Many previously installed Coordinate Measuring Machines (CMMs) are equipped with only a touch trigger probe. Data collection rates are slower than one point per second, and geometric error correction was applied only after the data point was recorded. Most commonly, the CMM structure was a bridge style, supported by air bearings above a granite table, and with the corresponding axis driven from only one side. It is assumed that the low speeds and accelerations do not introduce an uncorrected dynamic error. With the introduction of continues analog (scanning) touch probes and non-contact laser digitizers, CMM system improvements are required to support the new sensors, provide higher data point collection rates, and to manage dynamic error. This paper describes a retrofit open architecture system that provides these improvements. The control computer is based on an Intel Pentium processor, uses the Phar Lap ETS real-time operating system, and is implemented as an embedded system. Setup, including PID tuning, is accomplished remotely using ethernet and an external graphical user interface. Geometric error compensation is applied continuously along the entire motion trajectory. Uncorrected dynamic yaw error was significantly reduced by adding a parallel drive to the opposite side of the CMM bridge, together with a cross-coupling control algorithm that adjusts the reference position signals that are sent to the two parallel motion control loops. The effect is to speed up the lagging side, and slow down the leading side so as to minimize the differential error. Experimental results illustrate the improved dynamic performance of the system.

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