Electromagnetic Surface Micromotor Speed ​​Control Based on ML4426

Electromagnetic plane motors cannot be installed with position sensors due to their size. Therefore, position sensorless control methods are generally used. This not only reduces the system cost, reduces the system volume, but also improves the system's anti-jamming capability and reliability. However, it is difficult to detect back electromotive force when the motor is running at a low speed. In general, a control method in which the motor operating mode is combined with the back electromotive force commutation technology is adopted, and the synchronous motor operation mode is used when starting, and the back electromotive force is enough to be accurately detected by the controller. Then switch to back-EMF technology to control the commutation. The structural principle frame is shown in Fig. 2. The controller uses a special control chip ML4426 produced by American Microlinear. Its characteristics are simple structure, small size, low cost and short development cycle. This article introduces the main features, working principle of ML4426 and its application in electromagnetic plane micro-motor control.

The main features of the 1ML4426 ML4426 is a dedicated control integrated chip designed for drive control of a star- or delta-connected sensorless three-phase brushless DC motor, using a 28-pin dual-listing package. Its main features are shown in Figure 1.

National 973 Supported Project: No.: G1999033102 Next: It can be operated by a single unit, with few peripheral circuit components, simple circuit, high reliability, free from the impact of PWM noise and motor inrush current, and can provide the minimum jitter-free torque; using PWM control In this way, the motor power is increased and the size of the drive module is reduced; current and speed dual closed-loop control can be achieved; by setting a current sense resistor, the power level and the maximum current of the brushless DC motor can be adjusted; the soft start function can limit the start Current over-current detection and under-voltage lockout function, to achieve a positive, reverse and brake braking function on the motor.

2ML4426 structure and working principle can block diagram and its application circuit. Control functions include: start-up circuit, back-EMF sampling circuit, commutation control logic, PWM speed control, high-side and low-side gate drive, current limit, and under-voltage protection. Now when the system is just powered on, the motor is in a stationary state. At this moment, the stator passes a constant current, pulls the rotor of the motor to a certain position, and then starts in a synchronized manner until its back potential can be detected. Start is mainly divided into three steps: calibration reset, open loop acceleration, closed loop acceleration.

2.1.1 Calibration Reset When the ML4426 chip is powered on, the chip starts charging with a constant 0.75uA current to the 17-pin external capacitor Crst. When the 17-pin potential reaches 1.5V, the calibration reset phase ends.

During reset calibration, pins 17 and 19 of the chip are in a low state and pin 10 is in a high state, causing the P-type channels P1 and 3 to be turned on, the N-type channel N2 to be turned on, and the motor to be turned on. The torque is larger, so that the rotor of the motor is calibrated to a 30° electrical angle position before the first commutation state enters the center position. The reset time is set by the 17-pin grounding capacitance Crst: (1'0*V7)5uA: The RST calibration state must be long enough, otherwise it will fail to start. The reset time depends on the motor, load, friction damping, eddy current 2.1.2 open-loop acceleration 75uA current to 19-pin grounding capacitor Cen charge, while 0. 5uA current will be 20 pin external RC filter RC network charging, making The frequency of the VCO (Voltage Controlled Oscillator) begins to increase linearly. When the Cen pin potential reaches 1.5V, the open-loop acceleration phase ends and the system enters a closed-loop acceleration mode.

The open-loop acceleration time is set by the 19-pin grounding capacitor Cen: If the closed loop fails while the motor is running, increase the value of Cen and extend the time t until it can be closed.

2.1.3 When the closed-loop acceleration is 5V, the motor speed exceeds 100r/min, the back electromotive force is enough to be detected, and the motor enters the closed-loop commutation state. At this time, the frequency of the 20-pin voltage and the VCO keeps increasing linearly. When the 20-pin voltage is close to the voltage set by the 8-pin, the PWM current control module starts to work, controlling the motor current and keeping the motor speed constant, and the system enters a stable PWM speed. The chopper frequency of the PWM is set by the 6-pin grounding capacitor Csc. Set, control mode. The smaller the value, the lower the chopping frequency. The chopper turn-off time is set by the 26-pin grounding capacitor Cos: t= 2.2 PWM speed control When the 20-pin voltage is equal to the voltage set by the 8-pin, the 5-pin Cpwi capacitor starts charging, and the 6-pin grounding capacitor produces sawtooth Wave comparison, can generate a certain duty cycle, thus generating a PWM signal to control the power switch of the N-channel, achieving PWM speed control general value less than 0.5us. Speed ​​control accuracy is adjusted by the 5-pin RC RC circuit, its value The formulas are as follows: J-Motor and Load Moment of Inertia Sum (Kgm2) Ke - Back-EMF Constant (V/Rad) R-Motor Rotor Impedance (A) freq - Speed ​​Loop Bandwidth (Hz) Ucc - Supply Voltage (V) N ― Motor Rotor Poles 2.3 Cross Conduction Comparator ML4426 is turned on during calibration reset phase. When it switches to ramp acceleration mode, P3 turns off and N3 turns on, which may lead to cross conduction of three-phase power bridges. In addition, there is a difference in the turn-on and turn-off times of the power devices, which may also lead to cross conduction. To solve this problem, a comparator is used to compare the P3 level with the Vdd-3V voltage only when the P3 level is lower than Vdd- At 3V, N3 turns on to avoid cross conduction.

