Control device for coordinate measuring tools.

0001The invention relates to a method and an arrangement for automatically measuring a workpiece on a coordinate measuring machine, the stylus of the measuring machine being guided continuously along the workpiece surface and the signals emitted by the transducers in the probe when the stylus is deflected, both for forming the measured value and for Control of the machine drives can be used.

0002Such measuring methods continuously scanning the workpiece are referred to as "scan" methods or "scanning". In known scanning methods, the stylus of the probe is clamped with respect to two coordinate directions and moved in the direction of one of the clamped axes, while the stylus in the third coordinate is essentially perpendicular to the workpiece surface and slides along it. To maintain contact between the workpiece and the stylus and to maintain a constant probing force, the transducer used to measure the stylus deflection in this third coordinate is switched into the position control loop for the machine axis in question.

0003Since with this method the scanning direction is always bound to a machine axis, when the angle between the traversing direction and the workpiece surface becomes steep and exceeds about 60 °, it must be "clamped", ie the scanning direction and the measuring direction are interchanged, which means that the measuring process first is interrupted. In addition, the measuring force during scanning changes due to the not negligible friction between the stylus and the workpiece depending on the angle between the scanning direction and the workpiece surface.

0004From DE-OS 29 21 166 it is known to automatically switch between the scanning direction and the measuring direction when the control voltages in the control loops of the drive axes are the same for the measuring direction and the scanning direction.

0005Since the scanning direction is also bound to a specific drive coordinate of the machine in this method, changes also occur here in the measuring force exerted on the workpiece, which influence the measuring result. Because the resulting total measuring force is not kept constant but only the component of the measuring force in the axis regulated by the probe. In addition, the scanning process does not run smoothly, since only the speed in one drive direction, but not the speed along the actual path dependent on the workpiece geometry, is kept constant.

0006It is the object of the present invention to specify a scanning method and an arrangement suitable for carrying it out, which ensures the most uniform possible scanning process with constant total measuring force.

0007This object is achieved according to the characterizing part of the main claim in that<ul id="ul0001" list-style="none"><li>(a) the amount |<o>v</o>_should| the path speed resulting as a vector sum from the individual speeds v in the coordinate directions is specified as a setpoint and kept constant,</li><li>b) the stylus deflection or its amount |<o>A</o>_should| is also specified and with the measured stylus deflection or its amount |<o>A</o>is | is compared</li><li>c) one of the deviation Δ<o>A</o> formed between the measured and predetermined stylus deflection corresponding signal and a signal corresponding to the direction of the stylus deflection A and for correcting the direction <o>V</o>_e the web speed <o>v</o>_is is used.</li></ul>

0008According to the method according to the invention, the rigid separation between the scanning direction and the measuring direction is avoided. Rather, the speed on a path following the workpiece contour is kept constant, whereby the individual speeds in the drive axes of the machine may well fluctuate over time.

0009The method ensures that even in connection with probes in which the stylus is returned to its zero position by a spring, for example, and in which the measuring force exerted on the workpiece is proportional to this deflection, the resulting total measuring force is constant during the scanning process remains. Because with every deviation from the specified target value of the amount of the stylus deflection, the direction of the scanning movement changes in a manner which counteracts this deviation.

0010However, the method can also be advantageously carried out in conjunction with probes of the type described in DE-PS 22 42 355, which has active moving coil systems for applying a predetermined deflection-independent measuring force to the stylus of the measuring head. It is then expedient to steer the stylus in advance and with a constant measuring force that is independent of the amount of deflection and essentially perpendicular to the direction of the_Vexperience speed to apply. The entire deflection range of the stylus can thus be permitted as a control deviation, which enables higher travel speeds without loss of measuring accuracy.

0011The arrangement for carrying out the method comprises an arithmetic unit to which the output signals of at least two transducers of the probe assigned to the affected machine axes are fed and the amount of the stylus deflection and the amount of the total speed in the machine axes can be entered as a setpoint. This computing unit feeds the signals corresponding to the components of the newly calculated path speed to the speed control loops of the movable machine axes as guide variables.

0012The signals of all transducers assigned to the three spatial directions x, y, z are preferably fed to this computing unit in the stylus and the computing unit acts on the speed control loops of all three machine axes so that the position of the plane in which the workpiece is to be scanned can be freely selected.

0013The computing unit can be, for example, a process computer that controls all machine functions. However, it is also possible instead to use an autonomous microprocessor connected to the position and speed control loops to calculate the change in the scanning direction depending on the stylus deflection, which in turn is preceded by a control computer from which the amount of the web speed, the starting direction of the scanning movement, the Measuring force and the end position etc. can be specified.

