Position determining apparatus for coordinate positioning machine

Abstract: Described is a device which enables toolsetting on a machine tool. The device includes a light emitting unit (10) and a light detecting unit (14) . A light source (30) causes a light beam (12) to propagate from the emitting unit (10) to the detecting unit (14). A light detector (40) detects the presence or absence of the beam. A trigger signal is produced when occlusion occurs and so the position of an object can be determined by reference to the machine's coordinate readings. The beam (12) may be uncollimated and thus provide a device with an easier set-up and a greater resistance to vibration in use. The light detector (40) and/or light source may be protected from contamination by windows (34,44) or a protector having an aperture therein. The light emitting and detecting units each have an aperture for the light beam. The apertures may be oblique conduits (104) for pressurised air flow, thereby keeping the conduits free from contamination, while minimising turbulence in the path of the beam which could cause noise in the resulting signal.

Description: The present invention relates to an apparatus which enables a coordinate positioning machine (such as a machine tool) to determine the position of an object relative to a reference point. It may, for example, be employed on a machine tool for toolsetting operations. A known tool setting device for use on a machine tool includes a light source which generates a fine beam of light which is incident upon a detector. During a toolsetting operation, the machine is operated to move the tool in a direction transverse to the direction of propagation of the light beam until a part of the tool interrupts passage of the light beam. Detection of this interruption results in the generation of a trigger signal in the detecting unit, which is used by the machine to establish the relative position of its moving parts in order to determine dimensions of the tool. Such devices are known, for example, from German Patent Nos. DE 42 385 04 and DE 42 448 69, French Patent No. 2,343,555, European Patent No. 98,930 and US Patent Nos. 4,518,257, 3,900,738 and 3,749,500. The devices may be used additionally for measuring the length or diameter of a tool to monitor tool breakage or wear. The devices disclosed in the above-mentioned patent specifications use a narrow light beam into or through which the tool is passed. The detection units detect when the tool breaks into the beam from the resulting drop in the intensity of the light falling on them. The accuracy with which these devices can measure tool position is dependent on the diameter of the laser beam, the smaller the diameter the more accurate the measurement. For example, French Patent No. 2,343,555 describes a system in which the laser produces a coherent beam the diameter of which is of the order of 0.7 to 0.8mm. European Patent No. 98,930 proposes the use of a laser light source because lasers supply a sharply defined light beam for high measurement accuracy. US Patent No. 4,518,257 describes a system in which the laser beam is focused to a small examination zone at which all measurements are made. One problem with all of these devices is that the smaller the laser beam, the more difficult is the task of aligning the laser beam with the detector system. Another problem with the prior art systems exemplified in European Patent No. 98,930 is that the optical system has to be kept clean to avoid build up of contaminants on the optical components which can reduce the intensity of light received by the receiver and cause the signal from the detector to be emitted early. This is particularly so in the case of devices which rely on the reduction in intensity produced by the smallest obstruction of the beam to produce a measurement signal. European Patent No. 98,930 describes two methods of keeping the optical system clean, one of which involves directing an air blast at the external glass faces of the housings of the emitter and receiver, the other of which involves the use of movable mechanical shutter. These methods require the provision of additional cost and complexity to the devices. A problem with a focused system, as in US Patent No. 4,518,257, is that the tool can only be measured in the small examination zone. Unfocused systems do not suffer from this problem, but the beam must be accurately aligned with an axis of travel of the machine tool to permit accurate measurements anywhere along the beam path. Prior unfocused systems have been awkward to align, because each time an adjustment is made to the direction of the beam path, the position of the beam must be checked at two different positions along the path, followed by further adjustment as necessary. Embodiments of the present invention provide alternative devices which are suitable for toolsetting on a machine tool (or other applications on other coordinate positioning machines). The devices include a number of technical aspects in which they differ from the prior art devices discussed above and which enable various ones of the problems which arise with those prior art