Sensorless Control of Brushless DC Motor Drives

Brushless motors are a growing technology that is used to replace old, less performing DC motors. Their structure is based on a three-phase synchronous stator with a rotating magnetic field supplied by the stator windings and permanent magnets hooked to the rotor structure. However, the term Brushless refers to an electrical drive with position feedback, which includes power conversion stage, motor, load, sensor and all the accessories. The work brought on aimed to implement a sensorless control, independent from the use of position sensors on the motor and therefor flexible and versatile for a multiplicity of motors. The drive was completed with a control board equipped with all the interfaces needed to obtain the requested performances. The work was carried out following the usual approach in the development of a control system for electrical drives. First a study on the board has been done, the hardware structure was thoroughly analysed with a reverse engineering approach, researching components data, observing the performances offered in different configurations, analysing the previous operating status to understand which parts improve, integrate or maintain. Before working on the actual control algorithm some preliminary considerations were made, a series of modifications to the setup and the board configuration were needed in order to proceed with the next steps. Some of these adjustments were PWM settings according to the requested specifications, signal management and conditioning, sector determination for the definition of the rotor position, duty cycle computation and signal transition from the microprocessor to the power stage. Another important work concerned the brake bench construction, using a modelling software a braking structure for variable load performances analysis was first designed and then prototyped by using a 3D printer, it allowed to observe the electrical and mechanical quantities evolution even under load condition and the drive system behaviour in different working situations. From there the work consisted of software implementation for different control strategies. Open loop, which was the starting point for diagnosis of the correctness of sector definition (position definition), current sampling, speed calculation and signal conditioning for sensorless operations. Then the current loop, a constant torque control based on a PI scheme. The regulator tuning and the verification of the correctness of the preliminary work were a precondition for the correct functioning of the loop. The final step for the senored motor control was the speed loop, again tuning of the PI scheme and then digital filtering were needed for the implementation of a strategy for constant speed, load insensitive operations. Once completed these operations was possible to add the sensorless control. After studying different position estimation methods and making sure everything is correct, a feasible logic for open loop, constant current and constant speed operations was developed and the mechanical and electrical performances of the whole drive were verified. A parallel study was made with a MATLAB and Simulink simulation of the whole system, from the motor electrical and mechanical model to the power electronics stage and control logic implementation. The purpose was to obtain comparable results from both the situations and observe differences and similarity offered from the two approaches.