Method and arrangement for increasing a throughput in workpiece measurement

Abstract

A method and an arrangement for increasing the throughput in a workpiece measurement with a coordinate measuring machine (CMM) is provided. The CMM measures a workpiece, and the measurement is described by at least one measurement parameter, a value of which is variable. The method includes setting an initial value of the at least one measurement parameter, the initial value being a predetermined value of the at least one measurement parameter valid for measuring the workpiece, measuring the workpiece with the initial value, determining a value of at least one predetermined test characteristic based on results of the measuring of the workpiece, determining whether the at least one predetermined test characteristic satisfies a predetermined iteration criterion; and changing the initial value of the at least one measurement parameter and repeating the prior steps upon determining that the at least one test characteristic satisfies the predetermined iteration criterion.

Claims

1. A method for increasing a throughput in a serial workpiece measurement by a coordinate measuring machine (CMM), the CMM being configured to measure a workpiece, the serial workpiece measurement being defined by at least one measurement parameter, and a value of the at least one measurement parameter being a variable value, the method comprising: (a) setting an initial value of the at least one measurement parameter, the initial value being a predetermined value of the at least one measurement parameter valid for measuring the workpiece; (b) measuring the workpiece with the initial value; (c) determining a value of at least one predetermined test characteristic based on a result of the measuring of the workpiece; (d) determining whether the at least one predetermined test characteristic satisfies a predetermined iteration criterion; and (e) changing the initial value of the at least one measurement parameter and repeating steps (b) to (d) upon determining that the at least one test characteristic satisfies the predetermined iteration criterion.

2. The method as claimed in claim 1, further comprising: increasing the initial value of the at least one measurement parameter in step (e) when the predetermined iteration criterion is satisfied.

3. The method as claimed in claim 2, wherein the predetermined iteration criterion is satisfied when an admissible value of the at least one test characteristic is determined.

4. The method as claimed in claim 1, further comprising: decreasing the initial value of the at least one measurement parameter in step (e) when the predetermined iteration criterion is satisfied.

5. The method as claimed in claim 4, wherein the predetermined iteration criterion is satisfied when an inadmissible value of the test characteristic is determined.

6. The method as claimed in claim 1, further comprising: changing the initial value of the at least one measurement parameter at least once when in step (d) the predetermined iteration criterion is not satisfied; and choosing a value of the at least one measurement parameter that lies between the initial value of the at least one measurement parameter not satisfying the predetermined iteration criterion and a value of the at least one measurement parameter which most recently satisfied the predetermined iteration criterion.

7. The method as claimed in claim 1, wherein the initial value lies between a predetermined lower limit value for the at least one measurement parameter and a predetermined upper limit value for the at least one measurement parameter.

8. The method as claimed in claim 7, further comprising: determining at least one of (i) how the initial value of the at least one measurement parameter is changed based on a measuring accuracy achievable with the initial value, and (ii) determining how the predetermined iteration criterion is defined based on a measuring accuracy achievable with the initial value.

9. The method as claimed in claim 1, further comprising: setting a measurement parameter value for subsequent workpiece measurements when at least one of the following preconditions is satisfied: a prescribed highest number of changes of the initial value of the at least one measurement parameter has been reached; an admissible limit value of the changed initial value of the at least one measurement parameter has been reached or exceeded; a measurement parameter value is selected with which the predetermined iteration criterion is satisfied again after a prior failure to satisfy the predetermined iteration criterion; and an admissible overall time period has been reached.

10. An arrangement for increasing a throughput in a workpiece measurement, the arrangement comprising: a coordinate measuring machine (CMM) configured to measure a workpiece, the workpiece measurement being defined by at least one measurement parameter, and a value of the at least one measurement parameter being a variable value; and a controller configured to: (a) set an initial value of the at least one measurement parameter, the initial value being a predetermined value of the at least one measurement parameter valid for measuring the workpiece; (b) measure the workpiece with the initial value; (c) determine a value of at least one predetermined test characteristic based on a result of the measuring of the workpiece; (d) determine whether the at least one predetermined test characteristic satisfies a predetermined iteration criterion, and (e) change the initial value of the at least one measurement parameter and repeat (b) to (d) upon determining that the at least one test characteristic satisfies the predetermined iteration criterion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The disclosure will now be described with reference to the drawings wherein:

