GB2392503A

GB2392503A - Inductive position detector including a train of magnetic balls

Info

Publication number: GB2392503A
Application number: GB0220365A
Authority: GB
Inventors: Frederick Mark Hudman
Original assignee: Elliott Industries Ltd
Current assignee: Elliott Industries Ltd
Priority date: 2002-09-02
Filing date: 2002-09-02
Publication date: 2004-03-03
Anticipated expiration: 2022-09-02
Also published as: GB2392503B; GB0220365D0

Abstract

An inductive position detector 10 comprises a longitudinally extending scale 20 and a transducer 30 movable along the scale. The scale 20 comprises a tube 21 of non-magnetic material housing a train of magnetic balls 22. The transducer surrounds the tube 21 and comprises transmission and pick up coils. The transmission coils induce a magnetic field along the line of the balls 22 and the pick up coils detect variation in the field as the balls 22 move relative to a pick-up coils. Positional information is found from this variation. The transducer 30 is couped to the ball tube 21 by a thread (40, figure 2). A magnetic marker 23 may be used to act as a datum point. A tool head may mounted on the threaded tube (which forms a lead screw) and may be integral with the transducer.

Description

1 - INDUCTIVE POSITION DETECTORS

The present invention relates to inductive position detectors, particularly of the type described 5 in GB 1513567 and developments thereof. Such devices are typically attached to machine tools and can be used to determine the position of the tool head with respect to a workpiece.

10 Typically, the tool head is mounted on a threaded lead screw, the rotation of which causes the tool head to translate axially along the lead screw, thus altering its position relative to a workpiece. It is known to use rotary position detectors to monitor, 15 directly or indirectly, rotation of the lead screw thus to determine the position of the tool head.

However, rotary detectors have a number of inherent limitations. They cannot compensate for backlash or pitch errors, nor for the effects of thermal 20 expansion. Therefore, linear detectors are preferred which monitor the actual position along the lead screw.

Optical linear detectors are known which require light 25 to pass through or be reflected from a grating or scale. However, such optical detectors are easily damaged or obscured by coolants, swarf etc. which are present in machine tool environments.

30 Inductive position detectors such as described in GB 1513567 do not involve any optical systems and are more robust since they can be entirely sealed, overcoming these disadvantages. Detectors of this type comprise an elongate magnetic element which has a 35 periodically varying dimension in a direction perpendicular to the longitudinal axis of the element.

The element is typically a train of steel balls

2 - arranged in a line, in point contact. A transducer, which surrounds the magnetic element and travels along its length, is used to induce a magnetic field in the

element. The periodic variations in the dimension of 5 the element result in detectable periodic variations in the magnetic field and as such provide

corresponding periodic varying signalling which can be used to determine the relative position of the element with respect to the transducer.

In use, the magnetic element and the transducer must be mounted on the machine tool with the magnetic element exactly parallel to the lead screw and this requires very careful installation. Furthermore, 15 since this type of detector is usually retro-fitted to an existing machine tool, rather than installed during the original assembly, space may be limited and it may be necessary to fit the detector remote from the lead screw, for example on the opposite side of the 20 machine. This further increases the difficulty in ensuring that the axes of the magnetic element and the lead screw are parallel, and in coupling the transducer to the tool head so that movement of the tool head causes a corresponding movement of the 25 transducer.

The present invention provides an inductive position detector comprising a first member having a longitudinal axis and an element of magnetic material 30 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 means to 35 induce a magnetic field in the element, further

comprising means to couple the second member to the first member such that rotation of the first member

- 3 about the longitudinal axis causes the second member to translate axially along the first member.

Preferably, the means to couple the second to the 5 first member comprise a thread formed on the exterior surface of the first member and means to key the second member to the thread.

The means to key the second member to the thread 10 may comprise at least one nut on the second member engageable with the thread.

Advantageously, a reference marker may be provided on the first member to provide a datum point 15 detectable by the second member.

The reference marker may consist of a magnetic marker mounted on the first member and the second member may further comprise a sensor operable to 20 detect the magnetic marker.

The thread formed on the first member may consist of a multi-start thread.

25 The invention also provides a machine tool incorporating an inductive position detector of the type described above, wherein the first member is rotatably mounted on the machine tool and the tool comprises a tool head mounted on the first member for 30 axial movement therealong in response to rotation of the first member.

Conveniently, the second member may be formed integrally with the tool head.

The invention will now be described in detail, by way of example only, with reference to the

- 4 - accompanying drawings in which: Figure 1 is a schematic crosssectional view of an inductive position detector in accordance with the 5 present invention; and Figure 2 is a perspective view of the detector of Figure 1.

