DE19730471C5 - Method for scanning with a coordinate measuring machine - Google Patents

Abstract

A method for scanning with a probe of a coordinate, wherein first controlled scanning without predetermined nominal contour with a predetermined surface which cuts a workpiece surface and in which a Tastkugel is guided, and then controlled scanning with a generated target contour in a single scan is performed that the Switching between both types of scans is performed automatically and the following steps are performed in a scan:
a) a fixed starting point is approached;
b) it is scanned in a fixed direction controlled with a given control surface;
c) it is calculated from the actual data (I ₁ ) of the already measured contour by extrapolation desired data (S ₂ ) for the contour to be measured;
d) there is a transitional phase with a controlled movement of the probe head (1);
e) it is scanned controlled by the probe head (1) along a reference path (T _2), which (from the target data S ₂₎ is calculated for the contour, is guided;
f) the ...

Description

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

State of the art

Coordinate measuring machines essentially distinguish two operating modes:
The Einzelpunktantastung, wherein the stylus is brought at a point in contact with the workpiece surface to receive a measuring point and the scanning, wherein the stylus is guided in continuous contact with the workpiece surface and thereby receives measuring points in the machine cycle.

When scanning two different methods are known:
In "controlled" scanning, the deflection of the probe is regulated in a control loop to a setpoint. For control, only a surface is predetermined, which cuts the workpiece surface and in which the probe ball is guided. This method is universally applicable, but susceptible to vibration, since in this case a loop must be closed over the entire coordinate, in which any frequencies are coupled from the scanned contour. The achievable speed is therefore limited by the dynamics of the overall system given the accuracy.

On the other hand, with "controlled" scanning guided the probe along a predetermined path by target data and the deviation of the actual from the nominal contour measured. This method is regulatory easier to master and usually allows greater speeds as the controlled scanning, but must first be the target contour and the position of the workpiece be known. In limited Dimensions may vary Form or position compensated by adaptation of the predetermined target data be, for example, by switching on a variable Size that the original Target scan line changed.

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

According to the prior art ( DE 195 29 574 A1 A method is known in which only controlled scanning is performed. The probe of the measuring device moves exclusively according to target data. This means that the coordinate measuring machine is predetermined from the outset a target contour for probing. If necessary, a correction or adaptation of the desired contour is made, for example, if the probe according to the DE 195 29 574 A1 the edge ( 18a ) starts. In this case, all other points defining the desired path also experience a shift.

Also this belongs to the prior art method has the disadvantage that a Target path must be specified in each case.

The prior art ( DE 42 12 455 A1 ) is another method of measuring features on a coordinate measuring machine. Also according to this prior art, only a controlled scanning is done with the disadvantages mentioned above.

task

The The technical problem underlying the invention is that specify a scanning method that does not have these disadvantages, that is, that On the one hand, not how the controlled scanning, the specification of a Target contour requires, but on the other hand has a higher stability and thus higher Scan speeds allowed as the regulated scanning.

These Object is achieved by a method having the features in claim 1 solved.

According to the invention finds a smooth transition takes place between the two scan types. The smooth transition from controlled to controlled scanning can be done by interposing a transitional phase be achieved, within which the probe to 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.

In a scan, the following steps are performed in the specified order or in a different order:

• a) A fixed starting point is approached;
• b) it is scanned in a fixed direction controlled with a given control surface;
• c) it is calculated from the actual data of the already measured contour by extrapolation setpoint data for the contour to be measured;
• d) there is a transitional phase between regulated and controlled scanning;
• e) it is scanned under control by guiding the probe along a desired path calculated from the target data for the contour;
• f) the actual data of the contour are monitored;
• g) the nominal data for the contour to be measured are adjusted, if necessary;
• h) the scan is aborted after the abort criterion has been met or continued with another step.

In the following, the execution of the method according to the invention on a 3D coordinate measuring machine with a given basic accuracy and a given dynamics in the drive and scan control circuits. The starting point is a measurement object with a contour which is not known in detail and which is to be measured as quickly as possible with a given accuracy. The operator will therefore opt for the operating mode "scanning", wherein the contour line to be traveled is given as the intersection line of the workpiece surface with a control surface, for example a plane.

The inventive method is now running in following steps:

1st start:

According to the invention needs the operator to make no further specifications than the starting point, the control surface and the start direction of the scan within the control area. The Tastkugel is placed manually or automatically at the starting point on the workpiece surface, and the probe deflection will automatically close to the setpoint, this is usually the middle of the measuring range of the probe, set.

