Research On Self-sensing Control Method Of Piezoelectric Micro-gripper

In this thesis, the structural improvement and finite element analysis including static anddynamic characteristics of a developed piezoelectric micro-gripper were presented. Furthermore,the methods of self-sensing and control for gripper displacement instead of external sensors wereinvestigated. The main works are listed as follows:1) The structure of the gripper based piezoelectric bimorph actuators was designed inconsideration of electrode leading, installment localization and fingertip shape. The static anddynamic characteristics of the improved gripper were simulated by ANSYS. The results show thatthe maximum displacement of the fingertip was about98.4μm when maximum driving voltage of90V was applied to the gripper. The first order natural frequency was at661Hz. The steady statedisplacement was about21.5μm and the response time of the gripper is0.08s under a step voltageof20V. A prototype micro-gripper was produced according to design and analysis above.2) The griping force and displacement self-sensing method based on the integral charge wasproposed according to the relation between piezoelectric crystal deformation and its surface charge.A mathematical model of self-sensing for griping force and displacement was established based onthe Smits’ piezoelectric cantilever model. Then the circuit was designed. The balance condition ofself-sensing was derived, which shows that the product of piezoelectric crystals’ equivalentcapacitance and leakage resistance is equal to that of integrating capacitor and balancing resistors.Further, the effectiveness was verified by the corresponding tests. The results show thatdisplacement value obtained by the self-sensing method had a good consistency with the result ofexternal sensor.3) The feed-forward controller of piezoelectric micro-gripper was designed base on theimproved Prandtl-Ishilinskii model. The validity was tested by experiments. The results showthat feed-forward control system can significantly increase the response speed of the gripper andreduce the hysteresis error. The response time to5μm desired displacement was0.12s. Thehysteresis error of the gripper was decreased from3.2μm (without control) to1.1μm (withfeed-forward control) in the displacement range of15.2μm. The charge obtained by self-sensingmethod had a good consistency with the outcome from the external sensor.4) The PID feedback controller with a parabola integral and differential forward for thepiezoelectric micro-gripper was designed by improving conventional PID control and its validitywas tested by experiments. The experimental results show that it can completely eliminate thesteady-state error under external sensors feedback, but the response time to5μm referencedisplacement was0.18s which was longer than that of the feed-forward control. The steady-statedisplacement error of the gripper under self-sensing feedback was less than that of feed-forwardcontrol. But the response time to0.35μC reference charge (corresponding to5μm referencedisplacement) was0.4s which was longer than feed-forward control. 5) A compound controller combining the feed-forward control based on the improved PImodel with the modified PID feedback control was designed for the piezoelectric micro-gripper.The effectiveness was validated by the experiments. The experimental results show that whateverthe gripper was under sensors feedback or self-sensing feedback, the hybrid controller had theadvantages of both the feed-forward and the PID feedback control, faster response speed andhigher steady-state accuracy.