Pulse width modulation is an analog control method that modulates the bias of the base of the transistor or the gate of the MOS tube according to the change of the corresponding load to change the conduction time of the transistor or MOS tube, thereby changing the output of the switching power supply . In this way, the output voltage of the power supply can be kept constant when the working conditions change, and it is a very effective technology that uses the digital signal of the microprocessor to control the analog circuit. Pulse width modulation is a very effective technology that uses the digital output of a microprocessor to control analog circuits, and is widely used in many fields from measurement and communication to power control and conversion.
With the development of electronic technology, a variety of pulse width modulation (Pulse width modulation, PWM) technologies have appeared, including: phase voltage control PWM, pulse width PWM method, random PWM, SPWM method, line voltage control PWM, etc. The pulse width PWM method used in the Ni-MH battery smart charger is to use a pulse train with the same pulse width as the PWM waveform. The frequency can be adjusted by changing the period of the pulse train, and the voltage can be adjusted by changing the pulse width or duty cycle ，Appropriate control methods can make the voltage and frequency coordinately change. The purpose of controlling the charging current can be achieved by adjusting the period of the PWM and the duty cycle of the PWM.
The value of the analog signal can be continuously changed, and the resolution of time and amplitude is not limited. 9V battery is an analog device, because its output voltage is not exactly equal to 9V, but changes with time, and can take any real value. Similarly, the current drawn from the battery is not limited to a set of possible values. The difference between an analog signal and a digital signal is that the value of the latter can usually only belong to a predetermined set of possible values, for example, in the set of {0V, 5V}.
The analog voltage and current can be directly used for control, such as controlling the volume of the car radio. In a simple analog radio, the volume knob is connected to a variable resistor. When you turn the knob, the resistance value becomes larger or smaller; the current flowing through this resistor also increases or decreases, which changes the current value that drives the speaker and makes the volume correspondingly larger or smaller. Like a radio, the output of an analog circuit is linearly proportional to the input.
Although analog control may seem intuitive and simple, it is not always very economical or feasible. One of the points is that analog circuits tend to drift over time and are difficult to adjust. Precision analog circuits that can solve this problem can be very large, bulky (such as old-fashioned home stereo equipment), and expensive. The analog circuit may also generate severe heat, and its power consumption is proportional to the product of the voltage and current across the working element. Analog circuits may also be very sensitive to noise, and any disturbance or noise will definitely change the magnitude of the current value.
By controlling the analog circuit digitally, the cost and power consumption of the system can be greatly reduced. In addition, many microcontrollers and DSPs already include a PWM controller on the chip, which makes the implementation of digital control easier.
The basic principle of pulse width modulation (PWM): The control method is to control the switching of the switching device of the inverter circuit, so that the output terminal obtains a series of pulses with equal amplitude, and these pulses are used to replace the sine wave or the desired waveform. That is, a plurality of pulses are generated in a half cycle of the output waveform, and the equivalent voltage of each pulse is a sinusoidal waveform, and the obtained output is smooth and less low-order harmonics. Modulating the width of each pulse according to certain rules can not only change the output voltage of the inverter circuit, but also change the output frequency .
For example, by dividing the sine half-wave waveform into N equal parts, the sine half-wave can be regarded as a waveform composed of N pulses connected to each other. These pulses are equal in width and are equal to π/n, but the amplitudes are not equal, and the top of the pulse is not a horizontal line, but a curve, and the amplitude of each pulse changes according to the sine law. If the above pulse sequence is replaced with the same number of rectangular pulse sequences of equal amplitude but not equal width, so that the midpoint of the rectangular pulse coincides with the midpoint of the corresponding sinusoidal equal part,
PWM actual waveform
And make the rectangular pulse and the corresponding sinusoidal area (that is, impulse) equal, you get a set of pulse sequence, this is the PWM waveform. It can be seen that the width of each pulse changes according to the sine law. According to the principle that the impulse is equal and the effect is the same, the PWM waveform and the sine half-wave are equivalent. For the negative half of the sine, you can also use the same method to get the PWM waveform.
In the PWM waveform, the amplitude of each pulse is equal. When you want to change the amplitude of the equivalent output sine wave, you only need to change the width of each pulse according to the same proportional coefficient. Therefore, in the AC-DC-AC inverter, The pulse voltage output by the PWM inverter circuit is the amplitude of the DC side voltage.
