The basic principle of loop detector is: a group of induction coils are embedded in the road subgrade section of the same lane, and each induction coil is connected with the multi-channel vehicle detector.

When the vehicle passes through two coils, due to the change of coil inductance, the passing state of the vehicle will be detected, and the state signal will be transmitted to the vehicle loop detector for collection and calculation. This method has the advantages of accurate detection, stable equipment and excellent performance in bad weather conditions. In addition, the low cost is one of the reasons why it is widely used in the world.

Vehicle loop detector is a kind of loop detector based on the principle of electromagnetic induction. Its sensor is a ring coil buried under the road with a certain working current. When the vehicle passes through or stops at the ring buried coil, the iron of the vehicle cuts the magnetic flux line, which causes the change of the inductance of the cord circuit. The loop detector can detect the existence of the vehicle by detecting the inductance change. Generally speaking, there are two ways to detect the change of inductance: one is to detect the change of phase by using phase latch and phase comparator; The other is to detect the oscillation frequency by using the coupling circuit composed of the loop of the ring buried coil.