DE19730471B4 - Method for scanning with a coordinate measuring machine - Google Patents

Abstract

Method for scanning with a probe of a coordinate measuring machine, wherein optionally first controlled scanning without a predetermined target contour and then controlled scanning with a predetermined or generated target contour or in reverse order in a single scan run, and that the switching between the two types of scan is carried out automatically.

Description

The invention relates to a method for scanning with a coordinate measuring machine.

There are two main operating modes for coordinate measuring machines:
The single point probing, in which the stylus is brought into contact with the workpiece surface at one point in order to record a measuring point and the scanning, wherein the stylus is guided in continuous contact with the workpiece surface and in the process takes measuring points in the machine cycle.

Two different methods are known for scanning:
In "controlled" scanning, the deflection of the probe is regulated in a control loop to a setpoint. For control purposes, only a surface is specified that intersects the workpiece surface and in which the probe ball is guided. This method can be used universally, but is susceptible to vibrations, since a control loop must be closed across the entire coordinate measuring machine, into which any frequencies can be coupled from the scanned contour. The achievable speed is therefore limited, given the accuracy, by the dynamics of the overall system.

With "controlled" scanning, on the other hand, the probe guided along a path predetermined by target data and the deviation of the actual from the target contour measured. This method is easier to master in terms of control technology and allows in Usually higher speeds than the controlled scanning, however, the target contour must be and the position of the workpiece be known. In limited Dimensions can vary Shape or position compensated by adapting the specified target data be, for example, by changing a variable Size that the original Target scan line changed.

Both known methods are subject thus inherent limitations in practical use.

According to the state of the art ( DE 195 29 574 A1 ) a method is known in which only controlled scanning is carried out. The probe of the measuring device only moves according to target data. This means that the coordinate measuring machine is given a target contour for probing from the outset. If necessary, a correction or adjustment of the target contour is carried out, for example if the probe is in accordance with the DE 195 29 574 A1 the edge ( 18a ) starts. In this case, all other points defining the target path are also displaced.

This also represents the state of the art belonging has the disadvantage that a Target path must be specified in each case.

State of the art ( DE 42 12 455 A1 ) is another method for measuring shaped elements on a coordinate measuring machine. According to this prior art, too, only a controlled scanning process is carried out with the disadvantages already mentioned.

The basis of the invention technical problem is to specify a scanning method that does not have these disadvantages, that is, on the one hand, not how controlled scanning, the specification of a target contour requires on the other hand, a higher one stability owns and therefore higher Scanning speeds allowed as controlled scanning.

This task is accomplished through a process solved with the features in claim 1.

Because the two scanning methods alternate possible are, and that ever automatically and smoothly switched between the two scanning methods as required The advantages of both methods can be combined in one method united.

According to the invention takes place between the two Scan types a smooth transition instead of. The smooth transition from regulated to controlled scanning can be switched a transition phase can be reached, within which the probe on the probe target path guided becomes.

• a) Es wird ein festgelegter Startpunkt angefahren;
• b) es wird in einer festgelegten Richtung mit einer vorgegebenen Steuerfläche geregelt gescannt;
• c) es werden aus den Ist-Daten der bereits gemessenen Kontur durch Extrapolation Soll-Daten für die zu messende Kontur berechnet;
• d) es ist eine Übergangsphase zwischen dem geregelten und dem gesteuerten Scannen vorgesehen;
• e) es wird gesteuert gescannt, indem der Tastkopf entlang einer Soll-Bahn, die aus den Soll-Daten für die Kontur berechnet wird, geführt wird;
• f) die Ist-Daten der Kontur werden überwacht;
• g) die Soll-Daten für die zu messende Kontur werden, falls notwendig, angepaßt;
• h) der Scanlauf wird nach Vorliegen eines Abbruchkriteriums abgebrochen oder mit einem anderen Schritt fortgesetzt.

The following steps are carried out in a scan run in the specified or in a different order:

• a) A specified starting point is approached;
• b) it is scanned in a defined direction with a predetermined control surface;
• c) target data for the contour to be measured are calculated from the actual data of the already measured contour by extrapolation;
• d) there is a transition phase between regulated and controlled scanning;
• e) it is scanned in a controlled manner by guiding the probe along a target path which is calculated from the target data for the contour;
• f) the actual data of the contour are monitored;
• g) the target data for the contour to be measured are adjusted if necessary;
• h) the scan run is terminated after an abort criterion has been met or is continued with another step.

In the following, the execution of the method according to the invention is described on a 3D coordinate measuring machine with a given basic accuracy and a given dynamic in the drive and in the scan control loop. The starting point is a measurement object with a contour that is not known more precisely, with a given accuracy should be measured as quickly as possible. The operator will therefore opt for the "scanning" operating mode, the contour line to be traversed being the cutting line of the workpiece surface with a control surface, for example a plane.

