Low-frequency compensation of piezoelectric force sensors for servo systems

This paper presents a force/disturbance observer that compensates for the low-frequency response of a piezoelectric force sensor. Although the piezoelectric force sensor has many salient features, it is unable to measure dc and quasi-dc forces. The proposed observer compensates for this low-frequency deficiency, enabling the piezoelectric force sensor to be used in a servo system. Compared with previous studies, the proposed scheme requires redesigning neither the sensor structure nor sensing circuitry and can be easily integrated into an existing measurement system. As a by-product, the proposed observer also produces a disturbance estimate that can be used to enhance the robustness of a servo system. This paper reports experimental results of force control and position control for a linear motion platform equipped with a piezoelectric force sensor and a strain gauge-based load cell. The force control results indicate that the control system using the proposed observer can achieve a wider bandwidth than that using the load cell as a feedback-sensing device. In a comparative study, the position control results further demonstrate the proposed observer’s effectiveness in diminishing the piezoelectric sensor’s low-frequency deficiency. Moreover, the disturbance estimate produced by the proposed observer is experimentally introduced to the position control system, showing an enhanced tracking performance in terms of the root-mean-square error.