Super-precision programmable current source for coil/magnet actuators
This thesis describes the design and development of a super-precision programmable current source that can deliver up to about ±100 rnA to an inductive load. The load is intended typically to be a coil in a coil/magnet actuator that provides a force which is proportional to the current, and results in a linear and well defined movement of an elastic flexure mechanism. The particularly demanding application of long-range x-ray interferometry required two tracking current sources that offered a resolution to better than 1 part in 500,000 and this could not be satisfied by commercially available instruments. Consequently it was necessary to design, construct and test two identical supplies (or drives); a non-trivial and very demanding task since exceptionally slow drives scans needed to be accommodated. Temporal stability is therefore critical. Although the operational bandwidth can be kept small, noise up to over 1 kHz must be rigorously suppressed to avoid exciting resonances in the system being driven. Commercial 20-bit digital-to-analogue converters could not be utilised to provide a resolution of 1 part per million, because they are invariably designed for audio applications and have unacceptable drifts with temperature and time. The integral non-linearity had to be less than ±O.0007% (15 ppm) and the design actually achieves ±O.5 ppm by using an embedded precision analogue-to-digital converter to form a servo-loop within each drive. A desk-top computer (PC) accepts setpoints via a serial communications channel, and simultaneously controls the servo-loops for two drives by the exchange of simple messages via optically isolated links. The major components within each drive are, an embedded 8-bit micro-controller, two DAC's providing coarse and fine voltage settings, a precision voltage-to-current converter, a precision ADC and an ADC which monitors critical nodes, all of which are discussed in considerable detail together with the algorithms and software in the PC and microcontroller. Circuit simulations were an important part of preliminary studies and are presented along with measures of actual performance. It is shown that the drives achieve not only a resolution of 1 ppm but that all other operational parameters are of a similar order. A number of proposals are made for alternative methods which represent the foundations for future work.