GB2114755A

GB2114755A - Displacement transducer

Info

Publication number: GB2114755A
Application number: GB08301311A
Authority: GB
Inventors: John Harrison Asbery
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1982-02-12
Filing date: 1983-01-18
Publication date: 1983-08-24
Also published as: GB8301311D0

Abstract

A displacement transducer comprises a plurality of magnetic strips 13 spaced apart in the direction of movement of a sensor. Alternate strips are of opposite magnetic polarity so that the field strength between adjacent strips reduces to zero. The strips may be formed of magnetisable material and connected to the opposite poles of a permanent magnet. <IMAGE>

Description

SPECIFICATION Displacement transducer This invention relates to displacement transducers of the kind comprising a support on which is located a series of spaced magnetised strips and a sensor responsive to the magnetic field produced by said strips, the support and sensor being moved relative to each other during the displacement to be measured and said strips being spaced apart in the direction of relative movement, the sensor in use being connected to an indicator whereby the passage of the strips past the sensor can be indicated.

In known transducers of this type the strips have the same magnetic polarity and hence the sensor produces an output which varies as the strength of the field. The magnetic field strength is at a minimum when the sensor is half way between a pair of strips and at a maximum when it lies above a strip. The resolution of the transducer since observations of the maxima and minima are made, is equal to half the width of a strip pulse half the width of the spaces between the strips. The problem with this type of transducer is to be able to detect accurately the maxima and minima positions.

The object of the invention is to provide a displacement transducer of the aforesaid kind in which the problem expressed above is minimised.

According to the invention in a transducer of the kind specified adjacent strips have opposite magnetic polarity whereby the magnetic field strength between adjacent strips reduces to zero half way between adjacent strips.

In the accompanying drawings: Figure 1 is a diagrammatic side view of the form of transducer, Figure 2 is a plan view of the transducer of Figure 1, Figure 3 is a view similar to Figure 2 showing the modification in accordance with the invention, Figure 4 shows the variation of the field strength which occurs during operation of the transducer of Figure 1, Figure 5 shows the variation of magnetic field strength with the transducer shown in Figure 3, and the remaining figures show various graphs which can be obtained in the processing of the signals produced by the transducer.

The transducer shown in Figure 1 of the drawings is a known form of transducer and it comprises a support 9 on which is defined a series of magnetic strips 10 and also forming part of the transducer is a sensor 11. The support and the sensor are movable relative to each other in the direction of the arrows during the measurement of the displacement. The strips 10 are spaced apart in the direction of movement and are conveniently formed from magnetisable material, the ends of the strips being magnetically connected as shown in Figure 2, to one pole of a permanent magnet 12. The sensor 11 has a sensing head which is very much narrower than the strips and also the spaces between the strips and as the relative movement occurs the field strength detected by the sensor varies as shown in Figure 4.It will be observed that the field strength varies between maxima and minima corresponding to the sensor being over a strip 10 and half way between the pair of adjacent strips 10 respectively. If the sensor 11 is of the magnetoresistive type then the output of the sensor will correspond to the curve shown in Figure 4. It will of course be appreciated that the distance between the maxima and minima in Figure 4 is not to the same scale as the diagram of Figure 1. With this arrangement there is a difficulty in detecting the maxima and minima.

Turning now to Figure 3 the strips 13 shown therein are of alternate magnetic polarity and for this purpose the adjacent ends of alternate strips are connected to magnetically conductive members 14, 1 5 respectively these in turn being connected to the poles of a magnet 1 6. A side view of the modified transducer would be substantially the same as Figure 1. Figure 5 shows the variation in the magnetic field strength which occurs and once again the horizontal scale of the graph is larger than that of the diagram. It will be observed that between adjacent strips the magnetic field strength falls to zero and if the sensor is of the type which can detect magnetic polarity, the output of the sensor will correspond to the graph of Figure 5.If on the other hand the sensor 11 is of magnetoresistive type that is to say one which can detect field strength only, its output will be as shown in Figure 6. In each case however the output of the sensor when it lies between adjacent strips, would be zero.

In Figure 7 there is shown the output obtained with a sensor which is responsive to the magnetic polarity. If this output is applied to one input of a comparator the other input of which is at zero, the comparator output will be as indicated in Figure 8, making it particularly easy to identify the positions of zero field strength.

If the output is as shown in Figure 6, then the signal as shown in Figure 9 will be obtained by differentiation and by futher differentiation the signals shown in Figure 10 will be obtained. Thus it is particularly easy to distinguish the positions of zero field strength with the transducer as modified as shown in Figure 3.

In Figure 3 a separate magnet is shown to polarise the strips however, the strips themselves can be formed from magnetically polarised material.

It will be understood that in order to achieve the same resolution (considered in terms of output pulses/unit distance of movement) as is obtained with the transducer of Figure 1, the transducer as shown in Figure 3 will require twice the number of strips.

