Touch trigger probes are commonly used on coordinate measuring machines (CMMs) and computer-controlled machine tools in modern manufacturing environments. Lobing or pretravel variation in touch trigger probe applications is well-known. However, in commercial applications, effective error correction methods are not available.
Sensors and probes are devices through which coordinate measuring machines (CMMs) and computer numerically controlled (CNC) machine tools collect measurement data in modern manufacturing environments. Hard probes (ball, tapered plug, and edge probes) have been used for more than twenty years. See D. Genest, "Coordinate measuring machines (CMMs)", Quality in Manufacturing, 5 (2), 1994, pp. 21-23. The major shortcoming of these probes is that they require a certain amount of feel by machine operator. As such, they may result in inconsistency from one operator to another. A common solution is the touch trigger probe, which is used in manual and automated CMMs.
Touch trigger probes are available with interchangeable probe tips and extensions and the touch trigger probe automatically closes an electronic switch when the probe tip contacts a workpiece. See C. Butler, "An investigation into the performance of probes on coordinate measuring machines", Industrial Metrology, 2, 1991, pp. 59-70; T. Jarman et al, "Performance Characteristics of Touch Trigger Probes", Proceedings of Precision Metrology with Coordinate Measurement Systems, 1990, SME, Troy, Mich.; and J. Bosch "Evolution of Measurement", in Coordinate Measuring Machine and Systems, 1995, Marcel Dekker Inc., New York.
Touch trigger probes have been used on four- and five-axis machine tools. See G. Herrin, "Five axis probing", Modern Machine Shop, 65 (8), 1992, pp. 66-68 and, M. Lynch, "How a Touch Probe Works", Modern Machine Shop, 66 (10), 1993, pp. 142-144. The probe has become one of the basic building blocks for supporting untended machining in manufacturing cells and systems. They can be used to detect errors in part setup, part fixturing, improper tool use and tool wear, etc.
Touch trigger probes represent a key innovative technology in coordinate metrology in modern manufacturing processes and systems. The vast majority of probes used on CMMs are touch trigger probes. See C. Reid, "Probe Technology - Beyond Accuracy", Proceedings of Applying Imaging and Sensoring Technology to CMM Applications, 1993, SME, Nashville, Tenn. Most of these probes employ a kinematic seating mechanism for the probe stylus. However, there are errors associated with touch trigger probe application. Lobing or pretravel variation is a well-known type of probe error associated with touch trigger probe applications. See T. Hopp, "Computational Metrology", Proceedings of the 1993 International Forum on Dimensional Tolerancing and Metrology, Detroit, pp. 207-218 and S. Phillips, "Performance Evaluation", in Coordinate Measuring Machine and Systems, supra. Pretravel, caused by bending deflection of the stylus shaft, accounts for the majority of touch trigger probe errors. Pretravel errors are highly repeatable but can be adversely influenced by force variations encountered in different probe approach directions. This can lead to measurement errors caused by variations in stylus shaft bending prior to the trigger instant, i.e., when the threshold value is exceeded. Hysteresis and repeatability are other types of probe errors associated with touch trigger probe applications. See ANSI/ASME B89.1.12M-1990, "Methods for Performance Evaluation of Coordinate Measuring Machines", ASME, 1990, New York.
Errors introduced by the probes remain largely uncompensated although there have been attempts to do so. See Butler, supra and Jarman et al., supra. Commonly used methods dealing with probe lobing include probe calibration (probe datuming) and error mapping/vector compensation. Unfortunately, both methods are not effective in reducing probe lobing. Furthermore, advances in machine accuracy and tighter part tolerances make the probe error one of the major error sources in the measurement processes on CMMs and machine tools. See Bosch, supra and Phillips, supra.
The development of touch trigger probes greatly improved the versatility and usefulness of CMMs. See Bosch, supra. Touch trigger probes are also called switching probes or touch probes. Most touch trigger probes employ a kinematic seating arrangement for the stylus. FIG. 1 shows a typical touch trigger probe design. See Reid, supra. The tripod, which also works as the stylus holder, is seated on a kinematic seating arrangement. The tripod can be tilted when a force is applied to the stylus. Each tripod leg is supported by a kinematic seat formed by two cylinders, and the whole tripod is supported by six supporting cylinders. A spring is installed on top of the tripod to provide a spring force to make the tripod sit at the rest position before and after the probe stylus touches the workpiece during the measurement process. The stylus, which is usually made of steel or ceramic, serves as the means to contact the workpiece in the coordinate measurement process. The probe tip is a ruby ball with high-quality sphericity.
In the measurement process the probe is commanded to approach the workpiece at a constant speed (probe approach rate) when the probe is within the probe approach distance. See ANSI/ASME, supra. Before the probe is in touch with the workpiece, the tripod is held in its rest position by the spring force. No trigger signal is generated when the probe tip initially contacts the workpiece. Instead, the probe continues to move and the probing force, between the probe tip and the workpiece, will build up gradually until the force is large enough to tilt the tripod and cause a physical quantity to reach a threshold setting. A trigger signal is generated when the physical quantity exceeds a threshold in the trigger circuit. The trigger signal is used by the CMM or machine tool to latch the position counters or transducers to record the point coordinates at the triggering instant. The distance travelled by the probe from initial workpiece contact to generation of the trigger signal is known as probe pretravel.
The major constituent of probe pretravel is stylus bending. See Jarman et al., supra. One major characteristic of the touch trigger probe is that pretravel distances vary when the probe approaches the workpiece from different directions. See Reid, supra. This is because the magnitude of the trigger force varies when the probe approach direction varies. Pretravel-influencing factors include stylus length and material, spring force setting, and probe configuration, etc. See Butler, supra. Probe stylus length is usually decided by application needs. For example, a long stylus is needed when deep internal features are to be measured. Stylus material determines the deflection extent caused by the applied force. Spring force setting affects the force needed to trigger the switch of the probe. Other factors influencing probe pretravel include the form of the probe tip, the geometry of the stylus, and the cleanliness of probe and part to be inspected. Current practices in compensating probe pretravel include probe calibration and error mapping.
Probe calibration is also called probe qualification or probe datuming. It is done by measuring a number of points on a standard artifact, and the measurement data is used fit the feature of the artifact. High-quality spherical artifacts (reference balls) are typically used in the probe calibration process. Since the diameters of the reference ball and the probe tip (usually a ruby ball) are known, an "effective probe tip diameter" can be calculated by fitting the measurement data to a sphere. Usually the least-squares algorithm is used in the sphere fitting program. The effective probe tip diameter is slightly smaller than its true size because of probe pretravel. The effective probe tip diameter is the foundation for probe pretravel correction in the probe calibration method. It is simple and can be done quickly. However, it is not effective since pretravel distances are assumed to be the same in every probe approach direction.
The error mapping approach is similar to the probe calibration approach in that it takes measurements on a high-quality spherical artifact, but it needs a look-up table and interpolation to compensate for pretravel associated with various approach directions. See Jarman et al., supra. It typically needs many measurements on a spherical artifact to establish a good error map since the compensation effectiveness depends on the quality of the interpolation. This method takes more data-collection time and it needs more computation time and memory space to carry out pretravel prediction through interpolation.
There is a need for a pretravel model which can predict probe pretravel by taking its causes into account. The model should enable the compensation of probe pretravel by using a software correction system implementing the pretravel model.