WO1991009277A2

WO1991009277A2 - Position sensor

Info

Publication number: WO1991009277A2
Application number: PCT/GB1990/001928
Authority: WO
Inventors: Neil Stuart Craigie
Original assignee: British Technology Group Ltd
Priority date: 1989-12-11
Filing date: 1990-12-11
Publication date: 1991-06-27
Also published as: WO1991009277A3; JPH05506091A; DE3940894A1; DE3940894C2; EP0505422A1

Abstract

The invention relates to a position sensor for two relatively movable parts of which one part is connected to at least one coil and the other part is connected to at least one element arranged close to the coil so as to influence the magnetic field of the coil in dependence on the position of the element. Such a position sensor can be constructed in a highly compact form and integrated in or with an actuating cylinder, if at least two coils are present, one of which acts as a primary or driving coil and the other acts as a secondary or sensor coil, with the two coils being magnetically coupled together at least in part by the said element and the element being such that it exhibits a graduated permeability and/or electrical conductivity in its direction of movement.

Description

POSITION SENSOR

The invention relates to a position sensor for two relatively movable parts of which one part is connected to at least one coil and the other part is connected to at least one element arranged close to the coll so as to influence the magnetic field of the coll 1n dependence on the position of the element.

Known position sensors of this kind consist, for example, of a coll into which a core dips to a greater or lesser degree in dependence on the movement of the two parts. In this way the inductance of the coll arrangement Is changed and an inductive signal dependent upon the position of the core is received. A disadvantage of a position sensor of this kind lies in the fact that such an arrangement can be integrated into an operating cylinder only at considerable expense In terms of apparatus.

It is an object of the invention to provide a position sensor of a kind which can be integrated in simple manner, constructionally, into an adjustable drive mechanism, and which allows, for a small outlay in terms of apparatus, precise detection and determination of position.

This object is achieved in a position sensor according to the invention, by the provision, connected to one of two relatively movable parts, of at least two coils, of which one is formed as a primary or drive coll and the other as a secondary or sensor coil, the two coils being magnetically coupled together at least in part by an element connected for movement with the other of the two parts and exhibiting a graduated permeability and/or electrical conductivity in the direction of movement.

This position sensor is easily installed in a drive mechanism, for example a hydraulic or pneumatic actuating cylinder, by arranging the two coils as rings around the actuating rod of the cylinder and providing that the actuating rod exhibits a permeability which is graduated in the direction of movement. This can be brought about, for example, by giving the actuating rod a slightly conical form and overlaying the conical section with an inversely conical outer layer made, for example, of nickel, so that the cylindrical shape of the actuating rod is preserved overall. The arrangement of the sensor coils as rings around the actuating rod is only one of the possible ways of achieving the desired result. The graduated-permeabillty element co-operating with the sensor colls may, for example, be a separate rod-shaped element extending through the colls and being mounted on the control rod parallel therewith and for movement therewith in their common axial direction.

The connections of the drive coil and the sensor coil are linked to control and evaluation circuitry which supplies the drive coil with an alternating current voltage and evaluates the signal induced in the sensory coll.

If the drive and sensory coils are disposed sufficiently close to each other and the gaps between them and the actuating rod of the cylinder are small enough, very large changes may be made to occur 1n the induced voltage in the sensor coil in dependence on the position of the actuating rod relative to the cylinder casing.

The evaluation circuitry may be linked to programmable control means which in turn controls valves which actuate the cylinder. The compact design of the position sensor also enables it to be installed in very small adjustable drive mechanisms and hydraulic cylinders.

It is possible to linearise the voltage induced in the sensor coil, as a function of actuating rod position, by appropriate selection of the rate of increase in the thickness of the conical outer graduated layer. The required rate of increase in thickness as a function of axial position can be determined in advance by computer or by calibration measurements. In production, the corresponding profile can be applied by a computer-controlled machine tool to an actuating rod made for example from V2A steel, and the nickel layer, to be given a shape complementary to the generally conical profile of the steel rod, may be applied by means of a computer-controlled immersion during a subsequent electrolytic deposition process. Production may also be carried out by application to the cylindrical actuating rod of a special surface treatment which results 1n the basically conical external graduation of the permeability, for example by hardening with varying depth of penetration, possibly by means of an appropriately controlled laser hardening process.

