Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B3/00—Measuring instruments characterised by the use of mechanical techniques
  • G01B3/30—Bars, blocks, or strips in which the distance between a pair of faces is fixed, although it may be preadjustable, e.g. end measure, feeler strip
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B21/00—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant
  • G01B21/02—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant for measuring length, width, or thickness
  • G01B21/04—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant for measuring length, width, or thickness by measuring coordinates of points
  • G01B21/042—Calibration or calibration artifacts

Definitions

• This invention relates to position determination apparatus such as co-ordinate measuring machines, inspection robots and machine tools. More particularly, it relates to bars for testing the accuracy or repeatability of such apparatus.
• test bars It is known to check the volumetric accuracy and repeatability of positioning of such apparatus, in particular co-ordinate measuring machines, using a test bar. Examples of test bars are shown in DE-A-2603376 (Bendix) , US-A-4,435,905 (Bryan), US-A-4,437,151 (Hurt), and in Tooling and Production, vol. 49, No. 2, May 1983, pages 40-47, "Measuring Up: Coordinate Measuring Machine Survey” by B Nagler, see page 43.
• test bars usually have a fixed length.
• the apparatus is checked by touching the end, on both ends of the test bar with the probe, and calculating the length of the bar from the readings thus taken. This is repeated with the test bar in a number of different positions within the working volume of the apparatus, the results are compared, and any differences noted. In this way the entire working volume of the apparatus can be checked.
• the test bar may be pivotally mounted to facilitate repositioning within the working volume.
• the prior art test bars just described have an accurately- spherical test ball at one end or both ends, so that the centre of the ball can be calculated after taking four or more readings of the co-ordinates of points on the surface of the ball. The length which is measured is the length between the centre of one ball and the pivot point, or between the centres of two such balls on opposite ends of the bar.
• the present invention provides a test bar for a position determining apparatus, comprising: an elongate member, a probe-contacting element at at least one end of the elongate member, and means forming part of the test bar for detecting contact between the probe-contacting element and a probe.
• the detecting means comprises one or more piezo-electric elements or strain gauges, which may suitably be arranged between the probe-contacting element and the elongate member.
• the elongate member has bearing means for pivotably locating the elongate member with respect to the apparatus.
• Fig. 1 is a schematic diagram of a co-ordinate measuring machine with a first embodiment of test bar
• Fig. 2 is a view of the test bar of Fig. 1, in the direction of arrow II,
• Fig. 3 is a block circuit diagram of part of an interface circuit for the test bar of Fig. 1,
• Figs. 4,5 and 6 are partial views illustrating alternative constructions for the test bar of Fig. 1
• Fig. 7 is a view on the line VII-VII in Fig. 6
• Fig. 8 is a view corresponding to Fig. 1 but showing a further embodiment of test bar.
• Fig. 9 is a detail of Fig. 8, and
• Fig. 10 is a view in the direction of the arrow X in
• the co-ordinate measuring machine shown in Fig. 1 is generally conventional and comprises a bed or table 10 upon which (in normal operation) workpieces to be measured are placed.
• A. gantry 12 is mounted on uprights 14 (only one of which is shown) for movement in a direction Y.
• a carriage 16 is slideable in a direction X on the gantry 12, and a quill 18 is slideable in the vertical, Z direction in the carriage 16.
• the bottom end of the quill 18 carries a touch probe 20, which has a workpiece-contacting stylus 22.
• the stylus 22 is articulated in the probe 20 to prevent damage when it is brought into contact with a workpiece.
• touch probe 20 is a touch trigger probe in which the stylus 22 is biased into a kine atically defined rest position which is precisely fixed with respect to the quill 18. It should be noted however, that since, in the present invention the ball bar produces a signal, any type of touch probe would suffice.
• the probe 20 can be positioned anywhere within a three dimensional working volume of the co-ordinate measuring machine, in order to touch desired surfaces of the workpiece, under the action of X,Y and Z motor drives 24 of the machine (not shown in detail) which control the above-noted X,Y and Z motions. This is done under the numerical control of the computer 26.
• the position in space of the stylus 22 at any given moment is determined from respective scales 28,30,32 which have corresponding read heads, feeding pulses to X,Y and Z counters 34,36,38, which are connected to the computer 26.
