ESA GNC Conference Papers Repository

Payloads on top of launchers are subject to different sources of disturbances during flight. Critical loads are coming from the ignition of the rocket motors, pressure oscillation in solid boosters and from separation shocks of stages or boosters. The dynamic characteristic of these disturbances can be step-wise or oscillatory. Passive isolation cannot deal with both types simultaneously. An active payload adapter, built on force feedback, is proposed and tested on a bread-board system with one active strut. The principal concept has already been described in [1]. This paper presents in detail the closed loop tests on the bread-board and the modifications in the control architecture, which have been applied in the course of the experiments. The original control architecture, as described in [1], was based on position and velocity feed. The closed loop performance of the bread-board system showed large degradation compared to the predicted one. The reason was found to be in a non-linear dynamics of the piezo actuator (see [2]) which was higher than expected. A Bouc-Wen model (see [3]) was used to model the hysteresis effect of the strut and system identification has been performed to establish the model parameter. This augmentation of the design model allowed predicting the degraded performance which had been observed in the breadboard. As a consequence the control architecture was changed and the feedback based on a force sensor following [4] was introduced. A control design based on mu-theory way is applied with the aim to achieve sufficient robustness to cover the non-linear effect of the hysteresis. The paper provides the details of the modelling, the controller design and discusses various choices for the weighting functions. Different weightings functions were tested on the bread-board and there influence on the performance is discussed. The reported results demonstrate that an active isolation over a wide frequency range can be achieved when robust control design is used with force feedback. The activity has been performed under ESA's GSTP program 'Active Payload Adapter'.