A Robust Pretravel Model for Touch Trigger Probes in Coordinate Metrology

Abstract

This paper describes a pretravel model for vertically-oriented touch trigger probes with straight styli widely used on coordinate measuring machines (CMMs). Advances in machine accuracy and tighter part tolerances leave probe error as one of the major error sources in dimensional measurement processes. Pretravel accounts for the majority of touch trigger probe errors and is caused by bending deflection of the probe stylus shaft. A trigger force model was first derived and used to model the probing force at the trigger instant. Experimental probing force data were used to validate the trigger force model. The trigger force model was then used to model the bending deflection of the stylus shaft at the trigger instant. Experimental x-y-z coordinate data were used to validate the pretravel model. It is shown that 24 points of equatorial data measured in the probe calibration process can be used to accurately identify the only model parameter. Also, 12 points of equatorial data provides reasonably good results. Model predicted pretravels associated with 1561 probe approach directions were compared with experimental pretravels (range: 16 μm, when using a 50 mm stylus) and the standard deviation of prediction errors can be reduced to around 0.6 μm.

Performance Evaluation and Compensation Method of Trigger Probes in Measurement Based on the Abbé Principle

“…According to Equation (11), the pre-travel of the probe is computed according to Equation (13). When the probe is triggered, the elastic compression displacement Lk is calculated according to Equations (11) and (15):…”

“…When the probe is triggered, the elastic compression displacement L k is calculated according to Equations (11) and (15):…”

“…The above analysis can be applied to any angle direction of the probe. Taking the evaluation of the probe's pre-travel characteristics in the XY plane as an example, and measuring the probe at different angle intervals within 360 • in the XY plane, the measurement results of the parameters of the probe are shown in Figure 8. ( ) T p r e F f L ′ = (14) When the probe is triggered, the elastic compression displacement Lk is calculated according to Equations (11) and (15):…”

“…In addition, similar experimental studies of probe error and compensation can be found. Shen et al describe a pre-travel model and a trigger force model for touch trigger probes to obtain the value of pre-travel and experimental validation [11]; P. A. Cauchick Miguel et al [12,13] report the error sources of probe performance [14,15] and the development of a test apparatus to be used for checking touch trigger probes, which uses a gauge block surface to contact the probe ball tip until the probe is triggered, at which point the coordinate position is displayed by laser.Mayer et al [16] proposes a 3D error correction model and an experimental method based on the use of a micrometer and a high magnification video camera. Li et al [17] proposes a compensation method for probe errors using a 3D error map based on the probe's mechanical model.…”

Measurement Errors of Coordinate Measuring Machine Produced by Touch Trigger Probe System

“…Among the errors associated with a touch trigger probe, pretravel variation is the most important type. Many researches on the pretravel variation have been carried out [1][2][3] and several models to evaluate the influence of the pretravel variation proposed [4][5][6][7]. In these researches, approaching speed of probe was limited at a low value and no extension bar used.…”