Controller Design for Z Axis Movement of STM Using SPM Control Software

Scanning probe microscopy is a branch of microscopy that forms images of surfaces using a physical probe. This paper is devoted to the control system design for high performance scanning tunneling microscope (STM). The distance between tip and sample should be kept constant at every point of sample for better scanning results. Note that many samples have roughness, curvature and tilt that deviate from a flat plane .So it is necessary to move the probe in z-axis to maintain the height of probe from sample constant. But a sudden change in probe position can damage the probe. SPM feedback loops usually employ a proportional-integral (PI) controller to control the vertical movement of tip (z-direction). Our aim is to implement an accurate tuning algorithm that can find PI values that gradually change the position of the probe for higher safety and precision. Moreover, better sample topography image can be obtained after auto-tuning the control gains during different scanning speed. The square wave is given as input to the PI controller which represents the sudden change of surface that cause damage to the system, and the output should be triangular wave in nanometer range by proper tuning of PI controller. Triangular wave represents the tip being moved by the controller in z direction at the time represented by the square wave input.