GB2195447A

GB2195447A - Position sensing transducer

Info

Publication number: GB2195447A
Application number: GB08622426A
Authority: GB
Inventors: John Karl Atkinson
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1986-09-17
Filing date: 1986-09-17
Publication date: 1988-04-07
Anticipated expiration: 2006-09-17
Also published as: GB8622426D0; GB2195447B

Abstract

A position-sensing transducer is a linearly variable differential transformer, which is mounted on a motor vehicle suspension damper. The transducer comprises a primary winding 20 movable relative to two secondary windings 22, 24 and a detector responsive to the difference between the magnetic coupling of the primary winding to the respective secondary windings to provide an indication of the relative position of the primary winding. In order to achieve output linearity, one secondary winding 24 overlies the other 22 and the number of turns in each winding per unit length tapers along the length of the winding, the direction of taper in one winding being the opposite of the direction of taper in the other. Provision of a primary winding which is movable relative to the two secondary windings is said to simplify construction. Current is driven through the transformer primary winding by a Colpitts oscillator. A pair of capacitors (52, 54 Fig. 2) in series with the primary winding form a series resonant circuit across the power supply. The oscillator frequency is 2 kHz. <IMAGE>

Description

SPECIFICATION Position sensing transducer The present invention relates to a position sensing transducer.

It has been proposed to perform several functions in a motor vehicle automatically in dependence upon the load carried by the vehicle and the most convenient manner of determining the load is to measure the degree of compression of the suspension springs, as indicated by the position of the suspension damper or shock absorber. The transducer of the invention is primarily though not exclusively intended for such an application.

The transducer of- the invention is based on a linearly variable differential transformer (LVDT). In such a transformer, an alternating current in driven in the primary winding of a transformer having two secondary windings spaced from one another. Typically, the coupling into each of the two secondary windings is dependent upon the position of a high permeability slug which is displaced from a central position at which the coupling into the two secondary windings is equal. Displacement of the slug causes an imbalance in the coupling and the difference between the output voltages of the secondary windings is representative of the position of the slug. The direction of displacement affects the phase of the difference signal thereby necessitating the use of a phase sensitive detector.

In the case of a position transducer for a suspension damper it is preferable for the primary winding itself to be movable relative to the two secondary windings. This makes for a considerably simpler construction.

It has previously been considered to mount the primary winding of an LVDT on the cylinder of the damper and the secondary windings on an insulating sleeve connected to the piston of the damper, the secondary windings being mounted spaced apart from one another and the primary winding lying between the two secondary windings in the rest position.

The problem with such a configuration of the LVDT is that of linearity, that is to say the variation of the difference signal is not linearly related to the position of the primary winding.

The variation of coupling with displacement is initially relatively small but then increases significantly. Control systems require a signal which can readily be interpreted to indicate the position of the damper and the lack of linearity gives rise to problems. Attempts to correct the non-linearity by mathematical manipulation are time consuming and cannot be carried out in real time and look-up tables for conversion require individual calibration which is not practicable.

The invention therefore seeks to provide a position transducer based on an LVDT but which can provide an output signal which varies substantially linearly with relative displacement of the primary and secondary windings.

According to the present invention, there is provided a position transducer comprising a primary winding movable relative to two secondary windings and means responsive to the difference between the magnetic coupling of the primary winding to the respective secondary windings to provide an indication of the relative position of the primary winding, wherein one secondary winding overlies the other and the number of turns in each winding per unit length tapers along the length of the winding, the direction of taper in one winding being the opposite of the direction of taper in the other.

The number of turns can be varied by alter ing the pitch of the coils along the length of the winding or by arranging for the thickness of the winding to reduce along its length.

The taper should be gradual and preferably linear in order to achieve the best linearity in the output. During automatic winding, this can be carried out by continuously varying the axial speed of the wire dispenser for a given rotational speed of the core, to achieve varying pitch, or preferably, by varying the speed of rotation of the core with a fixed speed of traverse of the wire dispenser to achieve a varying winding thickness. If manually wound, it is possible to form a stepped winding in which the thickness of a winding, wound with constant pitch, is reduced in discrete steps at regular intervals and it has been found that the steps do not have a marked effect on the linearity of the output.

Because the secondary windings are positioned one above the other on the same area of the sleeve surrounding the primary winding, -any stray signal picked up by the windings will be the same in both windings and will be cancelled when the winding voltages are sub tracted from one another. The design is there fore inherently more immune to interference from an electrically noisy environment.

The sleeve on which the secondary wind ings are wound should be of an insulating material and this presents no problem in the case of transducer for a suspension damper as the outer sleeve is not a load bearing component.

The inner sleeve on which the primary winding is wound may on the other hand be of steel.

It is preferred that the primary winding be recessed in the surface of the inner sleeve both for mechanical protection and to improve the electrical performance by avoiding end effects.

Within the context of a motor vehicle, the important criteria to be satisfied by the design of the oscillator for driving a current in the primary winding of the LVDT are cost, power consumption and amplitude stability. Fre quency stability is not a major constraint.

With these considerations in mind and hav ing regard to the expected low Q factor of the primary winding, it is preferred that the oscillator used for driving a current in the primary winding of the transducer should be a Colpitts oscillator wherein the primary winding constitutes a reactive element in the resonant circuit of the oscillator. This results in a cost saving since the resonant circuit requires only the addition of two capacitors and also in high amplitude stability. The amplitude stability is obtained at the cost of frequency stability, but as earlier mentioned the frequency is not of primary importance in particular if, as later proposed, a ratiometric technique is employed for the conversion.

The oscillator may conveniently comprise an amplifier element driving current into the primary winding and a pair of capacitors connected in series with one another and forming in conjunction with the primary winding a series resonant circuit connected across the power supply, the junction between the two capacitors being connected to an input of the amplifier element.

