GB2158240A - A null-balanced proximity sensor - Google Patents

Abstract

A null-balanced proximity sensor comprises a flux generating coil (2) and a nearby flux sensing coil (1) placed within the flux field produced by the generating coil so that in the absence of any metallic object in the flux field flux linkages between the two coils are substantially null and so no output appears at the terminals of the sensing coil. Any metallic object (4) introduced into the flux field will unbalance the field and cause an output from the sensing coil. The sensing coil 2 may be wound round a ferrite core (3) and may additionally be screened to eliminate capacitive coupling between the two coils. The two coils may be mounted on a common plate with a rotatable eccentric mounting to adjust their relative positions. Ferromagnetic adjustment elements may be provided for obtaining balance. The sensor may be used in a position sensing arrangement where a member having a coded sequence of magnetic and conductive elements is provided. <IMAGE>

Description

SPECIFICATION A null-balanced proximity sensor This invention relates to a null-balanced proximity sensor responsive to the presence of metal objects.

Known types of magnetic proximity sensor exploit a single coil of which the inductive field tranverses an external air, or other, path. The presence of a metallic object in this path is indicated by the resulting change of inductance value of the circuit.

According to the present invention there is provided a null-balanced proximity sensor comprising a flux generating coil and a flux sensing coil placed within the flux field produced by the generating coil when energised, the position and orientation of the sensing coil with respect to the generating coil being such that in the absence of any metallic object in the flux field flux linkages between the two coils are substantially null or reduced to a minimum so that no significant output appears at the terminals of the sensing coil.

In a preferred embodiment of the invention the sensing coil is provided with a ferrite core.

Embodiments of the invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a functional diagram of a null-balanced proximity sensor; Figures 2a and 2b are two views illustrating the disposition of the flux generating and sensing coils, and Figures 3a and 3b are views of a proximity sensor constructed with screening and with a balancing system.

In its basic embodiment, the null-balanced proximity sensor comprises a sensing coil 1 (Fig.

1) which is placed in a null orientation with respect to an alternating field generated by coil 2, that is, the orientation is such that flux linkages between the two coils are reduced to a minimum so that no output appears at the terminals of the sensing coil. To enhance the performance, the sensing coil may be wound on an open-ended core 3 of a material having a significant permeability, such as a ferrite rod. Assuming, first, that a ferrous object 4 is brought towards the sensor from direction A, its presence will offer a lower-reluctance path to that part of the generated field that has proximity to side A of the sensing coil.The field will consequently be distorted so as to follow the easier path through the ferrous material, with the result that the null condition will be destroyed and the unbalance flux may be regarded as passing through the sensing coil in the direction from A to B. In fact, the flux will be alternating, in phase with the drive current to the generating coil, so will produce an output at the terminals of the sensing coil which will bear a specific phase relationship to the drive current. If, on the other hand, the ferrous object 4 approaches the sensor from direction B, the resulting unbalance will be in such a direction as to generate an output in antiphase to that described above.

Since the output voltage from the sensing coil is proportional to the rate of change of the linking flux, the actual phase relationships will be such that, when an approach from direction A produces an output voltage waveform leading 90 in relation to the phase of the drive current, an approach from direction B will give rise to a voltage waveform lagging 90 . This effect may be eliminated by shunting the sensing coil with a capacitor 9 having a value such as to tune the coil to the frequency of the drive waveform. Tuning the coil has the advantages that the output voltage is enhanced by the Q of the tuned circuit, and interfering signals at other than the working frequency are reduced in amplitude.It also introduces an additional 90 phase shift which results in the sensing coil output voltage appearing either in phase or in antiphase with the drive waveform, depending upon the direction of approach. This latter feature simplifies the design of the synchronous detection circuit which must compare the input and output phases to determine the polarity of the final output signal.

In brief, the presence of a ferrous material in region A relative to the sensing coil may be made to generate a pluse (+) output, whereas a ferrous material in region B will generate a minus (-) output (or their equivalent, according to processing requirements). It follows that a suitably narrow strip of ferrous material passing from A to B will generate a + - sequence, whereas on passing from B to A the sequence will be - +. The direction of movement is thereby made apparent.

Again, in reference to Fig. 1, if the object 4 is made of a non-ferrous conducting material, such as copper, the alternating field of the generating coil will induce eddy currents in the conducting material such as to oppose the field and repel it away from the object. Assuming the conducting object to enter region A, the field in region A will be reduced in intensity relative to that in region B. The unbalance flux may now be regarded as passing through the sensing coil in the direction from B to A, that is, the exact opposite of the result achieved by the presence of a ferrous material. Hence, by the same mechanism as that detailed above, a suitably narrow strip of non-ferrous conducting material passing from A to B will generate a - + sequence, whereas a + - sequence will be generated by its passage from B to A.