(2) FIG. 1 shows a view of an arrangement with which methods can be implemented according to an exemplary embodiment of the disclosure;

(3) FIG. 2A shows representations of possible measurement parameter changes according to a first exemplary embodiment of the disclosure;

(4) FIG. 2B shows a flow chart according to the first exemplary embodiment of the disclosure;

(5) FIG. 3A shows representations of possible measurement parameter changes according to a second exemplary embodiment of the disclosure;

(6) FIG. 3B shows a flow chart according to the second exemplary embodiment of the disclosure;

(7) FIG. 4A shows representations of possible measurement parameter changes according to a third exemplary embodiment of the disclosure;

(8) FIG. 4B shows a flow chart according to the third exemplary embodiment of the disclosure; and

(9) FIG. 4C shows further representations of possible measurement parameter changes according to the third exemplary embodiment of the disclosure.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

(10) Exemplary embodiments of the disclosure are described below based on the accompanying schematic figures. Features that coincide in their nature and/or function are provided with the same designations throughout the exemplary embodiments.

(11) The arrangement 11 shown in FIG. 1 includes a CMM of a gantry type of construction, which has a measuring table 1, over which columns 2, 3, which together with a crossbeam 4 form a gantry of the CMM 11, can be moved. The crossbeam 4 is connected at its opposite ends to the columns 2 and 3, respectively, which are mounted longitudinally displaceably on the measuring table 1.

(12) The crossbeam 4 is combined with a cross slide 7, which is movable, by air bearings, along the crossbeam 4 (in the X direction). The current position of the cross slide 7 relative to the crossbeam 4 can be determined on the basis of a scale graduation 6. A quill 8, which is movable in the vertical direction, is mounted on the cross slide 7 and connected at its lower end to a sensor device 5 by a mounting device 10. Removably arranged on the sensor device 5 is a probe head 9, which scans in a tactile manner. Instead of the probe head 9, the sensor device 5 could similarly include a contactlessly scanning sensor, in particular a laser sensor.

(13) Arranged on the measuring table 1 is an additional rotatable measuring table 13, on which a measurement object can be arranged, which can be turned about a vertical axis of rotation by a rotation of the measuring table 13. Also arranged on the measuring table 1 is a magazine 14, in which various probe heads that can be exchanged for the probe head 9 may be arranged, or in which various styluses that can be exchanged for the stylus carried on the probe head 9 may be arranged.

(14) FIG. 1 also schematically shows a controller 12 of the CMM, which may be realized, for example, by a computer on which software instructions are executed and which includes at least one data memory 15. The computer is connected by signal and control lines to actuatable components of the CMM, in particular to drives. Furthermore, the controller 12 is connected by a measurement data connection to those elements of the CMM that are used for determining the coordinate measured values. Since such elements and devices are generally known in the field of CMMs they are not discussed in detail here.

(15) With reference to FIGS. 2A and 2B, a first exemplary embodiment of a method 200 is described which can be implemented with the arrangement 11 shown in FIG. 1. In FIG. 2A, possible measurement parameter changes are shown, whereas FIG. 2B contains a flow chart of the method 200. In the case of this exemplary embodiment, it is generally envisaged to increase a measurement parameter value by default from its lower limit value a.sub.min in the direction of its upper limit value a.sub.max. The lower limit value a.sub.min is selected here as the initial value a.sub.0. The increase takes place in this case with the same amounts (for example an increase by 10% in each case with respect to the previous value). The measuring speed, which, when increased from the lower limit value a.sub.min, makes a higher throughput possible, but also causes an increasing risk of measuring inaccuracies, is considered as the measurement parameter.

(16) It should be emphasized that, in all exemplary embodiments (but also in the disclosure in general), the iterative changing of the measurement parameter values does not have to take place with the same amount in each case. Instead, irregular intervals may also be provided between successive values (for example continuously increasing or decreasing intervals considered over the values in their entirety). A measurement parameter value to be applied may consequently also be flexibly calculated and not only changed according to always the same criterion. For example, the measurement parameter value may be interpolated from the previous values and/or be determined in accordance with a stored functional relationship.

