Absolute vs Relative Positioning: Core Semantics
An absolute move commands the axis to a position referenced to zero (or another defined reference): a command of 120 mm means go to the position 120 mm. A relative move commands a displacement referenced to a starting point, normally the current position: a command of 120 mm means move by 120 mm from where the axis stands. The two commands therefore do not produce the same motion even with identical values. A useful shorthand: absolute = go to a coordinate; relative = move by a distance.
When to Use Each Mode
Mode selection follows the geometry of the task. For a repeated fixed increment (10 mm steps), relative mode needs only one command — "move by 10 mm" — executed in a loop, whereas absolute mode requires every target coordinate to be written out (10, 20, 30, 40, 50 …). Conversely, to draw a 50 mm square somewhere on an X-Y plane, use absolute mode to travel to the first corner (fixing the square in the workspace), then relative moves of 50 mm in X and Y to trace the sides without recalculating absolute coordinates for each corner.
- Absolute: indexed positions, part features at known coordinates, first-corner placement.
- Relative: repeated pitches, incremental jogs, offsets from the current point.
- Mixed: systems generally allow switching between modes on the fly within a program.
Position Tracking and Accuracy
Independently of the move mode, the absolute position is continuously updated, so the controller always knows where the axis is. For accuracy, prefer a single absolute move over a chain of relative moves: successive relative displacements accumulate measurement/execution deviations, whereas each absolute move references the same zero and does not carry forward prior error. When a trajectory is built from relative steps, periodic absolute moves or a return to reference re-zeros any accumulated drift.
FAQ
What is the difference between absolute and relative positioning?
Absolute positioning moves the axis to a coordinate measured from zero or another reference (go to 120 mm). Relative positioning moves the axis by a distance from the current position (move by 120 mm), so identical values produce different motion.
Which is more accurate, absolute or relative moves?
Absolute moves are more accurate over long trajectories because each targets the same reference; chained relative moves add their individual measurement deviations together, accumulating error.
Can you switch between absolute and relative mode during a program?
Yes, mode switching is generally possible on the fly. Regardless of mode, the absolute position register keeps updating, so the controller always knows the axis position.