According to the above principle, after giving the sine wave frequency, amplitude and the number of pulses in a half cycle, the width and interval of each pulse of the PWM waveform can be accurately calculated. According to the calculation result, the switching of each switching device in the circuit is controlled, and the required PWM waveform can be obtained. The following figure shows the real-time waveform of the PWM wave output by the inverter.
From the perspective of the modulation pulse, PWM can be divided into unipolar and bipolar control modes [2]. The basic principle of generating unipolar PWM mode is shown in Figure 6.2. First, the triangular wave carrier signal ut of the same polarity. Generate a unipolar PWM pulse with the modulation signal ur (Figure 6.2(b)); then multiply the unipolar PWM pulse signal with the inverted signal UI shown in Figure 6.2(c) to obtain positive and negative halves Wave symmetrical PWM pulse signal Ud.
The bipolar PWM control mode uses positive and negative alternating bipolar triangular carrier ut and modulated wave ur. As shown in Figure 6.3, the bipolar PWM pulse can be directly obtained by comparing ut and ur, without An inverter circuit is required.
In addition to the above two classifications of modulation methods from different angles of principle, in recent years, the method of directly using the chip for pulse width modulation has been accepted by more users. The field of signal conditioning often needs to face the problems of analog signal transmission, acquisition, and control. Traditional signal chain circuits include analog-to-digital converters (ADC), digital-to-analog converters (DAC), operational amplifiers (OpAmp), and comparators ( Comparator), etc., they play the main role of analog signal processing. The function of the signal chain chip is strong and powerful. After careful design, it can form a variety of excellent signal processing circuits, but even so, in many applications, there are still bottlenecks and constraints, and it is impossible to achieve the ideal circuit performance and indicators. Therefore, in the field of signal chain, it is eager to appear more innovative analog circuit processing technology and chip products. A new type of special chip for analog signal processing, which realizes the high-precision conversion function of analog signal to PWM signal, we call it APC (Analog to PWM Convertor).
Suppose the carrier frequency of the SPWM wave is fc, the fundamental frequency is fs, and fc/fs is called the carrier ratio N. For a three-phase inverter, when N is an integer multiple of 3, the output does not include the third harmonic and the integer 3 Harmonics. And the harmonic concentration is near the integer multiple of the carrier frequency, that is, the harmonic order is: kfc±mfs, and k and m are integers.
The figure on the right is a Matlab simulation diagram of the SPWM waveform and spectrum of the fundamental frequency fs=50Hz, carrier frequency fc=3kHz, modulation ratio 0.8.
In the figure, the amplitudes of the 58th and 60th harmonics are 27.8% and 27.7%, respectively. The harmonics with the largest content are the 119th and 121st harmonics, and the harmonic amplitudes are 39.1% and 39.3%, respectively. That is, the maximum harmonic is around twice the carrier frequency.
As the harmonic frequency increases, the overall amplitude of the harmonics shows a downward trend. According to the GB/T22670 inverter-powered three-phase cage induction motor test method, the bandwidth of the variable frequency power transmitter should be 6 times the carrier frequency Above, when the carrier frequency is 3kHz, the bandwidth is at least 18kHz. In practice, it is recommended to use a variable frequency power sensor and a variable frequency power analyzer with a bandwidth of more than 30kHz.
In the actual SPWM wave, the carrier ratio is not necessarily an integer. At this time, in order to reduce the spectrum leakage, the Fourier window length can be appropriately increased, and the Fourier transform (FFT or DFT) of the PWM of multiple fundamental cycles can be performed.
Pulse width modulation (PWM) is a method of digitally encoding analog signal levels. Through the use of a high-resolution counter, the duty cycle of the square wave is modulated to encode the level of a specific analog signal. The PWM signal is still digital because at any given moment, the full-amplitude DC power supply is either fully on (ON) or completely off (OFF). The voltage or current source is applied to the analog load in a repeating pulse sequence of ON or OFF. When it is on, it is when the DC power supply is added to the load, and when it is off, it is when the power supply is disconnected. As long as the bandwidth is sufficient, any analog value can be encoded using PWM.
