| 摘要 |
PURPOSE:To remove an influence due to environmental noise and to detect a metal with high sensitivity by providing plural hall devices AC driven in an advancing direction of a body to be inspected in a DC magnetic field, detecting a signal synchronously and passing the signal through a filter. CONSTITUTION:The DC magnetic field is generated by a magnetic field generator 10, and an alternating voltage of a fixed frequency is supplied to the hall devices 11a, 11b from an AC source 12. Under this state, the body W to be inspected is passed therethrough. If magnetic metal such as iron is contained in the body W, a difference occurs between voltages generated in the devices 11a, 11b, and an output is generated in an arithmetic amplifier 15. Said output is detected by a synchronous detector 16, and inputted into a comparator circuit 18 through a bandpass filter 17 having a frequency approximating to that supplied by the source 12. Thereat, the output is compared with a voltage of a reference power source 20, and if the output is larger than said voltage, a controlling signal is outputted from a controlling circuit 19. |
