DE19739674B4 - Method for radius correction in coordinate measuring machines and device for carrying out the method - Google Patents

Abstract

Method for radius correction in coordinate measuring machines with scanning probing methods with measuring points arranged essentially along a measuring line, with a probe radius being used to conclude a surface point from the center of the probe in the direction of a local surface normal, with the following method steps:
1. Entering specifications for the scan run,
2. Execution of the scan run, with transverse movements being superimposed on the main scanning direction and continuous recording of axis and probe data,
3. Calculation of the local surface normals using local deviations of the measuring points from the main scanning direction,
4. Starting from the probe ball center, calculate the point of contact on the surface of the workpiece with the probe ball radius in the direction of the local surface normals calculated under point 3.

Description

The invention relates to a method for radius correction in coordinate measuring machines, whereby from the probe center towards the local surface normal the surface point is closed with the button radius, and an apparatus for performing the method.

According to the state of the art (VDI reports No. 751/1989) can with the coordinate measuring machines used today, any profile cuts Free-form surfaces or cuts and slope lines measured on general screw surfaces become. The result of such a measurement is initially the Coordinates of the stylus centers so that a back calculation on the workpiece surface can be carried out got to. to recalculation one can move from the center of the button to the point of contact passively measuring probe, in which the probing force by springs is generated, use the stylus deflection, which is essentially is perpendicular to the workpiece surface. However, there is friction between the probe and the workpiece surface, which is always the case when scanning the case is, this method is often too imprecise. At a active probe, in which the probing force is only achieved using electromotive actuators is generated, this method is used to calculate back to the touch point basically not applicable.

There is a mathematical description the area to be scanned before, either in closed form or as a set of points or even as a set of points with normal vectors, this can be used for back calculation from the probe center to the probe point. Lies such target information not before, according to the prior art, there are several neighboring ones Lines scanned to get the area and thus the local normal direction to determine.

When measuring along an intersection line the center of the button is guided on a control surface, for example on a Level for a flat profile cut or on a cylinder for the slope of a Screwing. In this case, the button center lies on the control surface or at least nearby the control surface. The touch point the touch element is generally not in the control surface, but dependent on the angle between the control surface and the workpiece surface more or less far from the control surface.

According to the prior art the normal direction using a second parallel measuring line determined and thus the spatial Radius correction carried out. The actual measuring point on the line you are looking for one by extrapolation or interpolation.

The accuracy of the profile point found depends on it strongly from the line distance, the point density, the surface curvature, the angle between surface normals and control surface and the button radius.

This prior art method has in addition to the relatively large Inaccuracy the disadvantage that along two parallel measuring lines must be measured whereby the measuring time clearly increased.

The prior art includes a method in which, in a scanning method, the contact points are forced into a predetermined cutting plane by moving the probe ball according to the inclination δ of the normal ( n ) to the measuring plane by the amount r _t · sinδ outside the defined cutting plane or by means of vector control directly on the normal ( n ) is led along until touching. With this method, however, the scanning line is offset parallel so that it lies outside the cutting plane. However, the measuring points of a scan line are still arranged in such a way that they cannot be used to determine a surface normal.

According to the state of the art (CH-Z: Technische Rundschau issue 50, 1992, pages 20 to 25) is a measuring one Buttons with several degrees of freedom known. According to this state of the art become the measuring points (P) with the help of an empirically determined (e.g. from three neighboring measuring points) or the normal direction (s) derived from the target profile the suspected Touch point (B) recalculated: These three neighboring measuring points have to flat be distributed and allowed do not lie on a straight line if they are used to determine a surface normal should be used. How this flat distribution is achieved and over the way of obtaining these neighboring measuring points, whether by single point probing or scanning, nothing is said in the scripture. If the points are generated by scanning according to the prior art, one needs two or more measurement lines laterally spaced apart, to a flat To achieve distribution from which from three neighboring measuring points backwards to the normal direction and to the presumed point of contact. The recording of two or more measuring lines has the disadvantage, among other things, that this is prior art belonging Procedure is relatively time consuming.

