4.3 Experimental principle
Before presenting the experiment principle, some relevant concepts should be given out firstly.
Digital signal: its means that its amplitude is discrete, and limited into a range, like binary code used widely in computer science. The digital signal has a strong anti-interference ability, and can be easily processed by digital signal processing. Nowadays, there are many digital signals, such as mobile signal, information handling by computer, and so on.
Analog signal: its ware changes continuously. Theoretically, we can get any of the value from the analog signal. Since it is interfered easily by the other signals, it is difficult to handling. Thus, generally, the analog signal should firstly be transformed into the digital signal for the convenient signal processing. The difference could be shown in Figure 4-1.
Figure 4-1 Difference of digital and analog signals
PWM is the abbreviation of Pulse Width Modulation. It means that we can equivalently get the required signal wave on the basis of a series pulse width, as shown in Figure 4-2. We can achieve a sine signal wave by a series of pulse signal by the different width. In fact, this principle can be illustrated by the equivalent area from mathematical integral. For example, the area of the first pulse is equivalent to the area surrounded by sine signal wave. So, we can change the duty cycle to get the voltage signal wave. Please image it. If we want to get a direct current voltage wave, the width of each pulse should be equal. In addition, PWM technique has been widely applied to the motor speed control, valve control, and so on. For example, PWM has been used to electronic cars.
Figure 4-2 Sine wave by PWM