Reference wire compensation method and apparatus

A method for compensating for sensor support frame stress distortion due to ambient stress conditions in a scanning system wherein the object being scanned by a sensor on said support frame is maintained a fixed distance from the frame, comprising the steps of;

• extending a thin flexible reference element in fixed relation to the object and between the sensor and the object in a manner unaffected by stress distortion of the frame,
• maintaining a constant tension on said reference element sufficient to maintain a constant configuration of the reference element independent of ambient stress conditions,
• obtaining a first measurement of the distance between the sensor and the reference element under stress free conditions of the frame,
• obtaining a second measurement of the distance between the sensor and the reference member during frame stress conditions,
• comparing the first and second distance measurements to obtain a correction value, and
• applying the correction value to measurements made between the sensor and object during frame stress.

In combination with a support frame including mounting members for mounting sensor apparatus in fixed relation with an object to be sensed, said mounting members being subject to stress distortion due to ambient conditions, a constant reference comprising;

• a thin elongated flexible reference member,
• mounting structure for positioning said reference member between said sensor in the sensing path thereof and the surface of the object to be sensed,
• tensioning means for maintaining constant tension on said reference member sufficient to maintain a constant configuration of the reference member independent of ambient conditions,
• whereby said reference member is unaffected by stress distortion and remains in fixed position independent of movement of said sensor mounting members.