From the state of the art ( DE 43 05 842 A1 ) a method for three-dimensional detection of a surface is also known. According to this prior art, the Z component of at least one further point is measured in the vicinity of each measuring point in two linearly independent directions. For this purpose, the measuring points are advantageously arranged on a spiral or meandering path, the two linearly independent directions on the one hand in Direction of web and perpendicular to it. This means that the determination of the local surface normals is carried out in such a way that the Z components of at least one further point are measured in two linearly independent directions. This method, which belongs to the prior art, has the disadvantage that this measurement requires at least one further, precisely defined measurement for the respective measuring point.

This also relates to the state of the art (special print from "Workshop and Operation ", 1989/10, Pages 849 to 854) belonging Method of measuring curved surfaces has the disadvantage that the measurement neighboring points to the actual measured values is required to to the surface normal close.

On the state of the art ("Werkstatt und Betrieb", 126 (1993) 11, pages 687 to 690) still a process in which digitizing with 3D touch probes he follows. In this method belonging to the prior art, there is none Determination of the local surface normals.

The basis of the invention technical problem is a method for accurate radius correction, Specify when scanning, for which only a single measurement line is required is. About that In addition, a method for scanning is to be specified in which the actual touch points lie more precisely in the control surface than in known methods. Furthermore, an apparatus for performing the method is to be specified become.

This technical problem is caused by a method with the features of claim 1 and by a Coordinate measuring device with the Features of claim 13 solved.

The method according to the invention consists in random or scheduled deviations the center of the button from the control surface to the local surface normal calculate.

Because local deviations of the measuring points from the measuring line used to set the local surface normal it is sufficient to determine that along only one measuring line workpiece to depart.

• 1. Eingabe von Vorgaben für den Scanlauf. Bei geregeltem Scannen sind dies insbesondere Scan-Startpunkt, Scan-Geschwindigkeit und Scan-Richtung und die Steuerfläche. Der Scan-Startpunkt kann bereits so bestimmt sein, daß der Berührpunkt möglichst nahe an der Steuerfläche liegt.
• 2. Veränderung der Hauptscanrichtung, so daß ihr Querbewegungen überlagert werden.
• 3. Ausführung der Scanbewegung. Es erfolgt eine kontinuierliche Aufnahme von Achsen- und Tastkopfdaten.
• 4. On-line-Berechnung der lokalen Oberflächennormalen und hieraus der von der Tastkugel berührten Oberflächenpunkte, das heißt der Tasterradius-korrigierten Berührpunkte.
• 5. Steuerung des Tastermittelpunktes, derart, daß der Berührpunkt möglichst nahe an der vorgegebenen Steuerfläche liegt.
• 6. Wiederholung der Schritte 3, 4 und 5 bis zur Beendigung des Scans.

The process advantageously proceeds in the following steps:

• 1. Entering specifications for the scan run. With controlled scanning, these are in particular the scan start point, scan speed and scan direction and the control surface. The scan start point can already be determined so that the point of contact is as close as possible to the control surface.
• 2. Change the main scanning direction so that its transverse movements are superimposed.
• 3. Execution of the scanning movement. There is a continuous recording of axis and probe data.
• 4. On-line calculation of the local surface normals and from this the surface points touched by the probe ball, ie the touch radius corrected touch points.
• 5. Control of the button center, such that the point of contact is as close as possible to the predetermined control surface.
• 6. Repeat steps 3, 4 and 5 until the scan is complete.

• a) Schritt 2 kann entfallen, und die Querbewegung kann durch entsprechende Ansteuerung der Achsen ausgelöst werden, ohne Vorgabedaten zu verändern.
• b) Schritt 2 kann auch dann entfallen, wenn die zufälligen oder durch Systemschwingungen erzeugten Abweichungen des Tasters aus der Steuerfläche heraus ausreichen, um die lokale Flächennormale auszurechnen. Es kann auch zu Beginn offenbleiben, ob nur zufällige oder auch planmäßige Querbewegungen stattfinden sollen, oder ob nur bei nicht ausreichenden zufälligen Querbewegungen zusätzliche Querbewegungen erzeugt werden.
• c) Die Berechnungen (Schritt 4) können off-line erfolgen. Dann entfällt Schritt 5. In diesem Fall können die Berechnungen auch anstelle vom Maschinenrechner vom Auswerterechner vorgenommen werden.
• d) Das Verfahren kann sowohl mit einem passiven als auch mit einem aktiven messenden Tastkopf, dessen Antastkraft nicht durch Rückstellelemente wie Federn erzeugt wird, sondern durch ansteuerbare Kraftgeneratoren, durchgeführt werden. Bei einem aktiven Tastkopf ist es vorteilhaft, die lokalen Oberflächennormalen nicht nur zur Tasterradiuskorrektur zu verwenden, sondern auch zur Ansteuerung des Tastkopfes zur Erzeugung der Antastkraft.