Most loads (whether inductive or capacitive) require a modulation frequency higher than 10 Hz, usually between 1 kHz and 200 kHz. Many microcontrollers contain a PWM controller. For example, Microchip's PIC16C67 contains two PWM controllers, each of which can select the on-time and period. Duty cycle is the ratio of on time to period; the modulation frequency is the reciprocal of the period. Before performing the PWM operation, this microprocessor requires the following tasks to be completed in software:
1. Set the period of the on-chip timer/counter that provides modulated square waves
2. Set the on time in the PWM control register
3. Set the direction of the PWM output, this output is a general purpose I/O pin
4. Start the timer
5. Enable the PWM controller
Nowadays, almost all the commercially available single-chip microcomputers have the function of PWM module. If not (such as the early 8051), they can also be implemented using timers and GPIO ports. The more general PWM module control process is (I have used TI's 2000 series, AVR's Mega series, TI's LM series):
1. Enable related modules (PWM module and corresponding pin GPIO module).
2. Configure the functions of the PWM module, specifically:
①: Set the PWM timer period, this parameter determines the frequency of the PWM waveform.
②: Set the PWM timer comparison value. This parameter determines the duty cycle of the PWM waveform.
③: Set a dead band (deadband), in order to avoid the bridge arm needs to be set dead zone, generally higher-grade single-chip microcomputers have this function.
④: Set the fault handling situation. Generally, the fault is to block the output to prevent overcurrent from damaging the power tube. The fault is generally detected by a comparator or ADC or GPIO.
⑤: Set the synchronization function, which is especially important when multi-arm, that is, multi-PWM module coordinated work.
3. Set the corresponding interrupt, write ISR, generally used for voltage and current sampling, calculate the duty cycle of the next cycle, change the duty cycle, this part will also have the function of PI control.
4. Enable PWM waveform generation.
One advantage of PWM is that the signals from the processor to the controlled system are all in digital form, without the need for digital-to-analog conversion. Keeping the signal in digital form minimizes the effects of noise. Noise can only affect digital signals when it is strong enough to change logic 1 to logic 0 or logic 0 to logic 1.
The enhancement of noise resistance is another advantage of PWM over analog control, and this is also the main reason why PWM is used for communication at certain times. Switching from analog signals to PWM can greatly extend the communication distance. At the receiving end, the modulated high-frequency square wave can be filtered and the signal restored to an analog form through an appropriate RC or LC network. In short, PWM is economical, space-saving, and strong in noise resistance. It is an effective technology worthy of the majority of engineers in many design applications.
There is an important conclusion in the sampling control theory: when narrow pulses with equal impulses and different shapes are added to the inertial links, the effect is basically the same. The PWM control technology is based on this conclusion as a theoretical basis to control the on and off of the semiconductor switching device, so that a series of pulses with equal amplitude and unequal width are obtained at the output end. These pulses are used to replace the sine wave or other The desired waveform. Modulating the width of each pulse according to certain rules can change the output voltage of the inverter circuit and the output frequency.
The basic principle of PWM control has been proposed for a long time, but due to the restriction of the development level of power electronic devices, it has not been realized before the 1980s. Until the 1980s, with the emergence and rapid development of fully-controlled power electronic devices, PWM control technology was really applied. With the development of power electronic technology, microelectronic technology and automatic control technology, as well as various new theoretical methods, such as modern control theory, the application of nonlinear system control ideas, PWM control technology has achieved unprecedented development, and there have been a variety of PWM control technology, according to the characteristics of PWM control technology, mainly has the following 8 types of methods.
The VVVF (Variable Voltage Variable Frequency) device was implemented in the early days using PAM (Pulse Amplitude Modulation) control technology. Its inverter part can only output a square wave voltage with adjustable frequency but not voltage regulation. The equal pulse width PWM method was developed to overcome this shortcoming of the PAM method, and it is the simplest kind of PWM method. It uses a pulse train with equal width of each pulse as a PWM wave, and achieves the effect of frequency modulation by changing its period. The voltage can be adjusted by changing the pulse width or duty cycle, and the voltage and frequency can be changed in harmony by using appropriate control methods. Compared with the PAM method, the advantage of this method is that it simplifies the circuit structure and improves the power factor at the input. However, the output voltage also contains large harmonic components in addition to the fundamental wave.
