GB2542843A

GB2542843A - Inductive position detector

Info

Publication number: GB2542843A
Application number: GB1517382.6A
Authority: GB
Inventors: Whitmore Christopher; Goodman Stephen; Stephens Alexander
Original assignee: Newall Measurement Systems Ltd
Current assignee: Newall Measurement Systems Ltd
Priority date: 2015-10-01
Filing date: 2015-10-01
Publication date: 2017-04-05
Anticipated expiration: 2035-10-01
Also published as: GB201517382D0; GB2542843B

Abstract

An inductive position detector (10) includes a first elongate scale member (12) with a longitudinal axis and a periodically varying dimension perpendicular to the longitudinal axis, and a second member (14) movable relative to the first member (12) along the longitudinal axis. The first member (12) includes a plurality of magnetic material elements such as steel balls (24). The second member (14) comprises first and second transducers arranged in fixed spaced relation in a direction along the longitudinal axis, each transducer having induction means, detection means, and magnet sensing means, for example transmission and pick-up coils and hall effect sensors. At least one magnetic bar (42) is arranged alongside the first elongate member (12), and each magnet (42) spans the connection between a pair of adjacentsegments (16).

Description

INDUCTIVE POSITION DETECTOR

The invention relates to inductive position detectors, particularly of the type originally described in GB 1 513 567 and developments thereof. Such devices are typically attached to machine tools and can be used to determine the position of the machine tool head with respect to a work piece.

Detectors of this type comprise an elongate magnetic element which has a periodically varying dimension in a direction perpendicular to the longitudinal axis of the element. The element is typically a train of steel balls arranged in a line in point contact within a tube. A transducer, which surrounds the tube and travels along its length, is used to induce a magnetic field in the train of magnetic balls. The periodic variations in the dimension of the magnetic element result in detectable periodic variations in the magnetic field and, as such, provide corresponding periodically varying signalling which can be used to determine the relative position of the element with respect to the transducer.

In some applications, the elongate magnetic element, often referred to as the scale, may need to be many metres long. However, it can be very difficult to ship, handle and install very long scales and current products are limited to a scale of about 12 metres in length.

The present invention provides an inductive position detector comprising a first elongate member with a longitudinal axis and an element of magnetic material extending in the direction of the longitudinal axis and having a periodically varying dimension in a direction perpendicular to the longitudinal axis, and a second member movable relative to the first member along the longitudinal axis and comprising induction means to induce a magnetic field in the element of magnetic material and detection means to detect variations in the magnetic field as the second member moves relative to the element of magnetic material; wherein the first elongate member comprises a plurality of segments connectable together to form the first elongate member, each segment connected to an adjacent segment by an attachment means of non-magnetic material, wherein the second member comprises first and second transducers arranged in fixed spaced relation in a direction along the longitudinal axis, each transducer having induction means and detection means and also having magnet sensing means; and at least one magnet arranged alongside the first elongate member, each magnet spanning the connection between a pair of adjacent segments.

By making the scale as a series of separate segments, very long scales can be constructed but shipping, handling and installation are made considerably easier. Use of two transducer heads and the magnetic bar allows the detector to operate seamlessly as it travels along the scale, despite interruptions in the magnetic element created by the nonmagnetic connections between the scale segments.

Preferably, each segment the first elongate member comprises a tube of nonmagnetic material containing a train of magnetic balls arranged in point contact along the longitudinal axis and constrained against relative movement, and the attachment means comprises co-operating mechanical fasteners formed of non-magnetic material for joining adjacent tubes together. The attachment means may further comprise calibration means which is adjustable to constrain the balls against relative movement.

Preferably, the spacing between the first and second transducers is greater than the length of the non-magnetic attachment means. It is also preferable if the length of the magnet is greater than the length of the non-magnetic attachment means.

Each transducer preferably includes two magnet sensing means which are spaced from each other in a direction along the longitudinal axis. Each magnet sensing means preferably comprises a hall effect sensor.

The present invention also provides a method of operating a position detector of the type described above, which comprises moving the second member along the first elongate member, obtaining position information from one or both of the first and second transducers while they are an adjacent part of the element of magnetic material, detecting the magnet using the magnet sensing means, and using information from the magnet sensing means to switch the measuring function between the two transducers as each transducer passes a non-magnetic attachment means.