2nd pre-acceleration phase:

The coordinate measuring machine then begins to descend the workpiece contour in the predetermined direction. The drives are controlled so that the deflection of the probe always remains within the measuring range, that is, the mode is the controlled scanning. The scanning speed is gradually increased in a predetermined by the dynamics of the overall system dimensions also to a predetermined first speed limit _V1. This speed v ₁ is determined so that controlled scanning with the specified accuracy is possible with all conventional stylus combinations and under the conditions encountered in practice. Thus, v _{1 is} lower than the rate possible with controlled scanning at the same accuracy requirement, and also generally lower than the maximum possible controlled scanning speed under the current conditions.

3rd control phase:

According to the invention Scanning line in the described manner now traversed until a enough great Piece of Workpiece contour has been measured in order to determine, with a given accuracy, the position the direction and the curvature of the to determine the scanned contour. For this purpose, 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 it a variety of parameters, such as the dynamics of the overall system or the measuring range of the Probe.

4th transition phase:

Out the measured position, direction and curvature of the previously scanned Contour becomes a piece extrapolated the contour to be scanned as a target scan line. In order to is the requirement for controlled scanning fulfilled. The coordinate measuring machine provides automatically changes the operating mode to controlled scanning. It should on a transition be respected, which does not lead to the coupling of vibrations. This Connection can according to the transitional procedure described below respectively. In practice, however, is often a filtering of measurement data suffice for transitional effects to avoid.

5. Control phase:

The probe of the coordinate measuring machine is now guided along the now known nominal contour, wherein the known corrections, such as stylus radius, deflection, centrifugal force and bending correction, are attached to the extrapolated scan line to set the target path of the probe. Since controlled scanning usually allows a higher speed than v ₁ , the scan speed is first increased in a further acceleration phase up to a second limit speed v ₂ , which is then maintained until the occurrence of a termination criterion. The process of calculating the desired contour from the known actual contour or from a piece of the actual contour is, as soon as a desired contour has been calculated for the first time continuously repeated to extend the target path of the probe. If deviations occur between the actual contour and the desired contour, it can also be known by known methods, for example according to the DE 197 01 693.6-52 , The target contour are adapted to prevent migration of the probe from 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 beginnende 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.

An abort or a changeover between control and control phase takes place if one of the following abort or switchover criteria that are continuously monitored is fulfilled:

• a) The starting point of the scan, optionally a predetermined end point or, for example, the edge of the measuring range of the coordinate are reached. In this case, the entire scan is ended.
• b) The vibration amplitude of the system exceeds a predetermined level. Such oscillations do not yet have to be expressed in the measured values, for example in the case of a control loop oscillation between the axes and the probe head but may indicate an incipient instability. In this case, the scan will continue at a lower speed, or it will switch 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 aborted or readjusted at a previous point of the scan line again with the pre-acceleration phase, possibly with changed parameters or extended control phase.
• d) The necessary adjustment of the nominal contour by deviation between actual and nominal contour exceeds a predetermined level. In this case, the scan is continued at speed v ₁ with the control phase, again ensuring a smooth transition.

By Specification of a variety of parameters, such as the speed or the process of web extrapolation can be the process of the invention to the properties of the coordinate, possibly also to the respective measuring task, customized become. Accordingly, you can also further abort or switchover criteria can be defined.

Further Details of the invention can the dependent claims be removed.

embodiment

On The drawing is an embodiment of the invention, and show:

1 a method sequence according to the invention with a transition from controlled to controlled scanning;

2 a modified embodiment;

3 a used device.

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

It is assumed that a part of the actual contour I _{1 has been} determined in previous scan phases. From this, due to the region B _1, the actual contour is determined by known evaluation methods, for example by averaging, smoothing, filtering, spline fitting and so on, an averaged actual contour I _M. This averaged actual contour I _M is extrapolated, likewise by means of known methods, preferably by continuous and twice continuously differentiable connection with constant spatial curvature, in order to determine a nominal 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, the usual corrections, such as Tastkugelradius-, geometry, bending, linearity, temperature or dynamic corrections, all or some can be performed, but can also be left to subsequent evaluation steps. From S ₂ is again under back-calculation of the desired deflection of the probe and, if appropriate, the said corrections, the target path of the probe T ₂ calculated at least in the region _B2.

