This paper studies repetitive control (RC) with linear phase lead compensation to precisely track periodic trajectories in piezo-based scanning probe microscopes (SPMs). Quite often, the lateral scanning motion in SPMs during imaging or nanofabrication is periodic. Dynamic and hysteresis effects in the piezoactuator cause significant tracking error. To minimize the tracking error, commercial SPMs commonly use proportional-integral-derivative (PID) feedback controllers; however, the residual error of PID control can be excessively large, especially at high scan rates. In addition, the error repeats from one operating cycle to the next. To account for the periodic tracking error, a discrete-time RC is designed, analyzed, and implemented on an atomic force microscope (AFM). The advantages of RC include straightforward digital implementation and it can be plugged into an existing feedback control loop, such as PID, to enhance performance. The proposed RC incorporates two phase lead compensators to ensure robustness and minimize the steady-state tracking error. Simulation and experimental results from an AFM system compare the performance among (1) PID, (2) standard RC, and (3) the modified RC with phase lead compensation. The results show that the latter reduces the steady-state tracking error to less than 2% at 25 Hz scan rate, an over 80% improvement compared with PID control.

No abstract

Abstract-Ionic polymer-metal composite (IPMC) actuators have many advantages, for instance, they: 1) can be driven with low voltages (<5 V); 2) are soft, flexible, and easily shaped; and 3) can operate in an aqueous environment (such as water). Important applications for IPMCs include active catheter devices for minimally invasive surgery, artificial muscles, and sensors and actuators for biorobotics. Due to inherent nonlinear behavior, dynamic effects, and external disturbances, sensing and feedback control are required for precision operation. A new method to sense the displacement of an IPMC actuator using resistive strain gages is proposed. The sensing scheme is low cost, practical, effective, and importantly, compact compared to existing methods such as lasers and charge-coupled device (CCD) cameras. The strain-to-displacement relationship is developed and experimental results are presented to demonstrate the effectiveness of the sensing scheme. Furthermore, the sensor signal is used as feedback information in a repetitive controller to improve the tracking of periodic motion. The stability condition for the controller is presented, and the sensing scheme and feedback control approach are applied to a fabricated perfluorinated ion-exchange-membrane-based IPMC actuator with lithium as its counterion. Experimental results show that the tracking error can be reduced by approximately 50% compared to PID control for tracking of periodic signals, including sinusoidal and triangular wave forms.Index Terms-Ionic polymer-metal composite (IPMC) actuators, repetitive control, strain gage sensors.

Ionic polymer-metal composites (IPMCs) are innovative materials that offer combined sensing and actuating ability in lightweight and flexible package. IPMCs have been exploited in robotics and a wide variety of biomedical devices, for example, as sensors for teleoperation, as actuators for positioning in active endoscopy, as fins for propelling aquatic robots, and as an injector for drug delivery. In the actuation mode, one of the main challenges is precise position control. In particular, IPMC actuators exhibit relaxation behavior and nonlinearities; and at relatively high operating frequencies dynamic effects limit accuracy and positioning bandwidth. A frequency-weighted feedforward controller is designed to account for the IPMC's structural dynamics to enable fast positioning. The control method is applied to a custom-made Nafion-based IPMC actuator. The controller takes into account the magnitude of the control input to avoid generating excessively large voltages which can damage the IPMC actuator. To account for unmodeled effects not captured by the dynamics model, a feedback controller is integrated with the feedforward controller. Experimental results show a significant improvement in the tracking performance when feedforward control is used. For instance, the feedforward controller shows over 75% reduction in the tracking error compared to the case without feedforward compensation. Finally, the integrated feedforward and feedback control system reduces the tracking error to less than 10% for tracking an 18-Hz triangle-like trajectory. Some of the advantages of feedforward control as well as its limitations are also discussed.

No abstract