Poor Positioning Accuracy? Stop Blaming Encoders! The Real Errors Come From Transmission Chains

Poor Positioning Accuracy? Stop Blaming Encoders! The Real Errors Come From Transmission Chains

For automation engineers involved in equipment commissioning, after-sales service, and machine retrofitting, this is an extremely common headache:

The machine suffers from inconsistent positioning, reversal drift, and over-travel tolerance errors, even with correct parameter settings and no electrical fault alarms. Most engineers immediately suspect the encoder is defective or insufficiently precise and rush to replace or upgrade it — only to find the problem remains unresolved, resulting in wasted time and unnecessary costs.

In fact, over 90% of positioning inaccuracies are not caused by the encoder itself.

An encoder is merely a signal feedback component that faithfully detects shaft rotation angles and outputs pulse signals. As long as the encoder hardware and wiring are intact, the feedback data is accurate and reliable.

The final positioning deviation, backlash drift, and cumulative travel error are transmitted and accumulated through the mechanical transmission chain — including installation misalignment, coupling clearance, reducer backlash, lead screw pitch tolerance, and gear meshing errors.

This article thoroughly clarifies industry misunderstandings, distinguishes encoder failures from mechanical transmission errors, and provides practical troubleshooting and optimization solutions.

01 Core Principle: Encoders Do Not Generate Errors — They Reflect Them

A widespread misconception in the industry is equating end-effector positioning accuracy with encoder detection accuracy.

The underlying logic must be clearly defined:

✅ Encoder — The Detection Terminal

It only reads rotational displacement and outputs ABZ pulse signals. Its role is to measure accurately.

✅ Mechanical Transmission — The Execution Terminal

The complete set of motors, couplings, reducers, gears, lead screws, and guide rails determines whether the machine can move accurately.

Key conclusion: Accurate measurement ≠ Accurate movement.

Even high-resolution, high-precision encoders cannot compensate for mechanical clearance, misalignment, and wear. If the transmission chain contains mechanical errors, the final positioning result will inevitably be out of tolerance.

02 Five Major Mechanical Error Sources — The True Causes of Poor Precision

Most on-site precision failures originate from the following five mechanical links, which should be prioritized during inspection and optimization.

1. Encoder Mounting Errors (Most Common & Easily Overlooked)

Most encoder-related precision issues are caused by improper installation rather than defective encoders.

???? Shaft Coaxiality Misalignment

Offset or tilted assembly between the motor shaft and encoder shaft causes radial runout and axial float during operation, leading to periodic positioning fluctuations, low-speed jitter, and repeated offset errors.

???? Improper Coupling Selection and Assembly

Aged elastic elements, excessive backlash, loose set screws, and incomplete locking result in idle travel during forward/reverse switching. In severe cases, shaft slippage occurs, causing pulse loss, positioning drift, and poor repeatability.

???? Insufficient Bracket Rigidity

Weak or poorly fixed mounting brackets produce micro-displacement under operating vibration, causing unstable signal feedback and fluctuating positioning values.

2. Reducer Backlash Errors (Main Cause of Reversal Deviation)

Gear meshing inevitably produces inherent backlash, which is a mechanical hard error that cannot be eliminated by encoders or electrical parameters.

During direction switching, gear clearance directly converts into positioning offset, resulting in inconsistent forward and reverse positioning values. Long-term operation aggravates gear wear, further increasing backlash and degrading precision.

3. Lead Screw Transmission Errors (Typical for Long-Travel Linear Axes)

Linear motion axes suffer from cumulative travel errors due to screw manufacturing tolerances.

???? Pitch Cumulative Error

Inherent screw pitch tolerances accumulate linearly over travel distance. Short-stroke positioning appears normal, while long-stroke movement clearly exceeds tolerance.

???? Failed Nut Preload

Loosened double-nut preload structures lead to internal clearance, generating reversal idle travel and inconsistent bidirectional positioning accuracy.

???? Screw Deformation and Support Misalignment

Bent screws or misaligned support seats cause operational jitter and eccentric load, significantly amplifying dynamic positioning errors.

4. Multi-Stage Gear Transmission Errors (Error Superposition & Amplification)

For multi-stage gear transmission systems, minor errors at each meshing stage are superimposed and amplified at the final output end.

Gear side clearance, tooth profile errors, and center-distance deviation accumulate step by step. The more transmission stages applied, the worse the overall positioning accuracy.

5. Overall Mechanical Rigidity and Fit Errors

Guide rail clearance, timing belt elongation, frame deformation, and load deflection also produce micro-displacement errors. These minor mechanical deviations superpose with transmission errors and further reduce overall machine precision.

03 On-Site Quick Judgment: Encoder Fault vs. Mechanical Transmission Error

Engineers can quickly distinguish error sources through simple on-site observation without professional testing instruments.

✅ Mechanical Transmission Errors (90% of Field Cases)

Errors follow regular patterns: periodic fluctuation, fixed reversal offset, and larger deviation over longer travel
Position values remain stable when the shaft is statically locked
Upgrading to higher-resolution encoders achieves little or no improvement
No pulse loss, signal distortion, or system alarms during operation

✅ Encoder Hardware Faults (Rare Cases)

Completely irregular errors with random position jumping and drifting
Position values drift automatically even when the machine is stationary
Signal inspection shows waveform distortion, phase loss, or missing pulses
Positioning accuracy recovers immediately after encoder replacement

04 High-Precision Optimization Solutions — Fix Errors From the Source

To fundamentally improve positioning accuracy, optimizing the mechanical transmission chain is far more effective than blindly replacing encoders.

1. Standardize Encoder Mounting

Adopt high-precision bellows couplings, strictly calibrate shaft coaxiality, and control radial runout within 0.02 mm. Reinforce mounting brackets to eliminate vibration-induced displacement.

2. Control and Eliminate Transmission Backlash

Apply zero-backlash reducers for high-precision scenarios; adopt double-nut preloaded lead screws to eliminate idle travel; regularly inspect and replace worn transmission parts.

3. Utilize System Compensation Functions

Enable backlash compensation and screw pitch compensation on the motion controller to offset fixed mechanical tolerance errors and significantly improve positioning and repeat positioning accuracy.

4. Complete Machine Coaxiality Calibration

Integrally calibrate the motor, reducer, and lead screw to reduce eccentric stress and dynamic runout, minimizing error transmission at the source.

Closing Remarks

There is a classic industry consensus in automation:

The encoder defines the upper limit of machine precision, while the mechanical transmission system defines the lower limit.

Most positioning inaccuracy issues are caused by accumulated transmission clearance, insufficient assembly precision, and mechanical wear — not encoder performance limitations.

Next time you encounter poor repeatability, reversal drift, or over-travel errors, always inspect the transmission chain first before replacing encoders. This avoids ineffective maintenance and unnecessary costs.

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???? Technical Welfare

This article systematically summarizes the error mechanisms, discrimination methods, and practical rectification measures for equipment positioning faults, serving as a reliable on-site debugging and optimization guide.

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