Design of DC motor stepless speed regulation system using pic single chip microcomputer
In modern industrial production, the electric motor is the main driving equipment. At present, the KZ-D drive system that supplies the thyristor (ie, silicon-controllable) device to the electric motor has been widely used in the DC motor drive system, replacing the bulky electric motor. The F-D system, along with the high development of electronic technology, has led to the gradual shift of DC motor speed control from analog to digital, especially the application of single-chip technology, which has brought the DC motor speed control technology to a new stage, intelligent And high reliability has become its development trend. The speed control system adopts PIC16F874 single-chip microcomputer as the central processor, which fully utilizes the characteristics of the PIC16F874 single-chip capture, comparison and analog/digital conversion module as the trigger circuit. Its advantages are: simple structure, synchronization with the main circuit, smooth phase shift and With sufficient phase shift range and control angle adjustment up to 10,000 steps, it can realize stepless smoothing control of the motor. The pulse front is steep and has sufficient amplitude, pulse width can be set, stability and anti-interference performance are good.
1 DC motor speed principle
In medium and small power DC motors, the armature loop resistance is very small, and the IaRa term in equation (4) can be omitted. It can be seen that the speed of the DC motor changes when the armature voltage is changed.
2 system working principle
The system mainly consists of a main control switch, a motor excitation circuit, a thyristor speed control circuit (including a speed measuring circuit), a rectifying and filtering circuit, a smoothing reactor and a discharging circuit, and a power consumption braking circuit. The system is controlled by a closed loop PI regulator. After the main control switch is closed, the single-phase AC power is controlled by the thyristor speed control circuit, and after the bridge rectification, filtering, and smoothing reactor, a small pulse, continuous DC is obtained, which is supplied to the motor, and at the same time, the AC power passes through the excitation circuit. After rectification, the motor is excited and starts working. Adjust the speed setting potentiometer RP1 in the trigger circuit, so that when the input voltage of AN1 decreases, the control angle of the output of the PIC16F874 single-chip microcomputer also decreases accordingly, the conduction angle of the thyristor increases, the output voltage of the main circuit increases, and the motor speed increases. At the same time, the output voltage of the speed measuring circuit also increases. After the action of the PI regulator, the motor runs stably within the set speed range.
3 system part of the circuit design
3.1 main circuit design
The parameters of each component in the main circuit are shown in Figure 1:
Press the start button SB1, the contactor KM coil is energized, the KM normally open contact is closed, the normally closed contact is opened, the start button is self-locking, and the main circuit is turned on. The thyristor speed control circuit controls the AC output by changing the control angle of the triac, and then through the bridge rectification and filtering, the DC is obtained. At the same time, the motor is rectified by the excitation circuit to obtain excitation and start working.
Press the stop button SB2, the contactor KM coil is de-energized, the KM normally open contact opens, the normally closed contact closes, the self-locking is released, the main circuit is de-energized, and the motor stops working.
In order to limit the DC current ripple, a smoothing reactor is connected to the circuit, and the resistor provides a discharge loop for the smoothing reactor when the main circuit is suddenly powered off.
In order to speed up braking and stopping, the device uses energy-consuming braking, and the resistor R4 and the main circuit contactor normally closed contact constitute a braking link. The motor excitation is powered by a separate rectifier circuit. In order to prevent the motor from being demagnetized and causing a flying accident, in the excitation circuit, the undercurrent relay KA is connected in series. The operating current can be adjusted by the potentiometer RP.
3.2 Thyristor Trigger Circuit Design
The thyristor trigger circuit and parameters are shown in Figure 2. The voltage from the two points A and B in the main circuit is transformed into -20V by the transformer. After the bridge rectification, a half-wave signal of about 100 Hz is generated at 2 points, and the R6 is passed. After R7 is divided, the NPN transistor is connected to amplify, and a zero-cross pulse is generated at the collector of the triode. The rising edge of the zero-pulse is first captured by the CCP1 module, and the time of occurrence is recorded, followed by the falling edge of the zero-pulse pulse. The time difference is the zero-crossing pulse width, and half of its value is the pulse midpoint. With this kind of capture method, the actual zero-crossing point of the alternating current can be accurately obtained, and the analog voltage of the PIC16F874 pin RA1/AN1 is converted by the ADC mode conversion module. The value is used as the set value of the thyristor control angle (motor speed set value), the set value of the potentiometer RP1 is changed, and the thyristor control angle is changed accordingly. At the same time, the output value of the speed measuring circuit is input by the PIC16F874 pin RC0/T1CKI, and is counted by the TMR1 counter. Calculate the rotation speed as the speed feedback value. The oscillation frequency of the single-chip microcomputer in this system adopts 4MHz. It is known from the characteristics of the instruction cycle of the PIC16F874 single-chip microcomputer that the resolution of the thyristor control angle is the reciprocal of one quarter of the oscillation frequency of the single-chip microcomputer, that is, 1us, and the half-wave time of the power frequency is 10ms. Said that the control angle can reach 10,000 steps, which can completely realize the stepless smoothing control of the motor.
The system software and hardware design make full use of the characteristics of PIC16F874 single-chip capture, comparison, analog-to-digital conversion module, and the advantages of high oscillation frequency and fast response of the single-chip microcomputer, and design the corresponding trigger circuit to make the analog/digital conversion module of PIC16F874 single-chip microcomputer. It can quickly and accurately convert the speed setting value; CCP1 module can accurately capture the zero-crossing point of AC; the timing counting module of the speed measuring circuit can accurately count and calculate the feedback speed; CCP2 module can compare the Tf value output trigger pulse in time. In the application of small DC motor speed control system, it has the characteristics of simple structure, reliable operation, wide adjustment range, good current continuity and fast response.
