Inspire AG together with Oelhydraulik Hagenbuch AG Ebikon is involved in a project for the performance improvement of a servo hydraulic test shaker. We focus on the quality of the FRF response of the system’s acceleration. Experimental measurements showed the presence of significant harmonic components at frequencies multiple of the exciting frequency. Fig. 1 presents an example for a commanded frequency of 160 Hz and a amplitude of 40 g, Fig 2 shows the corresponding frequency spectrum. A compensation algorithm has been developed and successfully incorporated into the control loop of the shaker. The control strategy uses a Kalman filter to estimate the harmonic content of the system response during operational conditions; simultaneously the control signal is corrected by higher order harmonic terms whose magnitudes and phases are optimized in order to minimize undesired components in the response spectrum.