SU-8 as a Torsional Spring: Viscoelastic Creeping and Closed Loop Control

The torsional viscoelastic creeping and recovery of an SU-8 spring is reported by studying the deflection angle variation of a vertical comb-drive actuator suspended by an SU-8 spring at three different stress levels. A maximum creep of 0.43° (corresponding to a relative change of strain of 5.7%) is observed under a constant stress of 8.3 MPa in 1 h (V = 22.7 V). A capacitive readout circuit is designed to sense the torsional deflection of the comb-drive actuator. The circuit is capable of actuating and sensing the capacitance change by the same combdrives with a resolution of 0.03° (corresponding to a capacitance resolution of 0.05 pF). The readout capacitance is then used as the input variable for a proportional-integral (PI) controller in order to eliminate the time and load dependent drift by creeping of the SU-8 spring. The PI controller is tuned by the Ziegler-Nichols method and has a settling time of ~2.3 s for the output. Compared with the open loop deflection of 0.43°, the closed-loop deflection is reduced by a factor of 7 down to 0.06°. This controller error is prominent at high actuation voltages, and is attributed to the actuator´s lateral and vertical cross-sensitivity.