On-orbit calibration of inductosyn error

The rotary-type inductosyn is a device used for measurement of angular displacement. Because of practical limitation in mounting and electronics tolerances, the readout from the inductosyn is corrupted with the errors, which need to be calibrated in order to satisfy the stringent angular measurement accuracy requirement. A ground calibration scheme using precision gyros as calibration references has been developed and tested at Hughes to calibrate the inductosyn error. However, since the error characteristics of inductosyn may vary due to launch environments and component aging of readout electronics, the calibration table generated on the ground may no long provide accurate corrections during on-orbit operations. Methods to calibrate the residual inductosyn error on orbit without using the gyros are required. This paper describes an on-orbit calibration scheme which corrects the deviated inductosyn cyclic errors without using the precision gyros. In this calibration concept, the scan mirror is commanded to follow a specific scan motion and a Kalman filter is implemented to estimate the residual inductosyn error using the sampled servo error signals. To illustrate the proposed calibration concept, computer simulation models were developed and used to evaluate the filter performance. Both covariance analysis and time domain simulation models are described in the paper along with the simulation results. The results indicate that the 0.5 micro radian inductosyn calibration accuracy requirement can be maintained by using the proposed on-orbit calibration scheme.