The cryogenic positioning system for a magnetic microscope

We are developing a magnetic microscope, based on a micro SQUID sensor, wit h an integrated coil of 10 mu m diameter. We present here the new features of this system. The scanning stage of the microscope is completely containe d in a cryogenic environment and is activated by piezoelectric actuators of the bimorph type. A capacitive transducer of new design which has been des igned and constructed in our laboratory, monitors the movement of the scann ing stage. The capacitive transducer detects the position of the moving sta ge by the change in the capacitance of two sliding electrodes. The x and y measurements are integrated in a single device. The electronic capacitance monitor is based on a null capacitance bridge with fast-acting digital feed back. In the measurement of variable capacitance, we obtain a 14 bit resolu tion with a bandwidth larger than 750 Hz by using a XSP56001 digital signal processor (DSP), fast analogue-to-digital converters and multiplying digit al-to-analogue converters. The DSP controls the scanning stage by driving t he piezoelectric actuators, monitoring the capacitance change due to the mo vements of the sample platform and correcting accordingly its position.