".

When the target C is remote from the sensor, the inverse effect is produced and the potential step "falls", as can be seen in Figs.

3b, c, d.

Measurement consists in comparing the deformed output potential of the power amplifier 10 with a potential of the same type taken as a reference, derived from the output of the potential amplifier 7 at its junction with the power amplifier 10.

The comparator 11, which receives at one of its inputs both the output potential VS of the power amplifier 10 and also the reference potential at the output of the potential amplifier, provides an impulse potential at its output having a duration proportional to the closeness of the target C to the sensor 1.

This impulse potential corresponds to a logic state = 1 when the first potential step S1 is greater than the second S2.

In order to take into account only the potential step in the increasing part of the triangular wave, the three-input "AND" logic circuit 9 only allows validation of the information during the interval t2-tr (Figs. 3c and 3d).

The output of the logic circuit 9 delivers a pulse of a size proportional to the proximity of the target C to the sensor 1.

The rising front of this pulse is utilised to reset the monostable circuit 12, the time constant of which is greater than the period of the triangular wave, so that its output remains at the logic level = 1 as long as the pulses are applied to its input.

Thus a continuous signal is provided which changes the logic state as a function of the distance of the target C from the sensor 1.

The output signal of the circuit 12 can be utilised as desired to activate a monitor, indicator or other device.

To localise the position of the target with respect to the sensor, the frequency of the oscillator VCO should initially be regulated so as to correspond to Figure 3b, which corresponds to the position of the target at the detection threshold.

If use of the device only requires measurement of a single value of the distance of the target C from the sensor 1, only the beginning of the pulse is used.

Because the oscillator is of the VCO type, the frequency of which can be adjusted by means of a continuous potential, it is possible to simulate a nearer or further position of the target, during calibration tests, whatever the real position of the target with respect to the sensor.

This characteristic is important, particularly in the case of use of the apparatus according to the invention on an aircraft, within the field of maintenance tests.

It may be emphasised finally that a triangular waveform has been chosen in order to avoid the appearance of parasite HF on the line connecting the sensor to the device.

Claims

1. Process for measuring the proximity of a target having magnetic properties with respect to an inductive sensor, characterised by detecting a perturbation in the potential in a triangular wave signal supplied directly to the sensor.

2. Proximity detection device for a movable target having magnetic properties, for carrying out the process defined according to claim 1, the inductive sensor being of the type comprising a resonant series circuit supplied by an oscillator, a comparator and a source of stabilized supply, characterised in that the oscillator 0 supplying the sensor (1) is a triangular wave oscillator connected to a power amplifier (10) directly suppiying the sensor (1) and arranged to produce an output potential signal having a perturbation when the impedance of the sensor varies and comprising a three-input logic circuit (9) and a resettable monostable circuit (12).

3. Apparatus according to claim 2, characterised in that the oscillator 0 is of the VCO type having a frequency which can be regulated by means of a continuous potential.

4. Apparatus according to claim 3, characterised in that the power amplifier (10) comprises an NPN transistor (14) and a PNP transistor (15), the bases of which are connected together and the emitters of which are also connected together, the collector of the NPN transistor (14) being connected at its base to a resistance (16) and the junction of the emitters being connected directly to the sensor (1).

5. Apparatus according to claim 2, substantially as hereinbefore described with reference to the accompanying drawings.

6. Process according to claim 1 substantially as hereinbefore described with reference to the accompanying drawings.