The Quantity That Moves When the Motor Changes
Home on a Fanuc axis is not a switch. It is an offset between two counters: the pulsecoder count coming back from the motor shaft and the machine coordinate the control believes that count represents. The pulsecoder is bolted to the motor. Swap the motor and the one-revolution marker on the new encoder sits at a different angle relative to the coupling, the ballscrew, and the deceleration dog. Nothing in the control knows this happened. The stored home offset is now wrong by some fraction of one motor revolution multiplied by the axis pitch (and divided by any gear ratio). On a 10 mm pitch direct-coupled screw that is up to 10 mm of positioning error that looks perfectly repeatable, because the new encoder is just as consistent as the old one; it is consistently referenced to the wrong place.
The number that matters is that angular offset. Every homing method below is a way of measuring it and writing it back into the control. The label "0-Mate" identifies the control series; the servo amplifier module fitted to it is identified from its own nameplate, and the maintenance manual for that amplifier is where the axis-side alarm list and parameter tables live. Have both manuals open before proceeding.
Two encoder families change the procedure completely. An incremental pulsecoder forgets position at power-off and relies on a dog-plus-grid reference return every start-up. An absolute pulsecoder holds position through power-off on a battery in the amplifier or a separate battery case; disconnect the motor cable and that stored position is gone, and the control flags the axis as "reference position not established" until you deliberately re-establish it. Read the motor nameplate and the encoder type parameter for the axis to find out which one you have.
Three Ways to Re-establish Reference
The control supports three distinct paths, and only one of them is correct for a given machine. Choosing the wrong one produces an axis that homes without alarm but stops in the wrong place.
| Method | Encoder type | What gets measured | What you write | When it is wrong |
|---|---|---|---|---|
| Dog-type grid return | Incremental (also absolute machines that keep a dog) | First one-rev marker after the deceleration dog releases | Grid shift / reference offset parameter for the axis, from the displayed grid value | Dog release lands within a few counts of the marker, so home flips by one motor revolution between cycles |
| Absolute position establishment (dogless) | Absolute pulsecoder only | Current position, jogged to a known mechanical reference | Reference position establishment bit for the axis, then power cycle | Done with the axis parked anywhere convenient instead of at a measured mechanical reference |
| Mechanical re-alignment | Either | Coupling angle set so the new marker matches the old one | Nothing in the control | Not repeatable without the original marker angle recorded; used only where fixtures or offsets cannot be re-taught |
Recommendation: with an absolute pulsecoder, use absolute position establishment. It measures the offset directly at a physical reference, takes minutes, and does not depend on dog timing. With an incremental pulsecoder, run dog-type grid return and correct the grid shift value. Skip mechanical re-alignment unless the machine has no dog, no battery-backed encoder, and a fixed tooling layout that cannot be re-taught.
Checks Before Writing Any Parameter
Homing errors after a motor swap are frequently masked by a problem introduced during the swap. Clear these first; each one changes the outcome of the homing procedure.
- Confirm the new motor model and pulsecoder type match the old nameplate. A different encoder resolution or type changes the feedback count per revolution and the axis will run away or fault on servo mismatch before it ever homes. If they differ, the axis-type and feedback parameters must be set for the new motor from the servo parameter manual before continuing.
- Verify the amplifier battery. On absolute systems a low or missing battery raises a battery alarm and blocks reference establishment. Replace it with the control powered on so existing data on other axes is not lost, then clear the alarm.
- Check the feedback cable path: motor-side connector seated, shield terminated, no swapped axes if two motors were disconnected at once. A crossed feedback cable homes the wrong axis to the wrong screw.
- Jog the axis in both directions in low override and watch the position display. The count must increase in the positive machine direction. A reversed phase sequence or a flipped direction parameter shows up here as the display counting backwards.
- Trip the deceleration dog by hand (incremental machines) and confirm the input toggles in the PMC diagnostic display. A dog that never releases makes the axis drive into the hard limit during reference return.
Read the axis alarm on the control screen and look it up in the maintenance manual for your amplifier. The servo alarm class for lost absolute position is distinct from a battery alarm and from a positioning error alarm; the code tells you which of the three methods you are being pushed toward.
