Abstract:
The invention relates to a measuring device  5  of the type which detects the interruption of a beam of radiation (e.g. light  20 ) by an object ( 50  FIG.  1 ) in the beam. The device includes a light emitter  12  and a light detector  32  each enclosed in a housing  10  and  30 . In order to accommodate different spacings between the housings different caps  14  having varying size apertures  11  are fittable to the housings. This allows varying amounts of light to be issued and/or detected. Other ways of adjusting the amount of light passing through the cap are disclosed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to the operation of a measuring device which utilises the breaking or restoring of a beam of radiation for detection of an object, particularly but not exclusively a measuring device for use as a cutting tool detector having a-radiation transmitter for transmitting a beam of radiation and a radiation detector spaced from the transmitter for detecting the beam of radiation.  
         [0003]     2. Description of Related Art  
         [0004]     Measuring devices of the type mentioned above are known. One such device is described in U.S. Pat. No. 6,496,273. The optical apparatus described is designed principally for use in detecting cutting tools on machine tools and is capable of very precise measurement.  
         [0005]     In order to increase the versatility of the device shown in the above patent the light transmitter and light detector are required to be spaced at varying distances. Large tools require a greater distance between the transmitter and the detector than the distance required for a small tool so that they can fit in between the transmitter and the detector. A large machine tool may need a large distance between the transmitter and the detector so that a tool can be measured anywhere in the beam without it being necessary to traverse the tool long distances to a specific measuring location. Small machine tools do not have the space to include a lengthy measuring device which has a large distance between the transmitter and the detector.  
         [0006]     Consequential adjustments in the gain of the amplifiers at the detector have to be made when the distance between the transmitter and the detector are varied. In practice gain is adjusted by removing a cover to obtain access to switches within the detector housing which are used to select the required gain based on the distance between the detector and the transmitter.  
         [0007]     Problems occur when the user fails to replace the cover correctly or leaves contamination within the housing. Also the user can cause damage to the circuitry inside the housing or set the switches incorrectly. Other known commercially available systems use potentiometers instead of switches to adjust the gain, but such systems suffer the same drawbacks.  
         [0008]     Thus a permanently sealed system would be advantageous but would not allow for conventional adjustment of detector amplifier gain.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention provides a measuring device for detecting objects which interrupt a beam of radiation for use on a machine the device comprising: 
        an emitter having a radiation emitter for emitting a beam of radiation;     a detector having a radiation detector for detecting the radiation beam; and     at least one of the emitter or the detector having a radiation restrictor which restricts the amount of radiation reaching the detector, the radiation restrictor including an adjustment for altering the amount of radiation reaching the detector.        
 
         [0013]     Preferably the radiation restrictor includes a radiation transmissive area and the area is an aperture and the size of the aperture is alterable to form the said adjustment.  
         [0014]     Preferably the radiation restrictor comprises a plurality of caps each fittable to the emitter and/or detector and each allowing different amounts of radiation through their respective radiation transmissive areas.  
         [0015]     Preferably the amount of radiation is dependent on the distance of propagation of the said radiation.  
         [0016]     Thus embodiments of the invention so characterised have the advantage that the emitter and detector can be spaced at any practical distance and caps can be fitted to one or both, which allow an amount of radiation (e.g. light) to pass which is appropriate to the distance between the emitter and detector.  
         [0017]     Preferably the radiation is light.  
         [0018]     Advantageously both the transmitter and the detector have housings which are fitted with a cap and each cap may be exchanged for another which allows more light to pass when the distance between the two housings is increased, or less light to pass when the housings are spaced closer together.  
         [0019]     Preferably at least one of the housings has an internal cavity which is pressurised in use to a pressure which is greater than the pressure outside the housing and the aperture is in fluid communication with the cavity so that a fluid flow is induced through the aperture from the cavity to the outside of the housing.  
         [0020]     Thus in embodiments of the invention the aperture has, in use, a flow of fluid issuing from it e.g. air, which keeps the aperture free from contaminants.  
