Control of image-shifting coils for vibration isolation of an electron microscope

This paper proposes a new approach to control of image-shifting coils for vibration isolation of a scanning electron microscope. Image-shifting coils move the electron probe of the microscope out of phase with undesirable motion of a specimen due to any disturbance source. Two acceleration sensors are located at the root of the specimen chamber of the microscope to detect the disturbance. The outputs of the acceleration sensors are fed forward into a controller to move the probe by the image-shifting coils. The feed-forward controller is based on a transfer function from the sensor outputs to the relative displacement of a specimen to the electron probe that is assumed to be at rest. The microscope is put on a table attached to a shaker. Sinusoidal excitation tests are made by the shaker to measure the transfer function data using a video signal and the sensor outputs. The relative displacement of the specimen is estimated by identifying the measured video signal with a simulated one in a least-squares sense. The controller is implemented as a digital filter running on a digital signal processor. The amplitude of the vibrating images is significantly reduced by the controller at the natural frequency of the system.