The coil is wound on the unclosed E-shaped magnet. When the speed measuring device passes the iron tooth tip, the E-shaped magnet is close to the gear (the gap is several wires), and the magnetic circuit is closed, resulting in electromotive force. However, when the backlash is formed, the magnetic resistance is too large to form a magnetic circuit closure and generate electromotive force. Therefore, when the speed measuring device passes through a tooth tip, it will produce alternating pulse electromotive force, which will be generated at the speed of 1500.