(17) With reference to the flow chart shown FIG. 2B, in step S1 (or in the course of measure S1) first the initial value a.sub.0 of the measurement parameter is selected and, as already mentioned, set as the lower limit value a.sub.min. This is followed in step S2 by measuring a workpiece based on the initial value a.sub.0 of the measurement parameter or in accordance with the initial value a.sub.0 of the measurement parameter.

(18) It should be emphasized that first a workpiece of which the properties are known and which in particular satisfies predetermined accuracy criteria is measured. It is accordingly a workpiece that has test characteristics which undoubtedly satisfy the desired accuracy. In the case shown, the workpiece takes the form of a shaft, which has cross-sectional regions with different diameters. Each of these diameters lies within a predetermined tolerance, and consequently has a desired accuracy.

(19) In step S2, the corresponding cross-sectional regions are selected as a test element of the workpiece and are measured. On the basis of the measurement results, a shape deviation of these regions from an ideal circle is subsequently determined in step S3 as the test characteristic. A shape deviation of no more than 2.5 μm is prescribed as the admissible tolerance.

(20) As described, the workpiece itself is within this tolerance. However, if the measuring speed is too high, the shape deviation may under some circumstances no longer be correctly recorded. Instead, shape deviations outside the tolerance would then be determined, but would then be attributable to an unsuitable choice of the measuring speed and not to the workpiece itself. The method therefore generally envisages determining suitable values of the measuring speed which make a particularly high throughput possible, but continue to allow a sufficient measuring accuracy.

(21) In step S3, it is consequently also checked whether the current measurement parameter value can achieve a measurement result with which the test characteristic of the shape deviation has the desired admissible value of no more than 2.5 μm. The criterion according to which the shape deviation is to be no more than 2.5 μm forms an iteration criterion. Generally, the iteration criterion consequently concerns a check as to whether admissible values (no more than 2.5 μm) for a predetermined test characteristic (shape deviation) are achievable with a measurement parameter value.

(22) If this is the case, in step S4, a renewed iteration cycle is initiated. Therefore, the value of the measurement parameter is changed, and in the example described is increased. On the basis of the initial value a.sub.0 in this case first an increase takes place to the value a.sub.n−1 (see FIG. 2A). Subsequently, steps S2 (measuring) and S3 (checking the iteration criterion) are performed once again on the basis of the measured parameter value changed in step S4 (see arrow depicted by dashed lines in FIG. 2B). In the example shown in FIG. 2A, admissible values of the test characteristic are also obtained for the measuring speed, with the value a.sub.n−1 and also with the value a.sub.n selected in the subsequent cycle so that the iteration criterion is satisfied in each case.

(23) Therefore, in the course of a renewed iteration, the measuring speed is increased one more time, to be precise to the value a.sub.n+1 (see FIG. 2A). In this case, however, it is established in the course of step S3 that the test characteristic of this deviation has a value of more than 2.5 μm, and is therefore inadmissibly high. In other words, the measuring speed at the value a.sub.n+1 is so high that a sufficient measuring accuracy can no longer be achieved, and incorrect measurement results are obtained with respect to the shape deviation. The iteration criterion checked in step S3 is therefore not satisfied.

(24) According to a first aspect, it is therefore possible to continue directly with step S5, in which the value a.sub.n, for which the iteration criterion was last satisfied, is set as the measurement parameter value with which future measurements of workpieces of the same type of construction are to be carried out. On account of its increased measuring speed with respect to the initial value a.sub.0, this value makes a higher throughput possible, but at the same time also ensures sufficiently accurate measurement results.

(25) According to a further aspect, however, it is envisaged to carry out a further iteration cycle, wherein in step S4 the value of the measuring speed is set at an intermediate value a.sub.n+2, which lies between the two previous values an and a.sub.n+1 (see FIG. 2A). To be more precise, this intermediate value a.sub.n+2 lies between measuring speed a.sub.n, at which the iteration criterion was last satisfied, and measuring speed a.sub.n+1, at which the iteration criterion was not satisfied for the first time.

