| 摘要 |
PROBLEM TO BE SOLVED: To obtain good responsiveness and excellent detecting characteristics with a simple structure, and to achieve expansion of the object to be detected, improved detection reliability as well as improved reliability for life and easier maintenance. SOLUTION: A hollow ring-shaped sensor case 23 accommodating a toroidal coil 22 consists of a ring-shaped case 23A of conductive metal having an opening throughout the entire corner of its circumference and a ring-shaped rim part 23B of insulating material closing the opening, both made integral with each other. Under the sensor case 23, there is provided an electrode 25 through a spacer 24 of insulating material. When a tool 28 advances into a space W enclosed by the sensor case 23, coupling capacitance C between the electrode 25 and the tip of the tool 28 increases whereby a current flows passing the coil 22 through the capacitance C and the tool 28, which is detected by a signal processing circuit 27.
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