Design of a DC Motor Control Circuit

The DC motor pulse width modulation (PWM) controller UC3637 is used to control the speed or position of an open-loop or closed-loop DC motor. It generates an analog error voltage signal and outputs two PWM pulse signals. These two PWM pulse signals are proportional to the amplitude of the error voltage signal and are related to its polarity. Therefore, a bidirectional speed regulation system is formed to achieve PWM dual output and a driving current capacity of 100 mA. The device also has the characteristics of current limiting protection, undervoltage blocking and temperature compensation. The driver integrated circuit IR2110 has a bootstrap function for PWM signals. There are two completely independent high-fidelity input and output channels, and these two channels have an interlocking function of slow opening and fast closing to prevent bridge arm direct pass, which can make the circuit work reliably. Here, UC3637 and IR2110 are used to design a DC motor PWM open-loop control circuit, and combined with a computer control system to realize the control of a certain rudder system DC motor, and then verify the correctness of the circuit.

2 PWM open-loop control circuit

The goal of the circuit design control system is to enable the motor to quickly reach the specified position under different given signals from the computer to meet the system performance requirements. The control principle block diagram is shown in Figure 1. The speed of the controlled DC motor M is measured by the tachometer generator G. The speed signal measured by the tachometer generator is compared with the given signal in the computer after A/D conversion. After calculation, the digital control signal is output, which is converted into an analog signal through D/A conversion and sent to the pulse width signal generating circuit of UC3637, thereby realizing the speed control of the DC motor.

Figure 2 is a DC motor PWM control circuit based on UC3637. The circuit is divided into four parts: pulse width signal generation circuit, bootstrap drive circuit, main circuit, and protection circuit.

This circuit generates a threshold voltage of 5 to 10 V. U2 = 10 V is connected to pin 1, and U1 = 5 V is connected to pin 3. In this way, the triangle wave changes within 5 to 10 V, that is, the voltage of pin 2 connected to capacitor CT changes within 5 to 10 V. UK is the voltage obtained from the computer output through digital-to-analog conversion, and its range is -10 to +10 V. UC3637 requires a control voltage of 5 to 10 V to connect pins 9 and 11 to control the duty cycle of the output. Use R2 to R5 to perform level conversion on the control voltage UK. Let R2 = 10 kΩ, R3 = 18 kΩ, and R5 = 20 kΩ. When UK = -10 V, UR = 5 V should be obtained. The circuit shunt can be obtained as follows:

Substituting the data into the solution, we get R4=2 kΩ.

To avoid the direct short circuit during operation, an RC delay circuit should be connected to the output pins 4 and 7 of UC3637. Assume that the delay time r=5μs, and the resistor R6 used in the delay circuit is 5Ω. The formula is:

In this way, the dual-channel complementary PWM pulse signal has a delay of several microseconds on the rising edge and no delay on the falling edge. Combined with the delay of the upper and lower signals set inside the IR2110, it can ensure that there is a dead time for the upper and lower MOS tubes of the same bridge arm in the "H" bridge, thereby ensuring the safe and stable operation of the circuit. Since the 15 V DC power supply contains a certain amount of AC noise, a 0.1 μF capacitor is connected in parallel before pin 1, pin 3 and 15 V power supply to filter out the interference of AC noise.

2.2 Bootstrap drive circuit

This circuit uses two IR2110s, which are connected by an "H" bridge circuit composed of four MOS tubes. The power supply voltage of IR2110 is the power supply voltage UVD of 15 V, and its output working power supply is a floating power supply, which is derived from the fixed power supply through the bootstrap technology. The bootstrap technology uses a boost diode and a bootstrap boost capacitor to superimpose the capacitor discharge voltage and the power supply voltage, thereby increasing the voltage. This technology can increase the power supply voltage value several times, so the withstand voltage of the charging diode VD must be greater than the peak voltage of the high-voltage bus. To prevent the voltage across the bootstrap capacitor from discharging, a high-frequency fast recovery diode is used. The capacitance value of the bootstrap capacitor C3 can be 0.1μF for a switching frequency above 5 kHz. In order to provide transient current to the capacitive load of the switch , two bypass capacitors should be connected between VCC and COM, VDD and VSS. A 0.1μF ceramic capacitor and a 1μF tantalum capacitor are connected in parallel on VCC, and a 0.1μF ceramic capacitor can be used on the logic power supply VDD, that is, capacitors C4 and C5 are 1μF and 0.1μF respectively. In the specific wiring, IR2110 is the interface between the logic part and the power conversion part . The logic signal ground wire and the main power supply ground wire should be reasonably laid out to make the lead of the load loop as short as possible to reduce the loop inductance, and at the same time avoid the common mode interference caused by the load current flowing in the signal loop due to wiring.