Since the coupling region between the coils and the actuating rod Is situated inside the cylinder casing, the entire arrangement 1s very well shielded against electro-magnetic interference; as a result, it 1s possible to take reliable measurements of the position.

The principle of the position measurement system in accordance with the invention may be described mathematically as follows. With two coils arranged adjacent one another and coupled to each other through an element having two superimposed metal layers of different eddy current skin depth, the eddy current skin depth δ may be defined as follows:

1 δ = trfσμ where f = eddy current frequency σ = electrical conductivity μ = magnetic permeability The voltage V_s induced in the sensor coil is dependent on the current i_τ through the driving coil in accordance with the following formula:

V_s = 2 . f. M₁₂.i_τ where M-_|2 = mutual inductance between driving and sensor coils. V_s is measured by evaluation circuitry with high input impedance, providing a relatively temperature-independent measurement signal. If the thickness of the outer metal layer is less than 507. of the eddy current skin depth of that layer and the inner layer of the actuating rod has a greater eddy current skin depth, the result is a stronger dependence of the mutual inductance M₁₂ and on the thickness of the outer layer. By having only a small gap between the coils and a small gap between the coils and the actuating rod, not only can the size of the sensor be kept very small but in addition a stronger measurement signal can be obtained. A further reduction in the size of the sensor can be achieved if the two-coil arrangement takes the form of a multi-strand winding on a ferrlte ring core. The sensor coll is then one strand of this multi-strand winding. The measurement signal obtained is further improved if a ferrite ring core with a focussing effect is used.

An adjustable drive mechanism or hydraulic actuating cylinder controlled by a position sensor and evaluation circuitry is especially suited to the operation of valves, by reason of its high level of precision.

The use of polyvinyl fluoride as the outer layer of the actuating rod is particularly advantageous, since this material has excellent elasticity and can thus undertake sealing operations and furthermore allows, by reason of its favourable electro-magnetic qualities, a very small gap between coil and actuating rod.

Advantageously, the graduation of the permeability of the element can be achieved by means of suitable surface treatment which, for example, causes a structural change. This likewise does not alter the circular cross-section of the cylindrical control rod.

The invention is further described below with reference to embodiments thereof illustrated by the accompanying drawings, in which:-

Figure 1 shows a partly cut away longitudinal section of an actuating cylinder with an integrated position sensor according to the invention;

Figure 2 shows a partly cut away longitudinal section of another actuating cylinder with a mechanically connected position sensor according to the invention; Figure 3 shows an evaluation circuit for the position sensor controlling an actuating cylinder as shown in Figure 1 or 2; and

Figure 4 shows a cross-sectional Illustration of a ferrite ring core for the driving and sensor coils of such position sensor.

