GB2226636A

GB2226636A - A method and an apparatus for sensing the movement of a local magnetic object

Info

Publication number: GB2226636A
Application number: GB8828353A
Authority: GB
Inventors: Julian Nolan Gannaway; Robert Charles Minchin
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1988-12-05
Filing date: 1988-12-05
Publication date: 1990-07-04
Anticipated expiration: 2008-12-05
Also published as: GB2226636B; GB8828353D0

Abstract

The close approach of a moving magnetic object is sensed by means of the difference between the respective emfs induced in a pair (1, 3) of parallel conductors by the changing magnetic field associated with the object. An amplifier feeds the difference emf to a voltage threshold circuit (17, 19 Figure 4) having a digital signal output that changes when the object is a set distance away. This form of sensor has particular sensitivity to changes in magnetic field gradient. With appropriate choice of voltage threshold it is thus possible to distinguish movements of a local magnet, for which the magnetic field gradient change is high at close range, from more distant and stronger moving magnets. The signal-to-noise figure for the apparatus can be improved by coupling many shorted conductor pairs in parallel. Sensitivity to the movement of the more distant strong magnets may be reduced further, and thus discrimination improved, by introducing twists or cross-over connections at intervals along the length of each pair of shorted conductors. The sensor can be implemented simply, using for example either twin feeder cable or cable ribbon. <IMAGE>

Description

A METHOD AND AN APPARATUS FOR SENSING THE MOVEMENT OF A LOCAL MAGNETIC OBJECT TECHNICAL FIELD The present invention concerns magnetic sensing and in particular the sensing of the movement of a local magnetic object.

BACKGROUND ART In the measurement of changing magnetic flux it is commonplace to employ a conductor loop or coil and monitor the electromotive force (e.m.f.) that is induced. Such technique however is relatively insensitive to magnetic field gradient and thus is not readily capable of discriminating between movements of small magnetic objects in the near vicinity of the sensing loop or coil and other movements of larger magnetic objects remote from the sensing loop or coil.

DISCLOSURE OF THE INVENTION The present invention is intended to provide a method and an apparatus for sensing the local movement of magnetic objects. As such, it is intended to provide discrimination between the movement of a small magnetic object in the near vicinity of a sensor and the movement of a larger magnetic object remote from the sensor.

In accordance with the invention thus there is provided a method for sensing the movement of a magnetic object which method comprises the following steps: sensing a difference e.m.f generated between a pair of spaced parallel conductors when a magnetic object is moved relative thereto; amplifying the sensed difference e.m.f. to produce an amplified signal; and comparing the amplified signal against a voltage threshold thereby to generate a logic-valued output signal indicative of the close approach of a moving magnetic object.

In the method just described the sensor is especially sensitive to magnetic field gradient. A magnetic object that is relatively close to the sensor will exhibit a magnetic field gradient that is significantly larger than the field gradient exhibited when that small object is much further away. By comparing the induced difference e.m.f. against a voltage threshold it is thus possible to discriminate between movements of magnetic objects in the close proximity of the sensor and movements of similar objects somewhat further away.

It is preferable to provide signal amplification such that the amplitude of the amplified signal is relatively insensitive to the velocity of the moving object. This may be achieved using an amplifier in which the gain is frequency dependent and in particular rolls of with increase in frequency. By chosing an appropriate value of gain slope it is thus possible to provide relative insensitivity to object velocity within a predetermined range.

In accordance with a further aspect of the present invention there is provided an apparatus for performing the aforesaid method, which apparatus comprises; a sensor comprised of a pair of spaced parallel conductors which conductors are electrically connected at one end; an amplifier connected between the two spaced parallel conductors such that a difference e.m.f. shall be applied to the input of the amplifier; a voltage source for providing a threshold voltage; and a digital comparator connected to the amplifier and to the voltage source for comparing the amplified difference e.m.f. with the voltage threshold in order to provide a logic value signal indicative of the close approach of a magnetic object.

In the apparatus aforesaid a better signal to noise performance may be achieved by connecting additional pairs of spaced parallel conductors in series with that first mentioned. Such a sensor may readily be constructed from ribbon cable in which the conductors are interconnected in an overlapping series sequence.

Alternatively, the sensor may be constructed from twin conductor feeder cable. Discrimination may in this case be improved yet further by inserting crossover connections at points along the length of the feeder cable.

