GB2506698A

GB2506698A - Detector to measure the relative position of bodies

Info

Publication number: GB2506698A
Application number: GB1222030.7A
Authority: GB
Inventors: Mark Anthony Howard; Darran Kreit
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-10-02
Filing date: 2012-12-07
Publication date: 2014-04-09
Also published as: WO2014053835A2; WO2014053835A3

Abstract

A detector to measure the relative position of bodies along a measurement path comprising a first inductive target 4a attached to a first body 1 and arranged to face a first winding 3a attached to a second body 2; a second inductive target 4b attached to the first body arranged to face a second winding 3b attached to the second body; wherein the targets extend along the measurement path and their extent orthogonal to the measurement path varies such that comparison of the windings' inductances indicates the relative position of the two bodies. A reference winding whose inductance is substantially unchanged by relative motion of the bodies may be provided for comparison to the windings inductance. Attached to the first body may be a third and fourth inductive target arranged to face respective third and fourth windings that may be on the second body and where the pitch of variation in the extent of the third and fourth targets is different to the pitch of variation in the extent of the first and second targets.

Description

DETECTOR

Field of the Invention

This invention rdates to an inductive detector for measuring the relative position of at least two bodies.

Review of the Art known to the Applicant A known form of inductive detector is the linearly vanable differential transformer (LVDT). in which a magnetically permeable core moves relative to primary and secondary windings. Linear forms are typically referred to as LVDTs and rotary forms are is variously referred to as rotary variable differential transformers (RVDTs), synchros or resolvers. Such detectors are typically space inefficient and require lots of fine wire, precisely wound spools. Consequently, they have only limited scope of application due to their high cost, bulk and heavy weight.

Patent US4,737,698 discloses an inductive detector in which an inductive target moves relative to an arrangement of transmit and receive windings. Application of an AC input to the transmit winding results in a modulated output from the receive windings which may be demodulated to provide a signal indicative of the target's position. The electronics required to operate the sensor is complex and therefore expensive.

The authors have previously disclosed various inductive detectors for the measurement of displacement or deformation.

The present invention encompasses the concept of a ow cost, hghtweight, compact, so accurate and robust detector which can detect the relative position of two or more bodies and which is applicable to a variety of geometries. It has particular application to the measurement of displacement in harsh environments and especially the measurement of rotation angle or speed of large diameter or structurally loaded shafts.

Summary of the Invention

Preferably, a detector is provided to measure the relative position of bodies along a measurement path comprising: a first inductive target attached to a first body and arranged to face a first winding attached to a second body; a second inductive target attached to the first body arranged to face a second winding attached to the second body; wherein the targets extend along the measurement path and their extent orthogonal to the measurement path varies such that comparison of the windings' inductances indicates the relative position of the two bodies.

Preferably, the windings' inductances are compared to a reference winding whose is inductance is substantially unchanged by relative motion of the bodies.

Preferably, the targets are made from a material taken from the list: electrically conductive; magnetically permeable.

Preferably, the windings are laminar.

Preferably, the variation in the extent of the targets is continuous.

Preferably, the variation of the targets is sinusoidal.

Preferably, the variation in the extent of the second target is shifted along the measurement path relative to the first target.

Preferably, the indicated position is unique along the measurement path.

Preferably, at least one winding is arranged around a magnetic flux concentrator.

Preferably, a partition is arranged between at east one winding and at least one target wherein the thickness of the partition is less than the partition's electromagnetic skin depth at the detector's operating frequency.

Preferably, the windings are covered by an electrically insulating encapsulant.

Preferably, the target is formed by a groove in a metallic rotor.

Preferably, the detector also comprises a third winding attached to the second body and arranged to face a third inductive target attached to the first body; a fourth winding attached to the second body and alTanged to face a fourth inductive target attached to the first body; wherein the pitch of variation in the extent of the third and fourth inductive targets is different to the pitch of variation in the extent of first and second inductive targets.

Preferably, the variation in the extent of the targets is such that the gap between the targets and the windings varies.

Preferably, the variation in the extent of the targets is across the measurement path.

Preferably, at least one of the windings is wound as a balanced pair.