From the 1970s to the early 1980s, since the high-power transistors were mainly bipolar Darlington transistors at that time, the carrier frequency generally did not exceed 5 kHz. Electromagnetic noise of the motor windings and vibration caused by harmonics attracted people’s attention. . In order to seek improvement, the random PWM method came into being. The principle is to randomly change the switching frequency so that the electromagnetic noise of the motor is approximately limited to white noise (in a linear frequency coordinate system, the energy distribution of each frequency is uniform), although the total decibel of the noise is unchanged, it is characterized by a fixed switching frequency The intensity of the colored noise is greatly reduced. Because of this, even when the IGBT has been widely used, the random PWM still has its special value for the occasion where the carrier frequency must be limited to a lower frequency; on the other hand, it shows that the best way to eliminate mechanical and electromagnetic noise is not blind To improve the working frequency, the random PWM technology provides an analysis to solve this kind of problem.
The SPWM (Sinusoidal PWM) method is a relatively mature method and is now widely used. An important conclusion in the aforementioned sampling control theory: when narrow pulses with the same impulse and different shapes are added to the inertial links, the effect is basically the same. The SPWM method is based on this conclusion, and the pulse width changes according to the sine law and the PWM waveform equivalent to the sine wave, that is, the SPWM waveform, controls the switching of the switching device in the inverter circuit, so that the area of the output pulse voltage and the It is hoped that the output sine wave has the same area in the corresponding interval, and the frequency and amplitude of the output voltage of the inverter circuit can be adjusted by changing the frequency and amplitude of the modulated wave. SPWM refers to a modulation method in which the output amplitude of the variable-frequency power supply is equal and the duty cycle of the sequence pulse changes according to the sine function during modulation. The larger the value of the sine function, the greater the corresponding pulse duty cycle, and the smaller the adjacent pulse interval. Correspondingly, the smaller the value of the sine function, the smaller the pulse duty cycle and the larger the interval between adjacent pulses [3]. There are several schemes to realize this method.
Equal area method: This scheme is actually a direct interpretation of the principle of the SPWM method, replacing the sine wave with the same number of rectangular pulse sequences of equal amplitude but not equal width, then calculating the width and interval of each pulse, and storing these data in the microcomputer In the way, the PWM signal is generated by looking up the table to control the switching device to achieve the desired purpose. Since this method is based on the basic principle of SPWM control, the on and off time of each switching device can be accurately calculated, and the resulting waveform is very close to a sine wave, but its calculation is cumbersome, the data occupies large memory, and cannot be controlled in real time Shortcomings.
Hardware modulation method: The hardware modulation method is proposed to solve the shortcomings of the calculation of the equal area method. The principle is to use the desired waveform as the modulation signal and the modulated signal as the carrier. The desired result can be obtained by modulating the carrier PWM waveform. Usually an isosceles triangle wave is used as a carrier. When the modulated signal wave is a sine wave, the SPWM waveform is obtained. The realization method is simple. The triangle wave carrier and sinusoidal modulation wave generating circuit can be formed by an analog circuit, and the intersection point of them can be determined by a comparator. At the intersection point, the on-off of the switching device can be controlled to generate the SPWM wave. However, this analog circuit has a complicated structure and it is difficult to achieve accurate control.
Software generation method: Due to the development of microcomputer technology, it is easier to generate SPWM waveforms with software. Therefore, the software generation method came into being. The software generation method is actually a method for realizing modulation by software. There are two basic algorithms, namely natural sampling method and regular sampling method.
Natural sampling method: use sine wave as modulation wave and isosceles triangle wave as carrier wave for comparison, and control the on and off of the switching device at the natural intersection of two waveforms. This is natural sampling method. The advantage is that the resulting SPWM waveform is closest to the sine wave, but because the intersection of the triangle wave and the sine wave is arbitrary, the pulse centers are not equidistant within a period, so the pulse width expression is a transcendental equation, the calculation is cumbersome, and it is difficult to control in real time.
