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Using the OPTRA NanoGrid for Machine Calibration

True Position Measurement

Unlike interferometers and linear encoders, which can only perform one-dimensional measurements, the NanoGrid measures true planar position. This removes the traditional Abbe and orthogonality errors associated with misalignment of discreet axes of motion. The NanoGrid can also track multiple errors in the same setup, such as positioning accuracy and axis straightness.

Interferometer-Level Repeatability and Resolution

The NanoGrid A High Resolution system can resolve position increments down to a noise-limited 0.3nm with a repeatability of better than 10nm. This places the NanoGrid in the same realm as traditional laser interferometry, but the NanoGrid suffers none of the instability problems such as drift, sensitivity to air turbulence and length of optical path; that are normally associated with lasers.

Single Measurement Point

Other planar encoders on the market today are no more than two discreet sensors assembled in one unit. This configuration makes installation tolerances very tight and hard to optimize. These types of sensors are also very sensitive to yaw motions of the read head. The NanoGrid uses a single laser spot to image the grid grating and then extracts the X and Y information from this single image. The sensor can therefore tolerate yaw motions on the order of 2° and can even be used to measure yaw when coupled with a second sensor.

Coupled with OPTRA's Hi-Resolution phase processor and signal interpolation ISA card, the NanoGrid system has demonstrated its power as a machine and stage calibration tool. The output of this system is two position words on a PC bus, eliminating the need for counting cards or external interpolators. Position measurements can be sampled at a rate of 375kHz, which allows the NanoGrid to perform demanding tests such as scanning velocity stability, dynamic straightness, in-position stability and dynamic yaw. Furthermore, OPTRA can provide software tools for data analysis, such as device support through National Instruments? LabView software, or we can be contracted to develop specific customer test algorithms.

Real-time Calibration

Our customer in the automotive industry developed a precision ?pick & place? system to operate at a speed of 300 to 400 insertions per minute over an area of 15cm x 15cm. This customer discovered they were encountering long term positioning errors and needed help in resolving this problem.

Utilizing the NanoGrid system, OPTRA worked with the customer to design a metrology system that would sense when there was relative motion between the placement head at each insertion point and the substrate into which components were inserted. As these nascent positioning errors were sensed by the NanoGrid system, the corresponding coordinate locations were updated in software so that the ?pick & place? machine would continue to insert components in their proper locations. OPTRA provided the customer with the necessary hardware and software to accomplish this calibration and also assisted in the installation of the NanoGrid system.

The OPTRA components included a NanoGrid XY encoder, high resolution processing electronics and a customized 213mm x 213mm with accuracy of greater than +/- 1 micron.