"Author: you Zhiyu; Du Yang; Zhang Hong; Dong Xiucheng
1 Introduction
For a long time, DC motor has become the best choice for most variable speed motion control and closed-loop position servo control systems because of its good linear characteristics and excellent control performance. Especially with the development of computer in the control field, high switching frequency, fully controlled second-generation power semiconductor devices (GTR, GTO, MOSFET, IGBT, etc.), and the application of pulse width modulation (PWM) DC speed regulation technology, DC motor has been widely used. In order to meet the needs of small DC motors, semiconductor manufacturers have introduced special integrated circuits for DC motor control to form a DC motor servo system based on microprocessor control. However, the output power of DC motor driver composed of ASIC is limited, which is not suitable for the driving demand of high-power DC motor. Therefore, the H-bridge is constructed by using n-channel enhanced FET to realize the drive control of high-power DC motor. The drive circuit can meet the needs of various types of DC motors, and has the characteristics of fast, accurate, high efficiency and low power consumption. It can directly interface with microprocessor, and PWM technology can be used to realize DC motor speed control.
2 overall structure of DC motor drive control circuit
The DC motor drive control circuit is divided into four parts: photoelectric isolation circuit, motor drive logic circuit, drive signal amplification circuit, charge pump circuit and H-bridge power drive circuit. The circuit block diagram is shown in Figure 1.
As can be seen from the figure, the peripheral interface of the motor drive control circuit is simple. The main control signals include motor running direction signal dir, motor speed regulation signal PWM and motor braking signal brake. VCC provides power for the driving logic circuit, VM is the motor power supply voltage, and M + and M - are DC motor interfaces.
In the high-power drive system, the drive circuit is electrically isolated from the control circuit to reduce the interference of the drive control circuit to the external control circuit. The isolated control signal generates the motor logic control signal through the motor drive logic circuit to control the upper and lower arms of the H-bridge respectively. Because the H-bridge is composed of high-power n-channel enhanced FET, it can not be directly driven by the motor logic control signal. The control signal must be amplified by the drive signal amplification circuit and charge pump circuit, and then the H-bridge power drive circuit is driven to drive the DC motor.
3 H-bridge power driving principle
The most widely used DC motor drive is the H-type full bridge circuit. This drive circuit can easily realize the four quadrant operation of DC motor, corresponding to forward rotation, forward rotation braking, reverse rotation and reverse rotation braking respectively. The schematic diagram of H-bridge power drive is shown in Figure 2.
The four switches of H-type full bridge drive circuit work in chopper state. S1 and S2 are one group and S3 and S4 are one group. The two groups are complementary. When one group is on, the other group must be off. When S1 and S2 are turned on, S3 and S4 are turned off, and forward voltage is applied at both ends of the motor to realize forward or reverse braking of the motor; When S3 and S4 are turned on, S1 and S2 are turned off, the two ends of the motor are reverse voltage, and the motor is braked in reverse or forward rotation.
In actual control, it is necessary to continuously switch the motor between the four quadrants, that is, between forward rotation and reverse rotation, that is, between S1 and S2 on and S3 and S4 off to S1 and S2 off and S3 and S4 on. In this case, the two groups of control signals are required to be completely complementary in theory, but due to the on and off time of the actual switching devices, the absolute complementary control logic will lead to the direct short circuit of the upper and lower bridge arms. In order to avoid direct short circuit and ensure the coordination and synchronization of each switch action, the two groups of control signals are required to be inverted to each other in theory, but in practice, there must be a long enough dead time difference. This correction process can be realized by hardware, that is, adding delay between the two groups of control signals of the upper and lower bridge arms, or by software.
The four switches in Figure 2 are freewheeling diodes, which can provide freewheeling circuit for coil winding. When the motor operates normally, the driving current flows through the motor through the main switch tube. When the motor is in the braking state, the motor works in the power generation state, and the rotor current must flow through the freewheeling diode, otherwise the motor will be heated and even burned in serious cases.
Design of DC motor drive control circuit
From the block diagram of DC motor drive control circuit, it can be seen that the drive control circuit has a simple structure and is mainly composed of four parts. Among them, the photoelectric isolation circuit is relatively simple, which will not be introduced here. Other parts of DC motor drive control circuit are introduced in detail below.
