According to the Doppler effect, the frequency of waves encountering moving objects changes. The Doppler detector is installed on the pipeline, and the detector emits electromagnetic waves. When there is material flow in the pipeline, the frequency of the reflected wave received by the detector is different from the frequency of the emitted wave. When there is no material flow or blockage in the pipeline, the frequency of the reflected wave will not change. The detector determines whether there is material flow based on whether it receives a reflected wave with a change in frequency and outputs the signal through a relay.