• the probe 20 provides a trigger signal to the computer 26 via an interface (not shown) whenever it touches a workpiece surface, and at that time the computer 26 freezes the outputs of the counters 34,36,38 and reads them in order to determine the X,Y and Z coordinates of the point which has been touched.
• Fig. 1 also shows a test bar 40 according to one embodiment of the present invention. It comprises an elongate bar 42 which is mounted at a central pivot point 44 upon a pillar 46 which is placed on the table 10. At the pivot region 44, the pillar 46 has a ball 48 which is received in a corresponding socket in the underside of the bar 42, thus enabling the bar 42 to pivot in any direction, either horizontally or vertically.
• the pivot region 44 has a clamping means (not shown) so that once pivoted into a desired orientation, the bar 42 can be clamped into that position.
• each ball 50 having an accurately spherical surface. As seen more particularly in Fig. 2, each ball 50 is received in a conical recess 52 in the end of the bar 42. It is mounted in the conical recess upon three spaced pressure sensitive elements. Specifically the elements 54 are piezo-electric elements. In the view shown in Fig. 2, the ball 50 has been omitted for clarity, but its position is indicated by a phantom line.
• the piezo-electic elements 54 are sensitive to the shock wave generated in the associated ball 50 by the impact whenever the stylus 22 touches the ball 50, and also to the subsequent stress caused by the contact.
• the shock wave is the earliest indication of probe-test bar contact, and so it is preferable to use piezo-electric elements since they are capable of detecting the shock wave. It is possible however to use as an equivalent, one or more strain gauge arrays to detect stress.
• the elements 54 are connected to the computer 26 via an interface 56, part of the circuitry of which is shown in Fig. 3. To improve noise immunity, it is advantageous to include at least part of the circuitry of Fig. 3 within the bar 42 itself, rather than in a separate remote interface unit.
• the outputs of the three piezo-electric elements 54 are first amplified by respective charge elements 58, and then taken to respective absolute value circuits 60. These rectify the signals, and then feed them to respective comparators 62.
• the signals are compared with a threshold voltage from the reference souce 64, so that the comparators provide an output signal whenever the piezo-electric element output exceeds a certain value.
• the outputs of the comparators 62 are wire OR-ed, giving a combined trigger output on a line 70 which can be further processed in the interface 56 and taken to a trigger input of the computer 26.
• Resistors 66,68 in a feedback circuit around the comparators 62 provide a degree of hysteresis for the trigger output.
• the trigger output on the line 70 is caused by whichever of the piezo elements 54 is first to exceed the given threshold.
• the absolute value circuits 60 ensure that an immediate trigger signal is generated irrespective of whether the initial acceleratioin applied to a given piezo element is positive or negative.
• the mounting of the balls 50 in the bar 42 as described in respect of Figs. 1 and 2 can be modified if desired.
• One modification is shown in Fig. 4, where the end of the bar 42 and the ball 50 have opposing flat surfaces 72,74, three spaced piezo-electic elements 54 being sandwiched between the flat surfaces.
• the three elements 54 may be replaced by a single piezo-electric element provided it is made responsive to shock waves resulting from contact between the stylus 22 and ball 50 in any direction.
• Fig. 5 shows an arrangement in which the ball 50 is located on the free end of a rod 76 which is located in a bore 78 within the end of the bar 42.
• Four piezo-electric elements are provided, one of these, 54', is located between the end of the rod 76 and bottom of the bore 78, while the other three, 54", are radially spaced around the rod 76, between the rod and the cylindrical surface of the bore 78.
• a similar circuit to that of Fig. 3 is used, but with four sets of components 58,60,62 instead of three.
• Figs. 6 and 7 show a modified version of Fig. 5, in which the same reference numerals denote similar parts.
• piezo-electric elements 54,54 there is a single annular piezo-electric element 90, surrounding the rod 76 in the bore 78.
• the element 90 is sensitive to shear, so as to be responsive to stylus contact from any direction.
• the test bar in Fig. l has a fixed length between the centres of the two balls 50.
• the stylus 22 is touched at each of four or more points on the surface of one of the balls 50, using manual control of the movement of the quill 18 or under the action of a numerical control programme in the computer 26.
