Patent ID: 7146743

Claim:
A length measuring instrument for measuring the length of a part being measured by the draw-out length of a measuring belt, comprising: a housing; a rotary shaft rotatably held inside said housing; the measuring belt wound around said rotary shaft so that one end portion thereof is fixed to the rotary shaft, comprising a latching tool on the other end portion, and causing said rotary shaft to rotate in the forward direction when the measuring belt is drawn out from the side of said other end portion to the outside of said housing; a rotary shaft drive unit provided inside said housing and rewinding the measuring belt by rotating said rotary shaft in the reverse direction; an optical modulating section provided in communication with said rotary shaft and used for converting light from a light source into optical modulation signals; a measurement unit provided inside said housing for converting said optical modulation signals into electric pulse signals, counting the number of said electric pulse signals, determining the draw-out length of the measuring belt from the counted number, and displaying the determined length as a measurement result; a latching section provided outside said housing and capable of latching with the latching tool of said measuring belt; and a guiding unit having the prescribed length and formed on the outer side surface of said housing, wherein said measurement unit comprises said light source, a photoelectric conversion unit for converting said optical modulation signals into photoelectric conversion signals, a pulse formation circuit for converting said photoelectric conversion signals into electric pulse signals, a length determination unit for determining the draw-out length from the number of said electric pulse signals and outputting the determined length, wherein said length determination unit comprises a forward/reverse rotation judging section for deciding as to whether said rotary shaft rotates into forward direction or reverse direction from said electric pulse signals, and a pulse counting section that addition counts the number of said electric pulse signals when said forward/reverse rotation judging section decides that the rotation is in the forward direction, subtraction counts the number of said electric pulse signals when said forward/reverse rotation judging section decides that the rotation is in the reverse direction, and outputs the final count number obtained as a result of said addition counting and subtraction counting as said measurement result; and wherein said guiding unit comprises a distal end section provided with said latching section, and the measuring belt that has been drawn out from said housing is guided till the distal end section is reached along said guiding unit, and said latching tool is latched with said latching section and cues the measuring belt.