Driving the LHC Collimators’ Stepping Motors over 1 km with High Accuracy avoiding EMI Effects

The LHC collimators are exposed to very high levels of radiation, which means that the power drivers must be installed far from the stepping motors that they drive. Due to the geometry of the underground installations, the distances can be up to 1 km. The long cables that connect the drivers to the motors behave as transmission lines modifying dramatically the impedance seen by the drivers and consequently jeopardizing the control performance of Pulse Width Modulation (PWM) drivers. In this paper we address this problem, provide an analytical model of the driver-cable-motor system and describe the analog solution we have developed to improve the performance of a typical off the shelf driver. Finally we characterize the improvement of the performances with measurements of positioning repeatability and show that electromagnetic emissions from the long cables are drastically reduced, making the use of stepping motors compatible with extremely sensitive instrumentation such as the LHC Beam Loss Monitors (BLM).