Preferably, a detector is provided comprising a first target arranged on a shaft to face first and second windings wherein the second winding is shifted along the measurement path from the first winding: a second target alTanged on the shaft and shifted along the axis of the shaft relative to the first target; wherein the second target is alTanged to face a third winding and a fourth winding wherein the fourth winding is shifted along the measurement axis from the third winding; wherein the extent of the targets along the axis of the shaft varies such that comparison of the windings' inductance values uniquely indicates the angular position of the shaft.

Brief Description of the Drawings

Figure 1 shows a first rotary embodiment of the detector wherein the extent of the targets varies radially.

Figure 2 shows a second rotary embodiment of the detector wherein the extent of the targets varies axially.

Figure 3 shows a side elevation and flattened plan view of a winding and target arrangement for angle measurement which excludes the reference winding and target.

Figure 4 shows a side elevation and flattened plan view of an alternative winding and target arrangement for angle measurement which excludes the reference winding and target.

is Figure 5 shows a side elevation and flattened plan view of an alternative winding and target arrangement for angle measurement which excludes the reference winding and target.

Figure 6 shows a side elevation and flattened plan view of an ahernative winding and target arrangement for angle measurement which excludes the reference winding and target.

Figure 7 shows a section of a target and a winding and magnetic flux concentrator encapsulated within a stainless steel housing.

Figure 8 shows a winding in the form of a winding in the form of a balanced pair on a 2-layer printed circuit board.

Description of the Preferred Embodiment

Figure 1 shows a first embodiment of the detector, arranged to measure the angular position of a shaft [I] which rotates about its x) axis and relative to its housing [2]. The detector comprises three windings [3a, 3b & 3c] attached to the housing [2], and three targets [4a, 4b, 4c] attached to the shaft [1] so as to face the windings. Each winding [3a, 3b & 3d is produced in a laminar form on a multi-layer printed circuit board (PCB) with conductive windings arranged as a spiral on an electrically insulating substrate so as to form an inductor. I,6mm thick FR4 grade printed circuit board with 1 ounce copper and plated through holes is suitable. The windings may he produced by a variety of means including winding wire on a spool or bobbin, printed circuit board, stamped or etched metal forms, For most applications, laminar windings produced using multi-layer printed circuit board techniques is preferred as this is a compact, accurate and inexpensive technique which produces windings with tightly controlled electrical and mechanical properties. The windings [3a, 3b & 3d are arranged across and along the measurement path such that they cover a portion of the targets along the measurement path and slightly overlap the targets athng the axis of the shaft. Each target [4a, 4b, 4c] is produced as a ring of conductive or magnetically permeable material so as to form a surface facing a corresponding winding [3a, 3b, 3d. Metallic materials such as copper, stainless steel, carbon steel, ferrite or aluminium are ideally suited. hmulating materials such as plastic, glass or ceramic may be used provided that their surface facing the winding is covered in a conductive or magnetically permeable material such as plated gold or copper. The radial extent of the first two targets [4a, 4b] from the shaft [I] axis continuously varies, in a similar way to a cam or eccentrically mounted ring, so that the gap between each target [4a, 4bj surface and its corresponding winding [3a, 3bj varies according to angular rotation of the shaft [1]. The second target [4b] is shifted by 90 degrees along the measurement path compared to the first target [4a]. The radius of the third target [4c] is constant. In other words, there is a variation in the extent of the first two targets [4a, 4bj in an axis orthogonal to the measurement path whereas there is no such variation in the extent of the third target [4c]. Preferably, the radius of the third target [4c] is between the maximum and minimum radii of the first two targets [4a, 4b]. The maximum gap between the targets' [4a, 4b] surface and winding [3a, 3b] is preferably small compared to the dimensions of the windings [3a, 3b] and target [4a, 4b1 along and transverse to the measurement path. For example, the gap between a winding measuring 25mm diameter and a coresponding mild steel target of average diameter 200mm should preferably vary from a minimum of 0,01 to a maximum of 4,0mm. The windings [3a, 3b, 3c] are energised with an AC signal preferably in the range 5KHz -5MHz by an electronic circuit. Magnetically permeable materials preferably use a lower frequency and conductive materials preferably use a higher frequency. There is a wide variety of electronic circuits which may be used but, preferably, the electronic circuit comprises a power supply, frequency generation circuit, signa' receive circuits, microcontroller and electrical output circuit. As the gap between the target [4a, 4b] and winding [3a, 3b] changes, the windings' inductance changes. If a conductive target is used then the winding's inductance reduces as the gap reduces, If a magnetically permeable target is used then the winding's inductance increases as the gap reduces. The gap between the target [4a, 4b, 4c] and winding [3a, 3b, 3d need not be an air gap. The gap may be tilled, or indeed contaminated, by any material -for example, oil, water or grease -provided it has sufficiently low conductivity and magnetic permeability that it does not unduly interfere with the measurement. The third winding [3cj is arranged so that its inductance does not vary with rotational angle of the shaft [1]. This third winding [3d is used as a is reference inductor against which the inductance of the other two windings [3a, 3b1 may be compared. This helps to produce a stable measurement since inductance values are also influenced by other parameters -notably temperature. Since the reference winding [3d will be similarly affected by parameters other than displacement, this ratiometric method is useful when precision measurement is required. In Figure 1, the reference winding [3d is shown in close proximity to the other two windings but this need not be so. Instead the reference winding [3c] may be located some distance away from the first two windings [3a, 3b] so that space in and around the sensing area is minimised. The inductance value of each of the windings [3a, 3b, 3d is measured by the same electronic circuit multiplexed to each winding. Accordingly, the ratio of the values of the first winding [3aj to the third winding [3c]; and the second winding [3b] to third winding [3cj in this embodiment is unique. Experiments have shown that the inductance value of a winding [3a, 3b, 3c] can be readily measured in <1 millisecond. For many applications, the sequential energisation and measurement of each of the windings [3a, 3b, 3d will produce a sufficiently quick measurement. Preferably, the output signal is an electrical analogue of the relative position of shaft [i] and housing [21 and may be produced in a variety of formats such as RS232, 4-2OmA, O-SV etc. In some apphcations it is desirable to provide low latency control at high speeds as well as measuring position accurately at lower speeds for fine control of position measurement. This can be achieved by using a dual output circuit with a coarse resolution but rapid, analogue output and a high resolution but slower speed, digital output.

