GB2045443A

GB2045443A - Device for generating position signals

Info

Publication number: GB2045443A
Application number: GB8008701A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-03-31
Filing date: 1980-03-14
Publication date: 1980-10-29
Also published as: DE2912913C3; JPS55134302A; IT8021058A0; DE2912913A1; FR2453394A1; DE2912913B2; IT1130375B

Abstract

A device for generating position signals in dependence upon the relative position of two machine parts which are linearly displaceable relative to one another has an information carrier (16) supported on one machine part such as the carriage (10) of a machine tool. This information carrier (16) comprises a plate (18) of plastics material which extends in the direction of displacement of the carriage (10). A metallic layer (20) extends over the whole surface of the plate (18) and a number of information tracks (1-4) extend along the layer (20). At least one strip (22<1>, 24<2> 26<3>, 28<3>, 30<4>) of metal foil is aligned along each information track such that each track has at least two areas having differing electrical conductivity and/or cross-section. A multi-limit switch (12) having an inductive proximity switch (40<1>-40<4>) associated with each track (1-4) is mounted on a stationary machine part (14). As the carriage (10) moves relative to the machine part (14) the proximity switches (40<1>-40<4>) scan the respective tracks (1-4) and generate position signals. <IMAGE>

Description

SPECIFICATION Device for generating position signals The present invention relates to a device for generating position signals in dependence upon the relative position of two parts which are linearly displaceable relative to one another.

Such position signals are required, for example, with NC machine tools so that the machine control system can be constantly supplied with information about the position of a carriage or similar item. For this purpose, linear displacement pickups are already known which work with a glass scale which is optically scanned in order to supply digital position signals.

It is also known to attached to a machine carriage a plate with a group of grooves which extend in the direction of displacement and in which bosses can be mounted in a displaceable and lockable manner. These bosses are scanned by mechanically-actuated electrical switches of electronic switches which can be activated in a contactless manner, in such a manner that the position and length of the bosses makes it possible to select the phase relationship and length of the position signals generated by the switches.

It is an object of the invention to provide an alternative to these known devices for generating position signals which can be produced more cheaply than the known constructions.

According to the present invention there is provided a device for generating position signals in dependence upon the relative position of two parts which are linearly displaceable relative to one another, the device comprising an information carrier arranged to be supported on one of the parts to extend in the direction of displacement, said information carrier carrying at least one information track on a substantially plane surface, and a scanning device arranged to be supported on the other part to scan the information track, wherein the information track has at least two sections aligned in the direction of displacement and having different electrical, magnetic, dielectric, or optical properties, said scanning device being arranged to be responsive to the change of the property of the information track and to generate position signals.

A device of the invention can be produced using very simple and commercially available components, for example, adhesive metallic foil tapes can be applied to a plastics material plate, the position and length of these tapes determining the phase relationship and duration of position signals generated by scanning devices which respond to the reflection capability or the electrical or magnetic properties of the metal foil. Furthermore, the information track sections can also differ in their dielectric properties so that they can be scanned by known capacitive pick-ups.

A device which has a particularly simple structure, is easily programmed, and with which the required position signals can be generated particularly accurately, has an information track formed by a magnetisable layer, particularly by known magnetic tape layer, and the scanning device is constructed as a magnetic read head. It would be quite sufficient to bond a conventional magnetic tape to a level surface of one part, to record magnetic markers on it in the known manner, and to connect a commercially available magnetic read head, as used, for example, in magnetic tape units to the other part. In this case, the magnetic read head could be constructed as inductive pick-up. Alternatively, a Hall-effect generator could be used as a magnetic read head.This has the advantage that the head can generate a read signal even if it is stationary with respect to the information track, whereas inductive pick-ups require a relative movement in order to generate the flux changes necessary for the generation of a signal.

In a preferred embodiment of the device according to the invention, the information carrier is provided with several adjacent information tracks which, in particular, are formed by a single magnetic layer, the scanning device being constructed as a magnetic multitrack read head. In this way the same programs can be generated as with the known boss-equipped fields of grooves, but with considerably less effort, with greater accuracy with regard to the phase relationship and duration of the position signals and with the possibility of being able to change the program in a simple manner. Such a device according to the invention can also be programmed remotely which has not been possible hitherto.

For recording data on the information track, advantageously a magnetic write head is provided which can be displaced relative to the information track in the longitudinal direction of the latter and which can be attached, for example, to the machine part carrying the scanning device. This magnetic write head can then be used to set "magnetic bosses" or record other data which cause position or control signals to be produced when the information track is read.