devices to be eliminated or reduced. US 3,900,738 shows an unfocused tool setting system without protection for its light emitter or receivers. US 3,749,500 shows an optical gauging caliper which describes the use of light beam apertures which have airflow to prevent contamination. According to the present invention there is provided a position determination device for a machine utilising coordinate positioning, the device comprising a light emitting unit and a light detecting unit, the light emitting unit having a housing within which a light source is situated for emitting a light beam which in use propagates towards the light detecting unit which in turn includes a housing to detect the light beam propagating from the light emitting unit, one or both of the housings including a light channel for the beam in the form of a conduit through which pressurised air is passable from the interior of the housing(s) to the exterior thereof; characterised in that the or each conduit extends transversely or obliquely to the general direction of propagation of the light beam. The or each conduit has preferably a cross-sectional area of 0.8mm 2 or less. More preferably a conduit has at its narrowest point a cross-sectional area in the range of 0.02mm 2 to 0.8mm 2 . Where a conduit is approximately circular such areas would provide a diameter between 50 microns and lmm. Small conduits of this size are often referred to as pinholes. Preferably the light beam is uncollimated such that the size of the resultant spot formed on the detecting unit is larger than the conduit at the light detecting unit. Embodiments of the invention and other useful information will now be described, by way of example, and with reference to the accompanying drawings, in which: Fig 1 is a perspective view of a first embodiment of the present invention; Fig 2 is a schematic illustration of aspects of the embodiment of Fig 1 which are not part of the invention; Fig 3 is a detail of a modification of the embodiment of Fig 1 which accords with the invention; Figs 4a-c are details of Fig 3; Fig 5 is a vertical section of a light emitting unit of an alternative configuration according to the invention; Fig 6 is a section on the line XIII-XIII in Fig 5; and Fig 7 is a schematic illustration of a method of beam alignment. Referring now to Fig 1, a toolsetting apparatus which is suitable for use on, for example, a machine tool includes a light emitting unit 10 which emits a beam 12 of light, and a light detecting unit 14, where the light beam 12 is detected. Power and signal control cables to the light emitting and detecting units 10,14 are routed via inlet ports 16, and both the units 10,14 are advantageously mounted, via pillars 18, on the base of the machine, either via an intermediate base 20, to which they are both mounted, or directly to the base of the machine upon which they are to be employed. In operation, the device is used for toolsetting by operating the machine on which the device is mounted to move the tool in a direction transverse to the direction in which the beam 12 is propagating. When a predetermined level of occlusion of the beam has been established, the detecting unit 14 emits a trigger signal which is used by the machine to determine the relative position of its relatively movable parts, thereby to enable dimensions of the tool to be determined. Referring now additionally to Fig 2, for clarification of the invention, the light emitting unit 10 comprises a laser diode 30 which generates the beam 12. The beam 12 passes through a light channel, in this case an aperture 22 provided by a screen 32, and subsequently through a translucent or transparent window 34 provided within the housing 36 of the light emitting unit. The beam 12, being laser light, is substantially parallel, but is typically divergent at a small angle, unless collimated. In the present example, the beam is deliberately uncollimated, and allowed to diverge. The light detecting unit includes a photodetecting diode 40, placed in register with an aperture 24 provided within a screen 42, which is in turn mounted behind a translucent window 44 within the housing 46 of the unit 14. By the time the beam 12 has propagated from the light emitting unit 10, and is incident upon the window 44, the spot created by its incidence is substantially larger than the aperture 22 within the screen 42. Typically, over a distance of one metre between the light emitting unit 10 and the light detecting unit 14, the beam diameter will be of the order of 4mm at the detecting unit 14, whereas the aperture 24 within the screen 42 might be of the order of 300 microns or less. The screens 32/42 may be a coating on windows 34/44 e.g. a chromium coating or may be an etched surface on the window. In order to ensure that both windows 34 and 44 have an outer surface which is free from contaminants, both windows are subjected to an air blast. High-pressure air is supplied to the emitting and detecting units 10,14 via inlet ports 60,62 respectively, and is directed onto the surface of the window via mushroom members 70. Referring now to Figs 3 and 4, in an embodiment according to the