Two Level Controller Scheme for Fast Magnet Cycling

At the Institution of Physics in Lund quantification of rare isotopes is being conducted with the method of Accelerator Mass Spectrometry (AMS). The procedure involves cycling magnetic fields in two large dipole magnets. The magnets consist of large coils with unlaminated iron cores. These cores give rise to eddy currents which make the magnets difficult to control. To improve the precision of measurement and reduce the total measurement-time per sample it is important to minimize the settling-time of these magnetic fields. Before this work started the settling-time for both magnets was in average 18 seconds causing almost one third of the total measurement time per sample being spent on waiting for the magnetic fields to be adjusted. Using a two mode controller scheme this time has been reduced to approximately 7.5 seconds. It was early learned that the process had to be modified if the settling-time was to be reduced. After having modified a D/A-converter the process became more stable and easier to control. The controller designed is a two mode controller consisting of an optimization based Iterative Learning Controller (ILC) and a conditional integrator. Each cycle the ILC stores the input/output values in vectors and uses these to calculate the next input vector. The idea is to use the knowledge of the error from the preceding cycle to improve the input signal for the coming cycle. After the first seconds of the step the controller changes mode. The conditional integrator makes the final input-adjustments and keeps the field from drifting out of range while the isotopes are being measured.