| 摘要 |
PURPOSE:To correct automatically the variation in earth magnetism and to detect a vehicle by winding a core with the primary winding connected to an exciting circuit, the secondary winding which is connected to a magnetism variation detecting circuit and generates an induced voltage, and the feedback winding. CONSTITUTION:An exciting current of specific frequency is supplied from the exciting circuit 2 to the primary winding 11 of a detection coil 1 to establish exciting magnetic fields around two cores 10a and 10b in the opposite directions. Voltages having the opposite characteristics are induced at winding parts 12a and 12b of the secondary winding 12. The induced voltage is outputted by the whole secondary winding 12 because of earth magnetism applied to the cores 10a and 10b and a circumferential magnetic field. The induced voltage is detected by the detecting circuit 31 of the magnetism variation detecting circuit 3 and fed back to the feedback winding 13 through an amplifier 32, delay circuit 51, and current limiting circuit 52, so magnetic field variation depending upon the environment is canceled. Thus, the vehicle is detected accurately. |
