GB2204691A

GB2204691A - Position measurement

Info

Publication number: GB2204691A
Application number: GB08710782A
Authority: GB
Inventors: Vicq Van Oosthuizen Andrew De
Original assignee: Advanced Manufacturing Technology
Current assignee: Advanced Manufacturing Technology
Priority date: 1987-05-07
Filing date: 1987-05-07
Publication date: 1988-11-16
Also published as: GB8710782D0

Abstract

There is disclosed a method of and apparatus for determining the position of one member (such as a lathe saddle) (13) relative to a second member (such as a lathe bed) (12), wherein a strip (14) of material is applied to the second member (12) and a write head (15) on the one member (13) is supplied, during relative movement between the two members, with accurate information indicating the position of the one member relative to the second member (12), the write head (15) writing corresponding information onto the strip (14) of write/read material. Thereafter a read head (15) on the one member (13) can, during relative movement of the members, indicate the position of each relative to the other. <IMAGE>

Description

POSITION MEASUREMENT This invention relates to the measurement of the position of an article relative to another article. The invention is specifically described in relation to the measurement of the position of a machine tool carriage relative to a machine tool bed, but there are many applications, for example the measurement of the position of a crane trolley relative to a boom, the position of a rotatable member, as a telescope support, relative to a base, in fact any situation where the relative positions of a pair of relatively movable articles needs to be measured accurately.

Heretofore the measurement of the position of a lathe saddle relation to its bed has been achieved by providing a scale on the bed and having a reading head on the saddle which reads the scale and provides positional and directional information to a processor. This information can be used either to determine where the saddle is or to move the saddle to a predetermined position by the processor activating a drive.

The provision of a scale on an article such as a lathe bed presents various problems. Firstly, the presence of oil, swarf and contaminants in an industrial environment means that optically readable scales, as in the manner of bar code scales cannot be used. Magnetic tape clearly is unsuitable where quantities of ferromagnetic material are present.

It is known to use induction scales wherein inductive formations are embedded in a matrix such as plastics strip fastened to the lathe bed. However, such strips are often available in only short lengths, for example, one or two metres and there can be discontinuities at the joints. Such known method also requires that the apparatus be calibrated after installation by use of external equipment. Usually, a laser arrangement is used to provide exact measurements of a saddles position along the bed and accurate signals defining such position are compared with actual signals generated by the read head as it travels along the track. Even with this equipment, however, strip joint discontinuities still give rise to errors which can not be wholly eradicated.

It is an object of the present invention to provide an improved method of measuring the position of one article relative to another.

Accordingly the invention provides a method of measuring the position of a first member relative to a second member including the steps of: providing writable/readable material on the second member; providing a writing head on the one member in register with said material; generating information accurately reflecting the position of the first member during travel of the first member relative to the second member; feeding that information to the writing head on the one member to cause the writing head to operate to write on the material on the second member during travel of the first member relative to the second member; providing a reading head on the one member capable of reading the writing on the material and generating signals representative of the position of the one member relative to the other.

The writing head and the reading head can be one and the same instrument providing both functions. Alternatively, after the initial writing operation the writing head can be replaced by a reading head provided care is taken to ensure that the replacement head is an exact positional replacement.

The accurate information can be generated whilst moving the first member from one end to the other of its path and measuring such member accurately by temporary external means.

In the case where the one member is a lathe carriage and the second member is a lathe bed, the temporary measuring means can include a laser mounted to direct a beam parallel to the lathe bed to impinge upon a remote interferometer mounted on the carriage as well as a reference retro-reflector mounted on a fixed part of the lathe bed.

Electronic signals generated in the temporary measuring means can be fed electrically to the write head on the saddle and activate it to write on the material as the saddle travels down the bed.

In a rotational system some other angular position measuring means can be used.

The readable/writable medium can be CD MATERIAL and the reading/writing head can be a laser.