10 The position detector 10 as shown in Figure 1 comprises a scale 20 which extends longitudinally, and a transducer 30. The transducer 30 encircles the scale 20 and is movable along the length of the scale 20. The scale 20 comprises a tube 21 of non-magnetic material which houses a train of magnetic balls 22 in point contact and constrained to prevent relative ball movement. The transducer 30 comprises transmission 20 coils and pick-up coils (not shown). The transmission coils are used to induce a magnetic field along the

line of point contact of the balls 22 and the pick-up coils are arranged to detect variations in the magnetic field as the balls 22 move relative the pick

25 up coils. Detector circuitry (not shown) is used to analyse the signalling to give positional information.

Such devices are well known in the art and one such device has been described in GB 1513567. In particular, an overall position value is determined by 30 counting the number of balls 22 which are traversed from a datum point, known as pitch count, combined with the position along each ball 22, known as bit count. The pitch count is a relatively low precision measurement and the bit count is a high precision 35 measurement which is absolute within the pitch.

After assembly of the transmission coils and

- 5 pick-up coils, the transducer head 30 is filled with resin to completely seal it against the ingress of coolants or other fluids it may encounter in use.

5 In accordance with the present invention the tube 21 of the scale 20 has a thread 40 cut into its outer surface. The transducer 30 is secured to the tube 20 in any convenient manner such that rotation of the tube 20 about its longitudinal axis causes the 10 transducer 30 to translate axially along the tube 20.

For example, the transducer 30 may be keyed onto the thread by a nut (not shown)at one or both ends.

A reference marker may be incorporated within the 15 tube 20 which can be picked up by the transducer 30 to provide a datum point from which further measurements are taken. For example, a magnetic marker 23 may be used, inserted between two of the balls 22 in the tube 21. This is detectable by a sensor 24, such as a Hall 20 - effect sensor, provided in the transducer 30.

l The threaded tube 20 may be mounted on a machine tool so as to form the lead screw itself, with the tool head also mounted on the tube 20. Alternatively, 25 a transducer 30 may be integrated within the tool head, providing a single unit to be mounted on the tube 20. In either case, rotation of the tube 20 causes the tool head, and the transducer 30 if separate, to translate along the tube 20 by the same 30 amount. Therefore, with suitable calibration to account for any axial spacing between the transducer and the critical part of the tool head, the transducer can produce a position signal for the tool head.

35 The thread 40 may consist of a single thread or a multi-start thread. This provides a coarser pitch and thus allows greater axial movement of the tool head

- 6 and transducer per revolution of the tube 21, while keeping the thread relatively small and maintaining the strength of the tube 21.

5 By combining the lead screw with the tube 20 of the detector, the measuring axis and the point of measurement are always exactly in the right place.

This allows for ease of installation-and improved accuracy. There are no longer backlash or pitch 10 errors to be compensated for. Furthermore, the system effectively compensates as the lead screw becomes worn over time in use, allowing accuracy to be maintained over the life of the machine tool.

15 The detector is incorporated with the machine tool at the point at the point of original manufacture avoiding awkward-retro fitting and providing a more compact unit overall.

20 The skilled reader will appreciate that the present invention provides an improved inductive position detector but that variations and modifications to the precise details described may be made with out departing from the scope of the claims.

Claims

- 7 Claims

1. An inductive position detector comprising a first member having a longitudinal axis and a element of 5 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 moveable relative to the first member along the longitudinal 10 axis and comprising means to induce a magnetic field

in the element, further comprising means to couple the second member to the first member such that rotation of the first member about the longitudinal axis causes the second member to translate axially along the first 15 member.

2. An inductive position detector as claimed in claim 1, wherein the means to couple the second member to the first member comprises a thread formed on the 20 exterior surface of the first member and means to key the second member to the thread.

3. An inductive position detector as claimed in claim 2, wherein the means to key the second member to 25 the thread comprises at least one nut on the second member engageable with the thread.' 4. An inductive position detector as claimed in any - preceding claim, further comprising a reference marker 30 on the first member detectable by the second member.

5. An inductive position detector as claimed in claim 4, wherein the reference marker comprises a magnetic marker mounted on the first member and the 35 -second member comprises a sensor operable to detect the magnetic marker.

- 8 6. An inductive position detector as claimed in any preceding claim, wherein the thread comprises a multi-

start thread.

5 7. A machine tool in combination with an inductive position detector as claimed in any preceding claim wherein the first member is rotatably mounted on the machine tool and a tool head is mounted on the first member for axial movement therealong in response to 10 rotation of the first member.

8. A machine tool as claimed in claim 7, wherein the second member is integral with the tool head.

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

10. A machine tool substantially as herein before 20 described and with reference to the accompanying drawings. 1 1