A new robust damping and tracking controller for SPM positioning stages

Abstract

This paper demonstrates a simple second-order controller that eliminates scan-induced oscillation and provides integral tracking action. The controller can be retrofitted to any scanning probe microscope with position sensors by implementing a simple digital controller or op-amp circuit. The controller is demonstrated to improve the tracking bandwidth of an NT-MDT scanning probe microscope from 15 Hz (with an integral controller) to 490 Hz while simultaneously improving gain-margin from 2 dB to 7 dB. The penalty on sensor induced positioning noise is minimal. For the Scanning Probe Microscope considered in this paper, the noise is marginally increased from 0.30 nm RMS to 0.39 nm RMS. Open- and closed-loop experimental images of a calibration standard are reported at speeds of 1 and 10 lines per second (with a scanner resonance frequency of 290 Hz). Compared to traditional integral or PID controllers, the proposed controller provides a bandwidth improvement of approximately ten times. This allows faster imaging and less tracking lag at low speeds.