Renishaw Plc

Ferranti International Plc

Erenishaw Plc

Alexander Tennant Sutherland

David Allan Wright

David R McMurtry

John Charles Ould

Tim Prestidge

Jean-Louis Grzesiak

A method of calibrating a measurement probe () mounted on a machine is described. The measurement probe () has a stylus () with a workpiece contacting tip (). The method comprises determining a probe calibration matrix that relates the probe outputs (a,b,c) to the machine coordinate system (x,y,z.). The method comprising the steps of scanning a calibration artefact () using a first probe deflection (d) to obtain first machine data and using a second probe deflection (d) to obtain second machine data. The first and second machine data are used to obtain a pure probe calibration matrix in which any machine errors are substantially omitted. Advantageously, the method determines the pure probe matrix numerically based on the assumption that the difference between the first and second machine position data is known.

Differential calibration

A method is described for calibrating apparatus including a measurement probe mounted on a machine, such as a machine tool. The machine is arranged to capture machine position data indicative of the position of the measurement probe and the measurement probe is arranged to capture probe data indicative of the position of a surface relative to the measurement probe. The measurement probe may be an analogue or scanning probe having a deflectable stylus. The first step of the method involves moving the measurement probe at a known speed relative to an artefact whilst capturing probe data and machine position data. In particular, the measurement probe is moved along a path that enables probe data to be captured that is indicative of the position of two or more points on the surface of the artefact relative to the measurement probe. A second step of the method comprises analyzing the machine position data and the probe data and determining from that data the relative delay in capturing probe data and machine position data (i.e. the so-called system delay).

Calibration method and apparatus

A method of calibrating a probe mounted on a machine in which the probe has a probe calibration matrix which relates the probe outputs in three orthogonal axes to the machine's X, Y and Z coordinate system. A datum ball mounted on the machine is bi-directionally scanned by the probe in one or more planes. For each plane, the mean direction of two approximate probe vectors in the plane is rotated about an axis orthogonal to that plane until the apparent material condition from the scan in each direction is the same. This process may be iterative. The mean values of the directions of the probe vectors for each plane are rotated, thus forming a corrected probe calibration matrix. The datum ball is preferably bi-directionally scanned in three orthogonal planes.

Calibration of an analogue probe

A method of measuring an artefact using a machine on which a measuring probe is mounted. The probe is brought into contact with the artefact and movement continued for a limited distance to deflect the stylus The machine and probe outputs are recorded whilst the probe is free and when the stylus is deflected. A model of the probe and CMM outputs during both contact and non-contact between the probe and artefact is fitted to the data to allow the contact position when the stylus contacts the artefact with zero force to be determined. The probe outputs may be fitted to the model individually to determine a single contact position. By using data during movement of the probe towards and away from the artefact, errors due to time delays may be corrected.

Use of surface measuring probes

A method of calibrating a scanning system comprising a machine and a measuring probe, includes the steps of error mapping the system statically and qualifying the stylus tip so that the system will provide accurate measurements, determining the positions of a number of datum points on the surface of an artefact with the probe stylus in contact with the workpiece and at zero deflection normal to the surface, scanning the surface through the datum points at a nominal stylus deflection and at the maximum speed which provides repeatable position measurements to make a second determination of the positions of the datum points, determining the errors attributable to the scanning process by subtracting the positions obtained in the first and second determinations, and storing the error values for correction of subsequent measurements of similar artefacts.

Method of calibrating a scanning system

An analogue probe having a stylus with a spherical tip of radius (r) is calibrated using a sphere of known radius (R) mounted on a machine. The stylus tip is driven into the sphere from a plurality of directions (at least 9), each nominally normal to the sphere surface, until the stylus has deflected a predetermined amount. The machine movement is then reversed, and probe (a,b,c) deflection outputs are recorded simultaneously with machine (X,Y,Z) axis positions until the stylus tip leaves the surface. The readings are extrapolated to obtain the (X,Y,Z) readings when the probe radial deflection is zero. The value of (R&plus;r) is determined from these readings along with the position of the sphere center giving a value with zero probe errors. Values of (R&plus;r) are also determined using a pre-selected radial deflection for each of the directions, by converting probe (a,b,c) outputs at that deflection to incremental machine (X,Y,Z) axis values using a trial probe transformation matrix. The differences in (R&plus;r) values from the value obtained by extrapolation are noted as an error in each case, and the trial probe matrix is then optimized until the errors are minimized using the transformation obtained by the calibration, the probe can be error mapped.

Calibrations of an analogue probe and error mapping

Velocity dependent measurement errors made by coordinate measuring machines (CMMs) are corrected by deriving a polynomial expression which relates components of the errors to the relative velocity between the probe and the workpiece. A calibration is performed to establish the constant of the polynomial for different stylus configurations of the probe and these constants are stored. During a measuring process the probe produce analogue output signals from which a trigger signal is generated to latch the output signals of the machine measuring devices. The probe and machine output signals are monitored and recorded at clocked intervals over a range of positions within which lies the position at which the machine readings were latched. Actual relative velocity values are calculated at each position and, using these values and the stored constants, the errors in the machine readings at each position can be calculated from the polynomial. Signalling delays introduced to reduce false triggering of the probe can thus be compensated.

Method of and apparatus for measuring workpieces using a coordinate

A workpiece surface 30 is digitized by scanning it with the stylus 8 of a scanning probe 5. Digitized coordinate data for many points P1-P6 on the workpiece surface are subjected to a filtering algorithm, in order to remove redundant data when the surface is relatively flat or planar. Top and bottom tolerance vectors TV0,BV0 are defined, starting from a first point P1 and lying at a given tolerance h above and below a second point P2. Next, a direct vector DV1 is defined between the point P1 and a third point P3. If the vector DV1 lies outside the tolerance band between the vectors TV0 and BV0, then point P2 is deemed to be required and is not filtered out. Otherwise, the data for point P2 is deemed to be redundant and is rejected. In this case, new tolerance vectors TV1 and BV1 are now defined, starting from point P1 and passing within a tolerance h above and below point P3. From the four vectors TV0,TV1,BV0,BV1, a pair of vectors TVn,BVn is selected for a further iteration of the process, based upon which of these vectors lies closest to the vector DV1.

Method for scanning the surface of an object

Method of controlling a coordinate positioning machine for measurement

Apparatus with a multi-axis mechanism to perform, say, measurements or tests, has at least one pair of temperature detectors individually for each of selected members of the mechanism, each pair of temperature detectors being arranged to sense temperature differences transversely across the associated member. The required positions of the temperature detectors are predetermined from a preliminary examination of the mechanism, so that the extent and direction of each predetermined aspect of thermal distortion within the mechanism, can be computed from simultaneously detected transverse temperature differences, employing algorithms devised as a result of the preliminary examination of the mechanism. Compensation for corresponding selected errors in the measurements or tests can be calculated from the computed extent and direction of each of the predetermined aspects of thermal distortion within the mechanism.

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Edinburgh, United Kingdom

Alexander Tennant Sutherland