Design of a brushless DC motor control system

1. Introduction
　　
The permanent magnet brushless DC motor is a new type of mechatronic motor that has matured rapidly in recent years. The motor consists of a stator, a rotor, and a rotor position detection element Hall sensor. Since there is no excitation device, it has high efficiency, simple structure, excellent working characteristics, and has the advantages of smaller size, higher reliability, easier control, wider application range, and more convenient manufacturing and maintenance, making the research of brushless motors of great significance.
　　
This system design uses voltage regulation and speed regulation, and adjusts the voltage by adjusting the duty cycle of the power supply PWM power supply. This design uses the brushless DC motor dedicated control chip MC33035, which can decode the position signal detected by the Hall sensor. It itself has auxiliary functions such as overcurrent, overheating, undervoltage, forward and reverse selection, etc. The peripheral circuit required for the system is simple, and the designer does not have to use discrete components to form a huge analog circuit, which makes the design and debugging of the system quite complicated and occupies a large area of ​​the circuit board.
　　
The two integrated chips MC33035 and MC33039 can also easily complete the forward and reverse rotation, operation start and dynamic braking, overcurrent protection, generation of three-phase drive signals, simple closed-loop control of motor speed, etc. of brushless DC motors. The brushless DC motor control system composed of dedicated integrated chips has the characteristics of high integration, fast speed and perfect protection functions. The drive circuit structure is simple, so the entire circuit has few peripheral components and simple wiring, which can greatly reduce the size of the inverter.

2. System principle
　　
The closed-loop speed control system uses three Hall integrated circuits as rotor position sensors. The 8-pin reference voltage (6.24V) of MC33035 is used as their power supply, and the output signal of the Hall integrated circuit is sent to MC33035 and MC33039. The system control structure block diagram is shown in Figure 1. The output of MC33039 is smoothed by a low-pass filter and introduced into the inverting input of the error amplifier of MC33035, while the speed setting signal is input into the non-inverting input of the error amplifier of MC33035 through the integral link, thereby forming a closed-loop speed control of the system.

Figure 1 System control principle

3. Control circuit design The
　　
operating power supply voltage range of MC33035 is very wide, between 10V-30V, and the chip contains a reference voltage of 6.25V. The rotor position decoder inside MC33035 is mainly used to monitor the three sensor inputs so that the system can correctly provide the correct timing of the high-end and low-end drive inputs. The sensor input can be directly connected to the open collector Hall effect switch or optocoupler. In addition, the circuit also contains a pull-up resistor, and its input is compatible with the TTL level with a typical threshold value of 2.2V. The three-phase motor controlled by the MC33035 series products can work under the four most common sensor phases. The 60°/120° selection provided by MC33035 allows MC33035 to easily control motors with sensor phases of 60°, 120°, 240° or 300°. Its three sensor inputs have eight possible input code combinations, six of which are valid rotor positions, and the other two code combinations are invalid. The six valid input codes enable the decoder to distinguish the position of the motor rotor within the window using the 60° electrical phase. The forward/reverse output of the MC33035 DC brushless motor controller can change the direction of the motor by flipping the voltage on the stator winding. When the input state changes, the specified sensor input code will change from high level to low level, thereby changing the rectifier timing to change the direction of rotation of the motor. The motor on/off control can be achieved by output enable. When the pin is open, the built-in pull-up resistor connected to the positive power supply will start the top and bottom drive output timing. When the pin is grounded, the top drive output will be turned off and the bottom drive will be forced to low, thereby stopping the motor. The working principle and operation method of the error amplifier, oscillator, pulse width modulation, current limiting circuit, on-chip voltage reference, undervoltage lockout circuit, drive output circuit and thermal shutdown circuit in the MC33035 are basically similar to those of other similar chips. The peripheral circuit of MC33035 is shown in Figure 2. [page]