Alternatively, in a suitable environment, for example where dirt and loose ferromagnetic material is not around; a length of magnetic tape can be used as the readable/writable material. In some cases, an optical reader could be used, the writing being generated by a printer or like marker generating optically readable characters.

When the read head is operating in practise it will need to "know" the direction in which the one member moves.

Such directional information can be supplied to circuitry associated with the head from a drive such as an electric motor, but desirably the writing is itself such as to generate directional information as the head moves along it.

For example by writing two tracks with a 900 phase shift so that a logical circuit in the read head can establish the direction of travel from the sequence of rising and falling edges (as is standard practise in optical encoders). The invention also provides, in apparatus which includes a first member movable relative to a second member, the method which includes the steps of: providing on the second member writable/readable material; providing a write head on the first member; generating signals representative of the position of the first member as it travels relative to the second member.

feeding those signals to the write head on the first member to actuate it, as the first member moves relative to the second member, to write a scale onto the material; providing a read head on the first member; and causing the read head to read the scale to generate signals representative of the position of the first member relative to the second member.

The invention further provides apparatus including a first member movable relative to a second member, the second member having a scale which can be read by a read head on the first member to generate signals representative of the position of the first member relative to the second member, the scale having been generated by providing writable/readable material on the second member; and generating signals representative of the position of the first member relative to the second member as it travels relative to the second member and feeding these signals to a write head on the first member to cause it to write a scale on the material.

As used herein the term scale includes any series of readable markings such as visible readable markings (for example bar codes), magnetically readable markings (for example magnetic tape written and read by tape heads or digitally) or laser readable markings (for example COMPACT DISK MATERIAL).

The invention will be described further, by way of example, with reference to the accompanying drawings, wherein the single figure is a schematic view illustrating a preferred method and apparatus of the invention.

The preferred method of the invention will be described in relation to a machine tool such as a lathe 10 having a headstock 11, a second member in the form of a bed 12 and a first member in the form of a movable carriage 13.

A strip 14 of CD MATERIAL is secured to extend longitudinally along the bed 12 and a write/read laser head 15 is provided on carriage 13 and positioned to traverse strip 14 as the carriage travels. In conventional manner positional-representative signals are generated by a laser interferometer system (separate from the machine 10 and only used in setting up). A laser head 16 directs a beam onto a remote interferometer 17 on carriage 13 and a retro-reflector 19 on headstock 11.

The signals generated by interferometer are fed via an electrical or like connection 18 to head 15 which is thereby activated to write a scale on the strip 14 as the carriage 13 moves along bed 12. Because the signals fed to the head 15 are precisely representative of the carriage position the writing on the strip 14 becomes a precisely calibrated scale.

After the creation of the scale, the interferometer 16 is removed and the head 15 thereafter merely reads the scale on strip 14, generating signals which can be used to control the position of the carriage 13 in normal use. The signals produced can be fed to a display to be read and/or to circuitry for processing. The lathe 10 will usually be numerically controlled.

The head 15 has been described as a dual write/read head. Clearly, the "write" facility is used only once (upon scale creation and calibration) and a "write" laser head is more expensive than a "read" only laser head. For this reason it would be economically effective to use a write only head for scaling and calibration, and to replace it with a read-only head for normal use. However, the physical replacement must be done in such a way as to reduce the possibility of errors being introduced in the change-over.

The scale writing on the strip will, conventionally be written to contain an indication of direction of travel readable by the head 15.

The invention is not limited to the precise details of the foregoing and variations can be made thereto. For example, the strip material can be magnetic, where conditions of use permit and written and ready by a magnetic head. An optically readable scale could also be provided where deterioration or contamination is unlikely.

The external position signal generating means can take any convenient form other than a laser interferometer.

The invention can be applied to a first member travelling in a circular or other non-linear path, but the external measuring means cannot be a straight beam laser interferometer, some other means must be employed.

Many other variations are possible.