2.3 Main circuit

The speed of the DC motor is measured by the tachometer generator. When the speed of the controlled DC motor is less than the given speed, the computer outputs the control voltage UK through the D/A converter, and then converts it into UR through R2~R5 level and inputs it to pins 9 and 11, making pin 4 conductive. The conduction signal of pin 4 is transmitted to pin 10 of IR2110 (1) and pin 12 of IR2110 (2) through the RC delay circuit, making the upper channel pin 10 and the lower channel pin 12 conductive respectively. At this time, VF1 and VF2 in the "H" bridge circuit between the two IR2110s are triggered to conduct, and the circuit provides positive current to the motor, and the motor speed increases. When the speed of the controlled DC motor is greater than the given speed, UR makes pin 7 in UC3637 conductive, and the conduction signal of pin 7 is transmitted to pin 12 of IR2110 (1) and pin 10 of IR2110 (2) through the RC delay circuit, making the lower channel pin 12 and the upper channel pin 10 conductive respectively. At this time, VF4 and VF3 in the "H" bridge circuit are triggered to turn on, the current flowing through the motor is reversed, and the motor slows down. Under the action of the upper and lower channel input signals output by the control circuit, VF1, VF2 and VF4, VF3 are turned on alternately to achieve the speed regulation of the DC motor. Since the internal driving impedance of IR2110 is very small, directly using it to drive the MOSFET device in the "H" bridge will cause fast switching, which may cause voltage oscillation between the drain and source of the MOSFET, thereby damaging the MOS tube. Therefore, a non-inductive resistor of about 20 Ω should be connected in series between the output end of IR211O and the MOS tube.

2.4 Protection Circuit

In this circuit, the current flowing through the DC motor needs to be limited to protect the components of the circuit. TA is the current sampling link, which constitutes overcurrent protection for IR2110. Current Sensor Samples the current from the motor bypass and inputs the sampled current to pin 11 of IR2110. When the current is too large, SD is high, the S end of the Schmitt trigger is triggered, Q is low, and IR2110 stops working. Vf is the voltage feedback signal, which constitutes a closed-loop voltage regulation network. In this network, RS is the motor current detection resistor, and the RS value is determined by the maximum allowable motor current. The detection signal is input from pins 12 and 13. Assuming that the comparator C/L has a threshold of 200 mV, then

Take Imax = 8 A, then: RS = 0.025Ω

When the motor current increases and the voltage on RS reaches the threshold, C/L outputs a high level, resetting SRA and SRB to a low level, and then AOUT and BOUT become a low level, stopping the output.

2.5 Experimental verification

In a certain rudder system experiment, a permanent magnet DC motor with a rated working voltage of 27 V and a rated working current of 1 A is selected. According to different requirements of the experiment, the motor system can complete step, sine and other movements. A precision resistor potentiometer is used to detect the motor position, which is fed back to the computer through A/D conversion and a given control signal is controlled by an adaptive variable structure to obtain an output signal, which is then sent to the UC3637 PWM controller through D/A conversion to drive the motor to the desired position. Among them, the given signal, feedback signal sampling, control algorithm and control pulse output are all completed by the computer. Figures 3 and 4 respectively show the digital simulation and experimental curves of the system, where curve 1 is the experimental curve and curve 2 is a digital simulation curve of an adaptive variable structure control. It can be seen from the figure that it is feasible to control the DC motor using this control circuit.

3 Conclusion

The designed DC PWM control and drive circuit can be used to control the speed control and position of the DC motor. It has good speed regulation and driving performance. It can be combined with different computer control algorithms to achieve different motion characteristics and has a certain universality.