4.1 H-bridge drive circuit design
In DC motor control, H-bridge circuit is often used as the power driving circuit of the driver. Because power MOSFET is a voltage controlled element, which has the characteristics of large input impedance, fast switching speed and no secondary breakdown, and meets the action requirements of high-speed switching, power MOSFET is commonly used to form the bridge arm of H-bridge circuit. The four power MOSFETs in the H-bridge circuit adopt n-channel type and p-channel type respectively, while the p-channel power MOSFET is generally not used for the driving motor of the lower bridge arm, so there are two feasible schemes: one is that the upper and lower bridge arms use two p-channel power MOSFETs and two n-channel power MOSFETs respectively; The other is that the upper and lower bridge arms use n-channel power MOSFET.
Relatively speaking, the scheme of using two n-channel power MOSFETs and two p-channel power MOSFETs to drive the motor has the advantages of simple control circuit and low cost. However, due to the processing technology, the performance of p-channel power MOSFET is worse than that of n-channel power MOSFET, and the driving current is small. It is mostly used in driving circuits with small power. On the one hand, n-channel power MOSFET has high carrier mobility, good frequency response and large transconductance; On the other hand, it can increase the on current, reduce the on resistance, reduce the cost and reduce the area. Considering the system power, reliability requirements and the advantages of n-channel power MOSFET, this design adopts the H-bridge circuit of four identical n-channel power MOSFET, which has good performance, high reliability and large driving current. The circuit diagram is shown in Figure 3. In the figure, VM is the power supply voltage of the motor, four diodes are freewheeling diodes, and a small capacitor C6 is connected in parallel at the output end to reduce the peak voltage generated by the inductive element motor.
4.2 charge pump circuit design
The basic principle of charge pump is to generate high voltage through the accumulation effect of capacitance on charge, so that the current flows from low potential to high potential. The earliest ideal charge pump model was proposed by J. Dickson in 1976. At that time, this circuit was to provide the required voltage for erasable EPROM. Later, J. Witters, Toru tranzawa and others improved J. Dickson's charge pump model, put forward a more accurate theoretical model, confirmed it by experiments, and put forward relevant theoretical formulas. With the continuous development of integrated circuits, based on the consideration of low power consumption and low cost, charge pump is more and more widely used in circuit design.
The schematic circuit diagram of a simple charge pump is shown in Figure 4. The a end of capacitor C1 is connected to VCC through diode D1, and the B end of capacitor C1 is connected to the square wave of amplitude Vin. When the potential at point B is 0, D1 is turned on and VCC starts charging capacitor C1 until the potential at node a reaches VCC; When the potential at point B rises to the high level VIN, because the voltage at both ends of the capacitor cannot change suddenly, the potential at point a rises to VCC + Vin. Therefore, the voltage at point a is a square wave. The maximum value is VCC + VIN and the minimum value is VCC (assuming that the diode is an ideal diode). After simple rectification and filtering, the square wave at point a can provide a voltage higher than VCC.
In the drive control circuit, the H-bridge is composed of four n-channel power MOSFETs. To control each MOSFET, the gate voltage of each MOSFET must be sufficiently higher than the gate voltage. Generally, MOSFET should be completely and reliably turned on, and its gate voltage is generally more than 10 V, that is, VCs > 10 v. For the lower bridge arm of H bridge, directly apply a voltage of more than 10 V to turn it on; For the two MOSFETs of the upper bridge arm, in order to make VGS > 10 V, it must meet VG > VM + 10 V, that is, the driving circuit must be able to provide a voltage higher than the power supply voltage, which requires the addition of a boost circuit in the driving circuit to provide a voltage higher than the grid 10 v. Considering the upper limit requirements of VGS, the VGS is generally 10 V ~ 15 V when MOSFET is turned on, that is, the control gate voltage changes with the change of gate voltage, that is, floating gate drive. Therefore, a charge pump circuit is designed in the drive control circuit to provide a voltage VH higher than VM to drive the conduction of the power tube. The circuit schematic diagram is shown in Figure 5.
