The sensors can be repaired during the next programmed machine technical stops, allowing for a continuous operation of the collimators and enlarging the collimator mechanics lifespan thanks to the smooth motion while the sensors are working.Ī non-negligible trend in research is using stepper motors with the position loop closed in different ways, whether it be with a position sensor or with sensorless techniques. With it, the motor can work in closed-loop position control, providing smooth motion profiles, but in the case of sensor failure, the loop is opened, continuing to work in standard open-loop position control.
This switch is an original contribution of this thesis. To overcome these problems and take advantage of the smoother motion profile that can be achieved using a sensor and closing the position loop, but not reduce the reliability of the system, an automatic closed-to-open position loop switch is proposed. This fact does not interfere with its use for the purpose of mechanical health diagnostics, since the torque estimation is good during the parts of the motion profiles with non-zero speeds.įigure 1: Full-step mode rotor position and speed acquisition In the case of the collimators, these are the most common cases, and therefore sensorless estimation is not a reliable option to close the position loop. Sensorless control as presented article is an attractive option, but it has the problem of the loss of observability when the rotor is stopped or at low speed. The cost of such an event, plus the required maintenance of the collimator, is not affordable. A sensor failure could lead to the malfunctioning of the collimator, and thus to the beam dump and stop of the machine. However, it was claimed that relying on position sensors, as needed for FOC, for critical operation, as is the case of the collimation system, is preferably avoided in the LHC collimators, since it would reduce the reliability of the whole system. To attain such a profile, closed-loop position control is proposed using Field Oriented Control (FOC). This could be achieved with a smoother motion profile applied with the motor. It is therefore desirable to attempt to extend its lifespan. The worm screw connected to the motor to move the collimator jaw linearly wears and after a few years of use can need to be replaced, an expensive intervention. In the LHC collimators, this jittery movement is tough for the mechanics. Looking at the speed, the rotor reaches again large values compared to the average speed, which in this case with 40 steps per second is 1.25 rad/sec, whereas the peak values reach 25 and -20 rad/sec. The large overshoot of over 50 % can be easily appreciated, followed by several oscillations. 8.1, where an acquisition of the rotor position and speed are shown. We saw an example similar to the one presented in Fig. Stepper motors are typically used in open loop in positioning systems.
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