The generation of recognisable codes, by the provision of a group of detectable strips, requires, in general, that the interval between adjacent strips should be sufficient to enable individual strips to be sensed. This requirement will be satisfied if the spatial separation of adjacent strips is such as to allow the sensor output to fall to zero (0) when it is sampling intermediately between the adjacent strips. The potentiality of this coding techniques may be illustrated by the following examples based on the use of groups of four strips.

Four ferromagnetic strips: 000000 + -0+ - 0 + -00000 Four conducting strips: 000000- + - +0 - + 0- 00000 Alternate Fe Cu Fe Cu: 000000 - 0 - O + - 0- 00000 Fe Cu Cu Fe: 000000 + -0 - +0 - + 0 + - 00000 The above examples omit the brief transitions through zero which occur as a given strip passes from region A into region B or conversely. It will be apparent that codes may be constructed that remain the same regardless of the direction of movement of the carrier, or, alternatively, can be made to show direction sensitivity. Special cases may be constructed, using Fe Cu alternations, in which the strip spacing is reduced, giving sequences such as: 0000 + - - + + - - + + - - + + - 00000.In such cases, the adjacent strips of dissimilar material have the effect of augmenting the amplitude of the final output.

These examples assume that all strips in a group are present and equidistant, as would be required for a self-clocking system. If, however, an independent clocking system is available, such as might be based on a known constant velocity of carrier movement, further codes can be constructed by omitting one or more strips from the ordered sequence, so as to generate conventional mark/space codes.

The elementary arrangement, shown for ease of description in Fig. 1, bears similarity to conventionai equipment used for geological prospecting, in which the two coils are separated by an appreciable distance by a strut or pole. Although this enables sensing distances of up to a metre or more, the separation of the two coils presents problems in achieving sufficient rigidity to maintain their critical angular relationship, upon which depends the accuracy of the null balance and hence the achievable sensitivity of the device. Therefore, for the type of application here envisaged, it is preferable to bring the two coils into much closer proximity, so that they may share a rigid common support.The relative coil positions now become as shown in Fig. 2 (a) and (b), where it will be seen that the sensing coil no longer lies in the axial field of the generating coil, but in the returning flux path external to the generating coil. Since the axial field extends some distance from the generating coil before returning to close the loop, a sensible object or strip 4 placed in this region will be detected as before. Typical dimensions, and the performance parameters of a prototype model, are given below.

The extremely close proximity of the two coils presents several special balancing problems.

The level of sensitivity to a detectable object is naturally limited by the amplitude of the residual unbalance output when no object is present. This unbalance must therefore be reduced to the minimum achievable. A first requirement is to eliminate any capacitive coupling between the two windings. This is achieved by completely enclosing the sensing coil with a non-ferrous conductive screen, 5 in Fig. 3, which is earthed via the screened output connections. Two holes, axial to the sensing coil, are provided to permit the insertion of a ferrite rod which, for optimum performance, must project significantly on either side of the screen. To prevent the screen acting as a short-circuited turn, which would couple detrimentally with the sensing coil, the circuit is broken by a narrow slit 6, extending from the core holes to the exterior of the screen.It will be apparent that a variety of forms of construction might be adopted to achieve the purpose described.

Normal mechanical tolerances are not necessarily sufficient to ensure assembling the coils in the correct relative alignment for optimum balance. This is partly due to the possibility that the 'lie' of the turns of the coil windings will result in a lack of coincidence between the magnetic and mechanical axes. Also, the generated magnetic field is not straight and uniform in flux density, as might be desired, but is strongly curved in the region of interaction with the sensing coil. As a first method for adjusting the balanced condition, the generating coil 2 may be made tiltable in any direction upon its supporting pillar 10, which must be non-magnetic and nonconducting; for example, a nylon bolt. An eccentric movement in relation to the supporting pillar might also be provided. In practice however, it has proved unnecessary to make special provision of this nature, since the magnetic 'tilt' imposed by the lie of the winding enables the coil to be mounted with its mechanical axis permanently aligned with that of the supporting pillar, yet simple rotation of the coil bobbin about this axis will adjust the tilted field to an orientation at which the desired balanced condition is approached.

At this stage, the critical balanced condition may be further enhanced by tilt adjustment of the sensing coil, although, in practice, it is sufficient either to make tilt adjustment of the inserted ferrite core or, more simply, to move the ferrite core axially through the stationary sensing coil assembly in the appropriate direction to achieve balance.

