An Improved Control Structure for the Tracking of Sine Command in a Motion Simulator

The evaluation of inertial sensor's frequency response is a crucial step during the development of such sensors (gyroscope, accelerometer.) or of embedding systems. An accurate measurement of the sensor's gain and phase requires a test equipment, usually a motion simulator, able to create accurately controlled motions over a wide frequency band, with minimum amplitude and phase uncertainty. State-of-the-art motion simulators use permanent magnet synchronous motors as actuators and optical encoders as angular position sensor. They also include a servo-loop whose bandwidth is necessarily limited either for theoretical reasons, like the Bode Integral Theorem, or for physical ones, such as the inevitable time-lags occurring in the loop, or even mechanical resonances. Nevertheless, the appropriate bandwidth is required to allow for an accurate inertial sensor characterization. A well-known way of coping with the intrinsic limitations of the feedback control structure in a servo-drive consists in introducing a specific filter (called feedforward) between the motion trajectory generator and the feedback loop, to provide an anticipation independently of the feedback structure. This compensation requires a good modelling of the controlled system transfer function but is never perfect. Moreover, in a motion simulator, the tested inertial equipment is subject to change, and a unique feedforward filter cannot provide an accurate enough compensation. Thus, iXblue has introduced an adaptive feedforward structure in the controllers of their motion simulators, leading to a more accurate tracking of sine commands, beyond the initial closed-loop bandwidth. The benefits of this control structure are quite significant: the sine tracking is very accurate, having very little amplitude attenuation and phase lag.