“PWM is widely used in modern Electronic devices. As one of the control technologies, PWM has realized its own value. In order to improve everyone’s understanding of pwm, this article will introduce pwm, pwm principle, pwm advantages and so on.
PWM is widely used in modern electronic devices. As one of the control technologies, PWM has realized its own value. In order to improve everyone’s understanding of pwm, this article will introduce pwm, pwm principle, pwm advantages and so on.
1. Introduction to PWM
Pulse width modulation is a very effective technique for controlling analog circuits using the digital output of a microprocessor and is widely used in many fields from measurement and communications to power control and conversion.
Pulse width modulation is an analog control method, which modulates the bias of the transistor base or the gate of the MOS tube according to the change of the corresponding load, so as to realize the change of the conduction time of the transistor or MOS tube, so as to realize the output of the switching regulated power supply. Change. 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 to control the analog circuit by using the digital signal of the microprocessor.
PWM control technology has become the most widely used control method in power electronic technology because of its simple control, flexibility and good dynamic response, and it is also a research hotspot. Since the development of science and technology has no boundaries between disciplines, combining modern control theory or realizing non-resonant wave switching technology will become one of the main directions for the development of PWM control technology. It modulates the bias of the transistor base or the gate of the MOS tube according to the change of the corresponding load, so as to realize the change of the conduction time of the transistor or the MOS tube, so as to realize the change of the output of the switching regulated 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 to control the analog circuit by using the digital signal of the microprocessor.
Second, the advantages of PWM
One of the advantages 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 a digital signal if it is strong enough to change a logic 1 to a logic 0 or a logic 0 to a logic 1.
The increased immunity to noise is another advantage of PWM over analog control, and it is the main reason why PWM is sometimes used for communication. Switching from analog to PWM can greatly extend communication distances. At the receiving end, the modulated high frequency square wave can be filtered out and the signal restored to analog form through an appropriate RC or LC network.
In conclusion, PWM is economical, space-saving, and has strong anti-noise performance, and is an effective technique worthy of engineers in many design applications.
Three, PWM pulse width modulation principle
The pulse width modulation technology realizes the control of the analog circuit through the on-off control of the switch of the inverter circuit. The output waveform of the pulse width modulation technology is a series of pulses of equal size, which are used to replace the required waveform. Taking a sine wave as an example, that is, the equivalent voltage of this series of pulses is a sine wave, and the output pulse is as smooth and smooth as possible. Has fewer low-order harmonics. According to different needs, the width of each pulse can be adjusted accordingly to change the output voltage or output frequency and the like, thereby achieving the control of the analog circuit.
4. Introduction to PWM Synchronous Modulation
Synchronous modulation – N equals a constant, and keeps the carrier and signal waves in sync during frequency conversion
1. Basic synchronous modulation mode, N does not change when fr changes, and the number of output pulses in one cycle of the signal wave is fixed
2. A triangular wave carrier is shared in the three-phase circuit, and N is an integer multiple of 3, so that the three-phase output is symmetrical
3. In order to make the positive and negative half-cycle mirrors of one-phase PWM wave symmetrical, N should be an odd number
4. When fr is very low, fc is also very low, and harmonics brought by modulation are not easy to filter out
5.fr, when it is very high, fc will be too high, making the switching device unbearable
Five, PWM synchronous modulation advantages and disadvantages
When fc is changed proportionally while changing f, so that K remains unchanged, it is called synchronous modulation.
The advantage of using synchronous modulation in PWM is that it can ensure the symmetry of the output waveform. For a three-phase system, in order to maintain the symmetry between the three phases and the phase angle with a mutual difference of 120b, K should be an integer multiple of 3; in order to ensure the symmetry of the positive and negative half-waves of each phase waveform during bipolar modulation, the multiple should be taken as odd number. Due to the symmetry of the waveform, there are no even harmonic problems. However, limited by the allowable switching frequency of the switching device, keeping the K value unchanged, when the inverter operates at a low frequency, the K value will be too small, resulting in a larger harmonic content.Increase the harmonic loss of the motor, and the torque ripple is relatively intensified
Six, PWM specific application
1. PWM software method to control the charging current
The basic idea of this method is to use the PWM port of the single-chip microcomputer, without changing the PWM square wave period, adjust the PWM control register of the single-chip microcomputer through software to adjust the PWM duty cycle, thereby controlling the charging current. The single-chip microcomputer required by this method must have two necessary conditions, the ADC port and the PWM port. In addition, the number of bits of the ADC is as high as possible, and the working speed of the single-chip microcomputer is as fast as possible. Before adjusting the charging current, the single-chip microcomputer first quickly reads the size of the charging current, and then compares the set charging current with the actual reading charging current. If the actual current is too small, adjust the PWM in the direction of increasing the charging current. If the actual current is too large, adjust the PWM duty cycle in the direction of reducing the charging current. In the adjustment process of software PWM, attention should be paid to the ripple interference introduced by ADC reading deviation and power supply operating voltage, and digital filtering techniques such as arithmetic mean method should be reasonably adopted.
2. The application of PWM in thrust modulation
In 1962, Nicklas et al. proposed the theory of pulse modulation, pointing out that the use of jet pulses to control spacecraft is a simple and effective control scheme, and at the same time can achieve optimal control of time or energy.
The PWM engine control method is to change the gas flow through the valve by changing the time the valve stays in the open or closed position in each pulse cycle, thereby changing the total thrust effect. For a system with a constant mass flow rate , the effect of variable thrust can be obtained by pulse width modulation technology.
There are usually two methods of pulse width modulation: the first is the overall pulse width modulation, the controller is designed for the control object, and according to the force required by the control, the dynamic mathematical solution transformation is performed on the entire system model, and a fixed value is obtained. The time that the force output should continue to act and the time to start acting; the second is the pulse width modulator, which does not consider the control object model, but performs “dynamic attenuation” accumulation according to the input, and then after some algorithm transformation, determines the output duration. This method is very simple and can also achieve approximately the same output effect.
The PWM control technology is simple in structure, easy to implement, and relatively mature in technology. Russia has successfully applied it to the control of the angle stabilization system of long-range rockets. However, when the modulation amount is zero, the forward and reverse control effects cancel each other, and the control efficiency is obviously lower than that of the variable flow rate system. Moreover, the system response has a certain lag, and the switching frequency must be much larger than the natural frequency of the KKV itself, otherwise not only will the modulation effect not be achieved, but even catastrophic consequences will occur.