A high-bandwidth closed-loop MEMS force sensor with system dynamics adjustment

This paper describes a high-bandwidth control system design procedure for a MEMS force sensor equipped with an adjustable stiffness mechanism. When the force sensor comes into contact with a sample that has a stiffness at least comparable to its longitudinal stiffness, the resulting mechanical contact causes the system to become stiffer. This change in stiffness can be seen through the rising resonance frequency and falling dc-gain. In order to maintain closed-loop stability of the system after contact, it is necessary to tune the controller parameters since they are originally designed for the nominal system. By implementing a control system that combines an inner damping loop with a tracking loop together with adaptive algorithms to re-tune the controllers after contact, we were able to obtain satisfactory closed-loop performance. Also, the stiffness adjustment mechanism provides additional means of tuning the system dynamics. The numerical and experimental results demonstrate that these control approaches significantly increase the force tracking bandwidth.