Disturbance observer design for tape transport control

This paper concerns disturbance observer design for a direct drive tape transport servo system. A gain-scheduled notch filter is used in the outer-loop controller design for the decoupled tension loop to cancel a lightly-damped resonance. The disturbance observer is used to attenuate the effects of both stiction during start-up and the periodic disturbances caused by reel eccentricities during steady running. This paper presents the sensitivity function analysis by transforming overall system into an equivalent loop gain with both feedforward compensator and feedback compensator. Contrary to common approximation used in disturbance observer, this analysis shows that the outer-loop controller could be adversely affected by the introduction of disturbance observer. The analysis also shows that noise is amplified by the introduction of the disturbance observer. For the tape transport servo design, the location of the notch is shifted by the introduction of the disturbance observer, Which results in small phase margin. The loop gain analysis also provides an insight for the disturbance observer filter selection for the tape transport servo system. Experimental data show the effectiveness of the new design