Crack opening displacement gage

DETAILED DESCRIPTION To accomplish the foregoing and other objects of this invention there is provided a crack opening displacement gage which comprises a differential capacitive type sensor and a pair of arm means each having ends adapted to be disposed in the crack opening.
A common pivot means intercouples the arm means at a location spaced from the ends of the arm means which are disposed in the crack opening.
The arm means each include deflection means with the deflection means of each arm means moving toward and away from each other as the crack opening opens and closes.
Each arm means may comprise an arm assembly referred to hereinafter as either an upper arm assembly or a lower arm assembly.
In the disclosed embodiment each arm assembly comprises a forward leg disposed in the crack opening and a rear leg forming the deflection means.
These legs are preferably intercoupled by a flexural pivot.
In conjunction with the flexural pivot, the arm pair means serve to transfer displacement in the crack to the differential capacitive sensor.
The differential capacitive sensor means includes a first plate means, at least a second plate means, means securing the first plate means to one of said deflection means and means securing the second plate means to the other deflection means so that as the deflection means move relative to one another so also do the first and second plate means to vary the facing area between these plate means.
In the preferred construction of this invention there is preferably provided also a third plate means also secured to the other deflection means.
The first plate means in the illustrated embodiment comprises a single symmetrically shaped sensing plate insulatedly supported from the rear leg of the upper arm assembly.
In the disclosed embodiment the second and third plate means each comprise a pair of plates spacedly disposed and providing a cavity within which the sensing plate displaces.
One of the pairs of plates defines an upper excitation plate assembly while the other pair of plates defines a lower excitation plate assembly