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This paper presents a new design concept for cascade control of brushless direct current (BLDC) motor drive. Cascade controllers are designed for definite separation of loops. This separation allows independent design of controllers with minimum interaction among loops. The resultant structure with inner loop and outer loop is a robust structure. Magnitude Optimum Multiple Integration (MOMI) tuning method is proposed to design the PID controllers used in inner and outer loops. This method provides a fast and non-oscillatory closedloop response for motor drives. The aim of the proposed method is to determine optimal Proportional-Integral-Derivative gains of the controller so as to force the rotor to follow the reference speed without any oscillations with zero steady-state error. MOMI method proceeds by determining the open-loop process gain to calculate the moments (areas), from which controller parameters can be calculated. The MATLAB/SIMULINK is used for simulation of the drive system. Several simulation tests performed to investigate the robustness of the controller and is justified by the simulation of BLDC drive under setpoint changes, input disturbance and load disturbances.
BLDC motor; MOMI; PID controller; Trapezoidal back emf; Cascade control
BLDC motor has wide range of applications because of its advantages such as high efficiency, flat speedtorque characteristics, and high speed range, smaller in size and lighter, longer life, low noise and good dynamic response when compared against brushed direct current motor. Both, DC brushed and brushless motors contain a combination of Permanent magnet and electromagnetic fields, which produce torque resulting in motion. In a brushless DC motor, the rotor consists of magnets and stator contains the windings and no brushes present as name suggests. Commutation is implemented electronically in brushless motors using semiconductor switches to change current in the windings based on position of rotor . The BLDC motor is an equivalent reversed DC commutator motor, as its magnet rotates while the conductors remain stationary, this facilitates the external or internal position sensors in BLDC motors to sense the actual rotor position. The rotor position position can also be sensed by measuring the changes in back emf, which is sensorless control . As sensors cannot be used in all applications, the back emf method of commutation is used to simplify the motor construction and can reduce the cost of the motor. This paper provides an approach for application of PID controllers in cascade control of BLDC motor drive system. The cascade control configuration consists of two loops. The inner loop is called current control loop (secondary loop) and outer loop is called speed control loop (primary loop). Each loop is associated with its PID controller. Therefore, it is complicated design than that of single loop controller. A new method, Magnitude optimum multiple integration (MOMI) is proposed for tuning two controllers in cascade control system.
In this paper, cascaded control of brushless motor drive is proposed with MOMI tuning method to determine the parameters of PID controllers used in inner loop and outer loop. The motor drive system has been simulated using MATLAB/ SIMULINK. Several simulation tests were performed to investigate the robustness of the drive system and simulation results demonstrate the capability of performing under set-point changes, inner loop and outer loop disturbances. The dynamic responses with different values of controller parameters Kp, Ti, and Td were effective in their control actions. Therefore the proposed cascade control opens a new platform for the control of brushless motor drives.
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