Claims

1. A method of measuring the position of a first member relative to a second member including the steps of: providing writable/readable material on the second member; providing a writing head on the one member in register with said material; generating information accurately reflecting the position of the first member during relative movement of the two members; feeding that information to the writing head on the one member to cause the writing head to operate to write on the material on the second member during relative movement, of the two members; providing a reading head on the one member capable of reading the writing on the material and generating signals representative of the position of the one member relative to the second member.

2. A method as claimed in claim 1 wherein the writing head and the reading head are one and the same instrument providing both functions.

3. A method as claimed in claim 1, wherein after the initial writing operation the writing head is replaced by a reading head.

4. A method as claimed in claim 1, 2 or 3 wherein the accurate information is generated whilst moving the first member from one end to the other of its. path and measuring such member accurately by temporary external means.

5. A method as claimed in claim 4 wherein the one member is a lathe carriage and the second member is a lathe bed, the temporary measuring means including a laser mounted to direct a beam parallel to the lathe bed to impinge upon a remote interferometer mounted on the carriage as well as a reference retro-reflector mounted on a fixed part of the lathe bed; signals from the interferometer generating said accurate information during said initial movement.

6. A method as claimed in claims 4 or 5, wherein electronic signals generated in the temporary measuring means are fed electrically to the write head on the saddle and activate it to write on the material as the saddle travels down the bed.

7. A method as claimed in any preceding claim wherein directional information is supplied to circuitry associated with the head from a drive.

8. A method as claimed in any of claims 1 to 6, wherein directional information is generated by the writing is itself such as to generate directional information as the head moves along it.

9. A method as claimed in claim 8 wherein the head unites two tracks with a 90C phase shift so that a logic circuit in the head can establish the direction of travel from the sequence of rising and falling edges.

10. In apparatus which includes first and second relatively movable members, the method which includes the steps of: providing on the second member writable/readable material; providing a write head on the first member; generating signals representative of the position of the first member during relative movement before the members feeding those signals to the write head on the first member to actuate it, as said relative movement occurs to write a scale onto the material; providing a read head on the first member; and causing the read head to read the scale to generate signals representative of the position of the relative positions of the two members.

11. A method of measuring the position of a first member relative to a second member substantially as hereinbefore described with reference to the accompanying drawings.

12. Apparatus including a first member movable relative to a second member, the second member having a scale which can be read by a read head on the first member to generate signals representative of the position of the first member relative to the second member, the scale having been generated by: providing writable/readable material on the second member; and generating signals representative of the position of the first member relative to the second member during relative travel between the members and feeding these signals to a write head on the first member to cause it to write a scale on the material.

13. A method as claimed in claim 12, wherein the scale is made up of a series of readable markings such as: visible readable markings; example bar codes; magnetically readable markings; magnetic tape; or laser-readable markings; or compact disk material.

14. A method as claimed in claim 12 or 13 wherein the readable/writable medium is CD material and the read/write head is a laser.

15. A method as claimed in claims 12 or 13, wherein a length of magnetic tape is used as the readable/writable material.

16. A method as claimed in claim 12 or 13 wherein an optical reader is used, the writing being generated by a printer or like marker generating optically-readable characters.

17. A method as claimed in claims 12 to 16, wherein directional information is generated and fed to the head.

18. A method as claimed in claim 17 wherein the directional information is supplied to circuitry associated with the head from a drive such as an electric motor.

19. A method as claimed in claim 17, wherein the writing is itself such as to generate directional information as the head moves along it.

20. A method as claimed in claim 19, wherein the head operates to write two tracks with a 90C phase shift so that a logic circuit in the read head can establish the direction of travel from the sequence of rising and falling edges.

21. Apparatus enabling the position of a first member relative to a second member to be determined, substantially as hereinbefore described with reference to the accompanying drawings.

22. A tool calibrated in accordance with the method and apparatus aforesaid.