         [0021]     The invention extends to a method of adjusting the amount of radiation received within a device for detecting objects which interrupt a beam of radiation having a radiation beam emitter and a radiation beam detector, at least one of the emitter and detectors having a radiation restrictor which restricts the amount of radiation reaching the detector, the method comprising the step of adjusting the amount of radiation passable through the restrictor dependent on the distance of propagation of the radiation from the emitter to the detector.  
         [0022]     Preferably the method includes the step of selecting an appropriate radiation restrictor in the form of a cap from a plurality of caps for allowing varying amounts of radiation to pass therethrough, the selected cap allowing an appropriate amount of light to be transmitted therethrough determined by the spacing between the emitter and detector.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     Various embodiments of the invention will now be described by way of example with reference to the drawings wherein:  
         [0024]      FIG. 1  shows a measuring device according to the invention mounted to a machine tool;  
         [0025]      FIG. 2  shows a simplified section through the centre of a measuring device of the type shown in  FIG. 1 ;  
         [0026]      FIGS. 3   a,b  and  c  show light restrictors for use with the measuring device of  FIG. 2 ; and  
         [0027]      FIG. 4  shows in detail the aperture through the light restrictor shown in  FIGS. 3   a,b  and  c.   
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]      FIG. 1  shows a tool measuring device  5  suitable for use on a machine tool for example a computer controlled machine tool having a machine tool controller  56  and an automatic tool changer  54 . In operation the device  5  emits a beam of light  20 . A light emitting unit  12  emits the beam and light detecting unit  32  detects the beam. The two units are disposed within housings  10  and  30  respectively and are each mounted to a base  40 .  
         [0029]     In use, a tool for, example tool  50  is moved by its holder  52  toward (or away from) the beam  20  until it obscures (or restores) the beam. Measurement of the tool takes place at the instant the light beam is broken or restored. Conventionally a trigger signal is sent from the device to the machine controller  56  to indicate that the beam is broken or restored. The trigger signal is produced when a voltage at the detector is exceeded.  
         [0030]      FIG. 2  shows a section through the centre of a measuring device of the type shown in  FIG. 1 . In this device the light emitting unit is sealed within its housing  10 . The unit has a laser diode  16  which emits light beam  20 . The width of the light beam  20  can be restricted by one of a number of light restrictor caps  14 . The caps  14  are shown in more detail in FIGS.  3 a,b and c, but in essence they have an aperture  11  for restricting the amount of light passing therethrough. The aperture  11  is open to an internal cavity  18  within the housing  10  which is pressurised by a pressurised air supply  42 . Thus air bleeds constantly from the aperture  11  to the outside of the housing, so that the aperture does not become blocked by contamination.  
         [0031]     The light beam may propagate to a further cap  34  mounted at the light detecting unit  32  housed in housing  30 . A photodiode  36  detects the light beam when it is present. The amount of light and thus the photodiode voltage is controllable in this embodiment by the size of aperture in caps  14  and/or  34 . An autogain circuit is provided to take account of variation in light levels so that fewer caps  14  have to be used.  
         [0032]     Air bleeds also from an aperture  31  in cap  34  from pressurised cavity  38 . Pressurised air, power and signals are supplied via supply  42  and cables  44  respectively within the base  40 . Each of the housings  10  and  30  are in this instance secured to the base  40  also. However, the housings may be fixed directly to a machine surface for example the machine tool surface  58  shown in  FIG. 1 .  
         [0033]     Now, for versatility the distance between the housings  10  and  30  can be varied. This can be done for example by re-securing the housings at different positions on the base  40  or by using different length bases  40 . A few millimetres to  5 m or more separation is possible.  
         [0034]     Changing the distance between the emitting unit  12  and the detecting unit  32  changes the amount of light incident at the detecting unit, possibly causing an excess or a deficiency of light at the detector. This problem is overcome by providing a range of light restricting caps shown in  FIGS. 3   a,b  and  c  which alter the width of the beam detected. In addition a narrower beam is more useful for measuring smaller tools. A range of caps allows just one type of emitter and detector to be made and fitted with an appropriate cap. No adjustments to the circuits of the emitter and the detector need be made after manufacture.  
