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CMM DYNAMIC ERROR ANALYSIS , CONTROL AND COMPENSATION

Many Coordinate Measuring Machines (CMMs) were originally designed for only quasi-static touch trigger probe based operation, and hence dynamic performance was a secondary consideration. With the emergence of analog scanning touch probes and non-contact laser digitizers, analysis, control and compensation of CMM dynamic errors must be re-examined. This paper discusses design improvements and experiments for an open architecture motion controller equipped 3-axis bridge CMM. To correct for yaw error, a parallel drive system, including cross-coupling control and continuous error compensation, is implemented. Position Sensing Detectors (PSDs) are used to confirm the yaw correction and, additionally, to correct for air bearing compliance based pitch error. Real-time laser interferometer data further confirms the faithfulness of the PSD signal. INTRODUCTION The touch trigger probe facilitated automation in coordinate metrology, and contributed profoundly to CMM structure design. Low measuring speeds and accelerations where anticipated, and software error compensation was not initially provided. To attain intrinsic accuracy, a rigid but massive steel structure was adopted, with compressed air bearings used to avoid stick/slip friction. Field adjustment of squareness, etc., further increased compliance. To avoid kinematic redundancy, the common 3axis bridge style design was motorized on only one side. This choice adversely affects dynamic accuracy above low accelerations. As the manufacturing community moved away from inspection based quality towards process control strategies, the need to measure more frequently lead to the need to measure more quickly. This requires accelerations that soon compromise accuracies achievable with conventional CMM structures and motion control systems. Analog scanning touch probes and laser digitizers collect data at much higher rates, but, particularly for analog probes, require that dynamic accuracy be maintained throughout changes in measuring speeds and directions. Analysis of CMM dynamics errors has previously been reported by Weekers and Schellekens [1], who used displacement sensors near the major machine joints, with separate laser interferometer correlation. For a bridge style CMM, using a speed of 100 mm/s, and acceleration of 170 mm/s. Mu and Ngoi [2] observed angular errors of up to 5 arcseconds. A Finite Element Analysis (FEA) model was proposed, but it was acknowledged that parameter uncertainties require inclusion of realtime sensing. Dong et al. [3,4] made similar observations, and noted the influence of air bearing design on bridge pitch error (Figure 1). The CMM bridge dynamic yaw error is usually the most significant factor. A center driven approach has been developed [5], but is not widely adopted. Alternatively, the bridge can simultaneously be driven from both sides [6], with risk of damage should a malfunction occur. The remainder of this paper presents experimental observation of dynamic errors for a typical bridge style CMM, and describes improvements achieved by adding a slave scale and slave drive motor. A new real-time compensation strategy based on Position Sensing Detectors (PSDs) is presented, with results confirmed by comparison to high speed laser interferometer data. Dynamic error effects on measurement accuracy, and improvements achieved with the proposed strategies, are presented. OPEN ARCHITECTURE CONTROLLER CMM The CMM motion controller used for the research is based on an Intel® Pentium® III processor, Servo-To-Go [7] interface card, and Phar Lap ETS [8] real time operating system. The loop closing frequency is 1250 Hz. Renishaw® RGH22 linear scales are used for position sensing. Pulse width modulated amplifiers, DC motors, rack and pinion gearing, and air bearings are used for all axes. Details were presented in an earlier paper [9]. PARALLEL SCALE AND DRIVE For a bridge style CMM, oscillating yaw errors arise during acceleration. This can be measured by adding a second linear scale at the slave side, and executing a trapezoidal velocity profile. As observed in Figure 3(a), there is first a positive error of 10 arcseconds during acceleration, oscillation from 4 to -6 arcseconds during constant speed motion, and then an error of -12 arcseconds during deceleration. To reduce this error, a parallel slave drive was added (Figure 4) together with a cross-coupling control algorithm [10] (Figure 5). Continuous geometric error compensation within the feedback loop is essential to maintain bridge squareness. Within the Cross-Coupling Controller, the formula for calculating the reference signal offset n M for period n is: ( ) sync smooth R n p i n d v ff M K K K S K D K V ε = + + +