Basic principle of PWM speed control system
The basic principle of a PWM speed control system is to achieve speed regulation by changing the pulse width of the power input signal. Specifically, the PWM speed control system utilizes different time ratios for turning the switching device on and off to control the average output voltage. In this process, the high-level duration of the PWM signal (i.e., the pulse width) determines the average voltage received by the motor, and thus the motor speed. A wider pulse width results in a higher average voltage received by the motor, and a corresponding increase in motor speed; a narrower pulse width results in a lower average voltage received by the motor, and a decrease in motor speed.
PWM Principle and Application
PWM, or Pulse Width Modulation, is a technique that converts DC voltage into analog signals with different duty cycles. In motor control, PWM is widely used to regulate motor speed. Its principle is that a higher duty cycle means a larger proportion of the cycle is high-level, thus achieving precise adjustment of the motor speed. When the output is high, the motor starts to rotate. When the high level suddenly transitions to a low level, due to the inductance, the motor does not stop immediately but maintains its original speed. Through this state of near-stop, near-full-speed rotation, we can achieve smooth adjustment of the motor speed. Therefore, the duty cycle directly determines the average output voltage and speed of the motor.
In motor control, voltage is directly proportional to speed; that is, the higher the voltage, the faster the motor speed. PWM technology regulates the motor's output speed by outputting different analog voltages. Of course, frequency is also a crucial factor in motor control. Too low a frequency may lead to unstable motor operation and even produce audible whistling noises; while too high a frequency may exceed the motor's response capabilities. Therefore, selecting an appropriate frequency range is essential for ensuring stable motor operation. Generally, the normal motor frequency should be controlled between 6-16kHz.
Differences in output voltage can cause changes in motor speed. In fact, besides using a variable resistor or replacing the power supply with one of different voltages, PWM technology can also achieve motor speed control, and in practical applications, PWM technology is clearly more convenient.
Components of a PWM speed control system
A PWM speed control system mainly consists of the following components:
Control signal generator: Used to generate control signals for speed regulation. Common control signals can be pulse signals or DC voltage signals. These signals typically originate from user input, sensor feedback, or commands from a host computer.
Comparator: Compares the control signal with the reference signal and outputs a PWM signal. The comparator is one of the core components of a PWM speed control system; it is responsible for generating PWM signals with different pulse widths based on the magnitude relationship between the control signal and the reference signal.
Switching drivers: Based on changes in the PWM signal, they control the switching devices to turn on and off, thereby modulating the power input signal. Switching drivers typically use high-power transistors and other devices capable of withstanding large currents and voltages.
Output filter circuit: Used to filter the modulated power input signal to obtain the average output voltage. The output filter circuit can effectively eliminate high-frequency noise and ripple in the PWM signal, improving the stability and accuracy of the output voltage.
In-depth exploration of the meaning of PWM
PWM, or Pulse Width Modulation, is a technique that controls voltage or current by changing the pulse width. It is widely used in motor control, lighting regulation, and battery charging, among other fields. By precisely adjusting the pulse width, PWM enables fine-grained control of voltage or current, thus meeting various application requirements.
Pulse Width Modulation (PWM) is a highly efficient technology that uses the digital output of a microprocessor to precisely control analog circuits. It has wide applications in various fields, including measurement, communication, and power control and conversion.
The frequency of a PWM signal refers to the number of times the signal cycles from high to low and back to high within one second; that is, the number of complete cycles. Therefore, a higher frequency means more PWM cycles per second. Its unit is Hertz (Hz), with common representations including 50Hz and 100Hz.
The period of a PWM, T, has a reciprocal relationship with the frequency f, i.e., T = 1/f. For example, when the frequency is 50Hz, one period lasts 20ms. This means there will be 50 such periods per second, or 50 PWM switching cycles.
In addition, duty cycle is another key concept in PWM technology. It represents the proportion of time that the high-level signal occupies within a pulse cycle. Duty cycle is expressed as a percentage, ranging from 0% to 100%. Different duty cycle settings will directly affect the magnitude and shape of the output voltage or current.
Period: Refers to the length of time a pulse signal occupies. The number of pulse cycles within one second is the frequency. Pulse width, or the duration of the high-level signal, determines the shape of the pulse signal. The proportion of the pulse width within a given period is the duty cycle. For example, if the period is 10ms and the pulse width is 8ms, then the low-level time is 2ms. Therefore, the total duty cycle is calculated as 8ms/(8ms+2ms) = 80%, which is a pulse signal with an 80% duty cycle. By adjusting the duty cycle, we can achieve precise control of the pulse width, which is the core of PWM technology. Meanwhile, frequency represents the number of pulse signal cycles per unit time. The higher the frequency, the more times the pulse signal switches per second. Taking 20Hz and an 80% duty cycle as an example, it means that 20 pulse signals are output within one second, and the high-level time of each cycle is 40ms.
In the diagram above, we defined the period T, the high-level time T1, and the low-level time T2. If the period T is 1 second, the frequency is 1 Hz. In this case, the high-level time lasts for 5 seconds, and the low-level time is also 5 seconds. Therefore, the duty cycle for the entire period is calculated by dividing the high-level time by the period, i.e., 5 seconds divided by 1 second, resulting in a duty cycle of 50%. This is the basic principle of PWM technology.
Taking a microcontroller as an example, its I/O ports typically output digital signals, providing only two states: high and low. However, through PWM technology, we can simulate different analog voltage signals by varying the duty cycle of a square wave. Specifically, when the high level is 5V and the low level is 0V, by adjusting the duty cycle of the square wave output from the I/O port, we can output any analog voltage within the range of 0 to 5V. The principle behind this technology is that the voltage is actually applied to the analog load, such as an LED or a DC motor, through a repetitive sequence of high and low pulses. By precisely controlling the connection and disconnection time ratios, we can theoretically output any analog voltage not exceeding the maximum voltage value. For example, a 50% duty cycle will result in equal high and low level times, thus generating a 5V analog output voltage at a specific frequency; while a 75% duty cycle will generate a 75V analog voltage.
The regulation effect of PWM is based on precise control of the "duty cycle" width. When the "duty cycle" is wider, the output energy increases accordingly, and the average voltage value also rises after passing through the RC converter circuit. Conversely, when the "duty cycle" is narrower, the average value of the output voltage signal decreases, and similarly, the average voltage value processed by the RC converter circuit also decreases.
Specifically, by adjusting the duty cycle at a constant signal frequency, we can obtain different analog output voltages. This is the basic principle of PWM for D/A conversion.
Furthermore, PWM technology plays a crucial role in LED lighting applications such as breathing lights. Since the human eye is insensitive to flickering at refresh rates above 80Hz, when the PWM frequency of an LED light exceeds 50Hz, the persistence of vision makes the light appear stable and flicker-free. By precisely controlling the time ratio of high to low levels (i.e., the duty cycle), we can achieve different brightness levels in LED lights, thus simulating the gradual brightness change of a breathing light.