invention, either or both windows 34,44 are positioned behind a further screen 100. Air is supplied to the chamber 102 formed between the windows 34,44 and the screen 100 at pressure e.g. 1.5 bar, and bleeds out of the chamber 102 via a conduit 104 within the screen 100. In this case conduit 104 forms a light channel and is configured such that it extends in a direction transverse or obliquely to the direction of propagation of the beam 12, (e.g. at 10-20° to the general direction of propagation of beam 12) and therefore the resultant air flow has minimal effect upon the beam propagation. If the conduit were parallel to the beam, then turbulence along the path of the beam could affect the accuracy of measurement. The beam 12 passes through the conduit 104 by virtue of the inner 110 and outer 112 apertures thereof lying fractionally in register with each other, as can be seen more readily in the front elevation and perspective views of Fig 4a and 4b respectively, and the sectional view of Fig 4c. A similar conduit 104 may be formed at the detector unit 14 as well as or instead of at the emitter unit 12. Typically this or these conduits are a pinhole extending at 15° to the general direction of propagation of the beam 12 and is/are 1mm or less in diameter or 0.8mm 2 in cross-sectional area or less. If the pinhole is inclined its effective cross-sectional area will be less, and it will present a non-circular cross-section to light travelling therethrough. Apertures 22 or 24 may be smaller in diameter than the conduit 104 and may be omitted altogether. Figs 4 show a fine beam 12 propagating through conduit 104 without touching the sides. In this case aperture 22 will have a smaller diameter than the conduit 104. If an aperture 22/24 is included it will have a cross-sectional area of 0.8mm 2 or less. The embodiments described previously all employ a light channel which is relatively small in cross-sectional area e.g. 0.8mm 2 or less. The small area allows a fine beam which gives improved position determination accuracy when an object occludes the beam. The use of a divergent beam does not detract from this accuracy provided a light channel of small area is used at, at least the detector. Accuracy is maintained in such an arrangement because the detector will then only "see" a column of light from the light source and not the remaining divergent beam. As a consequence of this, only this column is detected by the light detector and the remaining divergent light is ignored. Using divergent light has the advantage that the detector does not have to be exactly in line with the emitter. Slight misalignment is possible as is slight vibration. The detector will in these circumstances still see a narrow column of light which can be monitored. Next, the embodiment according to Figs 5-7 will be described. The light emitting unit shown in Figs 5 and 6 comprises a housing 300, containing a laser diode 302 and a collimating lens 304. Thus, this embodiment uses a collimated beam of light, rather than a diverging beam. However, the beam is not focused, and therefore (once the beam has been aligned with the axes of the machine tool) toolsetting measurements are possible anywhere along the length of the beam. As previously, the diameter of the beam is about 1mm or less. Consequently, there is no need for relatively large apertures to allow the beam to pass from the light emitting unit or into the light detecting unit. The laser diode 302 and lens 304 are mounted together in an adjustment unit 306. This is adjustably fastened to the housing 300, through a spherical bearing face 308. This is shown in the present embodiment as being a surface of the adjustment unit 306, but could equally be a surface of the housing 300 which mates with the adjustment unit. The adjustment unit 306 has adjusting screws 310. Using these, the optical axis along which the laser diode 302 and lens 304 project the laser beam may be adjusted through a small angle, both vertically and horizontally. The effect of this adjustment can be seen in Fig 7. This figure shows the light emitting unit 300 and also the light detecting unit 320. It can be seen that the spherical bearing surface 308 is centred at a point 312, located along the path of the laser beam between the light emitting unit 300 and the detecting unit 320. Fig 7 shows three possible beam paths 314, for three possible adjustment positions of the adjustment unit 306, and all these possible paths pass through the point 312. Still referring to Fig 7, the toolsetting device is adjusted as follows. With the laser diode 302 emitting light, the detecting unit 320 is first adjusted both vertically (arrows 322) and horizontally so as to receive the laser beam. Next, the beam is probed at the point 312 by a tool or other object 330 mounted in the machine tool spindle 332. The vertical and horizontal positions of the beam are noted at this point 312. For accurate toolsetting, it is necessary that the laser beam should be aligned with one of the axes of travel of the machine tool. To ensure this, the alignment of the beam must be adjusted so that, at a further position 318, it is located at the same