In a preferred embodiment of the device having several information tracks, the scanning device possesses one magnetic read and magnetic write head for at least two of these information tracks, each head transferring the information from the one to the other information track. In this way, "fading" of the contents written onto one of the information tracks can be avoided in that, for example, with each new switch-on of the machine, the recorded information is read out of the one information track and transferred onto the other information track; it is also conceivable, however, that the information is alternately transferred from the one to the other information track by being read out and recorded every time the information tracks are being scanned.

Above all, however, the device according to the invention is especially welt suited to providing a high-resolution displacement pick-up, to which end the information track is made to form a "magnetic scale" by recording a longitudinally periodic magnetic pattern on the magnetisable layer, this pattern advantageously having the form of a square-wave sig nal. Tests have shown that, with the inching speeds of the order of 3 x 10-2 mm/sec and rapid motion speeds of the order of 250 mm/sec usual for machine tools, with the customary inductive recording and playback heads a "magnetic scale" resolution of 10-2 mm can be achieved, the period of the magnetic pattern recorded on the information track being of the order of magnitude of 10-2 mm in this case.In order to generate the "magnetic scale", the magnetic write head is fed with a square-wave signal of, for example, 25 kHz and is moved along the information track at the rapid motion speed of 250 mm/sec.

If inductive magnetic read heads are used it is proposed to provide means by which the magnetic read head is moved to and fro in relation to the magnetisable layer so that the information track can be read during standstill. If the magnetic read head is oscillating, for example, in a vertical direction with respect to the longitudinal direction of the information track or in the longitudinal direction of the latter, the flux changes necessary for reading out the information track are generated in the read head gap even if the two machine parts in question are standing still with respect to one another.If with an oscillation of the magnetic read head in the longitudinal direction of the information track the amplitude of the read head movement is greater than the period of the recorded magnetic pattern, known electronic means are used in order to prevent the surplus pulses generated from being counted.

In order to be able to generate position signals which clearly reproduce the position of a machine part without needing to use a counter for counting position pulses, several information tracks may have different magnetic patterns recorded on them which together form a digital length-measuring code.

According to a further aspect of the present invention there is provided a device for generating position signals in dependence upon the relative position of two parts which are linearly displaceable relative to one another, the device comprising an information carrier arranged to be supported on one of the parts to extend in the direction of displacement, said information carrier carrying at least one information track on a substantially plane surface, and an electronic switch controllable in a contactless manner arranged to be supported on the other part to scan the information track, wherein the information track has at least two sections aligned in the direction of displacement, the electrical conductivity and/or the cross-section of the sections of the information track being different such that the electronic switch is loaded when in the area of a first said section but is not loaded when in the area of a second said section of the information track.

This device incorporating the electronic switch is particularly useful when it is required to replace known groups of bosses for controlling, programming and positioning machines by simpler instruction generators.

In this embodiment the electronic switch may be equipped with a pick-up coil which scans the information track and is fed by an oscillator.

By the states "loaded" and "not loaded" in respect of the switch it is meant that the bistable switch assumes its one switching state in the area of the one information track section and its other switching state in the area of the other information track section, that is to say, "not loaded" does not mean that the switch oscillator is not loaded at all, merely that the loading is under a threshold value determined by the circuit.

The above-mentioned known inductive proximity switches can be actuated by the approach of any part of an electrically conductive material. In a particularly simple embodiment of the device at least one section of the information track is formed by a foil-shaped metal strip. The bosses of the known group of bosses for activating mechanical switches can be replaced by, in particular, self-adhesive metal foil strips which are bonded to a plastics material plate such that the length of the metal strips determines the duration and the position of the metal strips determines the phase relationship of the control signals.

