Sensorless Robust Speed Controller Design of Pancake Axial Field PMDC Motor

Due to superior multi-dimensional features (e.g., large torque to inertia ratio, fast response, high power density, negligible maintenance requirement, etc.) the axial-field pancake permanent magnet dc (PMDC) motors are popularly used for many control applications. When driven by pulsewidth modulation (PWM) controllers, due to their negligible electrical time constant, the armature current tends to be discontinuous. When the current disappears the measured armature voltage [i.e., back electromotive force (EMF)] is linearly related to its speed. In this article, the armature voltage at zero current is sampled to be used as feedback for speed control of axial field PMDC motors for wide range applications. Negligible prospect of demagnetization makes these motors ideal candidate to be controlled sensorless. When PWM frequency is large the armature time-constant poses as constraint for the back EMF to be available for sampling. The choice of PWM frequency is an important design parameter for the proposed concept. Further, this article explores, in practical domain, the usability of different control functions for robust speed control of pancake PMDC motors. It also details product design for sensorless PMDC motor speed controller using modified second-order sliding mode control function.