A linear module can return to the same position consistently and still stop slightly away from the commanded coordinate. This is not a contradiction. Repeatability and positioning accuracy measure different aspects of motion, and both may appear in the same specification table.
When evaluating a linear module, engineers also encounter terms such as backlash, lost motion, running parallelism and straightness. Treating these values as interchangeable can lead to an axis that performs consistently but does not meet the actual process requirement.
The correct specification depends on the task. Repetitive pick-and-place motion, camera inspection, dispensing and precision machining do not evaluate axis performance in exactly the same way.
Core Difference
Repeatability describes how closely the module returns to the same position over repeated moves under the same test conditions. Positioning accuracy describes how closely the actual position agrees with the commanded position across the measured travel.
| Specification | What It Measures | Typical Influence | Why It Matters |
|---|---|---|---|
| Repeatability | Variation when returning repeatedly to one target position | Backlash, friction variation, control response, load and temperature | Consistent cycles at the same working points |
| Positioning accuracy | Difference between commanded and actual position over the travel | Lead error, compensation, thermal expansion and measurement reference | Correct absolute position at different coordinates |
| Backlash or lost motion | Position change that appears when motion direction reverses | Nut clearance, support bearings, coupling and joint rigidity | Bidirectional positioning and contour control |
| Running straightness | Lateral or vertical deviation while the table travels | Guideway accuracy, base geometry and assembly | Path quality, inspection alignment and tool position |
Repeatability
Consider a module commanded to move to 200.00 mm ten times. If the measured stopping positions remain tightly grouped, the axis has good repeatability. The group may be centred at 199.97 mm rather than 200.00 mm, but the small spread shows that the module repeats the motion consistently.
Manufacturers may express repeatability as a plus-or-minus value, a total range or a statistical value. These formats are not automatically equivalent. A value of ±0.01 mm should therefore be interpreted together with the stated test method rather than compared directly with every value printed in another catalog.
Repeatability is especially important when an axis revisits a limited number of taught positions. Typical examples include component loading, screwdriving, labelling, dispensing and sensor positioning. If an offset at a repeated point can be calibrated, consistency may matter more than absolute travel accuracy.
Positioning Accuracy
Positioning accuracy compares the commanded coordinate with the measured coordinate at multiple positions along the stroke. A module may be close to the target near one end and gradually develop a larger position error farther along the travel.
In a ball screw driven axis, one major source is lead error. If the actual travel per screw revolution differs slightly from the nominal lead, the difference accumulates with distance. This is why a positioning accuracy value may be stated over a defined travel, such as per 300 mm, instead of as one unrestricted value for every module length.
Positioning accuracy matters when the machine must reach many coordinates that cannot all be individually taught or corrected. Measurement equipment, multi-position drilling, electronic assembly and machining processes usually require closer attention to absolute position across the full working stroke.
Good Repeatability, Different Accuracy
The following simplified example shows why the two specifications should not be substituted for one another.
| Commanded Position | Repeated Measurements | Interpretation |
|---|---|---|
| 100.00 mm | 99.99, 100.00, 99.99 mm | Small spread and small position error |
| 300.00 mm | 299.95, 299.96, 299.95 mm | Still repeatable, but the absolute error has increased |
| 500.00 mm | 499.91, 499.92, 499.91 mm | Good grouping does not remove accumulated travel error |
The measured positions in this example remain close to one another at each target, so repeatability is good. However, the difference between commanded and actual position grows along the stroke. Calibration or controller compensation may reduce a systematic error, but compensation cannot correct unstable mechanical behaviour that changes from cycle to cycle.
Backlash and Lost Motion
Backlash is often used to describe clearance in a mechanical transmission. Lost motion is a broader measured effect that can include screw-nut clearance, elastic deformation, support-bearing movement, coupling wind-up and joint deformation when the axis reverses direction.
A module may show better repeatability when every target is approached from the same direction than when the same targets are approached from both directions. The difference indicates that reversal behaviour is contributing to the result.
