Diagnosing Mastering Loss on a FANUC Robot

Confirm the alarm is a genuine mastering loss

Read the exact code. SRVO-062 BZAL means the Pulsecoder absolute-position backup battery is empty and the counts are gone, and a pulse-mismatch alarm also calls for remastering. A plain low-battery warning is different: SRVO-065 BLAL reports battery voltage below the rating but the position data may still be intact.

Caution: Treat SRVO-065 BLAL as urgent. The manual warns that delaying battery replacement can let the condition escalate to a BZAL alarm, at which point the position data is lost and mastering becomes necessary.

Understand what mastering actually stores

Mastering associates each axis angle with the Pulsecoder count read at the zero-degree position. The controller then computes current position from three pieces of data: the pulse count per degree ($PARAM_GROUP.$ENCSCALE), the pulse count at the zero-degree position stored in $DMR_GRP.$MASTER_COUN, and the current pulse count read by calibration. Lose any of these and the reported position is no longer trustworthy.

Identify which event caused the loss

The robot keeps mastering data and Pulsecoder counts on two independent backup batteries: a controller memory (C-MOS) battery for robot and mastering data, and a mechanical-unit battery for the Pulsecoder counts. Mastering is needed when the controller mastering data is lost (low C-MOS voltage or a memory-erasing initial start), when the pulse counts are lost (low mechanical-unit battery or a replaced Pulsecoder), or when the mechanical unit was bumped so the counts no longer match the true angles. Pin down which of these applies before choosing a method.

Check the Pulsecoder backup battery and its cable

For a BZAL (SRVO-062) the probable cause is an empty battery, a broken battery cable, or missing batteries. Lock out and tag out the controller's power at the main disconnect before opening the battery box or handling the mechanical-unit cable. Replace the battery in the battery box at the robot base, then check that the mechanical-unit cable feeding power to the Pulsecoder is not disconnected or grounded, and replace it if an abnormality is found. Correct this hardware cause before you attempt any mastering.

Caution: Apply lockout/tagout before opening the battery box or handling arm cabling. Use the specified battery type and confirm the connector seats fully so the replacement does not fail again under vibration.

Rule out a Pulsecoder or wiring fault masquerading as loss

Some faults look like mastering loss but are really a bad Pulsecoder or cable: SRVO-068 DTERR (no serial data returned), SRVO-072 PMAL and SRVO-074 LDAL (likely Pulsecoder failure), and SRVO-073 CMAL (Pulsecoder abnormal or noise). These call for replacing the Pulsecoder or cable and then remastering, so remastering alone will not hold if the hardware is failing. Confirm the encoder and wiring are healthy first.

Enable the Master/Cal screen

The Master/Cal screen appears only when the system variable $MASTER_ENB is set to 1 or 2, which guards against accidental mastering. Open the SYSTEM Variables screen and set $MASTER_ENB to 1. After you press F5 DONE at the end, the controller sets it back to 0 automatically.

Caution: Mastering by mistake can make the robot move unexpectedly and is very dangerous. Only enable the screen when you are ready to perform a controlled, deliberate recovery.

Open the Master/Cal screen

Press MENU, select 0 NEXT, then 6 SYSTEM, press F1 [TYPE], and choose Master/Cal. The screen lists the recovery methods: 1 FIXTURE POSITION MASTER, 2 ZERO POSITION MASTER, 3 QUICK MASTER, 4 QUICK MASTER FOR SINGLE AXIS, 5 SINGLE AXIS MASTER, 6 SET QUICK MASTER REF, and 7 CALIBRATE.

Clear a pulse-not-established condition first

If SRVO-075 Pulse not established is present, the absolute position of the Pulsecoder cannot be established yet. Reset the alarm and jog the affected axis, roughly one motor revolution, until the alarm stops recurring. Mastering cannot be performed until the axis has rotated enough to establish a pulse.