In the operating cylinder shown in part in Figure 1, which may be, for example, a pneumatic or hydraulic cylinder and which is hereafter referred to as a hydraulic cylinder 10, a piston 12 is axially slidably mounted in a cylinder casing 14. In the centre of the piston 12, axial actuating rods 16, 18 are secured for actuating position setting members. The actuating rod 18 extends at the longitudinal end of the cylinder casing 14 through a guideway 20, so that the actuating rod 18 can move axially with only a very narrow radial play. The guideway 20 1s also provided with a seal (not illustrated), in order to seal the pressure fluid tightly inside the pressure chamber 22 between the piston 12, the cylinder casing 14 and the guideway 20. The supply of the pressure fluid to and from the pressure chamber 22 is regulated by means of an electrically actuable valve 24, which 1s arranged in a pressure-fluid line 26 connecting the pressure chamber 22 with a source of pressure fluid. Also connected, flanged for example, to the guideway 20 is a ring with a circular coil arrangement 28 which surrounds the actuating rod 18. The coil arrangement 28 consists of a ferrite ring core 30, e.g. with a preferably T-shaped profile, and two coils 32, 34. These coils are laid out as a driving coil 32 and a receiving coil 34 and are arranged coaxially in relation to the actuating rod 18. The coils 32, 34 are fitted over the actuating rod 18 with an interposed plastics layer 36, in order to avoid damage to the coils as the actuating rod slides through. The gap between the coils 32, 34 and the actuating rod 18 is between 0.1 and lmm. Preferably the gap between the coils is also small, less than lmm. The actuating rod 18 consists of a conically tapering, solid inner part 38 extending from the piston 12 outwards and made from V2A steel for example, together with a surrounding hollow cylindrical outer part 40 of which the inner surface fits the cone-shaped inner part 38 exactly. By means of these two complementarily conically shaped parts 38, 40 the actuating rod 18 retains the external shape of a circular cylinder.

The outer part 40 may, for example, be of nickel, the eddy current skin depth of which differs greatly from that of V2A steel. The eddy current penetration depth of the nickel 1s substantially less than that of the V2A steel. The thickness of the outer part 40 is approximately 1 μ at the piston and approximately 50 μm at the opposite end from the piston. With a frequency of between 100 and 200 kHz for the alternating current in the driving coil 32, the induced voltage in the sensor coil 34 varies by, for example 50% in dependence on the position of the piston 12 and thus of the actuating rod 18. By way of example, this may be the case if the rod has a diameter of 20 mm and the coils have a clearance of approximately 0.5 mm, the gap between the coils being approximately 0.3 mm. An appropriately dimensioned coll system can thus provide a position indication for the actuating rod which is accurate to within about 1% for a piston stroke of between 20 mm and 500 mm. The measurement signal received can 1n turn be applied by means of evaluation and control circuitry as described below, for the purposes of the precise control of the hydraulic cylinder 10.

Figure 2 shows a hydraulic cylinder 42 similar to the hydraulic cylinder 10 in Figure 1, with essentially identical parts which, accordingly, are indicated by the same reference numerals. The difference from the arrangement shown in Figure 1 results from the fact that the actuating rod does not itself act as the coil core with a graduated eddy current skin depth, but that, instead, a position sensor 43 is connected to an actuating rod 44 of the hydraulic cylinder 42 by a linking element 46. This position sensor 43 is again identical in its constituent parts to the arrangement shown in Figure 1, being made up of a fixedly located coil arrangement 28 and, carried by the linking element 46 so as to move with the actuating rod 44, a core rod 48, which, like the actuating rod 18 of Figure 1, is axially movable through the coil arrangement 28 and consists of two complementarily cone-shaped coaxial parts. In the configuration shown in Figure 2 the position sensor is not integrated within the hydraulic cylinder 10 as in the case in Figure 1, but may easily be removed from the hydraulic cylinder and exchanged, or installed 1n other apparatus, should that be necessary. Figure 3 illustrates schematically a signal evaluation circuit and control system 50 for the position sensors shown in Figures 1 and 2. The evaluation circuit and control system 50 is based on a single chip computer 52, for example, an 8-bit computer, provided with an output 54 which applies an excitation signal causing the circuit 56 to deliver an output signal, at predetermined amplitude and frequency, to the driving coil 32 of the position sensor 28. The V_s signal Induced 1n the sensor coil 34, with an amplitude dependent on the position of the actuating rod 18 or 48, is amplified in a receiver circuit 57 and applied via an input 58 to an analog-digital converter section of the microprocessor 52. The microprocessor 52 has, in addition, inputs and outputs for an interface or bus connection 60, through which the processor can engage in an exchange of data with a programmable control system. The signal at the Input 58 to the microprocessor 52 may then be checked by way of a theoretical/actual comparison in relation to the control of the hydraulic cylinder, for which purpose the microprocessor 52 has two outputs 62, 64 for drive circuits, by which the electrically operated valves or magnetic valves 24 can be actuated. In order to minimise temperature drift the drive circuit 56 may be equipped with a section ensuring a constant current. The microprocessor timing frequency ensures a constant frequency of the signal at the output 54. If the evaluation circuit is to be used to take very precise measurements over a broad temperature range, a corrective algorithm can be programmed into the microprocessor to compensate for any temperature variations in the circuit 56 and 57 and the coils 32, 34.

