Plant Inversion-Based Speed Control of a PMDC Motor

Abstract

Permanent Magnet Direct Current (PMDC) motors are widely used in applications requiring precise speed control due to their efficiency and high torque-to-inertia ratio. This work proposes a feedforward speed control strategy for PMDC motors based on model inversion, complemented by a disturbance rejection feedback term. A gray-box model is developed using only four concentrated parameters, avoiding the overdetermination problem of classical seven-parameter identification. These parameters are identified experimentally from step-response data using a nonlinear optimization approach. The proposed control law is validated on two commercially available PMDC motors with distinctly different dynamics: a fast-response motor (FC130SA) and a slower motor with a gearbox (GM25-370). Experimental results show that the proposed feedforward controller with disturbance rejection achieves lower or comparable Integral Squared Error (ISE) than optimally tuned PID/PI controllers, while significantly reducing overshoot (up to 66% in the fast motor) and maintaining lower or comparable Control Input Area (CIA), a metric commonly used in the literature to provide an indirect indication of control effort. Unlike classical controllers, the proposed method requires no per-reference gain tuning. These results show compelling evidence that inversion-based control with disturbance rejection is a viable, energy-efficient alternative to PID control for PMDC motor speed regulation.