When a satisfactory balance has been achieved, it is appropriate to seal the adjustments by immersing the assembly in an epoxy or other suitable potting compound 11. After such treatment, there remains the possibility that trimming adjustment may yet be provided. This may take the form of a small permalloy strip 8, which is adjusted to a position in relation to the sensing coil which achieves the best possible balance. Assuming that the coils are mounted on a cylindrical screening plate 7, as indicated by Fig. 3 (b), the strip 8 might be mounted on a nonferrous ring (not shown) so as to be rotatable about the axis of the plate 7. Further adjustment may be achieved by moving the ring-and hence the strip 8-axially in relation to the plate, in combination with the rotary movement.The effectiveness of this final adjustment depends upon the success achieved in making the earlier adjustments, and cannot compensate for gross errors in these.

It will be apparent that the generated flux emerging from the operative 'front' face of the generating coil is matched by an equal field emerging from the back. Hence, in the absence of any other provision, the sensor is equally sensitive to disturbances arising in the non-operative rear region. To prevent this, the plate 7 is provided to constrain the rear field to within the limits of the coil assembly. The plate may be made of a ferrous material, such as permalloy, in which case the backwardly emerging flux will enter the plate and be directed so as to re-emerge in the vicinity of the sensing coil. Alternatively, the plate may be of a conducting material, such as brass or copper. In this event, eddy currents in the plate will serve to constrain the field.When a conducting plate is used, it is desirable to allow a clearance between the coil 2 and the plate, approximately equal to the depth of the coil, to ensure a reasonable flux distribution. In either case, the screening effect of the plate will eliminate the unwanted rear sensitivity.

Normally, metallic parts of the main housing should not extend beyond the front face of the screening plate 7. Any enclosure for the coil assembly must be non-ferrous and non-conducting.

The following details relate to an experimental prototype. All voltage and current readings are peak-to-peak, and the operating frequency f= 7 KHz.

Relevant Dimensions Transmitting coil: 250 turns, 1 5 mm dia., length 6 mm, current 400 mA.

Sensing coil: 800 turns, 1 5 mm dia., length 10 mm, fully screened.

Ferrite core for sensing coil: 6 mm dia., length 25 mm.

Coil separation, axis to axis: 20 mm.

Sensing Coil Outputs Optimum balanced condition: 1 mV Total unbalance (permalloy strip in contact with coil): 800mV Total unbalance (copper strip in contact with coil): 500mV Maximum sensing distance for output ten times balance condition: 50mm Detecting code strips at 8mm sensing distance, permalloy: 200mV Detecting code strips at 8mm sensing distance, soft iron: 100mV (Code strips each 6mm wide, 1.5mm thick, 40mm long, separated 32mm centre-to-centre along direction of movement).

Detecting code rings at 25mm sensing distance, permalloy: 75mV Detecting code rings at 25mm sensing distance, copper: 15mV (Code rings 90mm OD, mounted on axially moving cylinder.

Axial separation of rings 50mm centre-to-centre.

Permalloy rings 8mm wide, 3mm thick.

Copper rings 6mm wide, 3mm thick.

Sensor mounted 25mm from one side of cylinder.

Claims (11)

1. A null-balanced proximity sensor comprising a flux generating coil and a flux sensing coil placed within the flux field produced by the generating coil when energised, the position and orientation of the sensing coil with respect to the generating coil being such that in the absence of any metallic object in the flux field flux linkages between the two coils are substantially null or reduced to a minimum so that no significant output appears at the terminals of the sensing coil.

2. A sensor according to claim 1 wherein the sensing coil is provided with an open ended ferrite rod core.

3. A sensor according to claim 1 or 2 wherein the generating and sensing coils are mounted on a common plate, the two coils being substantially equidistant from the plate with the sensing coil lying in the return flux path external to the generating coil and having its axis in a direction at right angles to the direction of the axis of the generating coil.

4. A sensor according to any preceding claim wherein the generating coil is enclosed in a cylindrical non-ferrous conductive screen coaxial with the coil.

5. A sensor according to claim 4 wherein the screen incorporates an axial slit whereby it does not constitute a single turn short circuited coil.

6. A sensor according to claim 3 including a rotatable eccentric mounting means for one or both of the coils whereby the positions of the coils can be adjusted relative to one another to obtain a balance condition.

7. A sensor according to claim 6 including one or more ferromagnetic adjustment elements mounted on the plate the position(s) of which relative to the coils are adjustable.

8. A sensor according to claim 3 wherein said plate is a ferrous plate.

9. A sensor according to claim 3 wherein said plate is a non-ferrous metal plate.

10. A null-balanced proximity sensor substantially as described with reference to the accompanying drawings.

11. A position sensing arrangement including a sensor as claimed in any preceding claim and a member movable relative to the sensor, said member incorporating a coded sequence of magnetic and conductive elements which pass sequentially through the sensor flux field during movement of the member relative to the sensor.