Design of a high-resolution programmable current source for coil-magnet actuators in a monolithic x-ray interferometric calibrator

Abstract

This paper describes the design, implementation and testing of a pair of digitally programmable current sources which permit the controlled movement of a silicon monolithic x-ray interferometer over a range of with a resolution of better than 1 part in 500 000. Ideally a single coil-magnet actuator placed in line with the main axis of movement would ensure translation but no rotation. However, a number of factors produce parasitic twist which reduces the contrast of x-ray fringes. Two coil-magnet actuators have been placed symmetrically off the main axis of the monolith and by independent control of the currents it has been possible to compensate for asymmetries and obtain linear translation with negligible rotation. The measurement of x-ray fringes demonstrates a controlled movement of about 0.6 nm over a period of 30 min. Various techniques are considered which provide a suitable source of mA culminating in a design where each source has coarse and fine digital-to-analogue converters (DACs) with local feedback provided by an embedded proprietary 22-bit analogue-to-digital (ADC) module. Each source has a single-chip microcontroller which accepts serial messages from a computer to coordinate activity. Finally, future designs of multiple programmable current sources are considered.