Minimum Incremental Motion and Holding Stability in Beamline Positioning

Background

Many beamline applications such as X-ray microscopy and Computed Tomography (CT) require positioning of samples, detectors, and optics in order to perform measurements. Microscopy applications often require imaging of the structure of matter at the sub-micrometer and even nanometer level. Good holding stability, both short-term and long-term, is required because movement of the sample or optics over the time of measurement will cause poor images. Also, the ability to make small mechanical movements on the order of nanometers is often critical for alignment and adjustment of samples or optics.

Stepper and servomotors are two common drive methods for controlling position in mechanical positioning systems used in beamline applications. Stepper motors are brushless DC devices that divide 360 degrees of rotation into equal steps. Motion is induced by energizing segments of the coil winding sequentially. A stepper motor can hold position by energizing one segment of the coil, without any feedback, making it a good choice for low-cost, precision applications requiring holding stability. Servomotors are typically brushless AC devices, but in contrast to stepper motors, most often use three phases of coil winding plus a feedback device like an encoder or resolver. This feedback device allows a controller to commutate the motor phases and close a PID servo loop around the actual position of the motor, providing a significant advantage in applications that require small step sizes and good holding stability.