Dyer, David Charles (1995) Super-precision programmable current source for coil/magnet actuators. PhD thesis, University of Warwick.

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Abstract

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.

Item Type:	Thesis (PhD)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Library of Congress Subject Headings (LCSH):	Actuators, Actuators -- Materials, Electric currents, Engineering mathematics
Official Date:	May 1995
Institution:	University of Warwick
Theses Department:	School of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Chetwynd, D. G. (Derek G.), 1948-
Extent:	vii, 230 leaves : illustrations
Language:	eng

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