Development of a Coil Driver for Magnetic Manipulation Systems

Pulsewidth modulation (PWM) is the most commonly used technique to drive electromagnetic coils in magnetic manipulation systems. Relatively low PWM frequencies generate high-magnitude current ripple and magnetic field fluctuation. In this letter, coils are powered by a driver at PWM frequencies close to their self-resonant frequencies to generate high-frequency magnetic fields and minimize current ripple and magnetic field fluctuation. In order to protect the driver against the penetration of stray electromagnetic and magnetic fields, a multilayer shielding enclosure is employed. The coil driver is used to study the effect of varying PWM frequencies on current, magnetic field, and ohmic loss using Helmholtz, air, and iron core coils. The current ripple magnitude is significantly minimized when the coils are driven at PWM frequencies close to their self-resonant frequencies. This results in reduction of magnetic field fluctuation and provides more accurate measurement of magnetic field magnitude. Our experiments show that increasing the PWM frequency from 100 Hz to 25 kHz decreases the current ripple and magnetic fluctuation by two orders of magnitude, with a negligible effect on the ohmic loss.