Digital tape rule

Measuring apparatus comprising:

• (a) a body member;
• (b) an elongated member adapted to be extended and retracted relative to said body member;
• (c) a series of markings along said elongated member comprising a plurality of areas of first and second characteristics in cyclic relations, there being transitions between said areas at constant intervals along said elongated member;
• (d) a pair of sensing means carried by said body member for sensing the movement of said elongated element in response to the movement of said markings thereby, each of said sensing means providing a signal of a first type in response to said marking areas of said first characteristic and a signal of a second type in response to said marking areas of said second type during relative movement between said elongated member and said body member, said sensing means and said markings being phase related to provide said signals in a two bit cyclic code indicative of the direction of movement of said elongated member;
• (e) logic circuit means connected to said sensing means for converting said signals in said two bit cyclic code to logic signals of first and second types representing increments of distance travelled by said elongated member in opposite directions;
• and
• (f) means operatively connected to said logic circuit means for converting said logic signals to an indication of the net distance said elongated member is extended from said body member.

Measuring apparatus comprising:

• (a) a body member;
• (b) an elongated member adapted to be extended and retracted relative to said body member, the relative movement between said members being at a speed over a relatively wide range;
• (c) a series of markings along said elongated member at constant intervals;
• (d) first and second sensing means carried by said body member for sensing the movement of said elongated member in response to the movement of said markings thereby, each of said sensing means providing output signals corresponding to increments of distance during relative movement between said body member and said elongated member, said output signals containing information as to the direction of movement of said elongated member relative to said body member;
• (e) converting means connected to one of said sensing means for converting said output signals to an indication of the net distance travelled by said elongated member relative to said body member;
• and
• (f) monitoring means operatively connected to said first and second sensing means for comparing said output signals to indicate when the difference between the number of output signals provided by said first and second sensing means exceeds a predetermined amount.