Laser Scanning Galvanometer: From Core Principles to Advanced Control

Laser Research

Exploring diverse applications and cutting-edge developments in laser technology

01 Introduction

In precision laser technology, the laser scanning galvanometer is undoubtedly the core execution unit. From millimeter-level industrial marking to micron-level additive manufacturing, galvanometers—with their unparalleled speed and precision—have become a driving force in technological progress.

However, for most engineers engaged in laser application development, galvanometers are both familiar and mysterious.

• When laser additive manufacturing faces corner overheating, how can we eliminate it at the control level?

• When seamless processing on large workpieces is required, why does the traditional “step-and-scan” mode introduce stitching errors, and how does IFOV (Image Field of View / scan-stage synchronization) technology solve this?

• Why are small-aperture galvanometers extremely fast, while large-aperture galvanometers are relatively slow? What physical contradictions lie behind this?

This article will explore the core principles, physics, and control logic of laser scanning galvanometers:

• How is the beam movement driven? What irreconcilable conflict exists between motor inertia and optical aperture?

• What gap exists between command and execution? Does the system merely passively chase the target (tracking error), or can it actively predict the path (feedforward control)?

• How is stability ensured in microsecond-level response? Where do jitter and thermal drift originate?

• When the galvanometer collaborates with a motion stage, how are commands intelligently decomposed and scheduled? How can stitching errors be fundamentally eliminated?

 scanning galvanometer

02 Scanning Galvanometer Basics: How Does It Work?

To understand a complex system, we must return to its core. This section focuses on the basic physics and concepts of galvanometer technology.

Parts of a scanning galvanometer

1. Everything Begins with Deflection

The essence of laser scanning is precisely controlled deflection of the laser beam. Without deflection, a laser emits only a static beam, with very limited applications. Scanning transforms it into a dynamic tool—able to go wherever commanded—vastly expanding its applications in machining, marking, imaging, and more.

Several deflection technologies exist, each with pros and cons:

• Rotating polygon mirror: Extremely fast, but usually limited to periodic 1D scanning (like drum printing).

• Acousto-optic/electro-optic deflectors (AOD/EOD): Very fast (nanosecond response), but limited in deflection angle and beam aperture.

• Scanning galvanometer: Offers superior speed, precision, and flexible vector addressing. It follows computer commands to quickly and accurately direct the beam to any position on the working plane.

Laser scanner = Galvanometer + EOD/AOD

2. Oscillating Motor

At its core, a galvanometer is a special, high-precision oscillating motor. Its principle is based on Lorentz force:

• When a current-carrying coil is placed in a magnetic field, it experiences torque proportional to the current, causing rotation.

• Like an ammeter: current deflects the coil, moving the needle.

• In galvanometers, the needle is replaced by a mirror. Drive current controls the mirror’s deflection angle.

Through a precise servo control system, the mirror does not rotate continuously but stabilizes at a commanded angle. The driver board converts input voltage signals into accurate drive currents, ensuring precise mirror control.

3. Building the Scanning Plane

A typical 2D scanning head has two galvanometers with perpendicular axes:

• Laser hits the first mirror (X-axis)

• Reflected to the second mirror (Y-axis)

• Independent control of both angles directs the laser to any (X,Y) coordinate in the working plane.

This vector-based random addressing is a major advantage over raster-type polygon scanning.

4. Jumping & Marking Modes

Galvanometer movement has two modes, synchronized with laser ON/OFF:

• Jump mode (laser OFF): Fast repositioning to the next start point, at maximum speed/acceleration.

• Marking mode (laser ON): Controlled speed to meet processing requirements.

Core design contradiction:

• Speed demands: Requires strong torque + minimal inertia (small, light mirrors).

• Power demands: Requires larger mirrors to handle bigger beams without clipping, especially for high-power lasers.

These are contradictory:

• Small-aperture mirrors: Low inertia, very fast → biomedical imaging, small-spot high-dynamics.

• Large-aperture mirrors: High inertia, slower → high-power lasers for welding, cutting, 3D printing.

Thus, galvanometer selection is always a trade-off between speed and power capacity.

03 A Complete Scanning Galvanometer System

1. System Architecture

Workflow:

1. Laser source generates beam

2. Beam collimated and shaped by beam expander

3. Beam enters scanning head → deflected by X/Y mirrors

4. Beam passes through F-Theta lens → focused and linearized

5. Focused spot processes the workpiece

All controlled by software + control card, forming a closed loop from digital command to physical processing.

System diagram

2. Galvanometer Motor

Decides system dynamics. Components:

• Stator & rotor

  • Moving magnet design: Magnet on rotor, coil fixed. Low inertia → higher resonance frequency, faster response. Mainstream.