The method according to the invention now runs in following steps:

1st start:

According to the invention, the operator does not need any to make further specifications than the starting point, the control surface and the start direction of the scan within the control surface. The Probe ball is placed on the workpiece surface manually or automatically at the starting point, and the button deflection will automatically be close to the set point, this is usually the middle of the measuring range of the probe, set.

2. Pre-acceleration phase:

The coordinate measuring machine then begins to traverse the workpiece contour in the specified direction. The drives are controlled so that the deflection of the probe always remains within the measuring range, which means that the operating mode is controlled scanning. The scanning speed is gradually increased to an extent predetermined by the dynamics of the overall system up to a likewise predetermined first limit velocity v ₁ . This speed v ₁ is determined so that controlled scanning with the specified accuracy is possible with all common stylus combinations and under the conditions that occur in practice. This means that v _{1 is} lower than the speed possible with controlled scanning with the same accuracy requirement and also generally lower than the maximum possible speed under controlled conditions for controlled scanning.

3rd control phase:

According to the scan line is described in the Now proceed until a sufficiently large piece of the workpiece contour has been measured in order to determine the position, the direction and curvature of the to determine the scanned contour. For this, known methods, such as the least squares method. Also in the pre-acceleration phase recorded measured values can used to determine the sizes mentioned become. The predetermined accuracy with which they are determined have to, depends on a variety of parameters, such as the dynamics of the overall system or the measuring range of the Probe.

4th transition phase:

From the measured position, direction and curvature the previously scanned contour becomes a piece of the contour to be scanned extrapolated as the target scan line. This is the prerequisite for controlled Scanning accomplished. The coordinate measuring machine provides the operating mode automatically switches to controlled scanning. It should on a transition be observed that does not lead to the coupling of vibrations. This Connection can according to the transition procedure described below respectively. In practice, however, filtering the measurement data sufficient to transition effects to avoid.

5th tax phase:

The probe of the coordinate measuring machine is now guided along the now known target contour, the known corrections, such as probe radius, deflection, centrifugal force and bending correction, being applied to the extrapolated scan line in order to determine the target path of the probe. Since controlled scanning generally allows a higher speed than v ₁ , the scanning speed is first increased in a further acceleration phase to a second limit speed v ₂ , which is then maintained until an abort criterion occurs. The process of calculating the target contour from the known actual contour or from a piece of the actual contour is repeated as soon as a target contour has been calculated for the first time in order to extend the target path of the probe. If there are deviations between the actual and target contours, it is also possible to use known methods, for example according to DE 197 01 693.6-52 , the target contour to be adjusted to prevent the probe from moving out of the measuring range of the probe.

• a) Der Anfangspunkt des Scans, gegebenenfalls ein vorgegebener Endpunkt oder beispielsweise der Rand des Meßbereichs des Koordinatenmeßgerätes sind erreicht. In diesem Fall wird der gesamte Scan beendet.
• b) Die Schwingungsamplitude des Systems überschreitet ein vorbestimmtes Maß. Solche Schwingungen müssen sich noch nicht in den Meßwerten äußern, beispielsweise bei einer Regelkreisschwingung zwischen Achsen und Tastkopfwerten, können aber auf eine be ginnende Instabilität hinweisen. In diesem Fall wird der Scan mit einer niedrigeren Geschwindigkeit fortgesetzt, oder es wird auch schon bei einer niedrigeren Geschwindigkeit als v₁ auf gesteuertes Scannen umgeschaltet.
• c) Es tritt eine Kollision ein, oder die Tastkopfauslenkung über- oder unterschreitet ein vorbestimmtes Maß. In diesem Fall kann der Scan abgebrochen werden oder an einem zurückliegenden Punkt der Scanlinie wieder mit der Vorbeschleunigungsphase neu angesetzt werden, gegebenenfalls mit veränderten Parametern oder verlängerter Regelphase.
• d) Die notwendige Anpassung der Sollkontur durch Abweichung zwischen Ist- und Sollkontur überschreitet ein vorbestimmtes Maß. In diesem Fall wird der Scan mit der Geschwindigkeit v₁ mit der Regelphase fortgesetzt, wobei wiederum auf einen ruckfreien Übergang zu achten ist.

