1 Brushless DC motors have a series of advantages such as small size, light weight, easy maintenance, high efficiency, energy saving, and easy control. They are widely used in various fields. The conventional brushless DC motor mostly uses a Hall element or other position detection element as a position sensor, but the position sensor is difficult to maintain, and the temperature characteristics of the Hall element are not good, resulting in deterioration of the system reliability. Therefore, the position sensorless brushless DC motor becomes an ideal choice, and has broad development prospects, but its control circuit is quite complicated. The emergence of the M L4428 control chip simplifies the design of the control circuit. The chip contains a back-EMF detection circuit, a start-up reversing logic circuit, and a protection circuit, so that the controller chip can realize DC power without a small amount of RC components. Brush motor control. 2M L4428 Schematic and Function Realization M L4428 motor controller can provide Y-shaped brushless DC motor (BLDC) with various functions required for starting and speed regulation without Hall sensor. It uses a 28-pin dual-list SO IC package. Its internal block diagram is shown in Figure 1. It uses phase-locked loop technology to detect the back-EMF from the motor coil and determine the commutation sequence using a special back-EMF detection technique to achieve three-phase no Brush DC commutation and free from noise and motor buffer circuit The use of start technology to check the rotor position and accurately accelerate the motor ensures that the motor will not reverse and shorten the start-up time when starting. 2. 1 Obtaining the back-EMF detection signal The fundamental difference between the position sensorless brushless DC motor control and the position sensorless brushless DC motor control is to use the back-EMF waveform to find the best reversal point. When a permanent magnet brushless DC motor is running, the back electromotive force (EM F ) of each phase winding is closely related to the rotor position. Because the windings of each phase are alternately conducting, some special points of the back-EMF waveform can replace the function of the rotor position sensor to obtain the required information when a certain phase is not conducting. As for the single-inverted electromotive force waveform diagram, the zero-cross point of the back EMF zero crossing delays the corresponding commutation signal of the winding at 30° to find the back EMF zero-crossing point, ie, the task of back-EMF detection [2]. Based on this principle, in the chip A unique back-EMF detection circuit was designed (see Figure 2). Because of the mid-point analog circuit, there is no need to draw the neutral line from the three-phase winding of the motor [1]. Among them, the multiplexer switch is connected in turn to generate the back-EMF. Windings, comparing the output of the midpoint simulator and the multiplexer, it can be concluded that the two outputs have similar waveforms and different amplitudes. The only micromotor in the 34th Vol. 1 (total phase) of 2001 is the intersection point of the back EMF. Zero point. These two outputs pass through the comparator output on the right side as the current phase signal of the rotor, determine the increase and decrease of the commutation frequency (V CO ), the commutation frequency is compared with the sampling back EMF phase, and the backward commutation makes the error amplifier filter to the loop. Charger to increase input. In contrast, early commutation will cause the capacitor on the loop filter to discharge, reducing the VCO input. With this phase-locked loop technology, a proper commutation moment is obtained. In addition, the signal taken from the RC pin is a voltage signal that represents the speed of the motor and can be used for closed-loop speed control. The frequency signal of speed can be obtained by monitoring the output of VCO, which is the signal that the phase-locked loop locks to the exact commutating frequency of the motor. 2. 2 Starting commutation technique The commutation is accomplished by the back-EMF signal sampling detected by the phase-locked loop control. When the motor is at rest and at low speed, the back EMF is zero or extremely low and cannot be detected. Therefore, it must be determined by other methods. The method starts with an open loop and generates enough back electromotive force to enter normal commutation. The control chip provides a full start commutation technology: ML 4428 has a RUN comparator inside (see Figure 1), the foot voltage signal represents the motor speed signal, the starting leg voltage is lower than 0. 6V, RU N comparator output is turned on Start the logic circuit, turn off the commutation logic circuit, M L4428 will send out 6 samples to measure the rotor position, and drive the corresponding coil to produce the required rotation, which will cause the motor to accelerate until the RC pin voltage reaches 0. 6V, the speed is high enough The detected back EMF is generated. At this time, the RUN comparator output turns off the start logic circuit, allowing the phase-locked loop circuit to start working and entering the normal commutation logic working state. After detection, the motor speed is the maximum rotation speed of the motor. 2. 3 The closed-loop speed control system The internal speed control system is a typical DC motor PWM double-closed-loop speed control system. As shown in Figure 4, two regulators are set in the system to adjust the speed and current respectively. Cascade connection, that is, the output of the speed regulator acts as a current regulator, and the output of the current regulator controls the switching devices. The double closed-loop system composed in this manner shows a constant current regulation system in the sudden transition and given a transitional process. In the steady state and near-steady-state operation, it also shows that there is no static differential speed control system, ie, the speed and current are exerted. The respective roles of the two regulators also avoid the disadvantages of the two feedback mutual restraints, such as the single-loop system, so as to obtain good static and dynamic quality. 2. 4 Internal Protection Circuit The ML 4428 has current sensing and current limiting. The source current of the external power device MO SFET flows through the R to obtain a voltage directly proportional to the winding current of the motor with the simple method of position sensorless brushless DC motor control. A filter that can filter out the monostable by the loop filter. The noise peak current of the circuit is generally selected within the time constant of 300ns to the positive terminal of the current comparator), and the negative terminal of the comparator has a diode with a clamping voltage of 0. 5V, so the maximum of the stator circuit of the motor can be limited. Peak Current When the current sense circuit's voltage is higher than the comparator's negative terminal voltage, the monostable circuit is triggered, turning off the output MO SFET, and the current drops until the monostable current is reset. M L4428 normal power supply is 12V, when the power supply is lower, all 6 output drivers will be turned off. 3 concludes the speech to use the controller chip, has simplified the control of the brushless direct current motor, it not only has the good current limit and the protection function, but also uses the ML 4428 double closed loop velocity modulation system performance which also will improve, uses this controller chip It solves the problem of commutation using back-EMF detection and open-loop starting at low speed. Experiments show that the control system has a simple structure and complete functions, which improves the reliability of the system. This method has important practical significance for the wide application of brushless DC motors.
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