A Test of the Lifetime of a Tip-tilt Platform

An imaging-stability control system is a key subsystem of a large-aperture space telescope. The system comprises a monitoring camera, a computation/control unit, and a correction unit. Both the monitoring camera and the computation/control unit are electronic systems. The correction unit comprises a mirror and a tip-tilt platform, which is the only moveable component in the system. The lifetime of the tip-tilt platform appreciably influences the lifetime of the entire system. We have designed and built a system to test the lifetime of a tip-tilt platform. The lifetime-test system consists of a tip-tilt platform, a platform-motion driver, a waveform generator (with output waveforms arbitrary), a capacitive non-contact displacement sensor, a dual-frequency laser interferometer, and a control computer. Based on our analysis of the space environments and operation conditions for the tip-tilt platform, we have designed signals to drive the tip-tilt platform. These signals are periodic and are random within its period of 200s. To make the test of the lifetime accomplishable in a reasonable time, we have intentionally shortened the motion period of the tip-tilt platform by 5.5 times. Because there are hysteretic and creeping effects in the tip-tilt platform the relationship between the platform displacement and the drive-signal voltage is nonlinear. We propose a method of evaluating cross correlations between data sampled at different moments to nearly eliminate the influence of the nonlinearity in the statistical analysis of the data in the test. We use the method to compare displacements driven by the signals in different time periods to judge whether the platform works properly, with the criterion of failure of the platform being that the standard deviations of measured displacements exceed a large value. We present preliminary results of the ongoing test of the platform lifetime. The results show that the platform has been working without failure for one year. Taking into account the shortening factor of 5.5 of the motion period, we expect the results to be equivalent to proper running of the tip-tilt platform on a satellite for 5.5 years. We find that environment temperatures appreciably affect platform displacements. We plan to include temperature control in follow-up tests.