In an embodiment of the device the known foil or skin effect is utilized: the higher the frequency of the alternating electromagnetic field formed in front of the pick-up area of the known inductive proximity switch, the thinner the surface layer of the metallic body in which eddy currents are generated by the alternating field. On the other hand, such an inductive proximity switch responds the earlier the more energy is drained by these eddy currents from the oscillator feeding the switch.The result of these two effects is that, if such a known inductive proximity switch is approached by pieces of copper foil of different thickness, the operating distance of the switch (the distance of the copper foil from the active area of the proximity switch at the instant it switches with the approach of the foil) is the greater the thinner the metal foil, that is to say, one and the same switch operates earlier if it is approached by a thin metal foil than if it is approached by a metal foil with a greater cross-sectional area. The law holds only for as long as no foils are used in which the thickness of the layer is greater than the penetration depth of the alternating field of the respective switch.For this reason it is not absolutely necessary to provide information track sections of different electric conductivity, that is to say, for example of metal and of plastics material, but an embodiment is also possible in which the information track sections are formed by metal layers of different cross-section.In a preferred embodiment one information track section is formed by a metallic surface and the other information track section by a metallic strip which consists of a different metal from that of the metallic surface of the one information track section and/or of a thickness which is different from that of the metallic surface of the one information track section. it is also possible to apply to a plastics material information carrier laminated with a thin metal foil, metal strips as elements which generate the position signal and which consist of a different metal from that of the information carrier or are merely thicker than the metallic layer of the information carrier; this is because, as soon as the thickness of such a metal strip is great enough for the proximity switch not to be adequately loaded with the given distance from the proximity switch, the metal strip cancels the loading effect of the metal foil lying underneath which, of course, must be of such dimensions that the given switch is switched with the given distance through the base foil. A point to be considered here is that the threshold values of the proximity switch can always be adjusted such that the thicker metal strip does not adequately load the switch but that the thin metallic base foil produces a loading which triggers the switching process.

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a diagrammatic representation of parts of a machine tool having a machine carriage and an instruction generator scanning the carriage; Figure 2 shows a section taken along line 2-2 of Fig. 1; Figure 3 shows a displacement pick-up device of the invention with a "magnetic scale", mounted on a machine tool; Figure 4 shows a section taken along line 4-4 of Fig. 3, and Figure 5 shows a section taken along line 5-5 of Fig. 3.

Fig. 1 shows parts of a machine tool having a carriage 10 which can be moved to and fro in the direction of double-headed arrow A.

The carriage 10 can move to and fro either together with the workpiece to be machined or with the tool machining this workpiece.

This carriage is associated with an instructions generator in the form of a known multi-limit switch 12 which is mounted on a stationary machine part 1 4. In order to trigger certain machine operations at predetermined positions of the carriage 10 and in order to maintain these machine operations until the carriage reaches other predetermined positions, a program memory 1 6 is attached to the carriage and controls the multi-limit switch 12. In the embodiment illustrated, the memory 1 6 comprises an information carrier in the form of a plastics material plate 18, a metal layer 20 extending over the whole surface of the plate 18, and metal foil strips 22', 242, 263, 283, 304 and so forth.The memory 1 6 comprises several information tracks 1 to 4, which, in the illustrated embodiment, are formed by the metal foil strips and areas, marked with dashed lines, of the metal layer 20. The metal foil strips 221 to 304 are selfadhesive metal foil strips which can be easily attached and exchanged.

As shown in Fig. 2, the multi-limit switch 1 2 contains for each of the four tracks a known inductive proximity switch 40', 402, 403 and 404, the active areas 42 of which face the program memory 1 6. Pick-up coils (not shown) of the switches are located behind the active areas 42. As can be seen from Fig. 2, the device of the invention has an extremly flat structure since the multi-limit switch 1 2 can be moved close to the carriage 10 because the metal foil strips 22'-304, in contrast to the normally used bosses, hardly project beyond the surface of the metal layer 20.

The material and thickness of the metal layer 20 are selected in such a manner, and are matched to the properties of the proximity switches 40'-404 and the distance of the metal layer 20 from the proximity switches in such a manner that these switches are loaded outside the information track areas formed by the metal foil strips 22'-304, that is to say in the information track sections 23', 252, 273, 293, 314 and so forth, and are, therefore, in a first switching state.Similarly, the material of the metal foil strips 22'-304, or the crosssection of these metal foil strips, and particularly their thickness, are selected in such a manner and are matched to the properties of the proximity switches 40'-404 in such a manner that with the given distance of the metal foil strips from the proximity switches, the switches are not loaded by the metal foil strips and are in their second switching state when their active areas 42 are located above one of the metal foil strips.

Thinner metal foil strips can also be at tached directly to an electrically non-conducting plate such as the plastics plate 1 8 in order to obtain a reversal of the switching states. In this case, the metal foil strips 22'-304 take the place of the bosses of known instruction generators with boss-actuated electromechanical switches of a multi-limit switch.

Alternatively, the surface of the program memory facing the multi-limit switch 1 2 may be provided with a metal layer which is galvanized or vaporized on or is applied in another suitable manner. Instead of the metal foil strips 22'-304 or the information track areas 23'-314, this metal layer may be provided with oblong windows in accordance with the required program, and these windows can be produced, for example, by etching. If such a metal layer is applied, for example, to a black plastics material plate such as the plate 18, it is quite clear that the inductive proximity switches 401-404 can be replaced by optical sensing elements which consist, for example, of a light-emitting diode and a photo-transistor for picking up the light reflected by the metal layer.