(26) As can be seen in FIG. 2A, this results in a measuring speed a.sub.n+2 that has been increased one more time with respect to the value a.sub.n with which potentially even faster measuring, and consequently a higher throughput, can be achieved. Whether a sufficient measuring accuracy can also be achieved with this measuring speed a.sub.n+2 is determined when steps S2 and S3 are performed once again. If the iteration criterion is satisfied, the value a.sub.n+2 is set in step S5 as the measurement parameter value for future measurement operations. If the iteration criterion is not satisfied, instead the value a.sub.n is set as the value for future measurement operations.

(27) Even if it is not shown separately, further iterations may be provided on the basis of the value a.sub.n+2, in order to further approach the measurement parameter value that makes the highest throughput possible, but still satisfies the iteration criterion. Therefore, the measurement parameter value may be alternately increased and reduced, depending on whether or not the iteration criterion is satisfied. In particular, whenever the iteration criterion is satisfied with the value a.sub.n+2, the measurement parameter value could be increased again as often as it takes until the iteration criterion is no longer satisfied for the first time. Subsequently, the measurement parameter value could again be reduced as often as it takes until the iteration criterion is satisfied again, and so on. This may be carried out as often as it takes until the highest number of iterations is reached. In this case, that measurement parameter value with which the iteration criterion was last satisfied and/or generally the highest measurement parameter value with which the iteration criterion was satisfied would then be selected.

(28) A method 300 according to a further exemplary embodiment is described below with reference to FIGS. 3A and 3B. FIG. 3A in this case again shows possible measurement parameter changes, whereas FIG. 3B shows a flow chart. This exemplary embodiment differs from the previous exemplary embodiment with regard to the changes carried out, of the measurement parameter and of the iteration criterion considered. Unless otherwise stated or evident, with respect to the further details it otherwise coincides with the method shown in FIGS. 2A and 2B.

(29) In the first step S10, first the initial value of the measurement parameter b.sub.0 is set. As a difference from the first exemplary embodiment, here, however, an upper limit value b.sub.max is selected as the initial value b.sub.0. This is used in step S20 to measure a workpiece with once again previously known properties and a preferred accuracy and to check the measurement result in step S30 with regard to the satisfying of an iteration criterion. The iteration criterion in this case concerns the reaching of inadmissible values of a test characteristic. The test characteristic is in this case once again defined as a shape deviation, which has inadmissible values if a shape deviation of more than 2.5 μm is established. In this exemplary embodiment, the iteration criterion is therefore always satisfied whenever there are inadmissible values with a corresponding shape deviation of more than 2.5 μm. In other words, the measurement parameter is to be changed as long as it takes and at least as many iterations are to be carried out as it takes before a test characteristic with admissible values is obtained.

(30) Once again, the measuring speed is considered as the measurement parameter. If it lies close to its upper limit value b.sub.max, a high workpiece throughput through the CMM is made possible. However, as expected, in this case inaccuracies that are too high occur, not allowing admissible values with regard to the selected test characteristic to be recorded. Therefore, it should be ensured by the iteration criterion that the measuring speed is reduced by default and successively until admissible values of the test characteristic can be recorded.

(31) For the example shown, it is determined in step S30 with the initial value b.sub.0 that the iteration criterion is satisfied because a shape deviation of more than 2.5 μm, and consequently an inadmissible value, is obtained. Since the iteration criterion is consequently satisfied, step S40 is then performed. In this step, the measurement parameter value b.sub.0 is changed, i.e., the measurement parameter value is decreased with respect to the initial value b.sub.0 to a value b.sub.n−1 (see FIG. 3A). Subsequently, the method returns to step S20, in order to measure the workpiece in accordance with this new measurement parameter value b.sub.n−1 (see the arrow depicted by dashed lines in FIG. 3B). Then, a new sequence of steps S20, S30 and S40 is performed. For the then-following value b.sub.n, the iteration criterion is likewise satisfied. In step S40, the value is therefore reduced further, to the value b.sub.n+1. For this value, it is however established in step S30 that the iteration criterion is not satisfied, since an admissible value of the shape deviation of less than 2.5 μm has been obtained. In principle, the method could immediately end in step S50 and the value b.sub.n+1 could be set as the measurement parameter value for future measuring operations. With this value, which is the first value with which the iteration criterion has not been satisfied, a high throughput but also a sufficient measuring accuracy can be achieved.