The entire circuit 50 can be accommodated by using surface mount device (SMD) technology, for example, in a container having dimensions of, say, 50 mm x 50 mm x 15 mm and may be connected directly to the sensor coil system or housed in a suitable part of the structure of the hydraulic or pneumatic cylinder. The hardware costs for the circuit 50 can be kept very low.

Figure 4 shows the cross-section of a ferrite ring core 70 which enables the magnetic field to be focussed in the axial direction of the colls.

The ferrite core 70 is seen 1n its axial cross-section to have a truncated conical shape, and preferably the sensor coil 34 is mounted surrounding the small-diameter of the ferrite core, with the driving coil 32 surrounding the large-diameter end.

Claims

1. Position sensor for two relatively movable parts of which one part is connected to at least one coil and the other part is connected to at least one element arranged close to the coll so as to influence the magnetic field of the coll in dependence on the position of the element, characterised in that at least two coils (32, 34) are provided, of which one (32) is formed as a primary or driving coil and the other (34) as a secondary or sensor coil, the two colls (32, 34) are magnetically coupled together at least 1n part by the said element (18, 48), and the element (18, 48) exhibits a graduated permeability and/or electrical conductivity in its direction of movement.

2. Position sensor according to Claim 1, characterised in that the element (18, 48) comprises at least two materials (38, 40) of different permeability and the assembly together of the materials in the region of the surface of the element (18, 48) varies progressively 1n the direction of movement.

3. Position sensor according to Claim 1 or 2, characterised in that the two coils (32, 34) are arranged closely beside one another in the direction of movement of the element (18, 48).

4. Position sensor according to any of the foregoing Claims, characterised 1n that the element (18) is constituted by an actuating rod of a pull-and/or-push cylinder (10), and the coils (32, 34) are rigidly connected to the cylinder (10).

5. Position sensor according to Claim 4, characterised in that the coils (32,_. 34) are arranged close against the outer circumference of the actuating rod, at least partially in mating relationship therewith.

6. Position sensor according to Claim 4 or 5, characterised in that the actuating rod consists of two parts (38, 40) joined together and concentric with one another, and made of materials with different permeability and/or conductivity, of which the diameter proportions vary progressively in the axial direction.

7. Position sensor according to Claim 6, characterised in that the boundary surface between the parts (38, 40) has the shape of a conical surface.

8. Position sensor according to Claim 6 or 7, characterised in that the material of the inner part (38) has a greater eddy-current skin-effect depth than the material of the outer part (40).

9. Position sensor according to any of Claims 6 to 8, characterised in that the thickness of the material of the outer part (40) is less than half the eddy-current skin-effect depth of that material .

10. Position sensor according to any of the foregoing Claims, characterised in that the coils (32, 34) are arranged in the form of a multistrand winding of which one strand constitutes the sensor coil (34).

11. Position sensor according to any of the preceding Claims, characterised in that the coils (32, 34) have a ferrite ring core (30).

12. Position sensor according to Claim 11, characterised in that at least the ferrite core of the driving coil is disposed in a focussing manner.

13. Position sensor according to any of the foregoing claims, characterised in that one of the materials of the said element, especially the outermost material thereof, is or contains nickel or polyvinyl difluoride.

14. Position sensor according to any of Claims 1 to 12, characterised in that the graduation of the permeability of the said element (18, 48) results from a suitable processing of the surface which produces, for example, a change in the structure.

15. Position sensor according to any of Claims 1 to 14, characterised in that the circular cross-section of the cylindrically shaped actuating rod (18) is not altered by the graduation.