Low emission, self-tunable DSP based Stepping Motor Drive for use with arbitrarily long cables

The control requirements of stepping motors used in high accuracy positioning systems installed in particle accelerators are generally quite unusual. No power electronics can be installed in the proximity of the motors to be controlled due to the high level of radiation. The power drives are normally located in safe areas that can be several hundred meters away from the motors. Nevertheless the positioning repeatability has to be guaranteed even in micro stepping mode and the electromagnetic field, emitted by the power cables, has to be kept below often very demanding limits. Dramatically increasing the PWM frequency leads to an effective reduction of the total energy emitted in the low side of the spectrum which is the most critical range for the very sensitive equipment installed in particle accelerators. A 50 kHz PWM DSP based stepping motor drive designed to work with cables of 1 km length or more is presented. An inverse dynamic current controller, fully configurable in terms of motor and cable parameters, is implemented on a TI TMS320F28335 floating point DSC (Digital Signal Controller). The cables, that at these lengths can be considered to be transmission lines, are modeled using Pade's approximants. The controller is tuned as a function of the cable length. The drive is equipped with a complete set of micro stepping modes and it also implements 3rd harmonic correction in order to optimize the positioning repeatability. The design has focused on the EMI restrictions as well and measurements are provided for comparison with other solutions working with similar cable lengths.