Modifications and Further Embodiments Figure 2 shows a second embodiment of the invention. As with the first embodiment, the detector is arranged to measure the angle of rotation of a shaft [1] relative to its housing [2]. In this embodiment, the extent of the first two targets [4a, 4b] varies axially (rather than radially) and the axial extent of the third target [4c] is constant. In other words, there is a variation in the extent of the first two targets [4a, 4b1 in an axis orthogonal to the measurement path whereas there is no such variation in the extent of the third target [4c].

As the axial extent of the first two targets [4a, 4b] facing the corresponding winding [3a.

3b] varies, the inductance of the winding vanes. If the target is conductive then is inductance reduces in proportion to axial extent of the target. If the target is magnetically permeable the inductance increases in proportion to axial extent of the target. In this arrangement the first two targets [4a, 4b], are shifted by 180 degrees relative to each other along the measurement path. In this alTangement, position measurement is absolute over degrees. The form of targets and windings shown in Figure 2 are such that a generally linear variation will be produced. lii such an arrangement, it is helpful to set the zero and span of the detector by taking readings at either end of the measurement scale, storing these values in the detector's memory and producing an electrical output which is scaled accordingly. For higher accuracy applications, further calibration points may be set by measuring against a reference device such as a precision mechanical jig or fixture and the corresponding readings stored in memory.

Figure 3 shows a simplified side elevation and flattened out plan view of the windings and targets of the second embodiment -showing how the extent of the targets [4a, 4b] vanes with rotational angle. The reference winding [3d and target [4c] are excluded from Figure 3 for clarity. Preferably, the winding [3a, 3b] overlaps the targets [4a, 4b] by >10% of the target's extent along the x-axis. This helps negate the effects of small displacements of the shaft [1] along the x-axis. due to effects such as bearing clearances or thermal expansionlcontraction.

Variations in the extent of the targets which are symmetrical about the centre of windings are generally preferred as this gives maximum sensitivity. This is sometimes not practical. Figure 4 shows! (fl alternative method of producing the targets! (and the variation therein. In this arrangement, variation in extent of a sted shaft's surface is achieved so as to produce inductive targets by machining a groove at an angle to the main shaft path.

This is an inexpensive technique and suitable for those applications where, for example, the mechanical strength of the shaft must be maintained as far as practical. The groove may also act as a grease way for lubrication purposes. The reference winding [3d and target [4c1 are excluded from Figure 4 for clarity. k harsh environments the gap may be filled with a material such as epoxy or plastic which has no inductive signature, so as to avoid ingress or entrapment of foreign objects. Preferably, the depth of the groove is such is that the material at the bottom of the groove has little or no effect on the inductance of the winding, compared to the surface of the target facing the winding.

