GB2034479A

GB2034479A - Proximity detectors

Info

Publication number: GB2034479A
Application number: GB7843775A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1978-11-09
Filing date: 1978-11-09
Publication date: 1980-06-04
Also published as: GB2034479B

Abstract

A proximity detector for indicating the presence of a non-magnetic electrically conductive body e.g. turbine blades 2 of a flowmeter consists of an oscillator circuit operating in a marginal mode i.e. oscillator is normally only just oscillating. The tuned circuit of the oscillator contains an inductor in the form of a coil 4, which is positioned adjacent to a thin stainless steel window 5 and movement of the body 2 results in eddy current losses which alter the effective inductance of the inductor 4 and hence the amplitude of oscillations. The oscillator output is demodulated, amplified and fed to a trigger circuit and pulse former. <IMAGE>

Description

SPECIFICATION Proximity detectors This invention relates to proximity detectors. The need often arises to detect the presence of a metallic body without making direct physical or electrical contact to the body itself. The invention is applicable, for example, to proximity detectors which are suitable for detecting the flow of a fluid along a pipe in which the fluid flow causes rotation of a turbine blade. The sensitivity of proximity detectors which can accurately monitor the rotation of the turbine blade without itself coming into contact with the liquid leaves much to be desired. Additionally, because known detectors generally require the presence of magnetic material in the body to be detected, the operation of the detector can adversely affect the position or motion of the body.In the case of the turbine blade, a magnetic dragging effect is particularly detrimental at low rotation speeds.

The present invention seeks to provide an improved proximity detector.

According to this invenion a proximity detector includes an oscillatorwhose amplitude of oscillation is dependent on the effective inductance of an inductor forming part of a tuned circuit, the inductor being in the form of a coil positioned adjacent to one side of a thin window composed of an electrically conductive non-magnetic material, movement of an electrically conductive non-magnetic body on the other side of said window being such as to cause a variation in the self inductance of said coil and an alteration in the amplitude of oscillation, and means for detecting the alteration in amplitude to indicate the presence of said body.

Where the proximity detector is arranged to detect the rotation of a blade driven by a fluid, preferably the thin window is composed of stainless steel and is arranged to prevent the fluid coming into contact with the inductor. Stainless steel is a particularly inert material and can withstand the action of caustic fluids, but it is necessary to choose stainless steel which is substantially non-magnetic.

Preferably said inductor comprises a coil which is wound on a ferrite cup core which produces a directional magnetic field directed through the window.

Preferably again said coil forms part of the resonant circuit of an oscillator arranged to operate in a marginal mode. An oscillator operating in a marginal mode is one in which the normally positive feedback signal necessary to maintain oscillation is of such an amplitude as to be only just capable of sustaining oscillations. Usually with an oscillator, the positive feedback signal is of such an amplitude as to produce saturation of the oscillator, that is to say, the amplitude of oscillation is not dependent on the magnitude of the feedback signal.

Preferably the oscillator is a Colpitts type oscillator having a tuned resonant circuit in which the positive feedback signal necessary to sustain operation is obtained from between a pair of capacitors forming part of the resonant circuit.

Preferably again the feedback signal is applied to an amplifying device forming part of the oscillator via an adjustable potentiometer, by means of which the sensitivity of the amplitude of oscillation of the oscillator to variations in inductance of the coil can be adjusted.

The invention is further described by way of example with reference to the accompanying drawings in which, Figure 1 illustrates diagrammatically a proximity detector which is used to measure the rate at which a fluid flows along a pipe, Figure 2 illustrates part of the circuitry associated with the proximity detector and Figure 3 is an explanatory diagram.

Referring to Figure 1, a pipe 1 along which a liquid flows contains a set of turbine blades 2 mounted on a spindle 3 so that as the liquid flows along the pipe 1 the turbine blades 2 are caused to rotate. The rotation of the turbine blades is monitored by means of a proximity detector, which includes a probe consisting of a coil 4, which is separated from the interior of the pipe 1 by means of a very thin window 5. The window 5 in this case forms an integral part of the wall of the pipe 1 and both the wall and the turbine blades are composed of non-magnetic stainless steel. The coil 4 is mounted on a ferrite cup core 6, which produces a directional magnetic field 7 when an a.c. current is passed through the coil.The presence of the window 5 and a turbine blade 2 results in eddy current losses associated with the coil 4 and these produce an effective decrease in the self inductance of the coil. The coil 4forms part of the resonant circuit of a Colpitts type oscillator and variations in the self inductance of the coil result in corresponding variations in the amplitude of oscillations produced by the oscillator, as the oscillator is arranged to operate in a marginal mode. The remainder of the proximity detector is referenced generally as 8 and is arranged to produce an indication of the rate at which the turbine blades 2 are rotated by measuring these amplitude variations.