(32) However, in order to make an even higher throughput possible, step S40 may alternatively be performed once again to increase the value b.sub.n+1, at which the iteration criterion was not satisfied for the first time. The increase takes place in this case by obtaining an intermediate value b.sub.n+2, which lies between the value b.sub.n, for which the iteration criterion was last satisfied, and the value b.sub.n+1, for which the iteration criterion was not satisfied for the first time.

(33) If it is established that the value b.sub.n+2 likewise does not satisfy the iteration criterion, since admissible values for the test characteristic are also obtained for it, this value b.sub.n+2 can be set as the measurement parameter value for future workpiece measurements. If, however, the iteration criterion is satisfied with the value b.sub.n+2 (i.e., inadmissible values of the test characteristic are once again obtained), it can be reduced one more time, but typically in such a way that a then-following and not separately shown value b.sub.n+3 lies between the values b.sub.n+2 and b.sub.n+1. This can be repeated as often as desired, in order to approach that measurement parameter value with which the highest throughput is achievable along with a still sufficient accuracy. Alternatively, the method may end when a prescribed highest number of iterations or workpiece parameter changes has been reached in step S40.

(34) A further exemplary embodiment is described below with reference to FIGS. 4A to 4C. FIG. 4A and FIG. 4C in turn represent diagrams showing possible changes of values of the measurement parameter, while in FIG. 4B, a flow chart of method 400 is shown. The method 400 of this exemplary embodiment differs from previous exemplary embodiments mainly with regard to the setting of the initial value. Unless otherwise stated or evident, the method otherwise coincides with the previous examples.

(35) First with reference to FIG. 4A, it can be seen in FIG. 4B that the initial value c.sub.0 is not selected in step S100 as one of the limit values c.sub.min or c.sub.max. Instead, it is set as a mean value between these limit values c.sub.min, c.sub.max, although other values between the limit values c.sub.min, c.sub.max are also possible. In step S200, on the basis of this initial value c.sub.0, a workpiece that once again has known properties, and in particular a typical accuracy, is measured for the first time. In step S300, it is then determined whether or not, with the value c.sub.0, the test characteristic of a shape deviation has a desired admissible value.

(36) In the exemplary embodiment shown in FIGS. 4A to 4C, the measurement parameter is once again the measuring speed. As shown in FIG. 4A, it is established in step S300 for the value c.sub.0 that a sufficient accuracy can be achieved with this measuring speed.

(37) When performing step S300 for the first time and reaching the desired measuring accuracy in the way described, it is therefore possible to change the measurement parameter in a way that increases the throughput and that the iteration criterion concerns the acquisition of admissible values of the test characteristic. To be more precise, it is specified that the iteration criterion concerns the acquisition of a shape deviation of no more than 2.5 μm.

(38) In step S400, the value c.sub.0 is then increased to c.sub.1, which in principle would make possible a higher throughput along with a serial measurement of multiple workpieces. Subsequently, as shown by dashed lines in FIG. 4B, a renewed measurement S200 of the workpiece is carried out and the iteration criterion is once again checked in step S300.

(39) Since this criterion is once again satisfied, in a further step S400 the measuring speed is increased to the value c.sub.n+1. Then, however, it is established in step S300 that, with this measuring speed, a sufficient accuracy can no longer be achieved. By analogy with the previous example, the last admissible measuring speed could then be set in step S500 as the measuring speed for future workpiece measurements. Alternatively, it may be attempted to further approach the measuring speed with the highest throughput along with a still sufficient accuracy and for this purpose to decrease the value c.sub.n+1 one more time to an intermediate value c.sub.n+2 and to continue in a way analogous to the previous exemplary embodiments.

