What causes FANUC Robot alarm SRVO-071?

Electrical noise on the pulse coder cable causing false speed readings — ground/shield connection issue. Pulse coder hardware failure generating spurious feedback speed data. This alarm may follow a SRVO-064 PHAL alarm — in that case, PHAL is the root cause. Servo amplifier serial interface fault. Robot interconnection cable (pulse coder signal) damaged.

How do I fix FANUC Robot alarm SRVO-071?

1. Check whether SRVO-064 (PHAL) also appeared — if so, address PHAL as the root cause. 2. Verify ground and shield connections on the pulse coder cable are properly made. 3. Enhance grounding and shielding if noise is suspected. 4. Replace the pulse coder cable. 5. Replace the pulse coder if the cable and shielding are confirmed good. 6. Replace the servo amplifier if the problem persists.

FANUC Robot codes

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FANUC Robot Controller

Servo/Axis · Intermediate · 15-60 minutes

Alarm SRVO-071

Fanuc Robot SRVO-071 - SPHAL alarm (excessive feedback speed)

SRVO-071 SPHAL alarm occurs when the servo system detects an abnormally high feedback speed from the pulse coder that exceeds safe thresholds. This is typically caused by electrical noise corrupting encoder signals, damaged pulse coder cables, or actual pulse coder failure. Often appears as a secondary alarm following SRVO-064 PHAL, indicating the root problem is position deviation rather than speed feedback itself.

FANUC RobotServo/AxisIntermediate15-60 minutes

SRVO-071

Fanuc Robot SRVO-071 - SPHAL alarm (excessive feedback speed)

SPHAL alarm (excessive feedback speed)

An excessively large feedback speed was detected by the pulse coder. The speed reported by the encoder is abnormally high, which may indicate a noise problem or hardware failure.

Root Cause Summary

Safety

Always verify robot is in a safe stopped state before entering the work envelope
Use T1 mode when testing after clearing servo or motion alarms

Causes and Fixes

1
Check alarm history for SRVO-064
From teach pendant: MENU → ALARM → F1 [TYPE] → select 'Alarm Log'. Review the chronological order. If SRVO-064 PHAL appears before or simultaneously with SRVO-071, the PHAL is the root cause and must be resolved first.
Note the exact timestamp differential. If PHAL occurs within 1-2 scan cycles before SPHAL, they're related. SPHAL alone without PHAL points to encoder/noise issues.
2
Identify which axis triggered PHAL
The alarm detail screen shows the axis number (1-6 or 1-9 depending on robot model). Cross-reference this with the SPHAL axis. If they match, proceed with PHAL diagnostics: check for mechanical binding, external collisions, or payload issues on that axis.
Do not jog the robot until mechanical interference is ruled out. Collision damage may worsen with movement.
3
Inspect axis brake and motor coupling
Power down the robot (MENU → SYSTEM → F1 [TYPE] → Master/Cal). Manually move the suspect axis through full range. Check for unusual resistance, grinding, or loose motion indicating brake drag or coupling failure. Verify brake release parameter $SCR.$GENOVERRIDE enables proper release.
A partially failed brake that doesn't fully release creates continuous position error, causing the servo to command high speeds to catch up, triggering both PHAL and SPHAL.
4
Verify payload and inertia settings
Navigate to MENU → SETUP → Payload. Verify the configured payload matches actual mounted tooling. Check system variable $PAYLOADS[GP:x]. Incorrect payload causes unexpected position deviations during acceleration, especially on J2 and J3. Update if necessary and perform slow test jog.
After payload changes on R-30iB, run controlled start (250ms) motion tests. Monitor $MOR_GRP[1].$FOLLOW_ERR to see real-time position error on each axis.

1
Verify cable shield and ground continuity
Power off and lockout the robot. Using a multimeter, verify continuity between pulse coder cable shield and robot ground at both motor connector and axis amplifier (on amplifier side, check continuity to FG terminal). Resistance should be <1 Ω. Check cable routing for proximity to power cables, weld guns, or transformers.
Always lockout/tagout before measuring cable continuity. Induced voltages from nearby equipment can damage meters and cause injury.
2
Inspect pulse coder cable for physical damage
Trace the pulse coder cable from motor through the robot arm to the amplifier cabinet. Look for pinch points at axis bends (especially J2/J3 wrist area), cracked insulation, oil contamination, or cable strain at connectors. Flex the cable while monitoring for intermittent alarm occurrence during controlled jog.
On older robots (10+ years), cable insulation becomes brittle. Even without visible damage, microscopic cracks allow noise ingress. The wrist area (between J4-J6) sees the most flexing cycles and fails first.
3
Review servo amplifier error counters
Navigate to MENU → SYSTEM → Variables → $SCR.$SVALM_GRP[1,axis]. Check serial communication error counters. Also view MENU → MAINTENANCE → Servo → F2 [AMPS] → select axis → check 'Com Error' count. Values incrementing during motion indicate noise on the serial pulse coder interface.
R-30iB controllers log serial communication statistics to $SVALM registers. Compare error rates between axes—the problem axis will show 10-100x higher errors than healthy axes.
4
Test with known-good pulse coder cable
If available, temporarily replace the suspect axis pulse coder cable with a verified good spare. Perform a series of test moves at varying speeds (25%, 50%, 100% override) across full axis range. Monitor for alarm recurrence. If alarm disappears, cable replacement is confirmed as the solution.
Keep the original cable for bench testing. Measure capacitance and insulation resistance—degraded cables show increased capacitance (>200pF/ft) and reduced insulation (<10MΩ). This data helps predict failures on similar-age cables.
5
Enhance shielding and routing if noise persists
Install ferrite cores (Fair-Rite 0443164251 or equivalent) on both ends of pulse coder cable if operating near high-interference sources. Re-route pulse coder cables to maintain 300mm separation from servo power cables. Verify amplifier cabinet FG terminal has <0.1Ω to facility ground.
In welding cells, run pulse coder cables through continuous metal conduit bonded to building ground at both ends. This creates a Faraday cage effect, reducing noise by 40-60dB.

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