Part a of the circuit is a square wave generation circuit, which is composed of RC and inverse Schmidt trigger to generate a square wave with amplitude VIN = 5 v. Part B is a charge pump circuit, which is composed of a third-order charge pump. When point a is at low level, diode D1 turns on and capacitor C1 is charged to make point B voltage VB = VM VTN; When point a is at high level, since the voltage of capacitor C1 cannot change suddenly, the voltage of point B is VB = VM + VIN VTN. At this time, diode D2 is turned on and capacitor C3 is charged, making the voltage of point C VX = VM + vin-2vtn; When point a is at low level again, diodes D1 and D3 are turned on to charge capacitors C1 and C2 respectively, so that point d voltage VD = VM + vin-3vtn; When point a is high again, since the voltage of capacitor C2 cannot change suddenly, the voltage of point D becomes VD = VM + 2vin-3vtn. At this time, diodes D2 and D4 are turned on to charge capacitors C3 and C4 respectively, so that the voltage of point E is ve = VM + 2vin-4vtn. In this cycle, a voltage VH = VM + 3vin-6tn = VM + 11.4 V higher than VM is obtained at point G. Where VM is the diode voltage drop, generally 0.6 v. So as to ensure the complete conduction of the upper arm of H bridge.
4.3 motor drive logic and amplifier circuit design
In the DC motor drive circuit, the motor drive logic and amplification circuit mainly realize the conversion and amplification from external control signal to drive H-bridge control signal. After passing through the photoelectric isolation circuit, the control signals dir, PWM and brake are decoded by the gate circuit to generate four control signals M1 ', M2', m3 'and M4', and then amplified by the triode to generate four signals M1, M2, m3 and M4 controlling the H-bridge. The circuit schematic diagram is shown in Figure 6. Where VH is the voltage of VM raised by the charge pump, and VM is the motor power supply voltage.
When the motor works, the upper arm of H bridge is in normally open or normally closed state, which is controlled by dir, and the lower arm is controlled by PWM logic level to generate continuously adjustable control voltage. In this scheme, the upper arm MOSFET switches only when the motor is commutating, and the commutation frequency of the motor is very low. The low end is directly controlled by the logic circuit, and the signal level of the logic circuit switches quickly, which can meet the requirements of different frequencies. The circuit has another advantage. Due to the slow opening of the upper arm and the fast closing of the lower arm, the upper and lower arms will not be turned on at the same time during the actual control, which reduces the current impact during the commutation and improves the service life of the MOSFET.
5. PWM speed regulation control of DC motor
DC motor speed n = (U-IR) / K φ
Where u is the armature terminal voltage, I is the armature current, and R is the total resistance of the armature circuit, φ Is the magnetic flux of each pole, and K is the structural parameter of the motor.
DC motor speed control can be divided into excitation control method and armature voltage control method. The excitation control method is to control the magnetic flux. Its control power is small. It is limited by magnetic saturation at low speed and limited by the structural strength of commutation spark and commutator at high speed. Moreover, due to the large inductance of excitation coil and poor dynamic response, this control method is rarely used. Armature voltage control is used in most applications. With the progress of power electronics technology, changing armature voltage can be realized in many ways, among which PWM (pulse width modulation) is a common speed regulation method to change armature voltage.
The basic principle of PWM speed regulation control is to turn on and off the power supply according to a fixed frequency, and change the "duty cycle" of the voltage on the armature of the DC motor according to the need to change the time ratio (duty cycle) of turning on and off in a cycle, so as to change the average voltage and control the speed of the motor. In the pulse width speed regulation system, when the motor is powered on, its speed increases, and when the motor is powered off, its speed decreases. As long as the on and off time is changed according to a certain law, the motor speed can be controlled. The stepless speed regulation system composed of PWM technology has no impact on the DC system when starting and stopping, and has the characteristics of low starting power consumption and stable operation.
When the motor is always connected to the power supply, the maximum motor speed is Vmax, and the duty cycle is d = t / T, then the average motor speed VD is:
Vd=VmaxD
According to the formula, when the duty cycle d = t / T is changed, different average motor speeds VD can be obtained, so as to achieve the purpose of speed regulation. Strictly speaking, the relationship between average speed and duty cycle D is not strictly linear. In general applications, it can be regarded as a linear relationship approximately. In the DC motor drive control circuit, the PWM signal is provided by the external control circuit. After passing through the high-speed photoelectric isolation circuit, motor drive logic and amplification circuit, the switch of the lower arm MOSFET of the H-bridge is driven to change the average voltage on the DC motor armature, so as to control the speed of the motor and realize the PWM speed regulation of the DC motor.
6 Conclusion
The drive control circuit based on H-bridge PWM control with n-channel enhanced FET as the core has good performance for forward and reverse control and speed regulation of DC motor. The experimental results show that the DC motor drive control circuit runs stably and reliably, and the motor speed regulation response is fast. It can meet the requirements of practical engineering application and has a good application prospect.
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