Abstract:
In a machine tool with a spindle capable of motion along at least three axes x, y, and z, the position of the spindle is checked by a device that operates utilizing electromagnetic signals sent out by four emitters, located externally of the machine tool, and picked up by a receiver mounted directly to the spindle. The signals are fed into a master control unit and processed by circuits in such a way as to produce an output that can be used to verify and if necessary correct the position of the spindle on the three axes X, Y, and Z.

Description:
This application claims priority to Italian Patent Application No. BO 2003 A 000141, filed Mar. 13, 2003 which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a device for checking the position of a spindle in a machine tool. 
     The invention has application in machine tools of the type that comprise a bearing structure composed of two longitudinal and parallel slide ways mounted on respective vertical stands, and a beam or gantry traversable on the ways along an axis designated X, supporting a unit that comprises an upright pillar aligned on a vertical axis, of which the bottom end carries a machining head equipped with a spindle to which a tool can be attached. 
     In general, the pillar can be traversed on the beam horizontally, along an axis designated Y, in a direction transverse to the traversing direction of the beam. The pillar is also slidable vertically along a third axis designated Z, in such a manner that the machining head can be raised and lowered relative to a machining station beneath, which is occupied by a fixture holding a part for machining. 
     For particular types of operation, finally, the machining head is able to rotate about an axis C parallel to the vertical axis Z, whilst the spindle is also pivotable on an axis A transverse to the selfsame axis Z. 
     In order to control a machine tool of the type described above, it must be known from one moment to the next exactly where the tool is positioned, during the entire machining process. Using prior art methods, the position of the spindle in space is identified indirectly by optical, mechanical or encoder type systems capable of measuring the x, y and z coordinates of the spindle relative to the point of origin in a Cartesian system established by the X, Y and Z axes and associated rigidly with the fixed structure of the machine tool. 
     In consequence, the reliability of the measuring system in question, which depends on the connection between fixed and moving parts of the machine, is based on the assumption that the structure will be correctly and stably positioned in relation to the machining station. Similarly, it is assumed that the geometrical positions of the various machine components, namely the slide ways, beam and pillar, will be maintained within prescribed tolerances. 
     It happens however that the geometrical positions in question cannot always be maintained, and there is also the factor of variations in the structure, caused for example by the effects of load-related heat and deformation, which introduce errors into the measurement. 
     The object of the present invention, accordingly, is to provide a device for checking the position of a spindle in a machine tool that will be unaffected by the aforementioned drawbacks typical of prior art devices. 
     SUMMARY OF THE INVENTION 
     The stated object is realized according to the present invention in a device for checking the position of a spindle in a gantry type machine tool positionable along at least three axes X, Y, and Z within a predetermined machining zone. The device comprises a plurality of emitters placed externally of the machine and generating electromagnetic signals, a receiver mounted to the spindle and capable of picking up the electromagnetic signals, also a master control unit, connected on the output side to the drive motors by which the spindle is set in motion along the axes and incorporating circuits designed to process and output signals indicating the distance between each of the emitters and the receiver, and further circuits serving to identify the position of the spindle on the basis of the signals indicating the distances between the emitters and the receiver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in detail, by way of example, with the aid of the accompanying drawings, in which: 
         FIG. 1  illustrates a machine tool equipped with a checking device embodied according to the present invention, viewed schematically in perspective and partly as a block diagram; 
         FIG. 2  illustrates the machine tool of  FIG. 1 , equipped with a checking device in an alternative embodiment, viewed schematically in perspective and partly as a block diagram. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the figures of the accompanying drawings,  1  denotes a machine tool in its entirety, comprising a bed  2  and presenting two longitudinal parallel slide ways  3  supported by corresponding vertical stands  4 . Mounted on the slide ways  3 , and capable of traversing thus along an axis denoted X, is a horizontal beam or gantry  5  set transversely to the ways  3  and supporting a unit  6  that includes a vertical pillar  7  positioned perpendicularly to the gantry  5 , of which the bottom end  8  carries a machining head  9  equipped with a spindle  10  such as can be fitted with a tool (not illustrated). The pillar  7  is traversable on respective slide ways  11  afforded by the gantry  5 , along a second horizontal axis Y extending transversely to the axis X first mentioned, and capable also of movement on vertical ways  12  along a relative vertical axis Z. 
     The longitudinal ways  3 , the gantry  5  and the vertical pillar  7  combine to create a structure  13  serving to support the spindle  10  and incorporating means by which to position the selfsame spindle  10 ; such means consist in motors  14 ,  15  and  16  able respectively to set the gantry  5  in motion along the X axis, and to set the pillar  7  in motion along the Y axis and the Z axis. 
     The motors in question might be of any given type, including conventional motors coupled to a mechanical rack-and-pinion drive, or alternatively, by way of example, linear electric motors. 
     The space between the longitudinal slide ways  3 , beneath the gantry  5 , accommodates a machining station  17  in which to position a fixture  18  (see  FIG. 2 ) that serves to support a part  19  for machining. 
     The machine  1  as described thus far falls into the category of three axis machine tools, whereas in a so-called five-axis machine, the machining head  9  is able to rotate about an axis C parallel to the vertical axis Z, and the spindle  10  can be made to pivot on an axis A transverse to this same axis Z through the agency of respective drive means not shown in the drawings. 
     The device according to the invention comprises emitter means represented as blocks denoted  20 , installed on a level above the machine tool  1 , for example at four fixed points of the building in which one or more such machine tools  1  are housed, such as will radiate electromagnetic signals that can be picked up by receiver means  21  installed on the spindle  10 . The emitter means  20  are furnished with respective time measuring means synchronized one with another and represented by a single block denoted  22 . 
     A block denoted  23  represents a master control unit comprising respective data processing and transmission means, denoted by a block  24  drawn with phantom lines and designed to generate signals indicating the distance between each of the emitter means  20  and the receiver means  21 . 
     The data processing and transmission block  24  is connected to a block  25  representing means by which to identify the position of the spindle  10  on the basis of the signals received from the processing block  24 . 
     The master control unit  23  is connected on the output side to the aforementioned motors  14 ,  15  and  16 , in such a manner that these can be piloted to correct the position of the spindle  10  by adjusting the positions on the three axes X, Y and Z. 
     In the event that the machining head  9  is able to rotate about the aforementioned vertical axis C and the spindle  10  is pivotable about the horizontal axis A, the device according to the invention will also comprise means, shown as a block denoted  26 , able to detect the orientation of the polar axes A and C and supply respective output signals to the master control unit  23  in such a way as to allow of piloting the operation of motors (not illustrated) by which the rotary motion of the machining head  9  and the pivoting action of the spindle  10  are generated. 
     The data processing and transmission means  24  which indicate the distance between the emitter means  20  and the receiver means  21  are designed to compute the distance D in question on the basis of the time taken by the signal to travel between each of the emitter means  20  and the receiver means  21 . In effect, the send time (T 1 ) and receive time (T 2 ) of the electromagnetic signals are known to the time measuring means  22 , whereas the velocity (c) of the signals is equal to the speed of light. 
     For each of the emitter means  20 , therefore:
 
