1992 Volume 28 Issue 12 Pages 1403-1410
This paper proposes a method for measuring a short-range linear displacement of an object using a magnetic dipole and Hall-Effect devices. To determine a target of the object, a circular plain permanent magnetic dipole is installed on the surface of the object. In the direction of the magnetic field which is vertical to the object's surface, three Hall-Effect devices are placed so that the magnetic flux density is sensed. Supposing the points where magnitudes of the flux density of the magnet are equal exist on a surface of an ellipsoid, the position of the magnet is determined as a cross point of the three elliptical surfaces. Experimental results show the measurement error is within 0.03mm in a box of 1.6×1.6×2mm3 volume. Also, a mathematical form for expressing the area of equal magnetic flux is derived and depicted by the use of a computer. Measurement range of the displacement is clarified under such condition that the surface of the area in which the output of the Hall-Effect device is equal is approximated by the surface of a spheroid. The error caused by using the approximated surface is analysed quantitatively. The sensor is useful in measuring the linear displacement from 0.5mm to 2.5mm without the affect of light.
