Yaskawa Robot Axis Drift Problem

Sigma Servo System & Encoder Feedback Diagnostic Guide

Axis drift in Yaskawa robots is typically not caused by mechanical wear or servo tuning issues.
In most Sigma-5 / Sigma-7 systems, it comes from servo feedback consistency degradation within the encoder–servo pack–controller loop.

When this feedback chain becomes unstable, the controller gradually accumulates position error, leading to slow but continuous axis deviation during operation.

Core Mechanism

Sigma Feedback Loop

Encoder → Servo Pack → Sigma Controller → Motion Output

When instability occurs:

• Actual axis position ≠ controller model position
• Small errors accumulate over cycles
• TCP accuracy gradually shifts

This is a progressive control error, not a sudden mechanical failure.

Typical Symptoms

• Gradual trajectory deviation during long runs
• Position shift after restart or warm-up
• Increasing correction values in SigmaWin+
• Reduced return-to-origin consistency

Key pattern:
Drift develops over time → indicates feedback degradation.

Related Alarm Codes

A.510 – Position Deviation Alarm (Most Important)

• Indicates mismatch between command and feedback model
• Shows accumulated system-level positioning error
  → Primary indicator of axis drift

A.410 – Encoder Communication Error

• Signal interruption between encoder and servo pack
• Often caused by cable or connector issues

A.020 – Absolute Encoder Backup Error

• Position memory instability
• Battery or multi-turn data corruption
  → Common in restart-related drift

Root Causes

1. Encoder Feedback Degradation (Primary Cause)

High-resolution encoders (up to 24-bit) are extremely sensitive.
Even minor signal noise can accumulate into measurable drift over time.

2. Servo Loop Instability

Under continuous operation:

• Thermal drift affects signal processing
• Control delay increases under load
• Feedback correction becomes inconsistent

Result: progressive positional deviation.

3. Absolute Encoder Data Issues

• Backup memory instability
• Multi-turn count corruption
• Voltage fluctuation in encoder circuit

Symptom: position shift after restart.

4. Mechanical Factors (Secondary)

• Backlash increase
• Brake holding weakness
• Gear wear

Mechanical issues usually amplify existing feedback problems, not cause drift alone.

Diagnostic Workflow

Step 1: Identify Drift Type

• Progressive drift → encoder/servo degradation
• After restart → absolute encoder issue
• Axis-specific → localized hardware fault

Step 2: Check Alarm History

Focus on:

• A.510 frequency trend
• A.410 signal interruptions
• A.020 backup errors

Step 3: Test Feedback Stability

• Cold start vs warm operation comparison
• Repeatability check under same motion path

Step 4: Inspect Signal Path

• Encoder cable condition
• Connector looseness / oxidation
• Electrical noise interference

Step 5: Exclude Mechanical Issues

• Backlash test
• Brake holding check
• Gearbox inspection

Recommended Fix Strategy

Core Solution: Restore Feedback Integrity

If recalibration does not stop drift:

→ Treat as encoder/feedback system degradation, not tuning issue

Typical resolution:

• Encoder signal restoration or replacement
• Servo motor assembly replacement (if integrated encoder is degraded)

System-Level Outcome

• Restores stable positioning reference
• Eliminates cumulative pulse loss
• Improves long-term repeatability
• Prevents recurrence of drift

Key Engineering Insight

• A.410 = communication problem
• A.510 = real positional deviation (most critical)
• Drift increasing with load = cumulative feedback error
• Drift after restart = encoder memory issue
• Warm-up drift = thermal instability

FAQ

1. Is this a tuning issue?
No. Most cases are encoder or feedback-related.

2. Why is A.510 more important than A.410?
A.510 reflects actual position deviation in the control model.

3. Why does drift get worse over time?
Because feedback errors accumulate during continuous operation.

4. Which axes are most affected?
 High-load and frequently moving axes.