Figs. 3 to 5 show an embodiment of the device according to the invention for generating position signals, which is arranged to determine at all times the correct position of a machine part such as a carriage, and to transmit this position in the form of digital signals, for example to a machine control system. The components used in this embodiment could be used equally weli for triggering continuous signals between predetermined positions of the carriage, as is the case in the embodiment of Figs. 1 and 2.

The embodiment shown in Figs. 3 to 5 is provided with a "magnetic scale" which takes the form of a magnetic tape 50 which extends in the longitudinal direction of a carriage 52 and is firmly affixed to the carriage. For this purpose an information carrier 54 is attached to the carriage 52, this carrier being, for example, a plastics material plate which extends in the direction of displacement of the carriage 52. Instead of the magnetic tape 50, a magnetisable layer such as on magnetic tapes can be applied directly to the carrier 54.

A multi-track read/write head 58 is arranged on a stationary machine part 56. In the embodiment illustrated the read/write head 58 has seven read/write heads 58'-587 which can be customary record/playback heads of magnetic sound recorders. Since such read/write heads are known their construction is not further described herein. The heads 58'-587 are each used to record information on one track of the magnetic tape 50, and to read it out immediately, as is customary also in magnetic sound recorders as so-called separate-head monitoring.

The information carrier 54 is covered by a cover 60 which also encompasses the multitrack read/write head 58. Flexible sealing lips 62 are attached to the cover 60 and to the information carrier 54 and these normally rest against each other in a sealing manner although they also permit the passage of a shaft 64 which supports the multi-track read/write head 58.

In Fig. 4 the tracks of the magnetic tape are shown in dash-dotted lines and are designated 501 to 507.

The device illustrated in Figs. 3 to 5 can be used, for example, as follows. On one of the tracks, for example track 501, a square-wave signal with a period of 10-2mm is recorded by way of head 58'. If an incremental system is used as the displacement pick-up, the output of the read head part of the read/write head 58' is fed via an equalising amplifier, a further amplifier and a Schmitt trigger to an up- and down-counter which can be connected to a data display, if necessary. The other tracks 502-507 of the magnetic tape 50 can then be used to store any other marks for triggering control instructions.

The different tracks 50', 502 and so forth of the magnetic tape 50, however, can also be used to implement a coded displacement measuring process in which square-wave signals are recorded on the different tracks, the length and relative phase relationship of these signals corresponding to the required code so that the output signals of the associated read heads can always be used to generate a clear digital position indication.

If inductive read heads are used it is recommended to move the read heads rapidly to and fro with a small amplitude relative to the magnetic tape 50 so that read signals can be generated even with a stationary carriage 52.

This principle is known from video recorders for stopped-motion playback. Such a movement of the multi-track read/write head 58 is suitably brought about by causing the shaft 64 to oscillate about its longitudinal axis or to swing back and forth in the direction of its longitudinal axis. It is also possible to remove the multi-track read/write head 58 rapidly from the magnetic tape 50 and to bring it close again, that is to say, to cause the head to swing perpendicularly with respect to the plane of the magnetic tape 50. The appropriate drive means could be attached to the stationary machine part 56.

Claims

1. A device for generating position signals in dependence upon the relative position of two parts which are linearly displaceable relative to one another, the device comprising an information carrier arranged to be supported on one of the parts to extend in the direction of displacement, said information carrier carrying at least one information track on a substantially plane surface, and an electronic switch controllable in a contactless manner arranged to be supported on the other part to scan the information track, wherein the information track has at least two sections aligned in the direction of displacement, the electrical conductivity and/or the cross-section of the sections of the information track being different such that the electronic switch is loaded when in the area of a first said section but is not loaded when in the area of a second said section of the information track.

2. A device as claimed in Claim 1, wherein one of the sections of the information track is formed by a metallic surface and the other section is formed by a metal strip which is of a metal which is different from that of the metallic surface and/or of a thickness which is different from that of the metallic surface.

3. A device as claimed in Claim 1, wherein one of the sections of the information track is formed by a plastics material surface, and the other section is formed by a metal strip applied to said plastics material surface.

4. A device as claimed in Claim 2 or 3, wherein the metal strip is removably attached to the information carrier.

5. A device as claimed in any of Claims 2 to 4, wherein the metal strip is a strip of metallic foil.

6. A device for generating position signals in dependence upon the relative position of two machine parts which are displaceable relative to one another, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.