The invention will now be described in detail, by way of example only, with reference to the accompanying drawings in which:-

Figure 1 is a schematic front view of part of an inductive position detector of the present invention;

Figure 2 is a cross-section of Figure 1 in the plane of the figure itself; and

Figure 3 is a cross-section of part of the scale, showing the attachment of adjacent segments. A position detector 10 in accordance with one embodiment of the present invention comprises an elongate scale 12 with a longitudinal axis. A transducer 14 encircles the scale 12 and can travel back and forth along it.

The scale 12 comprises a plurality of segments 16. A number of segments 16 are joined together end-to-end to form the elongate scale 12. In this example, each segment 16 comprises a tube of non-magnetic material. Attachment means 18 are provided so that an end of one segment 16 may be connected to an end of an adjacent segment 16. In this example, the attachment means 18 comprises a male threaded end cap 20 which can be fitted into the end of one segment 16 and a female threaded end cap 22 which is fitted into the end of the adjacent segment 16. The two segments 16 are then joined by the threading the end caps 20, 22 together. Typically, each segment 16 may be approximately 1.5m long.

Each segment 16 contains a train of magnetic balls 24 which are in point contact with each other along the longitudinal axis of the segment and constrained to prevent relative ball movement. Typically, at least one of the end caps 20, 22 is provided with a calibration screw 26 which can be adjusted in order to constrain the balls 24 in a fixed position.

When the required number of segments 16 has been assembled to form a completed scale 12, the ends are closed by further end caps which may also include calibration screws (not shown).

The transducer 14 comprises two transducer heads 28, 30 which are held at a fixed spacing from each other by a rigid connection of any convenient form. In this example, the transducer heads are 28,30 are secured to a common backing plate 32. The spacing between the two transducer heads 28, 30 is greater than the length of the non-magnetic attachment means 18 between two adjacent tube segments 16.

Each transducer head 28, 30 comprises a casing 34 defining an axial bore 36 which slidably receives the scale 12 so that the transducer 14 can travel back and forth along the scale 12.

In a known fashion, each transducer head 28, 30 comprises transmission coils and pick-up coils (which are not shown). The transmission coils are used to induce a magnetic field along the line of point contact of the magnetic balls 24. The pick-up coils are arranged to detect variations in the magnetic field, as the coils move relative to the balls 24 when the transducer 14 travels along the scale 12. Detector circuitry is used to analyse the signalling to give position information, for example, as described in GB 1 513 567. The transducer heads 28, 30 are joined by a cable 38, which extends to a control panel (not shown) which provides power to the transducer heads 28, 30 and data transmission.

In the present invention, each transducer head 28, 30 further includes a pair of magnet sensors 40, such as hall effect sensors. The sensors 40 are arranged one at each end of each transducer head 28, 30. A magnetic bar 42 is mounted alongside the scale 12, extending axially and arranged to span the connection between adjacent tube segments 16. Where multiple segments 16 are used to create a long scale 12, a magnetic bar 42 is provided alongside each join between adjacent segments 16. The magnetic bar 42 is longer than the length of the non-magnetic attachment means 18. As the transducer 14 travels along the scale 12 and passes the connection between adjacent tube segments 16, the sensors 40 will detect the presence of the magnetic bar 42. As each transducer head 28, 30 passes the nonmagnetic attachment means 18 between adjacent segments 16, there will be a point at which no magnetic field can be induced or detected. However, by detecting the presence of the magnetic bar 42, the measuring function can be switched from one transducer head, as it passes the non-magnetic area, to the other transducer head, which is still alongside some of the magnetic balls 24, in order to provide for continuous measurement. This is described in further detail below.

In the following description, it is assumed that the transducer 14 is moving from left to right in the figures and will approach a join between adjacent segments 16. Accordingly, the transducer head 28 will be referred to as the leading transducer head and the transducer head 30 as the trailing head.

