Ring wheel speed sensor is mainly composed of permanent magnets, induction coils and gear rings. The permanent magnet is composed of several pairs of magnetic poles. During the rotation of the ring gear, the magnetic flux inside the induction coil alternates to generate an induced electromotive force. This signal is input to the electronic control unit of the ABS through the cable at the end of the induction coil. When the rotation speed of the ring gear changes, the frequency of the induced electromotive force also changes.