Figure 2 MC33035 peripheral circuit
　　
is shown in the figure. We supply a 24V power supply, and F/R controls the motor direction. The forward/reverse output can change the motor direction by flipping the voltage on the stator winding. When the input state changes, the specified sensor input code will change from high level to low level, thereby changing the rectifier timing to change the direction of rotation of the motor.
　　
The motor on/off control can be achieved by output enable 7 pin. When this pin is open, the built-in pull-up resistor connected to the positive power supply will start the top and bottom drive output timing. When this pin is grounded, the top drive output will be turned off and the bottom drive will be forced to low, thereby stopping the motor.
　　
Since the 8th pin of MC33035 provides a standard voltage output of 6.25V, this voltage can be used to power Hall components and other devices. In this system, it is easy to generate PWM signals, and the frequency of the PWM signal can be adjusted by the external circuit. Its frequency is determined by the formula. R5 is a variable resistor. By adjusting R5, the frequency of the PWM signal can be changed. We only need to add a capacitor, a resistor and an adjustable potentiometer to the periphery of MC33035 to generate the pulse width modulation signal we need. Because the 8-pin output of MC33035 is a standard voltage of 6.25V, R6 and C1 form an RC oscillator, so the input of pin 10 is approximately a triangle wave, and its frequency is determined by. R5 is a potentiometer for controlling the speed of the brushless motor. The voltage of pin 11 to the ground is changed by the potentiometer to change the speed of the motor. Operational amplifier 1 is connected to a follower form from the outside, so the voltage of pin 11 to the ground is the inverting input voltage of comparator 2. The voltage of pin 11 to the ground is changed by potentiometer R5 to change the duty cycle of the output square wave of comparator 2, that is, the output of comparator 2 is the PWM signal we need.
　　
Pin 14 is the fault output terminal, and L1 is used as a fault indicator. When an invalid sensor input code, overcurrent, undervoltage, internal overheating of the chip, or a low level is detected, the LED will light up to alarm and automatically block the system. Only after the fault is eliminated can the system be reset to resume normal operation. R6 and C1 determine the internal oscillator frequency (i.e., the modulation frequency of PWM). The output of the speed setting potentiometer W is input into the in-phase input of the error amplifier of MC33035 through the integral link, and its reverse input is connected to the output terminal. In this way, the error amplifier forms a unit gain voltage follower, thereby completing the speed control of the system.
　　
Pin 8 is connected to an NPN transistor. When the voltage of pin 8 is high, the transistor is turned on to provide voltage for MC33039 and the Hall sensor. The electrolytic capacitor C2 is a filter to prevent current backflow.
　　
When the input voltage of MC33035's 17th pin is lower than 9.1V, since the input of 17th pin is connected to the in-phase input of an internal comparator, and the inverting input of the comparator is an internal 9.1V standard voltage, MC33035 will block all three outputs of the lower bridge through the AND gate, and all three power transistors of the lower bridge will be turned off, and the motor will stop running, which plays the role of undervoltage protection. Overheat protection and other functions are internal circuits of the chip, and there is no need to design peripheral circuits.
　　
The brushless DC motor of this system has three built-in Hall effect sensors to detect the rotor position. Once the motor's commutation is determined, the motor speed can be calculated based on the signal. The output of the sensor is directly connected to the 4th, 5th, and 6th pins of MC33035. When the motor is running normally, three overlapping signals with a pulse width of 180 degrees electrical angle can be obtained through the Hall sensor, so that 6 forced commutation points are obtained. MC33035 decodes the three Hall signals to make the motor commutate correctly.
　　
When the 11th pin of MC33035 is grounded, the motor speed is 0, and the brake can be applied.
　　
MC330399 is a brushless motor closed-loop speed controller specially designed by Motorola for MC33035. It is an 8-pin dual in-line narrow integrated circuit block. MC33039 processes the input rotor position signal code and generates a speed voltage signal proportional to the actual motor speed.
　　
The three-phase position detection signal (SA, SB, SC) sent from the motor rotor position detector is sent to MC33035 on the one hand. After the internal decoding circuit of the chip combines the control logic signal states of the forward and reverse control terminal, the start and stop control terminal, the brake control terminal, the current detection terminal, etc., after calculation, six original control signals of the three-phase upper and lower bridge arm switch devices of the inverter are generated. Among them, the three-phase lower bridge switch signal is also subjected to pulse width modulation processing according to the brushless DC motor speed regulation mechanism. After the processed three-phase lower bridge PWM control signal (AB, BB, CB) and the three-phase upper bridge control signal (AT, BT, CT) are driven and amplified, and then applied to the six switch tubes of the inverter to generate the three-phase square wave AC current required for the normal operation of the power supply machine. On the other hand, the rotor position detection signal is also sent to MC33039, and after F/V conversion, a pulse signal FOUT with a frequency proportional to the motor speed is obtained. After filtering through a simple resistor-capacitor network, a speed feedback signal is formed. Using the error amplifier in MC33035, a simple P regulator can be formed to realize the closed-loop control of the motor speed. In practical applications, various external PI and PID adjustment circuits can also be used to achieve complex closed-loop adjustment control, as shown in Figure 3. [page]

