Compared with traditional brushed motors, BLDC motors have higher energy efficiency, longer service life, more compact shape, lower noise and higher reliability. These advantages make BLDC appear more and more in cars. In applications, it replaces conveyor belts and hydraulic systems, providing additional functionality and improved fuel economy while eliminating maintenance costs. Since the electrical excitation must be synchronized with the rotor position, one or more rotor position sensors are typically required for BLDC motors to operate. Due to cost, reliability, mechanical packaging, especially when the rotor is operating in a liquid, the motor is suitable to operate without a position sensor, commonly known as sensorless operation.
For automotive BLDC control systems, it is expected to achieve small PCB size, low BOM cost, simple and reliable, low power consumption, etc. For the needs of this series, Freescale Semiconductor has introduced a single-phase brushless motor for automobiles. Chip solution S12ZVM family. The S12ZVM is the most integrated brushless DC (BLDC) motor control solution on the market, helping to accelerate the transition from direct current (DC) to BLDC motors.
1 S12ZVM Features
The Freescale S12ZVM family is a breakthrough technology that combines the four system elements of the MCU, MOSFET gate drive unit, voltage regulator and local interconnect network (LIN) physical layer into a single-chip solution, as shown 1. Usually two to four chips are required to implement these four functions. Compared to other discrete solutions, Freescale reduces the physical footprint of printed circuit boards by 50% through on-chip integration. At the same time, automakers are constantly looking for ways to reduce vehicle weight and reduce power consumption, as this helps improve fuel economy. Electronic system vendors and motor manufacturers are also catering to this trend, but when faced with customized solutions, the solutions they get are often not optimal or scalable. The S12ZVM family is available in a number of different product versions, supporting CAN and LIN communication protocols, with a variety of memory capacities and package options. This will allow customers to re-use hardware and software designs to develop true platform solutions for applications such as air conditioning fans, wipers, fuel pumps and pumps.
Figure 1 S12ZVM integration solution
2 Sensorless BLDC Control System Design
As shown in Figure 2, the three-phase BLDC motor control can adopt the three-phase six-shot control method, and the commutation control is performed every 60 electrical angles. At the same time, the three-phase bridge PWM control can adopt the unipolar control strategy, and the upper bridge adopts PWM control, the lower bridge can be directly connected to the ground, which has the advantages of simple control, low MOS tube switching loss and low EMC noise.
Figure 2 Three-phase six-shot unipolar control strategy
Figure 3 shows the block diagram of the sensorless BLDC control system using S12ZVM. Except for the three-phase bridge and sampling resistor, the whole control can be implemented internally by S12ZVM. When using the three-phase six-beat control strategy, only one sampling resistor is needed to detect the current. The S12ZVM has an internal op amp that can amplify the current signal and sample it through the AD module. At the same time, the amplified current signal can be passed through the comparator. Overcurrent protection is compared to a given voltage. The blue part of the figure is the hardware module of S12ZVM, while the green part is implemented by software. The AD module samples the phase voltage, DCBUS voltage and operating current. After the zero-crossing detection algorithm determines the commutation control and calculates the actual speed of the BLDC. The PI controller of the speed loop calculates the actual speed and the set speed difference to determine the PWM. The duty cycle controls the rotational torque of the BLDC motor to ensure that the actual speed operates at the set speed.
Figure 3 S12ZVM BLDC sensorless control block diagram
Since the initial position of the BLDC sensorless motor cannot be known exactly, the starting process is more complicated than the starting process with the Hall sensor BLDC motor. As shown in Figure 4, the startup process includes the Alignment phase, the Open Loop StarTIng phase, and the final Run phase. In the Alignment phase, the controller simultaneously applies the same duty cycle PWM to phase A and phase B, and phase C is connected to ground, thus stabilizing the BLDC motor to a known position. The magnitude and duration of the duty cycle depends on the BLDC motor characteristics and load size, typically between 100ms and 500ms. When the Alignment process is over, it enters the Open Loop StarTIng phase. Since the back EMF is proportional to the rotor speed, the magnitude of the back EMF is very low at very slow speeds, making it difficult to detect zero crossings. Therefore, when the motor is started from a standstill state, it must use open-loop control. When there is enough back electromotive force to detect the zero-crossing point, the back-EMF detection control is used and enters the Run phase. When entering the Run phase, the BLDC uses speed closed loop control, and the zero crossing is detected by the back electromotive force.
Figure 4 BLDC sensorless control startup process
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