A sensitive photoelectric force transducer with a resonant frequency of 6 kHz

A displacement-type photoelectric force transducer with a projection arm and a resonant frequency of 6 kHz is described. A vane on a moving element partially interrupts a light beam shining on a pair of phototransistors. Movement of the vane causes changes in the phototransistor currents, which are amplified to give an overall sensitivity of 10 mV/mg. Most of the half of each phototransistor not under the vane is covered by a mask, the edge of which is parallel to the edge of the vane. This increases the sensitivity of the transducer so that stiffer moving elements can be used. Compliance of the transducer at the tip of the projection arm is about 10 micrometers/g. The inertia of the lightweight moving element is minimized by having it rotate about a twisting strip hinge. The equivalent mass of the moving element at the tip, 6 mm from the pivot, is about 0.9 mg. The principal advantages of the transducer over other types is that it is nearly impervious to moisture.