(40) For the sake of completeness, it should be mentioned that in the previous exemplary embodiments, a value newly selected in step S400 can be selected in each case as a mean value between the values on either side of it, including the limit values. The value c.sub.n may for example be selected as a mean value between c.sub.0 and c.sub.max (c.sub.n=[(c.sub.0−c.sub.max)/2]4). The same applies to the value c.sub.n+1, which may be selected as a mean value between c.sub.n and c.sub.max (c.sub.n=[(c.sub.n−c.sub.max)/2]4).

(41) Furthermore, in this case (but also in the case shown in FIG. 4C) it may be envisaged first to use the upper and/or lower limit value c.sub.min, c.sub.max for a measuring process. Otherwise, a measuring procedure in which these limit values c.sub.min, c.sub.max are not reached at all may occur. By initially checking the limit values c.sub.min, c.sub.max, it can also be ruled out that a sought optimum solution coincides with these values, and therefore under some circumstances a more exact iteration on the basis of the initial value c.sub.0 is no longer required.

(42) FIG. 4C shows the situation when it is established that a sufficient measuring accuracy cannot be achieved for the initial value c.sub.0 (i.e., shape deviations of more than 2.5 μm have been recorded). When step S300 is performed for the first time, it is then specified that the measurement parameter is to be changed in a way that makes a higher measuring accuracy possible (i.e., the measuring speed is to be reduced). It is also specified that the iteration criterion concerns the obtainment of values of the test characteristic of the shape deviation that are not admissible (i.e., measurement parameter changes should take place at least as long as it takes until admissible values are obtained). In step S400, the speed is therefore reduced and measuring and checking according to steps S200 and S300 are performed once again.

(43) As shown in FIG. 4C, the iteration criterion is also satisfied for the next measurement parameter value c.sub.n, but not for the then-following value c.sub.n+1. Since admissible shape deviations are obtained with the value c.sub.n+1, this value could be set according to step S500 from FIG. 4B directly for being used in future workpiece measurements. By analogy with the explanation given above with respect to FIG. 3A, this value may however also be increased one more time to the intermediate value c.sub.n+2, which lies between the values c.sub.n and c.sub.n+1. In this way it can be attempted to further approach the measuring speed with the highest throughput along with a still sufficient accuracy.

(44) Also in this case, the measurement parameter values may be selected in each case in such a way that they form a mean value between the values on either side of them, as described above with regard to FIG. 4A.

(45) It goes without saying that the aforementioned exemplary embodiments can also be applied in a combined way to a single workpiece. If, for example, a workpiece has two regions or test elements to be measured, a parameter increase according to the first exemplary embodiment (see FIG. 2A and FIG. 2B) may be performed for the first region and a parameter reduction according to the second exemplary embodiment (see FIG. 3A and FIG. 3B) may be performed for the second region.

(46) Similarly, the case may occur where there are different iteration stages for each region or test element to be measured of an individual workpiece. For example, a successive increase of the measurement parameter may also take place for a first workpiece region to be measured, for example between the values a.sub.n−1 and a.sub.n in the first exemplary embodiment. For the second region, however, it may be that the iteration criterion has already no longer been satisfied, so that an intermediate value is selected there according to the value a.sub.n+2 from FIG. 2A. Consequently, the measuring speed can be increased still further for one region, while the measuring speed is at least slightly reduced for a second region.

(47) It also goes without saying that, for a given workpiece, a choice of values may be performed in the way described above not only for one individual measurement parameter. Instead, after the described setting of a suitable measuring speed value, other measurement parameters may also be considered, for example a probing force or an acceleration.

(48) Furthermore, the inverse value of the speed may also be considered as a measurement parameter, which would then have to be reduced as much as possible to obtain admissible values.

(49) The measurement parameter values that have been determined by any of the aforementioned examples, and in the end set for the future measuring of workpieces of the same type of construction, may furthermore be stored for later use. In particular, they may be stored locally in the data memory 15 of the CMM or generally in a so-called test plan.

(50) In addition or alternatively, a log of the determination of parameters may be created, on the basis of which a user can obtain further information and/or create a database.

(51) It is understood that the foregoing description is that of the exemplary embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.