A preloaded ball screw can reduce axial clearance and increase rigidity. It cannot compensate for loose support bearings, an unsuitable coupling, flexible brackets or poor assembly elsewhere in the drive train.
Straightness and Running Parallelism
Positioning values describe location along the commanded axis. They do not describe every geometric error of the moving table. A carriage can reach the correct X coordinate while moving sideways or vertically away from the intended line.
Running straightness describes deviation from a straight path. Running parallelism commonly describes how the moving surface behaves relative to a specified reference surface. These values depend on the guideway, module body, mounting base, assembly accuracy and measurement method.
This distinction matters in camera scanning, laser processing, precision dispensing and inspection. In these applications, an axis may repeat its encoder position while lateral or vertical path error still affects the process result.
Main Influencing Factors
| Factor | Repeatability | Positioning Accuracy | Geometric Motion |
|---|---|---|---|
| Ball screw lead error | Limited influence at one repeated point | Major influence over the stroke | Usually limited direct influence |
| Backlash and compliance | Important during direction changes | Creates bidirectional position differences | May allow table attitude change |
| Guideway and base geometry | May cause friction variation | Can create local position disturbance | Major influence on straightness and parallelism |
| Motor, encoder and control | Affects stopping consistency | Affects command tracking and compensation | Cannot correct mechanical path geometry alone |
| Temperature | Changes results as the system warms | Thermal expansion changes travel | Uneven heating may distort the structure |
| Load and overhung moment | May change friction and settling | Causes load-dependent displacement | Can tilt or deflect the carriage |
Measurement Conditions
A specification value is meaningful only when its test conditions are understood. Stroke length, target locations, approach direction, number of cycles, travel speed, payload, warm-up state, mounting method and measuring instrument can all affect the result.
For repeatability testing, the module is normally moved away from and returned to defined target positions several times. Bidirectional testing is more demanding because it also reveals reversal effects. For positioning accuracy, actual position is measured at multiple locations and compared with the commanded position over a stated travel.
The encoder position should not automatically be treated as the true table position. A motor encoder confirms motor rotation, but mechanical errors between the motor and carriage may remain. External measurement at the moving table provides a more complete evaluation of the assembled axis.
Application Priorities
| Application | Primary Concern | Additional Concern |
|---|---|---|
| Pick and place | Repeatability at taught positions | Settling time and payload change |
| Dispensing | Repeatability and path consistency | Straightness and speed stability |
| Vision inspection | Positioning accuracy and straightness | Vibration and settling time |
| Multi-position assembly | Accuracy across the working stroke | Bidirectional lost motion |
| Light machining | Accuracy, rigidity and backlash | Thermal stability and load deflection |
Reading a Module Specification
Suppose a module lists repeatability of ±0.01 mm, positioning accuracy of ±0.05 mm per 300 mm, backlash of no more than 0.02 mm and running straightness of 0.03 mm per 300 mm. These values describe four separate limits:
- The module may return closely to the same target during repeated cycles.
- The actual coordinate may still differ from the command as travel increases.
- Reversing direction may introduce an additional position change.
- The table may deviate laterally or vertically even when its axial coordinate is correct.
The four numerical values above are illustrative rather than universal product specifications. Real values must be tied to a particular module size, stroke, drive type, accuracy grade, payload and test method.
Conclusion
Repeatability answers whether a linear module can return consistently to the same position. Positioning accuracy answers whether the actual position agrees with the commanded coordinate across the travel. Backlash describes reversal behaviour, while straightness and running parallelism describe the geometric path of the moving table.
No single value represents the complete performance of a module. A reliable evaluation starts with the machine process, identifies which error directly affects the result and then compares specifications measured under compatible conditions. This prevents a strong repeatability figure from being mistaken for full-stroke accuracy.
References
- ISO 230-2, Test Code for Machine Tools - Determination of Accuracy and Repeatability of Positioning of Numerically Controlled Axes.
- ISO 3408-3, Ball Screws - Acceptance Conditions and Acceptance Tests.
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