Position and speed control of Brushless DC motor using sensorless technology and its application trend
With the advantages of high power density and solid structure, brushless DC (BLDC) motor plays an important role in many application fields, especially in high-performance applications. For example, using server position sensors in machine tools and robots can start and operate successfully. Due to the cost problem, low performance applications, space constraints and the reliability of position sensors, the research of sensorless control is promoted. In addition, rapid and continuous improvement, powerful and economical microprocessor and digital signal processor (DSP) have accelerated the development of sensorless control technology. Brushless DC (BLDC) motor is widely used in automotive air conditioning compressor, engine cooling fan, fuel / water pump, etc.
Brushless DC (BLDC) motor is a synchronous motor. It looks like a DC motor. It has a linear relationship between current and torque, voltage and speed. It is an electronically controlled commutation system rather than mechanical commutation, which is a typical feature of brush motor. In addition, the electromagnet does not move, the permanent magnet rotates, and the armature remains static, which solves the problem of how to transfer current to the moving armature. Replacing the brush system / commutator assembly with an intelligent electronic controller, the controller performs the same power distribution as the brushless DC motor. BLDC motor has more advantages than brush DC motor and induction motor, such as better speed and torque characteristics, high dynamic response, high efficiency and reliability, long service life (brushless corrosion), noiseless operation, higher speed range and reduced electromagnetic interference (EMI). The higher ratio of transmitted torque to motor size makes it particularly useful in applications where space and weight are key factors, especially in aerospace applications.
Based on Sensorless Control Technology of Brushless DC motor (BLDC), back electromotive force (EMF) detection method has been widely used in industrial and commercial fields. As we all know, the size of back EMF is directly proportional to the motor speed, so it is impossible to use back EMF to detect the correct operation of the motor at rest. In order to solve this problem, many methods have been developed, one of which is called three-step starting method. Firstly, align the rotor with the predetermined direction, then accelerate the motor, and use the back electromotive force to open the loop. This starting method is very simple and easy to implement, but it will be affected by the load, which may temporarily lead to reverse, and can not be applied to the field where rotation is not allowed.
There are many ways to obtain the rotor position information. In the sensorless control technology, the rotor position information is determined by indirectly detecting the back electromotive force (electromotive force) from one of the three motor terminal voltages. Only two of the three BLDC motor phase windings are connected at the same time, and the third non-conductive phase will be connected in the opposite direction, so the electromotive force of rotor position and speed can be calculated indirectly. BLDC motor drivers with sensors generally use three-phase PWM inverters with rotor position sensors to perform phase commutation and / or current control.
Sensorless control technology was initially considered a cost saving benefit. It can improve system reliability, reduce the number of electrical connections, eliminate mechanical alignment problems, and reduce the size and weight of the motor. Sensorless control of BLDC motors without rotor position sensors eliminates rotor position sensors (e.g., optical encoders, Hall effect sensors, resolvers, cables and decoding circuits) and reduces manufacturing costs by reducing reliability and durability. At present, sensorless technology has not been widely used, and it is expected to become the main BLDC motor control method in the future.