DSP-based stepping motor drivers for the LHC collimators

The control electronics of the LHC collimators stepping motors will be located in radiation safe zones up to 800 meters far from the motors. With such cable lengths the standard chopping drivers do not work properly because of the voltage losses on the cable and even more because of the high cable capacitance. The capacitance in fact produces a ringing phenomenon on the driver feedback current that limits the control chopping frequency to the point of being incompatible with the tight EM emissions requirements of the LHC tunnel. In some cases the feedback loop may even become unstable, and the driver would fail. The problem was solved by accurately modeling the overall motor/cable system taking into account non-linearities due to hysteresis and eddy currents, and by designing an adaptive digital controller, self-tuning to the real cable length. The controller will aim at increasing the chopping frequency to reduce the spectral density of the emissions, and at damping the oscillations of the feedback current to avoid instability. In this paper the model of the motor/cable system as well as the digital controller are described. Particular attention is devoted to the hardware implementation based on a TI Cx 2000 DSP.