Countering Sensor Drift in X-ray Microscopy with Fast and Robust Optimal Control

In X-ray microscopy it is imperative that the relative position between the optics stage, that carries the X-ray focusing optics, and the sample stage follow a certain trajectory while either the optics or sample stage is being scanned. Main challenges in achieving this requirement include - open loop drift, environmental disturbance, measurement noise, sensor drift, and control hardware limit. The state-of-the-art in X-ray microscopy at Advanced Photon Source (APS) at Argonne National Laboratory (ANL) features an H∞ control architecture applied to only the optics stage or both the optics and sample stage, achieving the objectives of large tracking bandwidth over 200 Hz, good positioning resolution on the order of nanometers, rejection of environmental disturbance, attenuation of measurement noise, good X-ray diffraction image resolution and increased imaging bandwidth. However, an unaddressed issue in our existing robust control design is that the sensors and the fixtures that hold the sensors drift with time due to changing air temperature in the APS beamline. Since the drift of sensor itself is not measured, it affects the relative position between the focusing optics and sample resulting into imaging artifacts and reduced image resolution. In this article, we demonstrate the rejection of the sensor drift by directly measuring the displacement of the sensor with respect to the global reference frame. Both the measured sensor displacement (i.e. sensor drift) and optics stage displacement are incorporated in the H∞ control architecture to achieve the above mentioned objectives in addition to minimal relative displacement between the optics stage and the sample stage. This will improve the X-ray image resolution and reduce image artifacts.