A termination or a switchover between the regulation and control phase takes place if one of the following termination or switchover criteria, which are continuously monitored, is fulfilled:

• a) The starting point of the scan, possibly a predetermined end point or, for example, the edge of the measuring range of the coordinate measuring machine has been reached. In this case, the entire scan is ended.
• b) The vibration amplitude of the system exceeds a predetermined level. Such vibrations do not yet have to be expressed in the measured values, for example in the case of a control loop oscillation between axes and probe values, but can indicate an instability that is starting. In this case, the scan is continued at a lower speed, or a switch is made to controlled scanning even at a speed lower than v ₁ .
• c) A collision occurs, or the probe deflection exceeds or falls below a predetermined level. In this case, the scan can be stopped or restarted at a previous point on the scan line with the pre-acceleration phase, if necessary with changed parameters or an extended control phase.
• d) The necessary adjustment of the target contour due to the deviation between the actual and target contour exceeds a predetermined dimension. In this case, the scan continues at the speed v ₁ with the control phase, again ensuring a smooth transition.

By specifying a variety of Parameters, for example for the speed or method of web extrapolation the inventive method to the properties of the coordinate measuring machine, if necessary also the respective measurement task, customized become. Accordingly, you can further termination or switchover criteria can also be defined.

If known, by default a target contour piece the scan run also start with controlled scanning.

Further details of the invention can do that dependent claims be removed.

In the drawing is an embodiment illustrated of the invention, namely:

1 a process flow according to the invention with a transition from regulated to controlled scanning;

2 a modified embodiment;

3 a device used.

According to 1 the method according to the invention is shown with a transition from regulated to controlled scanning.

It is assumed that part of the actual contour I _{1 has been} determined in previous scan phases. Based on the area B _{1 of} the actual contour, an averaged actual contour I _{M is} determined by known evaluation methods, for example by averaging, smoothing, filtering, spline fitting and so on. This averaged actual contour I _M is extrapolated, likewise with the aid of known methods, preferably by means of a constant and twice continuously differentiable connection with constant spatial curvature, in order to determine a desired contour S ₂ at least in a region B ₂ . I ₁ , I _M and S ₂ describe the coordinates of the workpiece surface in any, but fixed, coordinate system, with the usual corrections, such as probe ball radius, geometry, bending, linearity, temperature or dynamic corrections, all or some can be carried out, but can also be left to subsequent evaluation steps. The target path of the probe head T ₂ , at least in the area B ₂ , is in turn calculated from S ₂ , taking back the target deflection of the probe head and, if necessary, the above-mentioned corrections.

While the data used for the extrapolation only reach point P ₁ , the probe ball will currently be located at point P ₂ . At this point, if the previous phase was the control phase, a control movement of the probe will generally also take place due to the control deviation between the actual contour I and the target contour S ₂ in order to set the target deflection of the probe again. This means that the probe path T does not correspond to the desired path T _{2 in} terms of position, speed and acceleration in P ₂ .

In the transition phase, therefore, a regulated movement is inserted, which leads the probe over its path T ₂ to its target path T ₂ . This control can be constructed analogously to controlled scanning, except that it is not the deviation of the actual deflection of the probe head from the target deflection that serves as the input variable of the controller, but rather the deviation of the actual position of the probe head from the target path T ₂ . As with controlled scanning, the deflection of the probe is irrelevant, but can be monitored. As with the actual scan control system, suitable parameters can be used to ensure that the coupling of vibrations is as low as possible. If necessary, a corresponding transition of the controller input variables can be ensured by correspondingly shifting the desired path T _{2 of} the probe (not shown in FIG 1 ). In point P ₃ the probe is sufficiently close to the target path T ₂ so that the control phase can begin.

Instead of using arithmetic methods in area B ₁ to determine an averaged actual contour, the control loop can also be changed continuously in area B ₁ , for example by reducing the control loop gain, until at P ₁ the probe hardly reacts to the probe deflection, so that the jerk is almost jerky the tax phase can be switched ( 2 ).

With a suitable form of the control loop change, the probe head in area B ₁ follows the high-frequency fluctuations of the workpiece surface less and less and thus automatically generates a path that continuously and twice continuously differentiates into the desired path for the probe head for the control phase and from that by extrapolation the target data for the probe path valid from P ₁ are calculated.

The reverse transition from controlled to regulated scanning will usually be associated with the existence of an abort criterion. Depending on the type of termination criterion, the pre-acceleration phase from standstill or, after braking to a speed close to v ₁ , the control phase is started. In both cases In addition to the braking phase, which can be controlled, len no further transition phase is required.

According to 3 the components of a device used are shown.

The probe ( 1 ) is controlled by the drive controller ( 4 ) and drives ( 2 ) to a specified starting point or to a previous point on the scan line.

Subsequently, a predetermined control surface is scanned in a defined direction in a controlled manner, with a measured value acquisition system ( 3 ) records the delivered measured values. The actual data from the data acquisition system ( 3 ) the machine computer ( 8th ) which calculates target data for the contour to be measured from the actual data of the already measured contour by extrapolation.

