High-Precision Positioning: Grating + Encoder Dual-Loop Servo System Eliminates Mechanical Transmission Errors

High-Precision Positioning: Grating + Encoder Dual-Loop Servo System Eliminates Mechanical Transmission Errors

Introduction

Precision equipment such as CNC machine tools, semiconductor packaging devices, and 3C precision inspection instruments commonly faces a typical problem: the motor encoder shows accurate readings, yet the actual positioning and repeat accuracy of the worktable deviate abnormally. This issue mainly stems from mechanical transmission errors including lead screw backlash, coupling misalignment, and structural deformation. This article systematically breaks down the grating scale + motor encoder dual-loop servo system, the core solution for high-precision positioning in industrial scenarios.

1. What Is a Servo Dual-Loop System? Clear Division of Internal and External Loops

This system adopts a dual-position feedback architecture consisting of an internal motor encoder loop and an external grating scale loop, matched with a classic series controller (position loop → speed loop → current loop). The two feedback loops operate independently with clear responsibilities and no mutual interference.

1.1 Internal Loop: Motor Encoder Feedback (High Bandwidth, Core for Dynamic Response)

Detection Object: Real-time position of the servo motor shaft P_m
Hardware Parameter: 23-bit high-resolution encoder
Core Advantages: High sampling frequency and bandwidth, capable of responding to speed fluctuations within milliseconds
Functions: Suppress motor torque and speed disturbances rapidly, ensure system dynamic rigidity, and adapt to high-speed start-stop and acceleration/deceleration impacts
Limitations: Only measures the motor shaft, failing to identify mechanical errors such as clearance and deformation caused by couplings and lead screws

1.2 External Loop: External Grating Scale Feedback (High Precision, Core for Positioning Accuracy)

Detection Object: Real displacement of the load worktable P_g, directly measuring the actual position of the end workpiece
Hardware Parameter: Ultra-high precision grating scale with 1nm resolution
Core Advantages: Bypasses the entire mechanical transmission chain to directly acquire the true coordinate of the load
Functions: Real-time correction of positioning deviations caused by lead screw backlash, installation misalignment, and structural deformation
Limitations: Relatively low bandwidth with weaker dynamic response than motor encoders, unable to support high-speed operation independently

Complete Control Loop

Position command P_{ref} → Position controllerC_p(s) → Speed controller C_v(s) → Current controller C_i(s) → Driver and motor actuation

Dual feedback signals are transmitted back to the position loop for difference calculation. The internal encoder loop stabilizes dynamic speed, while the external grating scale loop guarantees ultimate positioning accuracy, achieving a perfect balance of fast response and high precision.

2. Sources of Positioning Errors: Error Transfer Chain Analysis

The total positioning error of equipment is the superposition of multiple error sources. The complete error transfer path is as follows:

Command Error$$e_{ref}$$: Deviations generated by upper computer quantification and command output
Internal Encoder Error$$e_m$$: Errors from encoder subdivision, interpolation, and servo tracking lag
Mechanical Transmission Errore_{mech}: The dominant source of positioning errors
1. Radial, angular and axial coupling installation misalignment
2. Lead screw pitch error and reverse backlash
3. Thermal deformation of mechanical structures during operation
Load Original Errore_L: Errors from grating scale resolution, installation deviation and ambient temperature drift
External Grating Correctione_g: The external loop eliminates all cumulative mechanical errors in real time

Key Conclusion

A single-loop system with only a motor encoder can merely correct electrical errors, yet mechanical transmission errors cannot be eliminated — this is the core bottleneck restricting the positioning accuracy of ordinary servo systems. The dual-loop system eliminates all mechanical errors at the load terminal via the external grating scale loop.

3. Foundation of Precision: Coupling Coaxiality Installation Specifications

Mechanical assembly deviations will directly amplify positioning errors. The standard allowable deviations for coupling installation are defined as follows:

Radial Deviation$$\Delta K_r: ≤0.03mm (≤0.02mm for ideal working conditions)
Angular Deviation$$\Delta K_a: ≤0.05°
Axial Deviation$$\Delta K_z: ≤0.1mm

Assembly Tips

Clean the motor shaft and lead screw hole thoroughly before installation to remove burrs and iron scraps
Tighten the coupling with standard torque to avoid extrusion deformation from over-tightening or slipping from under-tightening
Regularly recheck coaxiality, as high-speed long-term operation will cause offset and require timely calibration

4. Four Core Advantages of Grating + Encoder Dual-Loop System

4.1 Ultra-High Terminal Positioning Accuracy

The grating scale directly measures the true displacement of the worktable, bypassing the entire transmission chain of lead screws and couplings. It supports micron and nanometer-level positioning, perfectly matching high-precision scenarios such as optical inspection, semiconductor processing, and precision grinding machines.

4.2 Uncompromised High-Speed Dynamic Response

The high-bandwidth performance of the internal motor encoder loop is fully retained. The system maintains sufficient dynamic rigidity during high-speed acceleration, deceleration and reciprocating motion, avoiding response delay caused by external grating feedback, and balancing high speed and high precision.

4.3 Strong Anti-Interference Capability and Robust Dual-Loop Complementation

The internal loop suppresses high-frequency disturbances such as electromagnetic interference and sudden torque fluctuations of the motor
The external loop offsets low-frequency errors including mechanical clearance, structural deformation and temperature drift

Dual-loop mutual compensation ensures smooth operation without crawling or jitter at both high and low speeds.

4.4 Real-Time On-Line Mechanical Error Compensation

There is no need for shutdown calibration for lead screw pitch error and backlash compensation. The external grating scale continuously collects real load position data to correct cumulative transmission errors online. It effectively reduces long-term precision attenuation and equipment maintenance & calibration frequency.

5. Applicable Equipment for Dual-Loop Servo Systems

Precision CNC grinders, coordinate boring machines, gantry machining centers
Semiconductor packaging equipment, die bonders, probe stations, optical inspection platforms
3C industry precision dispensing, visual alignment and laser cutting equipment
Coordinate measuring machines (CMMs) and high-precision linear motion modules

For low-precision equipment such as ordinary conveyor lines and general spindles, a single-loop system is sufficient without additional grating scale costs.

Summary

A single-loop servo system only controls the motor shaft, while a dual-loop servo system precisely controls the workpiece. The internal loop guarantees dynamic performance, and the external loop ensures positioning accuracy. It eliminates full-link positioning errors from electrical control to mechanical assembly, serving as the standard configuration for high-end precision automation equipment. If your equipment suffers from lead screw backlash or poor repeat positioning accuracy, the grating + encoder dual-loop architecture is the most cost-effective upgrade solution.

Interaction

Do you encounter problems such as lead screw clearance or repeated positioning deviation in your equipment? Welcome to leave a comment and share your servo debugging experience!