Regular sampling method: The regular sampling method is a widely applied engineering practical method, which generally uses triangular wave as a carrier. The principle is to use the triangular wave to sample the sine wave to obtain the staircase wave, and then to control the on and off of the switching device at the intersection of the staircase wave and the triangle wave, thereby implementing the SPWM method. When the triangle wave samples the sine wave only at its vertex (or bottom point), the pulse width determined by the intersection of the step wave and the triangle wave is symmetrical within a carrier period (ie, sampling period). The method is called symmetric regular sampling. When the sine wave is sampled at both the apex and bottom points of the triangle wave, the pulse width determined by the intersection of the step wave and the triangle wave is generally within a carrier period (at this time it is twice the sampling period). Asymmetric, this method is called asymmetric regular sampling. The regular sampling method is an improvement on the natural sampling method. Its main advantage is that it is simple to calculate and facilitate online real-time calculation. Among them, the asymmetric regular sampling method is closer to sine because of the higher order. The disadvantage is that the DC voltage utilization rate is low, and the linear control range is small. The above two methods are only applicable to synchronous modulation.
Low-order harmonic elimination method: The low-order harmonic elimination method is a method for eliminating some main low-order harmonics in the PWM waveform. The principle is to expand the output voltage waveform according to the Fourier series, expressed as u(ωt)=ansinnωt, first determine the value of the fundamental component a1, and then make two different an=0. Three equations can be established and a1, a2 and a3 can be solved simultaneously, so that the harmonics of the two frequencies can be eliminated. Although this method can eliminate the specified low-order harmonics well, the amplitude of the remaining lower-order harmonics that have not been eliminated may be quite large, and it also has the disadvantage of complicated calculation. This method is also only applicable to synchronous modulation.
Trapezoidal wave and triangle wave comparison method: The various methods described above are mainly for the purpose of output waveform as close as possible to the sine wave, thereby ignoring the utilization rate of DC voltage, such as SPWM method, the DC voltage utilization rate is only 86.6%. Therefore, in order to improve the utilization rate of DC voltage, a new method-trapezoidal wave and triangle wave comparison method is proposed. This method uses trapezoidal wave as the modulation signal, triangular wave as the carrier wave, and equalizes the amplitude of the two waves, and controls the switching of the switching device at the intersection of the two waves to realize PWM control. Because when the amplitude of the trapezoidal wave and the amplitude of the triangular wave are equal, the amplitude of the fundamental wave component contained in it exceeds the amplitude of the triangular wave, which can effectively improve the utilization rate of the DC voltage. However, since the trapezoidal wave itself contains low-order harmonics, the output waveform contains low-order harmonics such as 5th and 7th.
The various PWM control methods described above are used to control the three-phase output phase voltage when the three-phase inverter circuit is used to make the output close to a sine wave. However, for three-phase asynchronous motors such as three-phase Without a symmetrical load on the neutral line, the inverter output does not need to pursue the phase voltage to be close to sinusoidal, but can focus on making the line voltage tend to sinusoidal. Therefore, the line voltage control PWM is proposed, there are mainly the following two methods.
Saddle-shaped wave and triangle wave comparison method: Saddle-shaped wave and triangle wave comparison method is the harmonic injection PWM method (HIPWM). Its principle is to add a certain proportion of the third harmonic to the sine wave, and the modulated signal will appear saddle-shaped, and The amplitude is significantly reduced, so when the amplitude of the modulated signal does not exceed the amplitude of the carrier, the amplitude of the fundamental wave can exceed the amplitude of the triangular wave, which improves the utilization rate of the DC voltage. In the three-phase neutral line system, since the third harmonic current has no path, the third line voltage and line current do not contain the third harmonic. In addition to the injection of the third harmonic, other waveforms that are three times the frequency of the sine wave signal can also be injected. These signals will not affect the line voltage. This is because the phase voltage output from the inverter circuit after PWM modulation must also contain the corresponding harmonics that are three times the frequency of the sine wave signal, but when synthesizing the line voltage, these harmonics in each phase voltage will cancel each other out. So that the line voltage is still a sine wave.
