The principle of PWM wave speed regulation and the speed regulation method of tracking trolley

I. The concept of PWM and duty-free ratio

①PWM: Also known as pulse width modulation technology, is an analog control mode, the PWM waveform as shown below.

② duty cycle: refers to the proportion of the high level in a period. For example, a duty cycle of 50% means that the high level accounts for half of the period.

Two, PWM fixed frequency speed regulation principle

In the PWM speed regulation system, it is generally possible to change the duty ratio of the control pulse by the 3 methods of fixed width FM, FM modulation and fixed frequency width modulation, but the first two methods change the period of control pulse width in the speed regulation, thus causing the change of control pulse frequency, which will cause oscillation when the frequency is close to the natural frequency of system. In order to avoid this, a fixed-frequency width-varying duty-ratio method is designed to regulate the voltage at both ends of the DC motor armature.

Constant frequency speed regulation is the frequency of the pulse waveform under the premise (the Pulse waveform period unchanged), by changing the time of the high level in a periodic waveform to change the duty cycle of the waveform, thereby changing the average voltage, adjust the motor speed. Assuming that the motor is always plugged in, the maximum speed of the motor is Vmax, the duty ratio is D = t/t, then the average speed of the motor VD =d*vmax, by the formula, when the change in duty ratio D = t/t, you can get different motor average speed VD, so as to achieve the purpose of speed regulation. As shown above, the adjustable pulse width refers to the time of the high level, and the constant pulse period is the fixed frequency.

Three, PWM car speed regulation method

For the single-chip microcomputer trolley DC motor Speed Regulation, we can set the timing initial value through the timer, in the Interrupt service function to adjust the duty ratio, thus realizing the motor speed control. As an example:

In the image above we define a PWM waveform with a period of 16ms, and divide the pulse waveform into 32 parts (equivalent to 32 high and low levels to form a periodic waveform), so that the time per part level is 16MS/32 = 0.5ms. When the timer is initialized, we define an initial value of 500US, which corresponds to TH0 = (65536-500)/256,

TL0 = (65536-500)%256. We define a variable count to calculate the number of times the timer enters the interrupt service function, for example, we interrupt 10 times, in which we set the corresponding enable port to high, and the number of interrupts greater than 10 is less than 32 o'clock is defined as low level, This completes the definition of a PWM waveform with a period equal to 16ms, with a duty cycle of 10/32, which enables the control of the motor speed.