The amplifier element may consist of a transistor with biassing resistors for the base and the emitter. The value of the capacitors is preferably chosen such that the nominal frequency of the oscillator is in excess of 2 kHz as the inductance of the primary winding changes significantly with frequency below this range.

In a Colpitts oscillator it is usual to include a base decoupling capacitor connected between the base of the transistor and ground. It has been found however that this capacitor when included in a circuit configuration as described above results in significant distortion and it is preferred that the decoupling capacitor be omitted. The omission of the decoupling capacitor affects the values of the capacitors in the resonant circuit needed to achieve a given nominal frequency but does not adversely affect the operation of the oscillator and result in a more symmetrical waveform.

The invention will now be described further, by way of example, with reference to the ac companying drawings, in which: Figure 1 is a section through an LVDT mounted on a suspension damper or shock absorber, and Figure 2 shows the circuit diagram of an oscillator for driving a current through the primary winding of the LVDT in Figure 1.

In Figure 1, there is shown schematically a suspension damper 10 comprising a cylinder 12 and a piston rod 14. For protection, the piston rod is fitted with a sleeve 16 which fits over the cylinder 12. The sleeve for the purposes of the present invention should be of a plastics or other insulating material. It will be noted that the sleeve is not load bearing and making the sleeve of an insulating material presents no serious problem. On the other hand, the casing of the cylinder 12 may be of steel, as is conventional and as is required to bear the mechanical load.

In order measure the load on the vehicle suspension, a linearly variable differential transformer is mounted on on the damper.

The transformer comprises a primary winding 20 wound around the cylinder 12 and two secondary windings 22, 24 wound around the sleeve 16.

The secondary winding 24 overlies the secondary winding 22 and extends over the same length of the sleeve 16. The two secondary windings are tapered, the inner winding 22 having fewer turns per unit length at its lower end as viewed and the outer winding 24 having fewer turns per unit length at its upper end.

In the centre position of the primary winding, the magnetic coupling into both windings is the same, in view of the symmetry, and the induced voltages cancel one another out.

However, with movement away from the centre position the coupling into one secondary winding increases while the coupling into the other decreases and the difference signal is linearly related to the extent of the displacement.

In Figure 2, the winding of the transformer have been allocated the same reference numerals as in Figure 1. The primary winding is connected in the collector circuit of a transistor 50 which may be of type BC182L. The two secondary windings 22, 24 are connected to a detector circuit described below.

A capacitor 52, with a value of for example 1/LF is connected across the emitter and collector of the transistor 50 and a further capacitor 54, having a value of for example 2.2,uF, is connected between the emitter and earth.

The transistor 50 has the usual bias resistors 56, 58 and 60 for setting the d.c. operating point at the base and the emitter. The absence of a decoupling capacitor from the base of the transistor 50 should however be noted.

The circuit is essentially a Colpitts oscillator in which the primary winding 20 and the two capacitors 52, 54 form a series resonant circuit connected across the power supply lines.

To maintain oscillation, positive feedback is achieved bysampling a signal at the tapping constituted by the junction between the capacitors 52 and 54 and feeding the signal back in phase after amplification by the transistor 50 into the primary of the transformer. The absence of a decoupling capacitor from the base of the transistor results in further feedback tending to cancel out signal distortion.

The primary winding may consist of 500 turns of 38 swg copper wire while the secondary windings may each consist of 276 of 30 swg copper wire. With the component values given above as an example, the nominal frequency of the oscillator is 2 KHz.

An advantage of the design of the secondary windings is that as they are both mounted on the same section of the sleeve they are subject to the same induced noise and the noise is consequently substantially eliminated when the difference signal is produced.

The oscillating voltage produced by means of the oscillator described above is still not symmetrical fully despite the omission of a base decoupling capacitor for this purpose.

The residual asymmetry is not in itself a problem when measuring signal amplitude as it does not affect the peak to peak value of the signal. However, since directional information cannot be inferred from amplitude measurement alone further steps must be taken if the sense of the movement is to be determined.

In conventional LVDT design, a phase discriminator provides the solution but this requires signal linearity which is more than provided by the oscillator described above and an alternative solution is therefore required.

The preferred method of detecting the difference between the voltages V1 and-V2 induced in the secondary windings 22 and 24 of the LVDT is to full wave rectify each of the output voltages separately and to produce both the sum V1 + V2 and the difference signal V1-V2. The ratio of the sum and difference signals is a measure of the position of the primary winding giving both the extent and the direction of movement. The so called ratiometric technique also reduces any problem caused by amplitude instability in the oscillator as it affects the values of both the numerator and the denominator.

Claims

1. A position transducer comprising a primary winding movable relative to two secondary windings and means responsive to the difference between the magnetic coupling of the primary winding to the respective secondary windings to provide an indication of the relative position of the primary winding, wherein one secondary winding overlies the other and the number of turns in each winding per unit length tapers along the length of the winding, the direction of taper in one winding being the opposite of the direction of taper in the other.

2. A transducer as claimed in claim 1, wherein the thickness of each winding is reduced progressively along its length.

3. A transducer as claimed in claim 1 or 2, fitted to a suspension damper, wherein the secondary windings are mounted on an insulating sleeve movable with the piston rod of the damper and the primary winding is mounted recessed into the surface of the damper cylinder.

4. A transducer as claimed in any preceding claim, wherein the primary winding is connected as a reactive element in an oscillator circuit.

5. A transducer as claimed in claim 4, wherein the oscillator comprises a series resonant circuit included a tapped capacitor connected in series with the primary winding of the transducer, and a amplifier element connected in a positive feedback path from the capacitor tapping to the primary winding.

6. A transducer as claimed in claim 5, wherein the amplifier element is a transistor.

7. A transducer constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.