Unit pulse width modulation method: Because the three-phase symmetrical line voltage has the relationship of Uuv+Uvw+Uwu=0, at any time, a certain line voltage is equal to the sum of the other two line voltage negative values. Now divide a cycle into 6 intervals, each interval is 60°. For a certain line voltage such as Uuv, the 60° interval on both sides of a half cycle is expressed by Uuv itself, and the middle 60° interval is expressed by -(Uvw+Uwu). When Uvw and Uwu do the same treatment, you can get two waveform shapes of three-phase line voltage waveforms with only 60° intervals on both sides in a half cycle, and they have positive and negative. Using such a voltage waveform as a reference signal for pulse width modulation, the carrier wave still uses a triangle wave, and the curves of each interval are approximated by a straight line (practice shows that the error caused by this is not large and completely feasible), you can get the pulse of the line voltage The waveform is completely symmetrical and has a strong regularity. The negative half cycle is the inverse of the corresponding pulse train of the positive half cycle. Therefore, as long as the pulse train in the 60° interval on both sides of the half cycle is determined, the modulated pulse waveform of the line voltage is unique determine. This pulse is not the driving pulse signal of the switching device, but since the pulse working mode of the three-phase line voltage is known, the driving pulse signal of the switching device can be determined. This method can not only suppress more low-order harmonics, but also reduce the switching loss and widen the linear control area, and at the same time bring the convenience of microcomputer control, but this method is only suitable for asynchronous motors and has a small application range. .
The basic idea of current control PWM is to use the desired current waveform as a command signal and the actual current waveform as a feedback signal. The instantaneous value of the two is used to determine the switching of each switching device, so that the actual output changes with the command signal. And change. The realization scheme mainly has the following three kinds.
Hysteresis comparison method: This is a PWM control method with feedback, that is, each phase current is fed back and the current setpoint is passed through a hysteresis comparator to obtain the switching state of the corresponding bridge arm switching device, so that the actual current tracks the given Changes in current. The advantage of this method is that the circuit is simple, the dynamic performance is good, and the output voltage does not contain harmonic components of a specific frequency. The disadvantage is that the switching frequency is not fixed and causes more serious noise. Compared with other methods, the output current contains more harmonics at the same switching frequency.
Triangle wave comparison method: This method is different from the triangle wave comparison method in the SPWM method. Here, the command current is compared with the actual output current to find the deviation current, which is amplified by the amplifier and then compared with the triangle wave to generate a PWM wave. At this time, the switching frequency is fixed, thus overcoming the shortcoming of the frequency of the hysteresis comparison method is not fixed. However, the current response in this way is not as fast as the hysteresis comparison method.
Predictive current control method: predictive current control is to predict the current error vector trend according to the actual current error, load parameters and other load variables at the beginning of each adjustment cycle. Therefore, the voltage vector generated by PWM in the next adjustment cycle will be Reduce the predicted error. The advantage of this method is that if more information is given to the regulator in addition to the error, a relatively fast and accurate response can be obtained. The limitations of this type of regulator are the response speed and the accuracy of the process model coefficient parameters.
Space voltage vector control PWM (SVPWM) is also called flux sine PWM method. It is based on the premise of the overall generation effect of the three-phase waveform, with the purpose of approximating the ideal circular rotating magnetic field trajectory of the motor air gap, the actual magnetic flux generated by the different switching modes of the inverter is used to approximate the reference circular magnetic flux. The result of the comparison determines the switching of the inverter, forming a PWM waveform. From the perspective of the motor, this method regards the inverter and the motor as a whole, and controls them in a way that the inscribed polygon approaches the circle, so that the motor obtains a circular magnetic field (sinusoidal flux) with a constant amplitude. The specific methods are divided into flux open loop type and flux closed loop type. The flux open loop method uses two non-zero vectors and a zero vector to synthesize an equivalent voltage vector. If the sampling time is small enough, any voltage vector can be synthesized. The output voltage of this method is 15% higher than that of sine wave modulation, and the sum of the effective values of harmonic currents is close to the minimum. Flux closed loop type introduces flux feedback to control the size and speed of flux. After comparing the estimated magnetic flux and the given magnetic flux, the next voltage vector is generated according to the error to form a PWM waveform. This method overcomes the shortcomings of the magnetic flux open-loop method, solves the problem that the stator resistance is greatly affected when the motor is at low speed, and reduces the pulsation and noise of the motor. However, because no torque adjustment is introduced, the system performance has not been fundamentally improved.
