Positioning Accuracy vs Repeatability: What Really Matters in Linear Modules?

1. Accuracy vs Repeatability: Similar Words, Very Different Meanings

1.1 Positioning Accuracy

Positioning accuracy describes how close the axis reaches an absolute target position.

Example:
If the controller commands 500.000 mm and the actual position is 500.020 mm,
the positioning error is +0.020 mm.

Accuracy is influenced by:

• Ball screw lead error
• Total stroke length
• Linear guide rail straightness
• Installation datum and calibration method

👉 Think of accuracy as “map correctness.”

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1.2 Repeatability

Repeatability measures how consistently the axis returns to the same position over multiple cycles.

Example:
If the axis stops at 500.018 mm, 500.019 mm, and 500.020 mm repeatedly,
repeatability is excellent — even if absolute accuracy is slightly off.

Repeatability is influenced by:

• lm guide rigidity
• Ball screw or timing belt backlash
• Load variation
• Coupling stiffness
• Control algorithm and servo tuning

👉 Repeatability is “parking in the same spot every time.”

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2. Why Repeatability Matters More in Most Automation Systems

In over 80% of real industrial applications, repeatability is more critical than absolute accuracy.

Why?

Because most automated systems rely on:

• Relative positioning
• Vision compensation
• Fixed tooling references

Typical examples:

• Robot gripper pick & place
• Dispensing and gluing
• Battery cell stacking
• Inspection positioning

As long as the axis stops consistently, the system can compensate for small absolute offsets.

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3. Transmission Type Comparison: Accuracy vs Repeatability

Drive Type	Accuracy	Repeatability	Speed	Load Capacity	Typical Use
Ball screw drive	High	Very high	Medium	High	Precision assembly, inspection
Timing belt drive	Medium	High	High	Medium	Long stroke, high-speed transfer
Electric cylinder	High	High	Medium	Very high	Vertical lifting, pressing

📌 Key insight:
A timing belt system may show lower absolute accuracy, but still deliver excellent repeatability for high-speed applications.

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4. Where Accuracy Truly Matters (And Where It Doesn’t)

Accuracy-Critical Scenarios

You need high positioning accuracy when:

• Absolute coordinate alignment is required
• No vision or sensor compensation exists
• Long-stroke cumulative error is unacceptable

Typical cases:

• Laser cutting alignment
• Large-format inspection platforms
• Multi-axis gantry referencing a fixed datum

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Repeatability-Critical Scenarios (Most Common)

Repeatability dominates when:

• Using robot grippers
• Running closed-loop visual correction
• Performing repetitive pick cycles

Typical cases:

• 3C electronics assembly
• Battery cell handling
• Automated packaging

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5. Real Application Examples from the Field

Case 1: 3C Electronics Pick & Place

• Stroke: 400 mm
• Load: 3 kg robot gripper
• Drive: Ball screw linear module

Customer concern: “Accuracy not sufficient”

Actual issue:

• Over-specified absolute accuracy
• Underestimated guide rigidity

Solution:

• Improved lm guide preload
• Optimized coupling stiffness

Result:

• Repeatability improved to ±0.005 mm
• Pick success rate stabilized

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Case 2: Battery Module Transfer Line

• Stroke: 1,200 mm
• Load: 25 kg
• Drive: Timing belt drive

Why belt drive?

• High speed
• Long stroke
• Vision-corrected placement

Key requirement:

• Consistent stop position, not absolute coordinates

Result:

• Stable repeatability
• Lower system cost
• Reduced maintenance

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6. What Engineers Should Really Evaluate When Selecting Linear Modules

Instead of focusing on one spec line, evaluate the system as a whole:

• lm guide stiffness and preload
• Ball screw drive vs timing belt drive suitability
• Load-to-stroke ratio
• Dynamic vs static load behavior
• Servo tuning capability

At W-Robot, we often find that improving mechanical rigidity delivers better real-world performance than chasing theoretical accuracy numbers.

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7. Where W-Robot Adds Practical Value (Without the Sales Talk)

Our engineers work backwards from:

• Application
• Load
• Motion profile

Not from a spec sheet.

What typically matters:

• Matching the right drive structure
• Proper linear guide rail configuration
• Custom stroke and mounting
• Fast delivery for non-standard modules

This is why many system integrators use W-Robot linear modules as functional building blocks, not just components.