vertical and horizontal coordinates as at the position 312. Therefore, the tool 330 is moved by the spindle 332 to the location 318, as indicated in broken lines, and the beam is again probed at this location 318. It is of course unlikely that the beam will have initially been correctly aligned, and consequently the horizontal and vertical coordinates of the beam at the location 318 will not agree with those taken at the point 312. The adjustment unit 306 is therefore adjusted, using the screws 310, to realign the beam. Because the bearing surface 308 is centred on the point 312, it is assured that the beam will continue to pass through the point 312. Therefore, as the beam is adjusted, it is only necessary to re-probe it at the location 318, making further adjustments as necessary to align the beam with the machine axis. During these adjustments, of course, the detector unit 320 will be repositioned as necessary in order to capture the beam. The method of adjustment just described is considerably simpler as a result of the spherical bearing surface 308. This is because it is necessary to re-probe the laser beam only at the location 318 after each adjustment, and not to re-probe it at the location 312 after each adjustment step. However, it is of course possible if desired to re-probe at the position 312, as a final check after all the adjustment has been performed. It can be seen from Figs 5 and 6 that the adjustment unit 306 includes an exit conduit 340 for the laser beam. This is supplied with pressurised air, as previously. Also as previously, the conduit 340 is directed transversely or obliquely to the laser beam, so that the flow of air causes minimum disturbance to the beam as it propagates towards the detector unit 320. The pressurised air to the conduit 340 is supplied through passages in the front wall of the housing 300, passing through an annular space 342 between the adjustment unit 306 and the housing 300. As it does so, it is in contact with a body part 344 of the adjustment unit 306, which is in thermal contact with the laser diode 302. This body part is made of a thermally conductive material such as aluminium, and acts as a heat sink. The result is that the pressurised air passing through the annular space 342 cools the laser diode. The adjustment unit 306 is provided with appropriate O-rings to seal the pressurised air, including an O-ring 346, and an O-ring 348. The O-ring 348 seals between the housing 300 and the spherical bearing surface 308. The O-ring 346 provides sufficient flexibility to permit the tilting of the adjustment unit 306 horizontally and vertically, while maintaining the seal. As in previous embodiments, the conduit 340 need only be small, since the laser beam is unfocused and is itself collimated to a small diameter. The air consumption is therefore low. Since the pressurised air ensures that the conduit remains clean, a shutter over the conduit may not be needed, though one can be provided if desired. Of course, both in the embodiments of Fig 4 and of Figs 5-7, it is possible to provide a transverse or oblique conduit leading to the photodetector in the detecting unit, and to keep it clean with pressurised air, in the same way as in the light emitting unit. The present invention has been described and illustrated using a laser diode as a light source from which the light beam 12 is created. However, other forms of electromagnetic radiation may be employed, and any light source may be used to create the beam 12 with the proviso that a sufficiently high luminosity may be obtained from the beam at the detecting unit 14.

Claims: A position determination device for a machine utilising coordinate positioning, the device comprising a light emitting unit (10) and a light detecting unit (14), the light emitting unit having a housing (36) within which a light source (30) is situated, for emitting a light beam (12) which in use propagates towards the light detecting unit (14) which in turn includes a housing (46) and a light detecting device (40) to detect the light beam (12) propagating from the light emitting unit (14), one or both of the housings including a light channel for the beam (12) in the form of a conduit through which pressurised air is passable from the interior of the housing(s) to the exterior thereof, characterised in that the or each conduit extends transversely or obliquely to the general direction of propagation of the light beam. A position determination device as claimed in claim 1 characterised in that the or each conduit has a cross-sectional area of 0.8mm 2 or less. A position determination device as claimed in claim 1 characterised in that the light beam (12) is uncollimated such that the size of the resultant spot formed on the detecting unit (14) is larger than the conduit at the light detecting unit.

Inventors: FUGE JONATHAN PAUL, MCMURTRY DAVID ROBERTS, MERRIFIELD BENJAMIN JASON, STIMPSON VICTOR GORDON, FUGE, JONATHAN PAUL, MCMURTRY, DAVID ROBERTS, MERRIFIELD, BENJAMIN JASON, STIMPSON, VICTOR GORDON
Applicants: RENISHAW PLC
Application year: 2000
Main classification: B23Q17
IPC codes: G01B11, B23Q5, H03K17, G01B21, B23Q17, G01V8