Vector control is also called magnetic field-oriented control. Its principle is to convert the stator currents Ia, Ib and Ic of the asynchronous motor in the three-phase coordinate system to three-phase/two-phase transformation, which is equivalent to the alternating current Ia1 in the two-phase static coordinate system. And Ib1, and then through the rotor magnetic field oriented rotation transformation, equivalent to the DC current Im1 and It1 in the synchronous rotating coordinate system (Im1 is equivalent to the excitation current of the DC motor; It1 is equivalent to the armature current proportional to the torque), Then imitate the control method of DC motor to realize the control of AC motor. The essence is that the AC motor is equivalent to a DC motor, and the two components of speed and magnetic field are respectively.
In 1985, Professor Depenbrock of Ruhr University in Germany first proposed the theory of Direct Torque Control (DTC). Direct torque control is different from vector control. It does not control the torque indirectly by controlling the current and flux linkage. Instead, it directly controls the torque as the controlled variable. It does not need to decouple the motor model, but instead Calculate the actual value of the motor flux and torque in the stationary coordinate system, and then generate PWM signals through the flux-band and torque Band-Band control to optimally control the inverter's switching state, to a large extent Solve the deficiencies of the above vector control, can easily realize the speed sensorless, has a fast torque response speed and high speed and torque control accuracy, and with a novel control idea, concise and clear system structure, excellent The dynamic and static performance has been rapidly developed. But direct torque control also has shortcomings, such as the increase in inverter switching frequency is limited.
The one-cycle control method [7], also known as integration reset control (IRC), is a new type of nonlinear control technology. Its basic idea is to control the switching duty cycle and make the average value of the switching variable in each cycle. It is equal to or proportional to the control reference voltage. This technology has the duality of modulation and control at the same time, through the reset switch, integrator, trigger circuit, comparator to achieve the purpose of tracking the command signal. The single-cycle controller is composed of a controller, a comparator, an integrator, and a clock. The controller can be an RS flip-flop, and its control principle is shown in FIG. 1.
One-cycle control does not require error synthesis in the control circuit. It can automatically eliminate steady-state and transient errors in one cycle, so that the error of the previous cycle will not be brought to the next cycle. Although the hardware circuit is more complex, it overcomes the shortcomings of the traditional PWM control method and is suitable for various pulse width modulation soft switching inverters. It has the advantages of fast response, constant switching frequency, and strong robustness. In addition, the single cycle Control can also optimize system response, reduce distortion and suppress power interference, which is a promising control method.
In the traditional PWM inverter circuit, the working mode of the hard switching of the power electronic switching device, the large switching voltage and current stress, and the high du/dt and di/dt limit the increase of the operating frequency of the switching device, and the high frequency is the power electronics One of the main development trends is that it can reduce the volume of the converter, reduce the weight, reduce the cost, improve the performance (especially when the switching frequency is above 18kHz), reduce the vibration, and make the noiseless transmission system possible. The basic idea of resonant soft-switching PWM is to add a resonant network on the basis of the conventional PWM converter topology. The resonant network is generally composed of a resonant inductor, a resonant capacitor, and a power switch. When switching, the resonant network works to make the power electronic device realize the soft switching process at the switching point. The resonant process is extremely short, which basically does not affect the realization of the PWM technology. Thus not only maintaining the characteristics of PWM technology, but also realizing soft switching technology. However, due to the existence of the resonant network in the circuit, resonance losses will inevitably occur, and the circuit will be affected by inherent problems, thus limiting the application of this method.
Pulse width modulation can be used to control the servo mechanism.
In telecommunications, pulse width modulation is a form of signal modulation. The width of the pulse wave corresponding to another specific data will be encoded at the transmitting end and decoded at the receiving end. Pulse waves of different lengths (the message itself to be transmitted) will be transmitted after a fixed time (frequency of the carrier).
Pulse width modulation can be used to control how much energy is transferred to a carrier without generating linear energy transfer losses due to impedance. The price to be paid by this method is that the energy lost by the carrier is not a constant and discontinuous (such as a buck converter), and the energy transferred on the carrier is not continuous. However, since the carriers may be high-frequency inductive, a passive electronic filter must be added to make these pulse waves smooth and can recover the average analog wave shape before the energy flows into the carriers. It will be continuous. The energy flowing from the supply end is not continuous, so in most cases additional energy storage space is required.
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