While the data used for the extrapolation only reach to the point P ₁ , the probe ball will actually already be in the point P ₂ . In this point, if the preceding phase was the control phase, due to the control deviation between the actual contour I and the nominal contour S ₂ , generally also a regulating movement of the probe takes place in order to set the desired deflection of the probe again. This means that the Tastkopfbahn T according to position, speed and acceleration in P ₂ does not correspond to the desired path T ₂ .

In the transition phase, therefore, a controlled movement is inserted, which leads the probe over a track T 'to its desired path T ₂ . This control can be constructed analogously to controlled scanning, except that the deviation of the actual position of the probe from the target path T _{2 is} not the deviation of the actual deflection of the probe from the target deflection is used as the input of the controller. As with controlled scanning, the deflection of the probe does not matter, but can be monitored. As with the actual scan control can be ensured in this scheme by suitable parameters that the vibration coupling is as low as possible. Optionally, by appropriate displacement of the target path T _{2 of} the probe for a steady transition of the controller input variables are provided (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 computational methods in the area B ₁ to determine an averaged actual contour, the control loop can be continuously changed in the area B ₁ , for example, by reducing the loop gain until at P _1, the probe barely reacts to the Tastkopfauslenkung, so that almost jerk-free the control phase can be switched ( 2 ).

With a suitable shape of the control loop change, the probe in the area B ₁ less and less follows the high-frequency fluctuations of the workpiece berfläche and thus automatically generates a path that merges steadily and twice continuously differentiable in the desired path for the control phase for the probe and from the valid by extrapolation from P ₁ valid target data for the Tastkopfbahn be calculated.

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

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

Thereafter, scanning is carried out in a fixed direction with a predetermined control surface, whereby a measured value acquisition system ( 3 ) detects the delivered measured values. The actual data from the measured value acquisition system ( 3 ) are sent to the machine computer ( 8th ), which calculates nominal data for the contour to be measured from the actual data of the already measured contour by extrapolation.

After a transitional phase, a controlled scan is performed by the probe ( 1 ) along a target path T ₂ calculated from the target data for the contour.

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

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

In the presence of an abort criterion, which is determined by a monitoring unit ( 7 ) is monitored, the scan is aborted or interrupted and resumed with another step.

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

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 area B ₁

I _M

averaged actual contour

B ₁

Area of controlled scanning (control phase)

B ₂

Area the transitional phase and controlled scanning

B ₃

Area with reduced control loop gain

S ₂

Target contour in area B ₂

T ₂

Target path of the probe it in the area B ₂

T '

Piece of track with regulated Movement of the probe

P ₁ , P ₂ , P ₃

Points

Claims (7)

A method for scanning with a probe of a coordinate, wherein first controlled scanning without predetermined nominal contour with a predetermined surface which cuts a workpiece surface and in which a Tastkugel is guided, and then controlled scanning with a generated target contour in a single scan is performed that the Switching between both types of scanning is performed automatically and that in a scan the following steps are performed: a) it is approached a fixed starting point; b) it is scanned in a fixed direction controlled with a given control surface; c) it is calculated from the actual data (I ₁ ) of the already measured contour by extrapolation desired data (S ₂ ) for the contour to be measured; d) it finds a transition phase with a controlled movement of the probe ( 1 ) instead of; e) it is scanned under control by the probe ( 1 ) along a target path (T ₂ ) calculated from the target data (S ₂ ) for the contour; f) the actual data of the contour are monitored; g) the nominal data for the contour to be measured are adjusted, if necessary; h) the scan is aborted after the presence of an abort criterion. Method according to claim 1, characterized in that the existence smooth transition between both scan types. Method according to Claim 2, characterized in that the smooth transition from controlled to controlled scanning takes place by interposing a transition phase, and in the transition phase the probe head ( 1 ) is guided on the probe target 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 or 4, characterized in that for the extrapolation a computational averaging, smoothing, filtering or spline fitting of the actual contour (I ₁ ) is performed, and that at least position, direction and curvature of this average actual contour (I _M ) on Transition to be used. Method according to claim 1 or 4, characterized that for extrapolation the projection of the extrapolated target data into the control surface is used, or that the Deviation of the target path out of the control surface in position, speed and / or acceleration is limited. Method according to claim 1 or 4, characterized that this Occurrence of a specific probe deflection or a collision as termination criterion.