• a contact is made, this is detected by the piezo-electric elements 54 and a trigger output on the line 70 is taken to the computer 26.
• This takes readings of the X,Y,Z co-ordinates of the quill 18 at the contact point from the counters 34,36,38, in the usual manner. From the X,Y,Z co-ordinates of the four or more points touched, the computer 26 then calculates the co-ordinate of the centre of the ball 50.
• the whole procedure is repeated to obtain the co-ordinates of the centre of the other ball 50 at the opposite end of the bar 42.
• the distance between the two centre points is then easily calculated.
• the procedure just described is repeated at many different positions and orientations of the test bar 42 within the three-dimensional working volume of the machine, and the results are compared.
• the piezo-electric elements 54 generate the signal immediately upon contact between the stylus 22 and the ball 50.
• the accuracy as measurred would depend hot only on the accuracy of the co-ordinate measuring machine itself, but also on the accuracy of the probe 20, including any pre-travel of the probe.
• the volumetric accuracy which can be determined with such a prior art arrangement is valid while a given probe is mouned in the machine, but will not be valid if the probe is changed.
• the accuracy as determined is completely independent of the accuracy of the probe 20.
• the present test bar can also be used in the conventional manner.
• the double-ended test bar of Fig. 1 may be replaced by a single-ended test bar having a ball 50 at only one free end.
• the other end of such a test bar has a ball which is received pivotably in a socket which is placed on the table 10. It can be magnetically clamped in a desired orientation in the socket, as is well known.
• the standard length of such a test bar is between the centre of the ball 60 at the free end and the centre of the socket in which the bar is received.
• the single ball 50 is supported on piezo-electric elements 54, the same as in the test bar 40 in Fig. 1.
• Figs. 8,9 annd 10 show an alternative test bar according to another embodiment of the invention, mounted on the same co-ordinate measuring machine as Fig. 1. Parts which correspond have been given the same reference numbers as in Fig. 1.
• the test bar 80 shown in Fig. 8 is a development of that shown in International Patent Publication No. WO 85/05176, incorporated herein by reference. It is intended to permit automation of the checking of the volumetric accuracy of the machine, as described in that publication, and will not be described fully here.
• the test bar 80 has a means for enabling both universal pivoting of the stylus 22 about the ball at its free end, and relative translational movement of stylus and bar. Specifically there is provided at a free end of the bar 80 a pair of parallel, horizontal, longitudinally extending prongs 82 which form a fork for engagement with the stylus 22. This enables the movement of the quill 18 to cause pivoting of the test bar 80 into any desired position.
• the end of the bar 80 also has a spherical ball surface 84 which is located below and between the prongs 82.
• the quill 18 When it is desired to take a co-ordinate reading, the quill 18 is moved inwards (preferably radially inwards) so that the stylus 22 contacts the ball surface 84, at a kinematically defined position on the ball surface because it is also in contact with the two prongs 82.
• Fig. 9 also shows that optionally, the piezo-electric crystal 86 and ball 84 are mounted to the end of the bar 80 via a layer of a rigid sound insulating material 88, such as a rigid foam ceramic material. This reduces any risk of false triggering of the piezo-electric crystal 86 as a result of noise travelling to it via the bar 80, such as machine noise or any noise caused by the sliding of the stylus 22 against the prongs 82.
• a rigid sound insulating material 88 such as a rigid foam ceramic material.
• the above embodiments of the invention have detected the contact using one or more piezo-electric elements. This is preferred beacuse piezo-electric elements can give the most accurate results without the need for correction of systematic errors.
• other means may be provided in or on the test bar to detect the contact, such as strain gauges, electromagnetic detectors, photoelectric detectors, or even electrical switching elements.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Length, Angles, Or The Like Using Electric Or Magnetic Means (AREA)
• Length Measuring Devices With Unspecified Measuring Means (AREA)

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• 1988
  • 1988-05-27 GB GB888812579A patent/GB8812579D0/en active Pending
• 1989
  • 1989-05-23 EP EP19890906157 patent/EP0391985A1/de not_active Withdrawn
  • 1989-05-23 WO PCT/GB1989/000567 patent/WO1989011631A1/en not_active Application Discontinuation
  • 1989-05-23 JP JP50564789A patent/JPH02504427A/ja active Pending

Non-Patent Citations (1)

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