Choose the correct mastering method

Pick the method that matches what was lost. Quick mastering works when only the Pulsecoder backup battery emptied and the controller still holds the reference, but it cannot be used when the Pulsecoder was replaced or the controller mastering data itself was lost. Single-axis mastering suits a subset of axes, zero-position mastering is a less accurate visual emergency measure, and fixture mastering with the jig gives factory-grade accuracy.

Quick mastering: jog to the stored reference position

Jog the robot to the quick mastering reference position, turning off brake control only if necessary; before releasing any brake, mechanically block or support that axis so it cannot drop under gravity, a qualified-technician step. The misalignment from the reference must stay within half a motor revolution for the count to resolve correctly. Select 3 QUICK MASTER and press F4 YES, and the Mastering Data values are set.

Caution: Keep clear of the arm envelope and use reduced speed while jogging with brakes released. A released brake lets gravity move the axis, so support or expect motion on vertically loaded joints.

Single-axis mastering: master interaction axes together

On the SINGLE AXIS MASTER screen set SEL to 1 for each lost axis and leave MSTR POS at 0 unless the procedure specifies otherwise, then jog to the mastering position and press F5 EXEC. Interaction axes must be mastered together: for example a J2/J3 or J4/J5/J6 pair influences its partner, so master the whole pair in one operation. The ST field reads 0 while data is lost and 2 once mastering completes.

Handle a Gravity Compensation model

On robots with Gravity Compensation enabled, a popup asks you to confirm the payload data is set correctly before executing. Such models may require mastering all un-mastered axes at once rather than one at a time. If interference prevents reaching every reference, perform a tentative zero-position mastering first, then repeat single-axis or quick mastering accurately at each reference.

Calibrate (perform positioning) after mastering

Select 7 CALIBRATE and press F4 YES so the controller reads the current pulse counts and recognizes the current position. Alternatively cycle the controller power, since calibration runs at every power-on. Press F5 DONE when the screen reports Robot Calibrated.

Reset the pulse-count-loss flag when a battery emptied

When the loss came from an emptied Pulsecoder backup battery, the manual directs you to correct the cause first, then set the system variable $MCR.$SPC_RESET to TRUE and turn the power on again, after which mastering is needed. Do this only after the fresh battery and sound cabling are confirmed. Then complete the mastering and calibration steps above.

Caution: This reset belongs after the root cause is fixed, never as a shortcut to run a robot whose position is still uncertain. Do not return the robot to program execution until the position is verified in the next step.

Verify position accuracy against a known reference

Jog the robot to a known witness point, dowel-pin fixture, or reference position and confirm the pose matches, rather than trusting that the alarm merely cleared. Zero-position mastering relies on visual alignment to the zero marks and is less accurate than other methods, so scrutinize it closely. If the position is off, remaster with a more accurate method before proceeding.

Caution: Do not run production programs until mastering accuracy is verified against a known reference. An incorrect method can introduce a subtle position error that is hard to detect later and may drive the tool into fixturing.

Re-store the quick-master reference and re-validate safety

After a successful quick mastering, run 6 SET QUICK MASTER REF so a good reference is on hand for any future recovery. Then re-check Dual Check Safety zones and joint position checks, since these depend on accurate axis position data and can behave differently after remastering. Record the cause of the loss and every recovery step in the maintenance log.

Caution: Have a qualified technician re-validate DCS zones and joint checks before releasing the cell. Safety-rated position limits are only as trustworthy as the mastering underneath them.

What can go wrong: the Master/Cal screen will not appear

If the Master/Cal option is missing from the SYSTEM menu, $MASTER_ENB has returned to 0, which happens automatically after you press F5 DONE. Reopen the SYSTEM Variables screen and set $MASTER_ENB back to 1 to show the screen again.

What can go wrong: mastering will not take

If the controller reports that mastering cannot be performed until the axis is rotated enough to establish a pulse, the Pulsecoder has not yet resolved an absolute position. Jog the axis about one motor revolution to establish the pulse, clearing any SRVO-075 condition, then retry the mastering selection.