2.4 Forward/reverse rotation of the motor The forward/reverse rotation of the motor is set by controlling the ON/OFF between pin 12 and ground.

2.5 Motor Braking When the ML4426's 25-pin BRAKE level drops below 1.4V, the internal comparator forces the 3 P-channels to close, the 3 N-channels to conduct, and the current limit is inhibited. move. If fast braking is not required, pin 25 and pin 17 of ML4426 can be grounded at the same time. When the motor is stationary, pin 25 and pin 17 of ML4426 can be disconnected from ground at the same time. ML4426 does not need to be powered on again to realize the motor. Restart.

3 Several problems that the design controller should pay attention to The rate of increase of the bookmark4 rate should be less than the acceleration rate of the motor, that is, in the open-loop stepping mode, the motor must be able to keep up with the change of the VC0 frequency, otherwise it will cause the start-up failure.

The wave frequency is too low, less than 20kHz, the magnetostrictive effect in the motor will produce noise; if the chopper frequency is too high, greater than 30kHz, then the power stage drive will have serious switching losses.

3) For the motor with high driving voltage, current limitation is prohibited during braking. If forced braking, the motor coil may be burned and the BRAKE pin must be used with caution.

The reversal signal is valid, otherwise it will cause the motor to reverse start failure.

high. If the motor inertia is large, the rotor of the motor may not keep up with the frequency of the voltage-controlled oscillator in the acceleration phase, resulting in failure to start.

At this point 21 feet must be connected to a capacitor to ground.

And this capacitor is only turned on during the acceleration phase, reducing the rate of acceleration. The size of this capacitor is: J - sum of moment of inertia of motor and load (KgXm2) Kv - gain of voltage-controlled oscillator (Hz/V) N - number of poles of motor Kt - torque constant (Nm/A) C1, C2* pin external resistance capacitance filter capacitor (F) The electromagnetic plane micromotor used in this experiment is a three-phase Y-type, no position sensor structure, using + 6V DC voltage supply, the maximum operating current is 1.5A, rated speed is 15000r/min, the number of pole pairs is 3, and the armature circuit resistance 6.4a is the speed-regulating characteristic curve of the electromagnetic plane micromotor in the case of Cvc=5F and Cvc=2.2F. The experimental results show that: 1) The motor is started in an open-loop step-by-step mode. At this time, the VC0 frequency (turning to page 46) is high and low, and the security is controlled by Jin Liangliang and Hong Weihua. Research on a New Type of Linear Permanent Magnet Motor for Compressor Drive . A summary of the development of micro-motor sciences The 5th micro-motor professional committee academic exchange conference proceedings, (North Zhang Guangyi. Research on the static lateral edge effect of linear motors. Micro-electron Ye Yunyue, Lu Kaiyuan. Linear motor PID control and fuzzy control. Journal of Electrotechnics, 2001,16 (3): 11-15 1963-), male, professor, research direction: motor and its control technology, power electronics and power conversion technology.

(Continued from page 39) ML4426 chip 8-pin control voltage Vcm and the motor turn n shows a good linear relationship; the chip's 15-pin grounding capacitor Cv, the smaller the capacitance value, the minimum speed is smaller, the maximum speed Larger, the wider the range of speed; The same control circuit is used to drive the motor to drive loads of different inertias. Under certain control voltage, the speed is basically unchanged, only affecting the calibration reset and starting time.

High speed accuracy, when the speed is greater than 2000r/min, the error is less than 0.1%, when the speed is greater than 5000, the error is less than 0.02%. By adjusting the 5 RC RC circuit, you can get a good control accuracy.

5 Conclusions The electromagnetic type planar micromotor speed control system using ML4426 chip is simple and practical, with good linearity and high control precision. It is easy to form a micro control system with a microprocessor. The ML4426 accepts different control signals from the microprocessor, with different The different motor speed control of the inertia moment load is optimized. This method has been applied in the project and has obtained satisfactory results.

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