Immersion and invariance disturbance observer based discrete time vector control of BLDC machines

Abstract

Nowadays, with the use of electric motors in every field of daily life, the amount of electricity consumed by these electric motors has increased and today's studies are focused on making both the control and driver designs of these electric motors more efficient and able to operate at higher power levels. Today, electric motors are not only limited to industry or production processes but also started to be used with the main traction drive motors in electric vehicles, and the efficiency of these motors has a significant effect on the total range of the vehicles, and more efficient motors have also started to be preferred in these vehicles. The relationship between efficiency and price is not a design criterion that can be ignored. Along with the increasing demand for efficiency in electric vehicles, the types of motors used also vary due to price reduction policies. Synchronous motors are mostly preferred in electric vehicles. Synchronous motors are motor types in which the magnetic field produced by the stator and the rotor rotate at the same speed. However, this is not the case in asynchronous motors, and the rotor speed follows behind the magnetic field produced by the stator. When these motors are compared in terms of efficiency and price, the following arguments emerge. Among asynchronous motors, the induction motor has become one of the first preferred motors in the electric vehicle sector. This type of motor has short-circuited conductive plates on the rotor. The working principle of this motor is that the magnetic field produced by the stator induces a current on these conductive plates and this induced current ensures that the rotor rotates. Induction motors provide a more affordable price advantage than synchronous motors due to their structure, only having windings on the stator and the simple structure on the rotor, but since there is no structure to produce a magnetic field on the rotor, they have lower power density and therefore less efficiency than synchronous motors. When focused on the PMSM or BLDC motors, which is the most used in the sector among synchronous motors, there are permanent magnets on the rotor parts of these motors. These magnets constantly produce a fixed magnetic field. Since the magnetic field is produced by the rotor, energy does not need to be spent to re-generate the entire magnetic field by the stator, so these machines are more efficient than induction machines. Thanks to the magnets in the rotor structures, these motors also have higher power density. PMSM and BLDC motors are similar to each other in their basic structures. Both motors have stator windings and permanent magnets on their rotors, and these magnets produce the necessary magnetic field. The difference between these two motors is the difference in the waveforms of their back EMFs. While the back EMF in PMSM machines is sinusoidal, this back EMF is square wave in BLDC motors. This thesis study focuses on developing a discrete-time vector control method based on immersion and invariance disturbance observer for BLDC motors. In this context, firstly the structures of BLDC motors are considered and electrical and mechanical equations are derived from these structures. Apart from electrical and mechanical equations, the electromechanical torque equation that acts as a bridge between these two structures and connects the two systems is derived. Using these equations, a state space equation is derived for the BLDC motor and then an immersion and invariance based disturbance or in other words, a disturbance torque observer is developed based on this derived state space equation. The main purpose of the immersion and invariance based disturbance observer is to observe and estimate the disturbance torque effects experienced by the system on the shaft of the BLDC machine. These disturbance torque factors are inherently dependent on variables that cannot be measured or are very difficult to measure. Examples of the parameters that produce this immeasurable distorting torque include friction in the motor's balls, unequal conditions experienced by the motor during the production phase, the ideal unequal phase resistances and inductances of the motor, and the unequal heating of the machine due to an unequal ventilation system. Later, the vector control structure, which is the most preferred method for three-phase synchronous motors, was studied and this control method was tested in a simulation environment. The main idea in the vector control method is to be able to control the magnetic field production and torque production in brushed DC machines separately. While brushed DC machines achieve this by having separate windings in their stators and rotors, this study cannot be achieved in BLDC machines because there are permanent magnets in the rotor section. At this point, the vector control method converts the three-phase rotating currents into two constant currents at ninety degrees to each other using the transformations called Clarke/Park. These currents are called d-q axis currents, and while the d current controls the magnetic field produced, the q current controls the torque produced by the machine. In this way, the components that produce the torque and magnetic field are separated from each other as in brushed DC machines. The most preferred external control loops in the vector control method are known as torque control and speed control. These controls actually send control signals to the d and a current controllers as reference values as muscle layers with the speed control being the outermost. In this study, the speed control structure was preferred instead of torque control. The disturbing torque estimated by the immersion and invariance-based observer was then connected to the vector control structure as a pre-fed term, thus eliminating the effects of the disturbing torque on the system and achieving a more robust control. Here, the estimated torque was added to the reference torque, which is the output of the speed controller, and fed to the system. The speed, d and q axis current controllers, an inverter model, a BLDC machine model and a load model required for vector control were modeled in the Simulink environment. The reason for including the inverter model in this modeling is that the system has an inverter in real-life operation. Therefore, it was aimed at creating a modeling environment that exhibits more realistic dynamics. All components modeled in this modeling environment were modeled modularly. In other words, each of these models was developed as a separate function and the components such as motor resistance, inductance, and number of poles they needed to operate were provided parametrically from outside. Thanks to this parametric structure, BLDC motors with different structures and parameters can be changed quickly without making major changes to the system, which increased the mentioned parametric structure of the system. The developed models were first tested one by one in the Simulink environment and their operation was verified, then all models were connected to each other and the entire system was first operated in open loop and then in closed loop. After verifying that the proposed method works, a test environment containing an isolation transformer, DC input capacitor, inverter, BLDC motor, controller, induction motor and load was designed so that this method could be tested in real life. Attention was paid to keeping the cost of this test environment low. The hardware and software design of the inverter used in this test environment was made within the scope of this thesis. At the same time, the system design for the test environment and the implementation of this test environment were provided. After this design was realized, the six-step control method, which is widely preferred in BLDC motors due to its simplicity, was first implemented and the commissioning and verification studies of the designed test environment were carried out. After the commissioning studies of the test environment, the proposed method was tested on this test environment. The tests carried out in the simulation environment were compared with the tests carried out on a real engine in the test environment and the results were presented with the study of the proposed method.