In precision laser technology, laser scanning galvos are the pivotal actuator units. From millimeter-scale industrial marking to micrometer-resolution additive manufacturing, they deliver unmatched speed and accuracy, powering technological advancements.
Yet for most laser application engineers, galvos are both familiar and enigmatic. How do we eliminate corner scorching in laser additive manufacturing via control? Why do conventional step-scan methods introduce stitch errors when processing large parts—while IFOV (mirror-platform coordination) solves them? Why do small-aperture galvos hit blazing speeds while large-aperture ones lag—what fundamental physics underlie this?
This article explores the fundamental principles and control logic behind laser scanning galvos:
Beam steering and motor-inertia trade-off
Galvos use Lorentz-force driven coils in magnetic fields to tilt mirrors. The trade-off between torque and mirror inertia dictates performance.
Command vs. execution gap
Systems don’t just passively track (feedback only); they use feedforward control to anticipate trajectories and reduce error.
Stability in microseconds
Addressing jitter and thermal drift via high-bandwidth servo loops, damping, and temperature compensation.
Coordinated motion for seamless large-area scans
Motion commands are decomposed between mirror and platform (IFOV), synchronizing to eliminate stitching errors.
How Scanning Galvos Work
Beam Deflection
Laser scanning is fundamentally beam steering. Without deflection, lasers produce static spots—a far cry from dynamic processing applications. Scanning galvos, unlike resonant polygon mirrors or AOD/EODs, offer high speed, precision, and arbitrary vector addressing under computerized control, enabling arbitrary (X, Y) positioning.
Galvo Motor Mechanics
At their core, galvos are precision swing motors. A current-carrying coil in a permanent‐magnet field experiences a torque proportional to the current—identical physics to analog meters, but instead of a pointer, a mirror rotates. Closed-loop feedback keeps the mirror at the commanded angle.
2D Scanning
A typical scan head uses two galvos with orthogonal axes. The beam reflects first on the X-mirror, then the Y-mirror, enabling full (X, Y) plane access—crucial for random vector scanning.
Jump vs Mark Motion
- Jump motion: Laser off—galvos move rapidly to the next mark start.
- Mark motion: Laser on—galvos follow controlled paths under strict acceleration/velocity control.
This highlights the acceleration vs inertia dilemma:
- Small mirrors = low inertia, high acceleration (ideal for fast, small-spot applications like biomedical imaging).
- Large mirrors = high inertia, slower response but higher power handling (welding, cutting, large-format 3D printing).
System-Level Coordination
A complete scan system includes:
- Laser source → beam expander (reduces divergence)
- X/Y galvos → F-Theta lens → focused spot
- Control card + software → closed-loop control
Galvo Motors
- Moving-magnet designs are common—they keep coils stationary, minimizing rotor inertia and boosting resonance performance.
- Moving-coil designs have better torque efficiency but increased inertia.
High-precision preloaded ball bearings ensure stiffness and longevity.
Mirrors
- Aperture: determines beam-handling capability.
- Substrate: Silicon (infrared/high-power), fused silica (UV/ultrafast).
- Coating: >99.5 % dielectric coatings for high reflectivity and low absorption.
F-Theta Lens
Corrects field curvature and ensures linear spot displacement (y′ ≈ f·θ), maintaining uniform focus and energy across the scan.
Expander Lens
Reduces divergence and beam waist to improve resolution. Keplerian (with focal point) and Galilean (compact, no focal hot spot) designs are common.