Figure 3 Circuit diagram of closed-loop control system composed of MC33039
　　
The number of pulses output from pin 5 of MC33039 is 12 pulses per motor revolution. Select the timing element according to the maximum speed of the motor. Assume that the maximum speed is 3500r/min, that is, 58r/s. At this time, the number of pulses output per second is 58×12=696. That is, its frequency is about 700Hz and the period is about 1.4ms. According to the MC33039 technical manual, take the timing element parameters R21=100KΩ, C4=0.01uF, and the pulse width generated by the monostable circuit is 1340ns. Pin 8 is connected to the reference voltage of MC33035. The output of pin 5 is connected to pin 12 (the inverting input terminal of the error amplifier) ​​of MC33035 through a 100k resistor. The amplifier gain is 10 times at this time, and the 0.1μF capacitor plays a filtering and smoothing role. MC33035 oscillator parameters: resistance is 5.1kΩ, capacitance is 0.1μF, PWM frequency is about 2.4kHz. The system uses a non-inductive resistor (0.04Ω, 0.5W) as current detection, and is connected to pin 9 through a 1.1kΩ resistor.
　　
Since pin 22 is grounded at a low level, the control circuit works under the 120° sensor electrical phase input state.
4. Drive circuit design

Figure 4 The drive circuit diagram
　　
is shown in Figure 4. The output of the lower bridge three-way drive signal can directly drive the N-channel power MOSFET IRF530, and the upper bridge three-way drive signal can directly drive the P-channel power MOSFET IRF9530. The signals of pins 1, 2, and 24 of MC33035 are amplified by IRF9530, and the signals of pins 19, 20, and 21 are obtained by IRF530 to drive the brushless DC motor to rotate. A, B, and C are connected to the three-phase winding of the brushless DC motor in a triangle.

5. Experimental results



Figure 5 Driving signal
　　
The two driving signals in Figure 5 belong to the driving signals of the upper and lower MOSFETs of a certain bridge wall. It can be seen from the comparison that each switch tube conducts 120° in one cycle, and the two driving signals cannot overlap.

6. Conclusion
　　
The DC brushless motor control system designed in this paper is implemented in a pure hardware way. It has the characteristics of simplicity, reliability, small size and low cost. Especially after the speed closed-loop control is formed with MC33039, the speed regulation performance is very excellent. However, since the PWM modulation method of MC33035 is to adjust the duty cycle, it is difficult to improve the waveform of the output current, and there is a certain torque pulsation when the motor is running. In short, MC33035 is very suitable for the control of small-power brushless motors, especially for servo mechanisms and mechatronic speed regulation equipment.

References:
　　
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