         [0035]      FIGS. 3   a,b  and  c  show examples of restrictor caps  14  and  34 . In  FIG. 3   a  a small distance ‘a’ between the caps is present so a narrower beam  20   a  can be used because light losses in the beam path e.g. due to divergence of the beam are small.  
         [0036]      FIG. 3   b  shows a larger distance ‘b’ between the caps  14   b  and  34   b . As a larger distance ‘b’ results in losses so a wider beam of light  20   b  is required. This is achieved in this embodiment by providing larger apertures  11   b  and  31   b.    
         [0037]     Likewise  FIG. 3   c  shows a yet larger distance ‘c’ and yet larger apertures  11   c  and  31   c.    
         [0038]     To the side of each  FIG. 3   a,b  and  c  is shown a view of each cap in the direction of the axis of the respective beams  20   a,b  and  c . The different sizes of apertures are apparent from these Figures.  
         [0039]     It will be noted that the axis of each of the apertures is offset relative to the axis of the beam  20  (by an angle θ shown in  FIG. 4 ) It is known that air passing in the same axis as the beam causes noise in the signal at the detector, whereas air passing through the aperture obliquely has far less effect.  
         [0040]     A typical aperture used is shown in  FIG. 4 . Until now a circular aperture has been used which has resulted in an eyelid shaped beam when viewed in the direction of the beam. The present invention provides an aperture or other light transmissive area having major and minor axes (for example x and y), where the major axis is longer than the minor axis, the aperture or other radiation transmissive area extending obliquely (e.g. by the angle θ) to the axis of radiation propagating through the aperture in use and the major axis extending also obliquely (e.g. at 90 degrees) to the axis of the radiation.  
         [0041]     Preferably the angle θ is about 5-45 degrees, more preferably about 15-30 degrees, yet more preferably about 30 degrees.  
         [0042]     Whilst the invention has been described and illustrated with reference to specific embodiments, variants of the invention will be readily apparent to the skilled addressee.  
         [0043]     Whilst a tool detector for use on a machine tool has been described and illustrated the invention extends to a detector for any object. Whilst use with a light detecting beam has been shown it will be apparent that any beam of electromagnetic radiation could be utilised, e.g. infra-red radiation. Rather than an aperture in a series of caps, a variable size aperture could be used. Alternatively a light transmissive window could be used e.g. glass. The window might be different sizes in a series of caps and/or might have varying degrees of light transmission so that varying amounts of light can pass through each window. A variable size window could be used for example in the form of a light valve of variable size e.g. a liquid crystal display which has segments operable to increase or decrease the amount of light passing therethrough.  
         [0044]     The embodiments show both housings having a cap  14  and  34 . However an aperture or window as described above may be provided on only one of the housings so as to achieve the desired amount of light at the detector. The cap shown in  FIGS. 3   a,b  and  c  might be replaced by an exchangeable planar sheet having an aperture or window therein.  
         [0045]     Adjustment of the spacing between the housings  10  and  30  may be achieved by any suitable means for example by securing those housings at various positions on a base  40 , by fixing at a desired spacing directly to a machine tool bed  58  or providing bases which have the desired spacing.  
         [0046]     A wide range of spacings between the housing (a few millimetres to 5 m or more) can be achieved with only three different cap apertures as shown. Autogain circuitry is used to correct the operating voltage when different spacings are used with the same cap. This circuitry helps to increase the spacing range which can be accommodated by one cap. Fewer or more caps could be used. Fewer or more than three light restrictors could be employed also. Different size light transmissive areas can be used where two areas are employed.  
         [0047]     The light emitting unit  10  and the light detecting unit may be disposed side by side, perhaps in the same housing. In this case the light detecting unit will be looking for light or other radiation reflected off an object as it passes the beam emitted by the light emitting unit  10 . Thus, it may be necessary to alter the quantity of radiation in the beam dependent on the approximate distance between the object and the emitter/detector 10/30.  
         [0048]     So, it is the distance of propagation of the radiation which defines the required quantity of radiation of the beam, not solely the distance between the two housings of the emitter and detector units.