Figure 5 shows a schematic of an alTangement which uses targets whose extent varies according to a trigonometric function along the measurement path. The first target (4a) is arranged as a repeating sinusoidal pattern, repeating at a pitch distance of 180 degrees, and the second target (4b) is shifted along the measurement path so as to form a target whose extent varies inversely. In this arrangement 4 windings and 2 targets are deployed to provide high measurement performance. If the pitch distance of the target is x the two sets of windings should preferably be separated by x14. Absolute measurement is maintained over x. As will be appreciated by those skilled in the art, position may be calculated using a vanety of methods but an arctangent calculation of ((3a+)-(3a- ))I((3b+)-(3b-)) is readily and elegantly accomplished. Such an arrangement offers superior measurement performance compared to arrangements with fewer windings.

The concept of using multiple targets can be extended to enable high resolution measurement over extended distances. This can be achieved by using a coarse pitch set of targets and windings in combination with a fine pitch set of targets and windings. For example, a coarse set of windings and targets may have a sing'e pitch over the measurement scale whereas a fine set of targets and windings has a repetitive pitch over the measurement scale. The fine pitch arrangement enables high resolution but ambiguous measurement whereas the coarse arrangement enables lower resolution determination of absolute position and hence of which pitch is in operation. Combining the two measurements enables high resolution measurement over extended scales, An alternative to the coarse and fine arrangement is a Vernier arrangement as shown in Figure 6, whereby a first pattern of windings and targets is used in tandem with a second pattern of windings and targets with a slightly different pitch. Accordingly. absolute position may be calculated over the lowest common multiple of the pitches.

For good environmental protection, the windings may be encased in a housing.

Preferably, this is made from a low conductivity material, such as plastic or stainless sted and forms a partition between the windings and targets. An example is shown in section in Figure 7. Typically, the thickness of the partition [7] between the winding [3] and the target [4] should be kept as thin as practical -typically <1,0mm for most applications.

An energization frequency should be chosen which enables the winding's inductive field to project through the skin depth of low conductivity metal materials such as stainless steel -typically <30kHz. Structural integrity of such arrangements can be improved by filling any cavity with a material such as a 2 part epoxy encapsulant [6]. Preferably, the encapsulant is secured by re-entrant features [9] in the housing such as undercuts or blind holes. Such monolithic constructions are helpful in locating and securing the connecting wires [8] between the winding [3] and the corresponding electronics circuit. In order to increase the sensitivity of the detector, the winding [3] may be wound around a flux concentrator [5] such as a ferrite cylinder.

Preferably, the windings are formed by two counter-wound loops so as to form a balanced pair as shown in Figure 8. In this way any noise due to electromagnetic emissions from a far field source entering the windings is negated. This arrangement produces a detector with high electromagnetic immunity.

The detector is useful in measuring position in a variety of geometries including but not limited to rotary, linear and curvi-linear forms.

