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.
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 in the figure. 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 power consumption because it helps To 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.
Design of sensorless BLDC control system
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.
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.
It is very easy to implement with the operation of the Hall sensor, but removing the Hall sensor can reduce system cost and improve reliability. When the BLDC motor rotates, each winding produces a voltage called back electromotive force, which is in the opposite direction to the main voltage supplied to the winding according to Lenz's law. The back EMF is mainly determined by three factors: the angular velocity of the rotor, the magnetic field generated by the rotor magnet, and the number of turns of the stator winding.
When a sensor is used, the MCU determines the BLDC commutation point based on the Hall signal. When using sensorless control, the back-EMF zero-crossing detection can be used to determine the correct commutation point, as shown in Figure 5.
When at a constant speed, the switching period is equal to the zero crossing period. The circle in the figure represents where the zero crossing occurs, generally in the middle of the two switching points. Therefore, through the timer to get the last zero-crossing time and the current zero-crossing time, you can calculate the correct commutation point.
Among them: - actual zero crossing time, - last zero crossing time, - next commutation point time, - constant ranging from 0.3 to 0.5 (depending on motor parameters).
Therefore, the successful detection of the zero-crossing of the back EMF determines the key to the success of the sensorless control of the BLDC. As shown in Figure 6, there are usually two ways to detect the zero-crossing of the back EMF in both hardware and software. The first is to use three hardware comparators. A phase that is not powered during a switching cycle can be compared with a 1/2 Udcb voltage by a corresponding hardware comparator. When the ON period is valid during the PWM period, the comparator can A change in the back electromotive voltage is detected to arrive at a position of the zero crossing. The second method can be implemented by software AD sampling. During the operation of the BLDC motor, the ADC is used to sample the unpowered phase. At the same time, the Udcb will be simultaneously sampled by another ADC module, and then the software can be based on real-time. The two sample values ​​are judged by the zero crossing.
The use of a comparator can reduce the burden on the CPU, but the way the software is sampled is more flexible, and the position of the zero crossing can be detected more accurately. The S12ZVM also integrates the hardware modules that implement the above two methods. The internal phase includes three phase comparators for hardware comparison. The AD module includes two independent ADCs for software sampling. The two ADCs can be used for the back electromotive voltage and the Udcb voltage. Simultaneous sampling ensures that the zero crossing detection is more accurate.
When using software sampling, you must select the appropriate sampling point. Figure 7 shows the variation of the back electromotive voltage in the case of power-on in one PWM period. When ON is active during the PWM cycle, the SAtop tube is opened to DCBUS and the SCbot is also connected to ground. Therefore, the current will flow from the DCBUS through the SAtop tube through the A phase and the C phase, and then into the SCbot tube into the ground. It can be seen that the intermediate point voltage of the three phases can be approximated as DCBUS/2. At this time, the DCBUS/2 voltage is generated on the B phase. The back electromotive force can be detected as positive and negative, which also means that the voltage at the zero crossing can be successfully detected. When it is OFF in the PWM cycle, the lower bridges of the A phase and the C phase are respectively connected to the ground, and the intermediate point of the three phases is also connected to the ground. In this case, it is difficult to detect the B opposite electromotive force. Zero point.
It can be seen that the detection of the back electromotive force can be realized only when the PWM period is in the ON state. For the AD module, it needs to be synchronized with the PWM. With the timing delay module, it is possible to accurately sample the ON state before the end of the PWM cycle. The S12ZVM has a PTU programmable trigger unit module, which internally contains a 16-bit counter, two independent trigger generators, which can be configured with up to 32 trigger events, and the PTU trigger process can be initiated according to the Reload event of the PWM module. Figure 8 can visually understand the sampling process of sensorless detection of back electromotive force. PMF/PWM generates a PWM Reload signal to the PTU unit. The 16bit Counter in the PTU unit starts counting. The PTU will generate a trigger event at the time point T2. It triggers ADC0 and ADC1 to simultaneously sample the back electromotive force and DC bus voltage respectively. When the ADC sampling conversion ends, an ADC interrupt is generated. In the ADC interrupt service subroutine, the back electromotive zero crossing can be judged.
to sum up
This paper introduces the application of Freescale S12ZVM hybrid integrated chip in automotive BLDC, including the sensorless control strategy and startup process of S12ZVM, and also introduces the method and strategy of zero-crossing detection of back EMF. With Freescale's S12ZVM single-chip motor control solution, designers can reduce product size, reduce noise and improve energy efficiency, and accelerate the development of automotive BLDC controllers.
Bi Directions Thyristor (Triac)
Bi Directions Thyristor (Triac) is equivalent to the antiparallel connection of two unidirectional thyristors, but only one control pole.
Bidirectional thyristors are made of N-P-N-P-N five-layer semiconductor materials, and three electrodes are also derived from the outside. Its structure is shown in the figure. Bi-directional thyristor volt-ampere characteristic curve Since the forward and reverse characteristics of the bidirectional thyristor are symmetrical, it can be turned on in any direction and is an ideal AC switching device.
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