 D=c ( T 2 −T 1)
 
     The four distances D thus obtained provide input information for a computing procedure by which to obtain the three coordinates x, y and z that will define the position of the tool univocally. 
     The computing procedure is one of triangulation, based on an operation involving the intersection of three spheres of which the radii are obtained by applying the above formula and the centers coincide with the spindle  10 . The procedure produces two possible solutions, and the correct solution is identified by using the intersection of the fourth sphere, from which the position of the spindle is identified univocally. 
     Where the machine tool is a five-axis type, in which case the device according to the invention would include means able to detect the orientation of the two polar axes A and C, the detection means in question will incorporate means able to sense a rotation of the head  9  about the one polar axis A, illustrated as a block denoted  27 , and means able to sense a pivoting movement relative to the other polar axis C, illustrated as a block denoted  28 . Such means  27  and  28  could consist, for example, in rotary encoders or gyroscope devices. 
     The means  27  and  28  in question are connected on the output side to a block denoted  29  serving to process the respective signals, which is connected in turn to the master control unit  23 . 
     To determine the position of the spindle  10  in the case of a five-axis machine tool, the device according to the invention utilizes the computing procedures as described above for a three-axis machine, integrated with the information provided by the processing block  29 . 
     To advantage, the four emitters  20  are able to monitor a plurality of machine tools  1  installed internally of the same building, or two or more spindles  10  of a single machine  1 . 
     The emitters  20  can be of whatever type, that is to say, emitters of electromagnetic waves in the radiofrequency part of the spectrum, transmitters of radar waves, or transmitters of laser signals. 
     Correspondingly, the receiver  21  will be a radio receiving antenna, a radar target, or in the case of laser signals, a reflective target. 
     Referring to  FIG. 2 , the fixture  18  serving to hold a part  19  in process is prepared by precision machining operations used to fashion the seats that will accommodate the parts  19 . 
     To ensure machining operations are carried out with the necessary precision, the positions of the part  19  must be maintained steady over time and for this reason the relative positioning of the machine tool  1  and the fixtures  18  holding the part  19  must also be known to the device disclosed. 
     The positioning coordinates of the fixture  18  relative to the machine  1  are computed by a machine operator and the data entered by the operator in a machine controller at the start of each machining cycle. 
     In the same way as mentioned with reference to the structure  13  supporting the spindle, the system serving to detect the position of the tool during the machining cycle can be affected by errors due to thermal expansion of the fixture  18 . 
     Accordingly, the fixture  18  will be furnished with at least one set of means  30  able to receive electromagnetic signals from the emitter means  20 . The computing procedure is the same as described previously.