BLDC motor control algorithm The
brushless motor is a self-converter (self-direction conversion), so it is more complicated to control.
BLDC motor control requires an understanding of the rotor position and mechanism of the motor for rectification and steering. For closed-loop speed control, there are two additional requirements, namely the measurement of the rotor speed and / or motor current and the PWM signal to control the motor speed power.
BLDC motors can use side-aligned or center-aligned PWM signals according to application requirements. Most applications only require speed change operation, and will use six independent side-aligned PWM signals. This provides the highest resolution. If the application requires server positioning, energy consumption braking or power reversal, it is recommended to use a supplementary central arrangement of PWM signals.
In order to sense the rotor position, BLDC motors use Hall effect sensors to provide absolute positioning sensing. This leads to the use of more lines and higher costs. Sensorless BLDC control eliminates the need for Hall sensors, but uses the motor's back-EMF (electromotive force) to predict rotor position. Sensorless control is essential for low-cost variable speed applications like fans and pumps. When adopting BLDC motor, refrigerator and air conditioner compressor also need sensorless control.
Dead zone insertion and replenishment
Many different control algorithms are used to provide control of BLDC motors. Typically, power transistors are used as linear regulators to control the motor voltage. This method is not practical when driving high-power motors. High-power motors must use PWM control and require a microcontroller to provide starting and control functions.
The control algorithm must provide the following three functions:
· PWM voltage for controlling motor speed
· Mechanism for commutation of the motor
· Method for predicting rotor position using back electromotive force or Hall sensor
Pulse width modulation is only used to apply variable voltage to the motor windings. The effective voltage is proportional to the PWM duty cycle. When proper commutation is obtained, the torque and speed characteristics of BLDC are the same as those of DC motors. Variable voltage can be used to control the speed and variable torque of the motor.
The commutation of the power transistors achieves the proper winding in the stator, which generates the best torque based on the rotor position. In a BLDC motor, the MCU must know the position of the rotor and be able to commutate at the right time.
Trapezoidal commutation of BLDC motor
One of the simplest methods for brushless DC motors is to use so-called trapezoidal commutation.
Figure 1: Simplified block diagram of a ladder controller for BLDC motors
In this schematic, each time a pair of motor terminals is used to control the current, and the third motor terminal is always electronically disconnected from the power supply.
Three Hall devices embedded in the large motor are used to provide digital signals. They measure the rotor position in a 60-degree sector and provide this information on the motor controller. Since the amount of current on both windings is equal each time, and the current on the third winding is zero, this method can only generate a current space vector with one of six directions. With the turning of the motor, the electric current of the motor terminal is switched once at 60 degrees per revolution (rectification commutation), so the current space vector is always at the position closest to 30 degrees with a phase shift of 90 degrees.
Therefore, the current waveform of each winding is trapezoidal, starting from zero to positive current to zero and then to negative current.
This creates a current space vector, and when it steps up in six different directions as the rotor rotates, it will approach equilibrium rotation.
In motor applications such as air conditioning and frost, the use of Hall sensors is not a constant choice. Back-EMF sensors induced in non-coupling windings can be used to achieve the same result.
This type of trapezoidal drive system is very common due to the simplicity of its control circuit, but they encounter torque ripple problems during the rectification process.
Sinusoidal commutation of BDLC motor
Trapezoidal commutation is not enough to provide balanced and accurate brushless DC motor control. This is mainly because the torque generated in a three-phase brushless motor (with a regular wave back-EMF) is defined by the following equation:
Sinusoidal commutation brushless motor controller strives to drive three motor windings, and its three-way current smoothly changes sinusoidally as the motor rotates. The relative phases of these currents are selected so that they will produce a stable rotor current space vector, the direction is orthogonal to the rotor, and has an invariant. This eliminates torque ripple and steering pulses associated with north-shaped steering.
In order to generate a smooth sine wave modulation of the motor current as the motor rotates, an accurate measurement of the rotor position is required. The Hall device only provides a rough calculation of the rotor position, which is not enough to meet the requirements. For this reason, angular feedback is required from encoders or similar devices.
Since the winding currents must be combined to produce a stable constant rotor current space vector, and each position of the stator windings is 120 degrees apart, the current in each line group must be sinusoidal and the phase shift is 120 degrees. The position information in the encoder is used to synthesize the two sine waves, and the phase shift between the two is 120 degrees. Then, these signals are multiplied by the torque command, so the amplitude of the sine wave is proportional to the required torque. As a result, the two sine wave current commands are properly phased, resulting in a rotating stator current space vector in the orthogonal direction.
The sinusoidal current command signal outputs a pair of PI controllers that modulate the current in two suitable motor windings. The current in the third rotor winding is the negative sum of the controlled winding current and therefore cannot be controlled separately. The output of each PI controller is sent to a PWM modulator, and then to the output bridge and the two motor terminals. The voltage applied to the third motor terminal is derived from the negative sum of the signals applied to the first two line groups, and is suitably used for three sinusoidal voltages separated by 120 degrees, respectively.
As a result, the actual output current waveform accurately tracks the sinusoidal current command signal, the resulting current space vector rotates smoothly, is quantitatively stabilized, and is positioned in the desired direction.
Generally, the trapezoidal rectification steering can not achieve the stable control sine rectification steering result. However, due to its high efficiency at low motor speeds, it will separate at high motor speeds. This is due to the increased speed, and the current return controller must track a sinusoidal signal of increasing frequency. At the same time, they must overcome the back-EMF of the motor with increasing amplitude and frequency as speed increases.
Because the PI controller has limited gain and frequency response, the time variable disturbance to the current control loop will cause phase lag and gain error in the motor current. The higher the speed, the greater the error. This will disturb the direction of the current space vector relative to the rotor, causing displacement in the orthogonal direction.
When this happens, a smaller amount of torque can be generated by a certain amount of current, so more current is needed to maintain the torque, and the efficiency is reduced. As the speed increases, this decrease will continue. To some extent, the phase shift of the current exceeds 90 degrees. When this happens, the torque is reduced to zero. Through the combination of sinusoids, the speed at this point leads to a negative torque, so it cannot be achieved.