After a transition phase, scanning is carried out in a controlled manner by 1 ) along a target path T ₂ , which is calculated from the target data for the contour.

A monitoring unit ( 5 ) monitors the actual data of the contour.

The target data are adapted for this contour to be measured. This task is performed by a computer module ( 6 ) accepted.

If there is an abort criterion that a monitoring unit ( 7 ) is monitored, the scan run is interrupted or interrupted and resumed with another step.

The actual measured values are sent to the evaluation computer ( 9 ) passed on. This is also used to specify scan parameters or target data for individual web sections.

1

probe

2

drives

3

data acquisition system for the actual

Data

4

drive controller

5

monitoring unit for the Actual data

6

unit for the Adjustment of the target

Data

7

monitoring device for the

termination criteria

8th

machine computer

9

evaluation computer

I

actual contour

I ₁

Actual contour in the area B ₁

I _M

averaged actual contour

B ₁

Area of controlled scanning

(Control phase)

B ₂

Area the transition phase and the

controlled scanning

B ₃

Area with reduced control loop ver

Strengthening

S ₂

Target contour in the area B ₂

T ₂

Target path of the probe there in the area B ₂

T '

Track section with regulated Movement of the

probe

P ₁ , P ₂ , P ₃

Points

Claims (21)

Procedure for scanning with a probe Coordinate measuring device, wherein optionally first controlled scanning without a predetermined target contour and then controlled scanning with a predetermined or generated one Target contour or in reverse order in a single scan run, and that switching between the two types of scans is carried out automatically. A method according to claim 1, characterized in the existence smooth transition between the two types of scan. Method according to Claim 2, characterized in that the jerk-free transition from regulated to controlled scanning takes place by interposing a transition phase, and in that in the transition phase the probe ( 1 ) is guided to the probe head path (T ₂ ). A method according to claim 3, characterized in that in the transition phase a controlled movement of the probe ( 1 ) takes place. A method according to claim 1, characterized in that the following steps in a scan run te are carried out: a) a specified starting point is approached; b) it is scanned in a defined direction with a predetermined control surface; c) target data (S ₂ ) for the contour to be measured are calculated from the actual data (I ₁ ) of the already measured contour by extrapolation; d) there is a transition phase with a controlled movement of the probe ( 1 ) instead of; e) it is scanned in a controlled manner by the probe ( 1 ) is guided along a target path (T ₂ ), which is calculated from the target data (S ₂ ) for the contour; f) the actual data of the contour are monitored; g) the target data for the contour to be measured are adjusted if necessary; h) the scan run is terminated after an abort criterion has been met or is continued with another step. A method according to claim 5, characterized in that the Steps a) to h) are carried out in a different order. Method according to Claim 5 or 6, characterized in that a computational averaging, smoothing, filtering or spline adjustment of the actual contour (I ₁ ) is carried out for the extrapolation, and that at least the position, direction and curvature of this averaged actual contour (I _M ) be used at the transition. Method according to claim 5 or 6, characterized in that for the extrapolation the control loop for the probe head deflection is adjusted so that the probe ( 1 ) automatically follows an averaged path in a limited area (B ₃ ). A method according to claim 5 or 6, characterized in that that for extrapolation the projection of the extrapolated target data into the control surface is used will, or that the Deviation of the target path from the control surface in position, speed and / or acceleration is limited. A method according to claim 5 or 6, characterized in that that this Occurrence of vibrations with a predetermined amplitude Abort or switchover criterion is. A method according to claim 10, characterized in that this Probe signal as an indicator of Vibrations is used. A method according to claim 10, characterized in that the measured actual contour is used as an indicator of vibrations. A method according to claim 10, characterized in that vibration sensors be used. A method according to claim 5 or 6, characterized in that that this Occurrence of a particular probe deflection or collision is the termination criterion. A method according to claim 5 or 6, characterized in that that after an abort of the scan run at a point of the measured contour is continued. A method according to claim 5 or 6, characterized in that that after aborting the scan run with changed parameters continued becomes. A method according to claim 16, characterized in that after aborting the scan run continued at reduced speed becomes. A method according to claim 1, characterized in that before Start of the scan run at least for individual pieces of the contour to be measured, target data are specified. A method according to claim 18, characterized in that before At the beginning of the scan run it is determined which pieces of the contour with which scanning method should be measured. A method according to claim 1, characterized in that as a button ( 1 ) a mechanical probe is used on the coordinate measuring machine. A method according to claim 1, characterized in that as a button ( 1 ) the coordinate measuring machine an optical button is used.