Claims

1. A displacement transducer comprising a support on which is located a series of spaced magnetised strips and a sensor responsive to the magnetic field produced by said strips, the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Displacement transducer This invention relates to displacement transducers of the kind comprising a support on which is located a series of spaced magnetised strips and a sensor responsive to the magnetic field produced by said strips, the support and sensor being moved relative to each other during the displacement to be measured and said strips being spaced apart in the direction of relative movement, the sensor in use being connected to an indicator whereby the passage of the strips past the sensor can be indicated. In known transducers of this type the strips have the same magnetic polarity and hence the sensor produces an output which varies as the strength of the field. The magnetic field strength is at a minimum when the sensor is half way between a pair of strips and at a maximum when it lies above a strip. The resolution of the transducer since observations of the maxima and minima are made, is equal to half the width of a strip pulse half the width of the spaces between the strips. The problem with this type of transducer is to be able to detect accurately the maxima and minima positions. The object of the invention is to provide a displacement transducer of the aforesaid kind in which the problem expressed above is minimised. According to the invention in a transducer of the kind specified adjacent strips have opposite magnetic polarity whereby the magnetic field strength between adjacent strips reduces to zero half way between adjacent strips. In the accompanying drawings: Figure 1 is a diagrammatic side view of the form of transducer, Figure 2 is a plan view of the transducer of Figure 1, Figure 3 is a view similar to Figure 2 showing the modification in accordance with the invention, Figure 4 shows the variation of the field strength which occurs during operation of the transducer of Figure 1, Figure 5 shows the variation of magnetic field strength with the transducer shown in Figure 3, and the remaining figures show various graphs which can be obtained in the processing of the signals produced by the transducer. The transducer shown in Figure 1 of the drawings is a known form of transducer and it comprises a support 9 on which is defined a series of magnetic strips 10 and also forming part of the transducer is a sensor 11. The support and the sensor are movable relative to each other in the direction of the arrows during the measurement of the displacement. The strips 10 are spaced apart in the direction of movement and are conveniently formed from magnetisable material, the ends of the strips being magnetically connected as shown in Figure 2, to one pole of a permanent magnet 12. The sensor 11 has a sensing head which is very much narrower than the strips and also the spaces between the strips and as the relative movement occurs the field strength detected by the sensor varies as shown in Figure 4.It will be observed that the field strength varies between maxima and minima corresponding to the sensor being over a strip 10 and half way between the pair of adjacent strips 10 respectively. If the sensor 11 is of the magnetoresistive type then the output of the sensor will correspond to the curve shown in Figure 4. It will of course be appreciated that the distance between the maxima and minima in Figure 4 is not to the same scale as the diagram of Figure 1. With this arrangement there is a difficulty in detecting the maxima and minima. Turning now to Figure 3 the strips 13 shown therein are of alternate magnetic polarity and for this purpose the adjacent ends of alternate strips are connected to magnetically conductive members 14, 1 5 respectively these in turn being connected to the poles of a magnet 1 6. A side view of the modified transducer would be substantially the same as Figure 1. Figure 5 shows the variation in the magnetic field strength which occurs and once again the horizontal scale of the graph is larger than that of the diagram. It will be observed that between adjacent strips the magnetic field strength falls to zero and if the sensor is of the type which can detect magnetic polarity, the output of the sensor will correspond to the graph of Figure 5.If on the other hand the sensor 11 is of magnetoresistive type that is to say one which can detect field strength only, its output will be as shown in Figure 6. In each case however the output of the sensor when it lies between adjacent strips, would be zero. In Figure 7 there is shown the output obtained with a sensor which is responsive to the magnetic polarity. If this output is applied to one input of a comparator the other input of which is at zero, the comparator output will be as indicated in Figure 8, making it particularly easy to identify the positions of zero field strength. If the output is as shown in Figure 6, then the signal as shown in Figure 9 will be obtained by differentiation and by futher differentiation the signals shown in Figure 10 will be obtained. Thus it is particularly easy to distinguish the positions of zero field strength with the transducer as modified as shown in Figure 3. In Figure 3 a separate magnet is shown to polarise the strips however, the strips themselves can be formed from magnetically polarised material. It will be understood that in order to achieve the same resolution (considered in terms of output pulses/unit distance of movement) as is obtained with the transducer of Figure 1, the transducer as shown in Figure 3 will require twice the number of strips. Claims

1. A displacement transducer comprising a support on which is located a series of spaced magnetised strips and a sensor responsive to the magnetic field produced by said strips, the support and sensor being movable relative to each other during the displacement to be measured, said strips being spaced apart in the direction of movement, the sensor in use being connected to an indicator whereby the passage of the strips past the sensor can be indicated characterised in that adjacent strips have opposite magnetic polarity whereby the field strength between adjacent strips reduces to zero half way between adjacent strips.

2. A transducer according to Claim 1 in which said strips are formed for magnetisable material and the transducer includes a permanent magnet, adjacent strips being connected to the opposite poles of the magnet respectively.

3. A displacement transducer comprising the combination and arrangement of parts substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.