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33 Engineering a More Sustainable Future | ADI Closed-Loop Motor Control Feedback System There has been a steady and constant move from traditional grid-connected motors to inverter- driven motors in the last couple of decades. This has been, and is continuing to be, a significant transition in industrial rotating equipment and has resulted in huge process and energy savings with motors and end equipment being used more efficiently. Higher quality motor control performance with variable speed drives and servo driven systems now offer enhanced quality and synchronization for the most demanding applications. Motor performance and efficiency are improved by using the power inverter, high performance position sensing, and current/voltage closed-loop feedback for the power stage as shown in Figure 1. Open-loop speed control of motors is possible by applying a variable frequency voltage to the motor using pulse width modulation in the inverter. In steady state or slowly varying dynamic conditions, this will work reasonably well, and many motor drives in lower performance applications utilize open-loop speed control, without requiring an encoder. However, there are several disadvantages to this approach: X Speed accuracy is limited as there is no feedback X Motor efficiency is poor, as the current control cannot be optimized X Transient response must be strictly limited so that the motor does not lose synchronization What Is a Position Encoder? An encoder provides closed-loop feedback signals by tracking a rotating shaft speed and position. Optical and magnetic encoders are the most widely used technologies, as shown in Figure 2. In general-purpose servo drives, encoders are used to measure the shaft position, from which the drive rotational speed is derived. In robotics and discrete control systems, precise and repeatable shaft position is required. Optical encoders consist of a glass disk with fine lithography slots. Photodiode sensors detect light variation as it passes through or is reflected off the disk. The analog output of the photodiode is amplified and digitized and then sent over wired cabling to the inverter controller. Magnetic encoders consist of magnets mounted to the motor shaft, with a magnetic field sensor providing sine and cosine analog output, which is amplified and digitized. The optical and magnetic sensor signal chains are similar as shown in Figure 2. Figure 1: The closed-loop motor control feedback system. 1 Figure 2: (a) An optical encoder and (b) a magnetic encoder. 2