The circuit is shown in greater detail in Figure 2 in which the oscillator 20 is a Colpitts type oscillator and contains a resonant circuit 21 consisting of a pair of capacitors 22 and 23 in addition to an inductor 24 which is constituted by the coil 4 shown in Figure 1.

The output of the resonant circuit 21 is taken off between the two capacitors 22 and 23 and applied to a variable potentiometer 25. In operation, the potentiometer 25 is adjusted to control the degree of positive feedback such that the oscillator is only just oscillating so that the amplitude of the oscillations is highly dependent on the effective value of the self inductance of the inductor 24.

As the turbine blades 2 rotate, corresponding variations in amplitude are obtained at output terminal 26 of the oscillator 20. This signal is passed to a demodulator 27, which detects the amplitude variations produced by the oscillator 20. The demodulating signal is passed via a very high gain amplifier 28 to a trigger circuit 29, which produces an output pulse of predetermined amplitude each time a variation in amplitude level is detected from the oscillator 20. A pulse former circuit 30 produces pulses of predetermined amplitude and duration from the trigger 29 and these pulses are used to provide an indication of the rate at which the turbine blades 2 are rotated.

In general, it is very difficult to obtain reliable variations in amplitude from an oscillator in which a frequency determining inductor is situated within a large mass of electrically conductive metal. Electric eddy currents are induced within the metal which are in such a sense as to oppose the magnetic flux creating them. The way in which the output amplitude of the oscillator 20 varies in dependence on the thickness of the conductive metal positioned in front of the coil 4 is illustrated in Figure 3. It will be seen that both frequency and amplitude are dependent on this thickness and typically the thickness of the window 5 is arranged to be about sixteen thousandths of an inch. Thus the presence of a turbine blade 2 increases the thickness of the metal sensed by the coil 4 and as can be seen from the curve shown in Figure 3 this produces an increase in oscillation amplitude and a decrease in oscillation frequency.

Claims

1. A proximity detector including an oscillator whose amplitude of oscillation is dependent on the effective inductance of an inductor forming part of a tuned circuit, the inductor being in the form of a coil positioned adjacent to one side of a thin window composed of an electrically conductive nonmagnetic material, movement of an electrically conductive non-magnetic body on the other side of said window being such as to cause a variation in the self inductance of said coil and an alteration in the amplitude of oscillation, and means for detecting the alteration in amplitude to indicate the presence of said body.

2. A proximity detector as claimed in claim 1 and wherein the thin window is composed of stainless steel.

3. A proximity detector as claimed in claim 1 or 2 and wherein the thin window provides a fluid-tight seal for the coil.

4. A proximity detector as claimed in claim 3 and wherein the thin window forms part of the wall of a hollow pipe carrying a movable electrically conductive non-magnetic body which is movable under the influence of a fluid flowing along the pipe.

5. A proximity detector as claimed in any of the preceding claims and wherein said inductor comprises a coil which is wound on a ferrite cup core which produces a directional magnetic field directed through said window.

6. A proximity detector as claimed in claim 5 and wherein said coil forms part of the resonant circuit of an oscillator arranged to operate in a marginal mode.

7. A proximity detector as claimed in claim 6 and wherein the oscillator is a Colpitts type oscillator having a tuned resonant circuit in which the positive feedback signal necessary to sustain operation is obtained from between a pair of capacitors forming part of the resonant circuit.

8. A proximity detector substantially as illustrated in and described with reference to the accompanying drawings.

8. A proximity detector as claimed in claim 7 and wherein the feedback signal is applied to an amplifying device forming part of the oscillator via an adjustable potentiometer, by means of which the sensitivity of the amplitude of oscillation of the oscillator to variations in inductance of the coil can be adjusted.

9. A proximity detector substantially as illustrated in and described with reference to the accompanying drawings.

New claims or amendments to claims filed on 9th Jan. 1980.

Superseded claims 1,6,7,8,9.

New or amended claims:- 1, 6, 7, 8.

1. A proximity detector including an oscillator whose amplitude of oscillation is dependent on the effective inductance of an inductor forming part of a resonant tuned circuit, the oscillator being arranged to operate in a marginal mode and the inductor being in the form of a coil positioned adjacent to one side of a thin window composed of an electrically conductive non-magnetic material, movement of an electrically conductive non-magnetic body on the other side of said window being such as to cause a variation in the self inductance of said coil and an alteration in the amplitude of oscillation, and means for detecting the alteration in amplitude to indicate the presence of said body.

6. A proximity detector as claimed in claim 1 and wherein the oscillator is a Colpitts type oscillator having a tuned resonant circuit in which the positive feedback signal necessary to sustain operation is obtained from between a pair of capacitors forming part of the resonant circuit.

7. A proximity detector as claimed in claim 6 and wherein the feedback signal is applied to an amplifying device forming part of the oscillator via an adjustable potentiometer, by means of which the sensitivity of the amplitude of oscillation of the oscillator to variations in inductance of the coil can be adjusted.