Linear Motors vs. Servo Motors – 5 Key Selection Guides

In industrial automation, "positioning accuracy" often determines whether equipment can accept orders: from bonding and dispensing to laser processing and optical inspection, errors range from tens of micrometers to units of micrometers and even nanometers. When designing linear axes, many engineers hesitate between direct-drive linear motors and servo motors paired with ball screws or belts. Which is more accurate? The answer depends on whether you prioritize resolution, repeatability, absolute positioning accuracy, dynamic tracking, or cost and maintenance. From an engineering practice perspective, we will break down the accuracy components and common error sources of both, providing design parameters and a selection calculation example to help you grasp accuracy and budget from the early stages of a project.

From resolution and repeatability to absolute positioning accuracy

1. Resolution: The smallest position increment that the system can distinguish.

• Rotary encoder to linear conversion:
  • Linear resolution = Screw lead (µm/revolution) ÷ Count per revolution (CPR).
  • Example: Lead 5 mm, encoder 20,000 CPR → 5,000 µm ÷ 20,000 = 0.25 µm/count.
• Linear encoder: Determined by the grid pitch and interpolation factor. With a grid pitch of 1 µm and an interpolation factor of 1,000, the theoretical resolution can reach 1 nm.

1. Repeatability: The discrete range of the final position when the same instruction position is reached multiple times.

• Key factors include friction, backlash, rigidity, and temperature. Direct drive systems have less transmission backlash and generally offer better repeatability.

1. Absolute positioning accuracy: the deviation between the commanded position and the measured position.

• It is affected by geometric errors (lead error, straightness), sensor accuracy, structural thermal expansion, and control compensation.
• Common error budget notation (RSS can be used for independent approximations):
  • E_total≈√(E_encoder²+E_mechanical²+E_thermal²+E_control²)

Dynamic accuracy: Error during acceleration/deceleration and constant-speed following (following error, overshoot). It is determined by servo parameters, rigidity, and driver bandwidth.

Accuracy and performance comparison

Practical judgment:

• Target repeatability ≤ ±1 µm, high dynamic tracking requirements → linear motors preferred.
• Target repeatability of 2–10 µm, cost-sensitive, high load capacity → Screw servo is a practical solution.
• Long stroke, high speed, medium precision requirements → Servo + belt is the most cost-effective.

Selection process and table lookup calculation

Objective: 300 mm stroke, repeatability ≤ ±1 µm, absolute positioning accuracy ≤ ±10 µm, maximum speed 1 m/s, load 8 kg (fixture + workpiece), level installation.

Step 1 | Breaking down accuracy requirements

• Repeatability ≤ ±1 µm → Belt conveyors are basically excluded; screw solutions require high-grade screws, preloaded double nuts + linear scale closed loop to be viable.
• Absolute positioning ≤ ±10 µm → 300 mm, equivalent stroke 33 µm/m; screw lead error and thermal expansion must be able to compensate for this level. This is relatively easy to achieve with a linear motor and a high-order linear scale.

Step 2 | Dynamic and Rigid Demands

• 1 m/s speed, general acceleration requirements (assuming 5 m/s²) → The critical speed of the screw may be approaching the limit, requiring a large-diameter, short-lead screw and a high CPR encoder.
• The workpiece weighs 8 kg, the estimated total moving mass of the machine is 15 kg, and the target acceleration is 5 m/s². Therefore, the required thrust is approximately 75 N. Adding friction and margin, we can estimate 150–200 N. Standard linear motor modules typically provide continuous thrust of 100–300 N, with higher peak values, which is sufficient.

Step 3 | Resolution and Encoder Conversion

A. Screw servo (lead 5 mm)

• 20,000 CPR → Linear resolution = 5,000 µm ÷ 20,000 = 0.25 µm
• 80,000 CPR → 0.0625 µm

*Note: Sufficient resolution does not guarantee good repeatability. Backlash and heat can widen the repeatability range.

B. Linear motor + optical scale

• A resolution of 5 nm to 10 nm is feasible with a gate pitch of 1 µm × interpolation of 200 (in practice, after controlling noise, a resolution of 50–100 nm is used to prevent jitter).

Step 4 | Error Budgeting

Assuming a target of ±10 µm (full range), estimated using RSS:

• Optical scale grade: E after laser compensation_encoder=4µm
• Geometry and Assembly: Straightness, Parallelism E_mechanical=6µm
• Thermal expansion: Aluminum base α≈23×10^-6/°C, ΔT=2°C → ΔL ≈ 300×23×10^-6×2 ≈ 13.8 µm; through temperature control and compensation pressure to Ethernet=5µm.
• Control (following error, compensating for residuals) E_control=3µm → E_total≈√(4²+6²+5²+3²The accuracy is approximately 9.5µm, but temperature control/compensation and proper assembly are required for safety.

Conclusion: With repeatability of ±1 µm and absolute accuracy of ±10 µm, linear motor direct drive is more reliable; screw servo requires high-specification parts and more compensation, and the overall risk and time are usually higher.

How to calibrate and measure

Control Adjustment

• The position/velocity loop gain should balance stability and bandwidth; linear motors can have higher bandwidth to reduce dynamic tracking error, but attention should be paid to the plateau resonance point.
• Feedforward control (velocity, acceleration FF) can reduce following errors, especially for constant velocity scanning or S-curve motion.
• Friction/backlash compensation: The screw system needs to undergo dead zone and friction model compensation, and the backlash compensation needs to be adjusted.
• Filtering: Add a notch filter to the resonant frequency band; measure the bode before proceeding, and do not blindly add filters to reduce bandwidth.

Organization/Environment

• Geometric accuracy: The straightness, parallelism, and torsional rigidity of the guide rail are directly written into the accuracy; the machining and assembly fixtures of the guide rail base must be in place.
• Thermal management: Motor and drive heat generation, ambient temperature drift; plus the difference in thermal expansion between aluminum and iron materials, compensation should be made accordingly.
• Wiring and noise: The signal quality of linear scales can be affected by the scale from hundreds of nanometers to micrometers. Pay attention to grounding, isolation and cable routing.

Measurement and verification

• Laser interferometer: Corrects positioning accuracy and pitch error, and establishes a compensation table.
• Golf cue/positioning ruler: Used for quick health checks.
• Repeatability test: Take 3σ by making N round trips from multiple points; the curve should be uniform along the journey, and the middle section should not be measured only.

When to Choose Whom? An Engineer's Decision Checklist

• Linear motor direct drive: repeatability ≤ ±1–2 µm; high dynamic tracking requirements (high-speed scanning), cleanroom, low particle count, low maintenance frequency; acceptable higher initial cost and calibration.
• Servo + ball screw: repeatability in the range of 2–10 µm; cost-effective and with high load capacity; vertical axis has self-locking advantage (does not fall when power is off); can approach higher accuracy through linear scale closed loop + compensation.
• Servo + Belt: Long stroke, high-speed handling and sorting; medium accuracy requirements (10–50 µm); lowest cost, easy maintenance.

Want to learn about precision transmission components such as ball bearings and roller screws?
Further Reading :How to Choose Between Ball Screws and Roller Screws? Three Key Indicators: Lifespan, Efficiency, and Preload