According to the invention, the method can be varied as follows:

• a) Step 2 can be omitted, and the transverse movement can be triggered by correspondingly controlling the axes without changing the specification data.
• b) Step 2 can also be omitted if the accidental deviations of the button or those generated by system vibrations from the control surface are sufficient to calculate the local surface normal. It can also be left open at the beginning whether only random or planned transverse movements are to take place, or whether additional transverse movements are only generated if there are not sufficient random transverse movements.
• c) The calculations (step 4) can be carried out off-line. Then step 5 is omitted. In this case, the calculations can also be carried out by the evaluation computer instead of the machine computer.
• d) The method can be carried out both with a passive and with an active measuring probe, the probing force of which is not generated by restoring elements such as springs, but by controllable force generators. With an active probe, it is advantageous not only to use the local surface normals to correct the radius of the probe, but also to control the probe to generate the probe force.

At the measuring points can be on-line or off-line further corrections are applied, such as bending, thermal, linearity, geometry corrections or dynamic corrections. It is also a filtering, averaging or smoothing both the measuring points as well as the surface normal possible.

The method according to the invention can be general on all cuts with geometrically determined surfaces such as plane, cylindrical, Cone or polynomial surfaces be used in which the probe are guided in this control surface should.

• a) es ist nur eine Meßlinie erforderlich, das heißt die Meßzeit ist reduziert,
• b) es ist eine stabilere und genauere Berechnung der lokalen Normalenrichtung möglich durch die Punktewolke in der Nähe des gewünschten Meßortes,
• c) es kann eine on-line Korrektur durchgeführt werden, das heißt, es ist nur eine minimale Extrapolation bei sich ändernder Oberflächenrichtung notwendig.

The method according to the invention has the following advantages:

• a) only one measuring line is required, i.e. the measuring time is reduced,
• b) a more stable and precise calculation of the local normal direction is possible through the point cloud in the vicinity of the desired measuring location,
• c) an on-line correction can be carried out, that is, only a minimal extrapolation is necessary with a changing surface direction.

Another advantage of the method according to the invention is the increase the resolution, if, for example, with straight-line scans that are almost parallel to the axis inadequate resolution of probe or axis displacement measuring systems arise in the measuring line "steps". When using of the method according to the invention is the measured measuring line even with higher ones Resolution, than the measuring systems deliver, smooth.

Further details of the invention are the dependent claims refer to.

In the drawing is an embodiment shown, namely:

1 a probe with a ball probe on an inclined surface;

2 a probe according to the prior art;

3 a detail X of 2 ;

4 a probing according to the inventive method.

According to 1 is a probe ball ( 1 ) with a probe ball center (M). If on a workpiece surface inclined by α = 45 ° ( 2 ) the probe ball center (M) is guided in a control surface (S), for example a target cutting plane, then the actual contact point (B) on the workpiece surface is at a probe ball radius (r) of, for example, four millimeters ( 2 ) offset by a distance (d) of four millimeters from the cutting line (L) with the control surface (S).

According to 2 becomes an area normal n to calculate, a second scan line traveled according to the prior art. The first scan line is identified by a probe ball center (M) and the second scan line by a probe ball center (M '). The connecting line (M-M ') between the probe center points (M, M') defines a local surface normal ( n '). Then by interpolation or extrapolation to the cutting line (L ') between surface ( 2 ) and control surface (S) are closed, but measurement errors δ arise depending on the curvature and surface (see 3 ). If, for example, the radius of curvature is 20 mm and the distance between the scan lines is 0.4 mm, the surface at the cutting line (L) with the control surface (S) is measured incorrectly by approximately one μm.

