A relative-position measuring system of a recording-medium library apparatus, said library apparatus including:
- a housing;
- a cell drum which is rotatably installed in said housing and has a plurality of cells each for accommodating a recording-medium cartridge;
- a drive unit for writing and reading out data into and from said recording-medium cartridge;
- an X rail which is installed in said housing and has an X axis extended in a horizontal direction;
- and an accessor for conveying said recording-medium cartridge between said cell drum and said drive unit by moving said accessor along said X axis, said accessor having a hand mechanism movable in the vertical direction along a Y column extended in the vertical direction and rotatable around a Y axis parallel to said Y column;
- said relative-position measuring system comprising:
- a first reference flag installed at a front lower portion of said housing;
- a second reference flag installed above said first reference flag;
- a third reference flag which is installed at a rear lower portion of said housing in such a way that a straight line connecting said third reference flag to said first reference flag forms a right angle with said X rail;
- a plurality of relative-position flags installed at said cell drum;
- a home sensor installed on said X rail for detecting a home position of said accessor;
- a flag sensor installed at said hand mechanism for detecting said first to third reference flags and said relative-position flags;
- a first motor for moving said accessor along said X rail and, at the same time, generating a train of first tachometer pulses;
- a first tachometer counter for counting said first tachometer pulses;
- a second motor for moving said hand mechanism along said Y column and, at the same time, generating a train of second tachometer pulses;
- a second tachometer counter for counting said second tachometer pulses;
- a third motor for rotating said hand mechanism around said Y axis and, at the same time, generating a train of third tachometer pulses;
- a third tachometer counter for counting said third tachometer pulses;
- a means for finding a gradient of said Y axis of said accessor with respect to an X direction and gradients of a Z axis defined as a straight line connecting a rotation start point to a rotation end point of said hand mechanism with respect to said X direction and with respect to a Y direction by detecting said first to third reference flags using said flag sensor;
- a means for finding first theoretical tachometer count values by correcting design tachometer count values of said reference-position flags using said gradients of said Y and Z axes of said accessor and storing said first theoretical tachometer count values;
- a means for storing first actual tachometer count values of said first to third tachometer counters which are obtained when said relative-position flags are detected by said flag sensor;
- and
- a means for finding difference data between said first actual tachometer count values and said first theoretical tachometer count values and storing said difference data as first correction values.