  • Moving coil design: Coil on rotor, magnet fixed. Higher torque efficiency but higher inertia → limited speed.

• Bearings: Precision preloaded ball bearings ensure rigidity, low friction, and long lifespan.

3. Mirrors

Affect optical quality and power handling.

• Aperture: Determines max beam size.

• Substrates:

  • Silicon: Good thermal conductivity, for mid/high-power IR (CO₂, fiber lasers).

  • Fused silica: Ultra-low thermal expansion, best for UV and ultrafast lasers.

• Coatings: High-reflectivity dielectric (>99.5%) at target wavelength.

4. F-Theta Lens

Without it → two fatal issues:

• Field curvature: Focused on a curved surface, not flat.

• Nonlinearity & distortion: Spot speed varies across field; square becomes pillow-shaped.

Solution: F-Theta lens

• Adds barrel distortion to cancel scanning distortion → nearly linear relationship y’≈f·θ.

• Functions:

  • Linear scanning

  • Flat-field focusing

  • Uniform spot size & energy density

Without F-Theta Lens

With F-Theta Lens: Introduces Barrel Distortion

5. Beam Expander

Used before scanning head.

• Function: Telescope system enlarges beam diameter.

• Effect: Reduces divergence → smaller focused spot, higher resolution.

• Types:

  • Keplerian (two positives, real focus, supports spatial filtering, but risk of air breakdown at high power).

  • Galilean (negative + positive lens, compact, no internal focus, most common in industry).

Beam Expander: Keplerian & Galilean Types

04 Closed-Loop Servo System

1. Why Closed-Loop?

• Open loop: Sends commands but doesn’t check results → drift, load change, interference.

• Closed loop: Compares command vs actual (feedback), corrects continuously → precision achieved.

Error = Command – Actual. Controller drives error → 0.

2. From Digital Command to Mirror Deflection

Flow:

• Design converted into (X,Y) points

• Control card sends signals (e.g., XY2-100) to driver

• Driver compares command vs feedback

• DSP calculates error → generates drive current via PID or advanced algorithms

• Current → torque → mirror moves → feedback updates

• Loop repeats thousands of times per second

Closed-Loop Control System of the Galvo Scanner

3. Position Detectors

Types:

• Optical (analog): IR LED + shadow mask on rotor → detected by photodiode.

• Capacitive (analog): Measures capacitance change with rotation.

• Digital encoder (grating): High-resolution disk read optically → digital output, highest precision, lowest drift.

4. Servo Driver

• Analog (old): Prone to noise, manual tuning, inflexible.

• Digital (modern):

  • DSP firmware runs control algorithms

  • Noise immune

  • Can use predictive feedforward → near-zero tracking error

Key difference:

• Analog = reactive, always chasing error

• Digital = predictive, plans ahead → solves corner overheating in additive manufacturing.

05 Key Performance Indicators

1. Speed

• Marking speed (mm/s or cps) → efficiency

• Jump speed → repositioning time

• Step response → stabilization time after deflection

2. Precision

• Accuracy = actual vs commanded

• Repeatability = consistency returning to same point

• Resolution = smallest deflection step

3. Stability

• Thermal drift = slow position shift due to heating

• Jitter = high-frequency oscillations at static position → poor edge quality

06 Control Architecture: Challenges & Solutions

1. Traditional Step-and-Scan

• Large stage inertia = slow

• Limited FOV = requires stitching (errors appear)

• Platform & galvanometer independent = simple sequence execution

• Control delays = trajectory distortion

Stitching Error

2. Solution 1: Distortion Correction

• Old: Lookup tables (LUT), many measured points, time-consuming

• Modern: Sparse data + physical modeling, e.g., only 25 points to model entire field

3. Solution 2: Infinite Field of View (IFOV)

• Also called galvo-stage synchronization

• Unified controller coordinates both systems

• Path decomposition:

  • Low-frequency, long moves → stage

  • High-frequency, fine details → galvo

• Error correction:

  • Predictive planning or real-time compensation

4. Solution 3: Precision Synchronization

• Direct drive (PWM): Controller bypasses drivers, talks directly to motor → reduces delay

• Position-Synchronized Output (PSO): Laser pulses triggered by distance, not time → consistent energy density, solves corner overheating

• Advanced power control: Real-time power adjustment ensures constant energy density

Goal: Stable process parameters → predictable physical results

07 Conclusion

The scanning galvanometer is no longer a simple 2D beam deflector.

• Past: Independent actuator (motor + mirror), following input signals.

• Now: Integrated subsystem with Z-axis focusing, built-in digital controllers, diagnostic data streaming.

• Future: Possible deep integration with robots, motion stages, real-time data exchange → true smart manufacturing.

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