Claims

CLAIMSClaim 1. A detector to measure the relative position of bodies along a measurement path comprising: a first inductive target attached to a first body and arranged to face a first winding attached to a second body; a second inductive target attached to the first body arranged to face a second winding attached to the second body; wherein the targets extend along the measurement path and their extent orthogonal to the measurement path varies such that comparison of the windings' inductances indicates the relative position of the two bodies.Claim 2. The detector of Claim 1, wherein the windings' inductances are compared to a is reference winding whose inductance is substantially unchanged by relative motion of the bodies.Claim 3. The detector of Claim I, wherein the targets are made from a material taken from the list: electrically conductive; magnetically permeable.Claim 4. The detector of Claim I, wherein the windings are laminar.Claim 5. The detector of Claim 1, wherein the variation in the extent of the targets is continuous.Claim 6. The detector of Claim 1, wherein the variation of the targets is sinusoidal.Claim 7. The detector of Claim 1, wherein the variation in the extent of the second target is shifted along the measurement path relative to the first target.Claim 8. The detector of Claim I, wherein the indicated position is unique along the measurement path.Claim 9. The detector of Claim 1, wherein at least one winding is alTanged around a magnetic flux concentrator.Claim 10. The detector of Claim I, wherein a partition is arranged between at least one winding and at least one target wherein the thickness of the partition is less than the partition's electromagnetic skin depth at the detector's operating frequency.Claim 11. The detector of Claim 1, wherein the windings are covered by an electrically insulating encapsulant.Claim 12. The detector of Claim i, wherein the target is formed by a groove in a metallic rotor.is Claim 13. The detector of Claim i, further comprising: a third winding attached to the first body and arranged to face a third inductive target attached to the second body, a fourth winding attached to the first body and arranged to face a fourth inductive target attached to the second body, wherein the pitch of variation in the extent of the third and fourth inductive targets is different to the pitch of variation in the extent of first and second inductive targets.Claim i4. The detector of Claim 1, wherein the variation in the extent of the targets is such that the gap between the targets and the windings varies.Claim 15. The detector of Claim 1, wherein the variation in the extent of the targets is across the measurement path.Claim 16. The detector of Claim 1, wherein at least one of the windings is wound as a balanced pair.Claim 17. A detector comprising: a first target alTanged on a shaft to face first and second windings wherein the second winding is shifted along the measurement path from the first winding; a second target arranged on the shaft and shifted along the axis of the shaft relative to the first target; wherein the second target is arranged to face a third winding and a fourth winding wherein the fourth winding is shifted along the measurement axis from the third winding; wherein the extent of the targets a'ong the axis of the shaft varies such that comparison of the windings inductance values uniquely indicates the angular position of the shaft.Amendments to the claims are made as follows:CLAIMSLA detector to measure the relative position of bodies along a measurement path comprising; a first mducbve target attached to a fir st body and arranged to face a first windiag attached to a second body; a second inductive target attached to the first body arranged to face a second.winding attached to the second body; wherein the targets extend along the measurement path arid their extent in a direction orthogonal to the measurement path varies such that comparison of the windings' inductances mdicates the relative position of the two bodies; and wherein a partition is arranged between at least one winding and at least one target.
2. The detector according to claim 1, wherein the windings inductances are compared to a reference winding whose inductance is sI3bstantially unchanged by relative motion of the bodies.
3. The detector acwrding to either claim I or claim 2, wherein the targets are made froni a material r then from the list: electrically conductive; magnetically permeable. r
4. The detector according to any of' the preceding claims, wherein the windings are laminar.
5. The detector according to any of the preceding claims, wherein the variadon in the extent of the targets is continuous,
6. The detector according to any of claims 1 to 4, wherein the variation of the targets is sinusoidal.
7. The detector according to any of the preceding claims, wherein the variation iii the extent of the second target is shifted along the measurement path relative to the first target.
8. The detector according to any of the preceding claims, wherein the indicated position is unique along the oleasuretnent path.
9. The deiector according to any of the preceding claims, wherein at least one winding is arranged around a magnetic flux concentrator,
10. The detector according to any of the preceding claims, wherein the thickness of the partition is less than the partitions electromagnetic skin depth at the detecto?s operating frequency.
11. The detector according to any of the preceding claims, wherein the windings are covered by an electrically insulating encapsulant.
12, The detector according to any of the receding claims, wherein the target is formed by a groove in a metallic rotor,
13. The detector according to any of the preceding claims, fi3rther comprising: a third winding attached to the first body and arranged to face a third inductive target attached to the second body, a fourth whidiog attached to the first body and ammged to face a fourth inductive 0') target attached to the second body, wherein the pitch of variation in the extent of the third and r fourth inductive targets is different to the pitch of variation in the extent of first and second inductive targets. r14. The detector according to any of the preceding claims, wherein the variation in the extent of the 0 targets is such that the gap between the targets and the windings varies, 15. The detector according to any of the preceding claims, wherein the variation in the extent of the targets is across the measurement path.I 6. The detector according to any of the preceding claims, wherein at kast one of the wiadings is 17, A detector comprising: a first target arranged on a shaft to face first and second windings wherein the second winding is shifted abug the measurement path from the first winding; a second target arranged on the shaft and shifted a'ong the axis of the shaft relative to the first target; wherein the second target is arranged to face a third winding and a iburth winding wherein the fourth winding is shifted along the measurement axk front the third winding; wherein the extent of the targets along the axis of the shall varies such that comparison of the windings inductance values uniquely indicates the angular position of the shaft. r rN