It is preferable that the amplifier has a gain that rolls off with frequency and has a slope chosen such that the amplitude of the amplified difference e.m.f. is relatively independent of the velocity of the moving object.

It is preferable to include a low pass or a band pass filter between the amplifier and the comparator.

The difference e.m.f. may be of either positive or negative polarity depending upon the direction of movement of the magnetic object and also depending upon the orientation of the magnetic poles of the magnetic object. It is therefore preferable to feed the amplified signal into a pair of comparators to which positive and negative voltage threshold signals are applied, respectively. The comparator outputs may then be OR-wired to provide the required logic signal. Alternatively the amplified signal may be passed through a rectifier and then onto a single comparator.

BRIEF INTRODUCTION OF THE DRAWINGS Figure 1 is a graph depicting conductor line e.m.f. as a function of object distance from a sensor; Figure 2 is a sketch showing an arrangement of conductors and magnetic object provided to illustrate the principle underlining the present invention; Figure 3 is a schematic block diagram showing an arrangement of a logic circuit and sensor arranged in accordance with the present invention; Figure 4 is a block circuit diagram of an electrical circuit that may be implemented as the logic circuit of the preceding figure; Figure 5 is an illustrative plan view of a ribbon cable modified to provide a sensor that may be used in place of the sensor shown in the apparatus of Figure 3 above; Figure 6 is a plan view of a length of feeder cable modified to provide an alternative sensor for use in the apparatus shown in Figure 3 preceding; and Figure 7 is a plan view showing a combination of two lengths of modified feeder cable arranged to provide an improved sensor for the apparatus shown in proceeding Figure 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order that this invention may be better understood embodiments thereof will now be described and reference will be made to the drawings. The description that follows is given by way of example only.

The principle underlying this invention will now be described with reference to Figures 1 and 2. A pair of parallel conductors 1, 3 is shown each of length 1 and spaced apart a constant distance Ax.

This pair of conductors lies in the path of a magnetic object 5 a distance x away which is moving towards the pair of conductors at a velocity v. As the magnetic object 5 moves towards the conductors I and 3, line e.m.f.'s are induced between points A, B and C, D respectively. The line e.m.f.'s 5 are plotted as a function of the distance x as shown in Figure 1. Inspection of this figure shows that the line e.m.f. first varies slowly as a function of distance as the magnetic object is moved from a great distance and then changes more rapidly as the conductors are approached. The line e.m.f. 5 is equal to the rate of change of the magnetic flux N and is thus related by the formula: 5 = - dN = - d Blvdt dt dt = - Blv - (I) In the above derivation the longitudinal variation of the magnetic field has been ignored by way of simplification. When the two conductors 1, 3 are connected at one end B, D, the line e.m.f.'s 5 are summed subtractively and a difference e.m.f. At is measurable at the other end A, C of the two conductors 1, 3. To this approximation, the difference e.m.f. per unit velocity depends upon the local field gradient dB dx and the conductor spacing Ax as follows: X, = Ax dB v dx (11) Thus where the difference e.m.f.At is compared with the threshold voltage AtT it is possible to detect the movement of the magnetic object closer than a distance xT: 4+ > t4T; x < xT t < tT; x > xT (III) A practical arrangement is shown in Figure 3 where the parallel conductors 1, 3 are provided in the form of a 300 ohm impedance twin conductor feeder cable. The two conductors 1, 3 are spaced a fixed distance apart by plastics insulation 7 and are connected at one end by a short circuit connection 9. At the other end the two conductors 1, 3 are connected to the input of a logic circuit 11. A suitable logic circuit is shown in Figure 4.At the front end of this circuit 11 there is a high gain operational pre-amplifier 13 the output of which is connected to a band-pass filter 15 and via this to the respective inputs of a pair of voltage comparators 17, 19. The amplified voltages applied to each of these comparators 17, 19 are compared against voltage thresholds +VT, -VT. The output of each of these comparators 17, 19 are fed to an OR-gate formed of a pair of diodes 21, 23. A logic signal is thus presented to the OR-gate output O/P. The output logic signal will have a value logic 1 or logic 0 dependent upon whether the amplified signal voltage lying above or below voltage threshold. Change of logic value and thus sensor indication is provided as the threshold voltage is exceeded corresponding to an approach of magnetic object closer than a critical distance XT.In the aforesaid circuit the filter 15 has a pass band 0.1 through 10 hertz. The pre-amplifier has a gain of 106 at the lower pass frequency 0.1 hertz and varies with frequency with a roll-off of 10 db/octave. The latter roll off function has been provided so that logic response is relatively insensitive to magnetic object velocities within a practical range.

