Operation
Mastering
Mastering is the procedure that teaches a robot the correspondence between each joint's encoder count and its known mechanical zero position, so the controller reports absolute joint angles correctly.
What it is
A robot's servo motors carry encoders (pulsecoders) that count how far each axis has turned. Those counts are meaningless on their own until the controller knows which count equals the joint's true zero. Mastering is what establishes that link: it stores a reference count at a known physical position for every axis.
Once mastered and calibrated, the controller can convert raw encoder counts into real joint angles, and from there compute where the tool actually is in space. Lose the mastering data and the robot still moves, but it no longer knows its own absolute position, so taught points and cartesian moves become wrong or unavailable.
How it actually works
Mastering is a qualified-technician procedure. Each axis is brought to a defined reference posture, and the controller records the encoder value at that pose as the zero reference. On many FANUC arms this is done at the witness marks stamped on each joint, where lines on the two sides of a joint line up at the calibration position. Single-axis mastering does this one joint at a time against its own witness marks, which is why it's useful when only one motor or pulsecoder was disturbed.
The stored reference is the offset that turns a live count into an angle. A pulsecoder tracks position through absolute counts backed up by a battery when the controller is powered down; if that backup fails, the counts are still valid relative to power-up but the absolute reference is gone. Mastering is lost when the encoder battery dies, when a pulsecoder or motor is replaced, when the mechanical unit is disassembled at a joint, or when the mastering data in the controller is corrupted or cleared.
How it differs
- Calibration · Mastering sets the encoder-to-zero reference; calibration is the follow-on step where the controller reads the current encoder values against that reference and rebuilds its position model. You master first, then calibrate. In practice the controller often calibrates automatically right after you complete mastering, which is why people treat them as one operation. They are separate steps.
- Home position · Home (or a reference position) is just a convenient, repeatable posture the program sends the robot to. It has nothing to do with absolute accuracy. Mastering is what makes any position, home included, mean the right thing. A robot can drive to its programmed home while being badly mismastered and land nowhere near where you expect.
Where you meet it in the field
- Single-Axis Mastering a FANUC Robot · The step-by-step procedure for restoring mastering on one axis at a time, the case you hit after a single pulsecoder or battery event.
- FANUC LR Mate 200iD · A compact six-axis arm that needs re-mastering after a dead encoder battery or a pulsecoder swap before it will run taught programs correctly.
Common questions
- Do I lose taught program points when mastering is lost?
- No. Taught points are stored as joint angles or cartesian positions and stay in the controller. The problem is that a mismastered robot interprets those stored positions against a wrong zero, so it physically goes to the wrong place. Restore correct mastering and the existing points become valid again.
- Why does mastering matter for accuracy if I only care about repeatability?
- If you never lose mastering and only run points the robot taught itself, repeatability carries you. Mastering bites the moment absolute position matters: reloading a backup onto a repaired arm, offline-programmed paths, sharing points between identical robots, or any move computed in cartesian space rather than replayed from a taught pose.
- Is replacing the encoder battery the same as re-mastering?
- No. Changing the battery before it fully dies preserves mastering because the pulsecoder keeps its absolute count backed up. If the battery goes flat while the controller is off, the absolute reference is gone and you have to re-master even after you install a fresh battery.