Development of a Low Resistance Micro Electro Magnetic Distance Sensor Using High Aspect Ratio Photo Resist

To maintain metal pipes, an electromagnetic coil is necessary as the detecting device of the eddy current change in metal due to pipe defects and as the distance sensor of the clearance between the detecting device and metal wall. This type of sensor should be thin and have a smaller diameter for detection by precise resolution and have large inductance and low electrical resistance for high sensitivity and high S/N. In this paper, for the above requirements, a new planar spiral type of coil for higher inductance, and thicker coil for reducing the electrical resistance by using ultra-thick photoresist SU-8 and Ni electroplating is proposed. Micromachining technology for the small size diameter and for automatic assembly was applied to microcoil fabrication. The Ni microcoil 300 μ m thick and 3mm in diameter was fabricated. The static distance characteristics and the frequency response were measured. Experiments show that the sensor of the 300 μ m thick coil has better output characteristics and linearity. High sensitivity, high signal-to-noise ratio and wide frequency response of the fabricated sensor have been measured to be 1.7v/mm, 75.6dB and 10Hz to 1000Hz, respectively, for 300 μ m thick coil. The position was controlled using the microcoil distance sensor.