According to the inventive method 4 from the transverse movement of the probe ball ( 1 ) to the local surface normal n closed, which is perpendicular to the connecting line (M'M ''). With the normal n and the touch sphere radius (r) is calculated from the center point (M) starting from the contact point (B).

In the case of an on-line probe radius correction using the method according to the invention, the center probe center (M) is controlled so that the center contact point (B) lies in the control surface (S) and thus with the cutting line (L) to be measured between the workpiece surface ( 2 ) and control surface (S) coincides, as in 4 shown. The measurement errors described then no longer occur, since then no interpolation or extrapolation is necessary.

If the described calculation of the spatial norms is carried out on-line during the measurement, it is thus possible to correct the position and direction of the current control surface so that the contact point (B) of the probe ball ( 1 ) is always moved on the line to be measured, or fluctuates only slightly around this line. The method according to the invention has the advantage that only one line has to be measured. On the other hand, it has the advantage that no more or hardly any extrapolation is required.

The process according to the invention results with an off-line calculation of the stylus radius correction (not shown) in principle the error δ described above, but is only one Scan line required. The length the scan line is not significantly enlarged by the transverse movement, because the Experience shows that already with an amplitude of 20 μm up to 100 μm the basis for the normal calculation is sufficient.

The normal information can be how in the case of two measuring lines, for the Extrapolation into the desired Profile cut can be used.

1

probe ball

2

Workpiece surface

M, M ', M' '

Tastkugelmittelpunkte

S

control surface

B B ', B' '

touch points (Measuring points)

d

offset

L, L '

cut lines

n . n '

surface normal

δ

measurement error

r

radius

α

tilt angle

Claims (15)

Method for radius correction in coordinate measuring machines in scanning probing methods with measuring points arranged essentially along a measuring line, whereby from the probe center point in the direction of a local surface normal with a Probe radius is concluded on a surface point, with the following procedural steps: 1. Input of specifications for the scan run, 2. Execution of the scan run, with the main scanning direction superimposed on transverse movements and a continuous recording of axis and probe data, 3. Calculation of the local surface normals with the help of local deviations of the measuring points from the main scanning direction, 4. starting from the probe ball center point, calculate the contact point on the surface of the workpiece with the probe ball radius in the direction of the local surface normals calculated under point 3. A method according to claim 1, characterized in that the local deviations of the measuring points from the measuring line in before the spin not generated by the measuring system. A method according to claim 1, characterized in that the local deviations of the measuring points from the measuring line are specified. Method according to claim 1, characterized in that the measuring line, along which the measuring points are essentially arranged, by cutting the workpiece surface ( 2 ) is defined with a given control surface (S). A method according to claim 4, characterized in that the local deviations of the measuring points from the measuring line through change the control surface (S) are generated. A method according to claim 1, characterized in that the Probe radius correction is carried out off-line. A method according to claim 1, characterized in that the Probe radius correction is carried out on-line. A method according to claim 4 and claim 7, characterized characterized that the Probe radius correction is carried out on-line and the probe is controlled in this way is that the touch points on the control surface lie. A method according to claim 1, characterized in that the measuring line is predetermined by target data. A method according to claim 3, characterized in that the local deviations from the measuring line have a given spatial or time frequency. A method according to claim 10, characterized in that the measurement data are filtered with the specified space or time frequency. A method according to claim 1, characterized in that this Method is carried out along at least two measuring lines. Coordinate measuring device with one Probe that carries a button, and a controller that contains the following hardware and / or software modules: - a normal calculation module, with which the local surface normal calculated with the aid of local deviations of the measuring points from the main scanning direction will run after of the scan run, the main scanning direction superimposed on transverse movements and a continuous recording of axis and probe data he follows, - on Radius compensation module that starts from the center of the probe ball contact point on the surface of the workpiece with the probe sphere radius in the direction of the calculated local surface normal calculated. Coordinate measuring device according to claim 13, in which the controller is a module for generating local deviations the measuring points from the main scan direction. Coordinate measuring device according to claim 13, in which the probe is controlled so that the actual touch points a given control surface lie.