Signal to noise performance may be improved by employing a number of shorted conductor pairs and summing the difference e.m.f.'s. Signal to noise performance is then improved as the square root of the number "m" of shorted conductor pairs employed. This principle is exploited in the modified design sensor head shown in Figure 5. This modified sensor head 25 is constructed from a length of conventional ribbon cable comprised of 10 conductors C1, C2 ......

C10 which are held in fixed spaced relationship by plastics insulation 27. Each end of the ribbon cable is terminated by a ribbon pin connector 29, 31. Each of the pin connectors 29, 31 are plugged into a respective printed circuit board 33, 35. The ten conductors C1 through C10 are connected in pairs; C1 and C6, C2 and C7 ... C5 and C10 by short circuit metallisation 37 defined on the underside of one of the printed circuit boards 35. The five conductor pairs, thus provided, are connected in series to sum the difference e.m.f.'s generated in each conductor pair. Looping connections are defined by further metallisation on the underside of the other printed circuit board 33. Looping connections therefore are thus provided between conductors C6 and C2; C7 and C3; ; and, C9 and C5.Conductors C1 and C10 are connected to metallisation pads 41 and 43 respectively and a summed difference e.m.f. extracted between these two pads 41 and 43.

This sensor 25 is very flexible, and can be rooted to provide coverage of a wide range of shapes and areas. The cable length can be increased to cover a very large area without increasing its sensitivity.

Further discrimination against distant fields can be obtained by twisting the cable periodically to make uniform fields cancel at the output of the looped arrangement. Alternatively crossover interconnects can be inserted at intervals along the length of the cable. This is illustrated for a length of feeder cable as shown in Figure 6. In this Figure, crossover interconnects 45, 47 are shown on the face and underside respectively of the feeder cable. In such an arrangement, however, sensitivity nulls correspond to each crossover interconnect. This problem may be obviated by using a pair of sensor heads which may be overlapped or arranged in close parallelism with the crossover interconnects arranged in a staggered relationship such that the crossovers of one sensor are arranged opposite to the parallel parts of the other sensor. Such an improved arrangement is shown in Figure 7.

Claims

CLAIMS:

1. A method for sensing the movement of a magnetic object which method comprises the following steps: a) sensing a difference e.m.f. generated between a pair of spaced parallel conductors when a magnetic object is moved relative thereto; b) amplifying the sensed difference e.m.f. to produce an amplified signal; and, comparing the amplified signal against a voltage threshold thereby to generate a logic valued output signal indicative of the close approach of a moving magnetic object.

2. An apparatus for sensing the movement of a magnetic object.

the apparatus comprising a sensor having a pair of spaced parallel conductors connected at one end thereof, an amplifier connected ta said conductors at the other end thereof. such that a difference e.m.f.

is applied to the input of the amplifier. a voltage source for providing a threshold voltage and comparator means connected to said amplifier and said voltage source for comparing the amplified difference e.m.f. with the voltage threshold in order to provide a logic-valued output signal indicative of the close approach of a magnetic object.

3. An apparatus according to claim 2. including an additional pair of spaced parallel conductors arranged in series with the firstmentioned pair of conductors.

4. An apparatus according to claim 2 or 3, including a low pass or band pass filter arranged between the amplifier and the comparator means.

5. An apparatus according to any one of claims 2 to 4, in which said comparator means includes a pair of comparators to which positive and negative voltage threshold signals are applied respectively, the amplified signal being applied to each of said comparators and the outputs of the comparators being OR-wired to provide said logic-valued output signal.

6. An apparatus according to any one of claims 2 to 5, wherein said amplifier has a gain that roles off with frequency and has a slope such that the amplitude of the amplified difference e.m.f. is relatively independent of the velocity of the moving object.

7. An apparatus according to claim 2, wherein said sensor is constructed from a ribbon cable wherein the conductor are interconnected in an overlapping series sequence.

8. An apparatus according to claim 2, wherein said sensor is constructed from a twin conductor feeder cable having cross over connections at points along the length thereof.

9. A method for sensing the movement of a magnetic object substantially as hereinbefore described with reference to the accompanying drawings.

10. An apparatus for sensing the movement of a magnetic object substantially as hereinbefore described with reference to the accompanying drawings.