Tracking algorithm for the gain of the phase modulator in closed-loop optical voltage sensors

The accuracy and scale factor of the optical voltage sensor are considerably affected by the gain drift of the phase modulator induced by temperature fluctuation. In this paper, we present a tracking system for the gain drift of the phase modulator based on four-state modulation which provides a significant improvement in the performance of the optical voltage sensor. We analyze the influence of the main closed-loop error of optical voltage sensor on the tracking system. The analysis results indicate that the main closed-loop error causes uncertain parameter and external disturbance for the tracking system. Then the model of the tracking system for the gain of the phase modulator is transformed into a dynamic system with uncertainty and stochastic disturbance by substitution of variable. The tracking system is accurately designed considering the main closed-loop error. To insure the tracking system achieves good transient performance, the closed-loop poles of the tracking system are constrained to locate in a desired circular region, while the gain error is mean-square exponentially stable via the robust control. The experiments show that the tracking system realizes precise compensation for the gain drift of the phase modulator.