Procedure: Absolute Pulsecoder Reference Establishment
This is heat-free, low-speed work, but the axis moves under manual control with a possibly wrong soft-limit table. Keep override low and stay clear of the stroke ends.
- Power up, note the "reference not established" indication for the axis, and clear any battery alarm.
- Enable parameter write on the control.
- Jog the axis to the mechanical reference the machine was originally homed to: a dial indicator against a datum, a fixture pin, or the dog-release point if the machine keeps a dog. Use a handwheel for the final approach. The accuracy of home is now exactly the accuracy of this positioning.
- With the axis stationary at the reference, set the absolute position establishment bit for that axis in the parameter table identified in the maintenance manual. The control will demand a power cycle.
- Power the control off and on. On restart the axis reports reference established and the machine coordinate reads the value the control assigns to home for that axis.
- If the original machine used a non-zero home value, or a deliberate offset from the mechanical reference, apply it now through the reference offset parameter rather than by re-jogging.
- Disable parameter write and back up the parameter set.
For an incremental pulsecoder the procedure differs at step 3: select reference return mode, drive the axis toward the dog at the reference feedrate, and let it stop on the grid. Then read the grid display for the axis, compare where it stopped against the mechanical datum, and enter the difference in the grid shift parameter. Repeat the return until the stop point matches the datum within the machine's positioning tolerance.
Verification
Homing is verified by repeatability and by absolute agreement, and both are needed. Repeatability without agreement means the offset is wrong; agreement without repeatability means the grid or dog is marginal.
- Cycle power three times (absolute) or run reference return five times (incremental). Indicate the axis against the datum after each. The spread should be inside the axis positioning tolerance; a jump equal to one motor revolution times pitch means the dog releases too close to the marker and the dog needs to be moved so release lands mid-grid.
- Jog to both soft limits and confirm they land inside the physical stroke. If a limit now lies past the hard stop, the home offset is wrong, not the limit table.
- Touch off a known fixture or tool-setter and compare against the stored work offset. Agreement here is the final proof the axis is back where the programs expect it.
- Run a warm-up program at reduced feed and watch following error in the servo diagnostic screen. A large or growing following error after a motor swap points to a servo parameter mismatch, not a homing fault.
Recurring Pitfalls on Fanuc 0-Mate Systems
The failure that repeats most on this generation of hardware is the battery. Absolute pulsecoders on older Fanuc amplifiers hold position only while the battery does; a motor swapped with the battery already weak produces a machine that homes correctly today and comes up with lost reference next week. Check battery voltage and date before declaring the job done.
Second is parameter backup. Setting the establishment bit and cycling power is easy to do on the wrong axis line of the table. Take a backup before the change so a mis-set axis can be restored instead of re-homed.
Third is the grid-shift trap on incremental machines. Adjusting grid shift by exactly half a revolution's worth of counts, or moving the dog by a similar distance, moves the dog release point onto the marker itself. The axis then homes to two positions one revolution apart on alternate attempts. Aim the release point mid-way between markers and confirm with the grid display.
Fourth is a replacement motor that is electrically equivalent but carries a different pulsecoder resolution. The control accepts it, the axis moves, and every position is scaled wrong by the ratio of the two resolutions. Match encoder type first; home second.
FAQ
Why does a Fanuc axis home consistently to the wrong position after a motor change?
The new pulsecoder's one-revolution marker sits at a different shaft angle than the old one, so the stored home offset is out by up to one motor revolution times the axis pitch. Re-establish reference at a measured mechanical datum and correct the reference offset or grid shift parameter.
Why does the control report reference position not established on an absolute pulsecoder axis?
Disconnecting the motor feedback cable, or a dead amplifier battery, erases the battery-backed absolute position. Replace the battery if flagged, jog to the datum, set the absolute position establishment bit for that axis, and cycle power.
Why does dog-type reference return stop one revolution apart on alternate homing cycles?
The deceleration dog releases within a few encoder counts of the one-revolution marker, so the control alternately catches this marker or the next one. Move the dog or adjust grid shift so release lands near the middle of the grid interval, then confirm with the grid display.
Stop and contact Fanuc's official service channel for your region when the axis raises a servo alarm that persists after the battery, cable, and encoder-type checks, or when the replacement motor does not appear in the servo parameter tables for the amplifier. Those cases require the correct servo parameter set from Fanuc rather than further homing attempts.