An online closed-to-open position loop switch for stepper motors in high reliability systems

Stepper motors are widely used in open-loop positioning applications. They are the choice of actuation for the collimators in the Large Hadron Collider, the largest particle accelerator at CERN. In this case the combination of precise positioning requirements and the highly radioactive operating environment is unique. The latter forces both the use of long cables to connect the motors to the drives and also prevents the use of standard position sensors. However, reliable and precise operation of the collimators is critical for the machine. A problem arises however from the use of stepper motors in open-loop. The mechanics of the collimators can wear due to the abrupt motion and torque profiles that are applied by the motors. Closed-loop position control allows smoother movements to be applied but it requires position feedback. The use of sensors in radioactive environments is very limited for reliability reasons. In order to allow the use of position sensors without reducing the long term reliability of the whole system, the possibility to switch online from closed to open loop is proposed and validated in this paper. It allows the use of closed-loop control when the position sensors function correctly and to switch online to open-loop when there is a sensor failure, without the need to stop the machine.