0014The method according to the invention is explained in more detail below on the basis of the exemplary embodiment illustrated in FIGS. 1 and 2 of the drawings:<ul id="ul0002" list-style="none"><li>Fig. 1 is a schematic sectional view of a workpiece contour with the scanning line of the probe along it;</li><li>Fig. 2 shows a block diagram of an arrangement for controlling a measuring machine according to the invention.</li></ul>

0015In FIG. 1, 10 denotes a workpiece that is to be scanned on a coordinate measuring machine along its contour line 11. The probe ball 1, which is in permanent contact with the workpiece 10, therefore follows the scanning path 14 parallel to the contour line 11.

0016To ensure that the scanning process is carried out evenly and as error-free as possible, a controller is provided that controls the amount |<o>v</o>| the scanning speed with which the probe ball 1 moves on the curved path 14 is constant, that is to say applies to the drawn positions of the probe ball<maths id="math0001" num=""><img file="EP0211202A1_D0001.tif" /></maths>

0017At the same time, the measuring force F directed essentially perpendicular to the contour line 11 is kept constant in terms of amount, ie it applies<maths id="math0002" num=""><img file="EP0211202A1_D0002.tif" /></maths>

0018The block diagram of an arrangement which achieves this is outlined in FIG. 2: three motors 8a-c, on whose shafts tachogenerators 7a-c sit, represent the drive of a measuring machine (not shown in more detail here) move the measuring machine in the three coordinates x, y, z. The position signals p_x) p_y and p the signals A from the length measuring systems 13a, b and c assigned to the three machine axes_x, A_y and A_e.g. the signal transmitter 12a, b and c of the probe 15 measuring the deflection of the stylus 2 is linked to one another in a known manner by the control computer 6 of the measuring machine. The formation of measured values ​​and the display of the measurement result should not be discussed in more detail here.

0019To control the drive of the three machine axes x, y and z, a microprocessor 4 terminates on a circuit board 5. This microprocessor supplies, among other things, the manipulated variables v_x, v_y, v to the speed control loops 9a-c for the drive motors 8a-c. The electronic part of the control loops 9a, b and c is also located on the circuit board 5.

0020To carry out the scanning process shown in FIG. 1, the microprocessor 4 receives the amount vsoll of the web speed and the amount from the control computer 6 <o>A</o>_should| the button deflection. It is assumed here that the stylus 2 is reset by a spring system, so that the force<o>F</o> between workpiece 10 and probe ball 1 the probe deflection A is proportional. At the same time, the microprocessor 4 receives the signals A._x, A_y and A_e.g. the transducer 12a-c in the probe 15, from which the actual value of the amount of the probe deflection |<o>A</o>_is| and the direction of the stylus deflection A_e is calculated.

0021The microprocessor 4 compares the predetermined value |<o>A</o>_should| and the actual value |<o>A</o>_is| of the amount of the stylus deflection and then changes the direction of the travel speed if there are deviations ΔA between these two variables<o>v</o> according to the calculation rule presented below in simplified vector notation:<maths id="math0003" num=""><img file="EP0211202A1_D0003.tif" /></maths> In this spelling is <o>A</o>_e the unit vector in the direction of the stylus deflection and results from<maths id="math0004" num=""><img file="EP0211202A1_D0004.tif" /></maths>

0022The newly calculated vector of the web speed <o>V</o>₂ is therefore at a constant amount compared to the vector of the previous speed <o>v</o>₁ rotated in the direction of the button deflection A.

0023The new process speed calculated by the microprocessor v₂ is then split into its components v_X, v_y and V_v fed as a manipulated variable to the speed control loops 9a, b and c of the machine drives.

0024Since the adaptation of the scanning direction to the stylus deflection takes place continuously in a cycle derived from the time base of the microprocessor system, the movement of the probe head 15 adjusts itself automatically to the contour line 11 of the workpiece 10, apart from the permissible control deviation ΔA. Because as soon as the button steering |<o>A</o>is | the target value predetermined by the control computer 6<o>A</o>_should| exceeds, the direction of the web speed V changes in a manner that counteracts the deviation Δ A. If the computing cycle is fast enough in relation to the web speed, the probe 15 moves quasi-continuously along the workpiece contour.

0025In the example described, it was assumed that the stylus 2 is reset by a spring system. With such a probe, the stylus deflection is automatically almost vertical₂the workpiece surface and, because its amount is kept constant, the measuring force also remains <o>F</o> constant between workpiece 10 and probe ball 1.

0026The arrangement shown in Fig. 2 can also be used with touch plugs of the type described, for example, in DE-PS 22 42 355, which has no return springs but active moving coil systems for applying a measuring force independent of deflection to the stylus. The "spring characteristic" can be electrically simulated there by supplying the measuring force coils with a signal which is proportional to the deflection in the respective coordinate.

0027With such probes, however, it is more advantageous to apply the measuring force independently of the deflection, specifically in a direction derived from the current trajectory perpendicular to the path speed. The default value |<o ostyle="rightarrow">A</o>_should| for the stylus deflection, 0 can then be set or kept very small, so that the entire deflection range of the stylus 2 is available as a control deviation. This allows higher path speeds during the scanning movement.