For motor control, duty cycle is a key parameter that directly affects control accuracy, and how to quickly analyze the trend of duty cycle change is the core of this problem. As long as there is a duty cycle change trend curve, the cause of abnormal motor operation naturally rises to the surface.

1, PWM duty cycle overview

Pulse Width Modulation (PWM) is a method of digitally encoding an analog signal. With the use of a high resolution counter, the duty cycle of the square wave is modulated to encode a specific analog signal. The PWM signal is still digital because at any given time, the full-scale DC supply is either fully (ON) or completely OFF. The voltage or current is applied to the analog load in a repeating pulse sequence of ON or OFF. When the DC power is applied to the load, the power is disconnected when the power is turned off. Any analog value can be encoded using PWM as long as the bandwidth is sufficient. The duty cycle is the ratio of the high level duration to the entire cycle time. The larger the duty cycle, the longer the high level lasts, and the longer the analog circuit is turned on, as shown in Figure 1.

2. What is the duty cycle trend measurement?

To measure the duty cycle trend, first measure the duty cycle. The duty cycle measurement method uses the ultra-high speed ADC to sample the PWM waveform, and then calculates the duty cycle by measuring the high-level pulse width t and period T through the programmable logic device FPGA. The duty cycle trend measurement measures and stores the duty cycle of each PWM wave, and finally displays it on the screen, as shown in Figure 2. A PWM wave corresponds to a duty cycle value, which is displayed on the screen and connected, so that the duty cycle of all PWM waveforms can be visually seen.

The storage depth of the oscilloscope has a great influence on the duty cycle trend measurement. The so-called trend trend is the long-term change trend. If the storage depth is small, the recorded time is very short, and the sampled waveform is very small. The trend is meaningless. At present, the ZDS4000 series oscilloscopes have a storage depth of up to 512 Mpts, which can store tens of seconds of waveforms at high sampling rates, and is more suitable for analyzing signal trends.

3. Example application and analysis

A DC motor driven by a PWM waveform has a slight jitter during the acceleration process. Observe that the motor speed waveform is as shown in Fig. 3. After the acceleration starts, a pull-down action occurs at about 3.6 seconds. The motor PWM speed regulation principle is to change the duty ratio of the voltage on the armature of the DC motor by changing the width of the on-pulse, thereby changing the average value of the armature voltage and controlling the rotation speed of the motor. Therefore, it can be basically determined that this speed pull-down action is caused by an abnormality in the duty cycle of the PWM wave.