While both transducer heads 28, 30 travel along a part of the scale 12 which contains an uninterrupted train of magnetic balls 24, either or both transducer heads 28, 30 may be active and providing position information. As the leading head 28 approaches the join between adjacent segments 16, the leading sensor 40 will detect the presence of the magnetic bar 42, alerting the system to the approach of the non-magnetic attachment means 18 which will interrupt the train of magnetic balls 24. Upon further travel of the transducer 14, both magnetic magnet sensors 40 in the leading head 28 will detect the magnetic bar 42 and the leading sensor in the trailing head 30 will also start to detect the magnetic bar 42. At this point, the leading head 28 will be switched to an inactive mode and the trailing head 30, which is still adjacent a portion of the magnetic balls 24, will provide the measuring function. Once the leading head 28 has passed the non-magnetic attachment means 18, due to the spacing of the two transducer heads 28, 30, for a short time, the non-magnetic attachment means 18 will be between the two heads 28, 30. At this point, both heads 28, 30 may be actively measuring. The trailing head 30 will then itself travel across the non-magnetic attachment means 18 and the measuring function will be switched back to the leading head 28. When the trailing head 30 has also passed the nonmagnetic attachment means 18, it too may resume a measuring function.

Thus, the magnetic bar 42 and pairs of magnet sensors 40 in the two transducer heads 28, 30 enable the detector 10 to detect the approach and passage of a nonmagnetic area. The measuring function can then be switched back and forth between the two transducer heads 28, 30 as required, in order to provide for continuous measuring and seamless position detection as the transducer 14 travels along the length of the scale 12.

In this way, a position detector with a scale 12 which is very long can be constructed, whilst ensuring that the detector is still able to provide seamless measuring along its length. Use of a simple magnetic bar adjacent the joins in the scale, and magnetic sensors encased within the transducer heads, ensure a simple and robust system.

Claims

CLAIMS:

1. An inductive position detector comprising a first elongate member with a longitudinal axis and an element of magnetic material extending in the direction of the longitudinal axis and having a periodically varying dimension in a direction perpendicular to the longitudinal axis, and a second member movable relative to the first member along the longitudinal axis and comprising induction means to induce a magnetic field in the element of magnetic material and detection means to detect variations in the magnetic field as the second member moves relative to the element of magnetic material; wherein the first elongate member comprises a plurality of segments connectable together to form the first elongate member, each segment connected to an adjacent segment by an attachment means of non-magnetic material, wherein the second member comprises first and second transducers arranged in fixed spaced relation in a direction along the longitudinal axis, each transducer having induction means and detection means and also having magnet sensing means; and at least one magnet arranged alongside the first elongate member, each magnet spanning the connection between a pair of adjacent segments.

2. An inductive position detector as claimed in claim 1, wherein each segment of the first elongate member comprises a tube of non-magnetic material containing a train of magnetic balls in point contact along the longitudinal axis and constrained against relative movement, and the attachment means comprises co-operating mechanical fasteners formed of non-magnetic material for joining adjacent tubes together.

3. An inductive position detector as claimed in claim 2, wherein the attachment means further comprises calibration means which is adjustable to constrain the balls against relative movement.

4. An inductive position detector as claimed in any preceding claim, wherein the spacing between the first and second transducers is greater than the length of the nonmagnetic attachment means.

5. An inductive position detector as claimed in any preceding claim, wherein the length of the or each magnet is greater than the length of the non-magnetic attachment means.

6. An inductive position detector as claimed in any preceding claim, wherein each transducer comprises two magnet sensing means which are spaced from each other along the longitudinal axis.

7. An inductive position detector as claimed in any preceding claim, wherein the magnet sensing means comprises a hall effect sensor.

8. A method of operating an inductive position detector as claimed in any preceding claim, comprising moving the second member along the first elongate member, obtaining position information from one or both of the first and second transducers while they are adjacent the element of magnetic material, detecting the magnet using the magnet sensing means as the transducer approaches a join between adjacent segments of the first elongate member, and using information from the magnet sensing means to switch a measuring function between the first and second transducers as each transducer passes a non-magnetic attachment means.

9. An inductive position detector substantially as hereinbefore described and with reference to the accompanying drawings.

10. A method of operating an inductive position detector substantially as hereinbefore described and with reference to the accompanying drawings.