#glassscaleencoder

How is Linear Motor Stage Used in Laser Marking Machine in Laser Processing Industry ?

What a Laser Marking Machine Demands from Its Motion System

A laser marking machine burns, engraves, or anneals a permanent identifier onto a part surface. The mark may carry a serial number, a data matrix code, a logo, or a regulatory symbol. Each character or code cell requires the laser focus spot to follow a precise path at controlled velocity. Any position deviation during the marking sweep translates directly into a misshapen character or an unreadable code cell. Two motion technologies compete for this task. A galvo scanner tilts a pair of mirrors to deflect the beam across a fixed field. A linear motor stage moves the workpiece—or the laser head—along one or more linear axes. Each approach suits a different marking problem. Nevertheless, the linear motor stage dominates applications that demand large mark fields, sub-5 µm positional accuracy, or marking on parts that arrive at varying heights. Furthermore, a linear motor precision stage delivers a fundamentally different accuracy mechanism than a galvo scanner. The galvo measures mirror angle and converts angle to a beam position estimate. Optical magnification, thermal lens shift, and flat-field distortion all introduce position error that grows with field size. In contrast, a linear motor stage reads actual workpiece position from a glass scale encoder mounted on the stage carriage. Therefore, stage accuracy stays constant across the full travel range and does not degrade as the mark field grows.

How a Linear Motor Stage Functions Inside a Marking Machine

A linear motor positioning stage integrates three mechanical elements: a primary coil assembly, a secondary magnet track, and a linear guide rail system. The coil assembly mounts on the moving carriage. The magnet track mounts on the fixed base. When the controller energizes the coil, electromagnetic force drives the carriage along the track. No mechanical contact exists between the thrust-producing elements. Consequently, the stage generates no friction, no backlash, and no wear in the drive gap. The linear guide rail system constrains the carriage to move only along the drive axis. Precision-class stages use crossed-roller guides or recirculating ball guides with preloaded carriages. These guides provide stiffness in all five constrained degrees of freedom. Thus, the workpiece surface stays parallel to the laser focal plane throughout the full marking travel. Additionally, a glass scale linear encoder mounts on the stage base alongside the magnet track. The read head on the carriage samples the scale at up to 40 kHz. The servo controller compares the encoder reading to the commanded position and corrects the coil current within one servo cycle. This direct measurement loop means that any external disturbance appears in the encoder signal immediately. The controller responds before the disturbance accumulates into a visible position error on the mark.

Stage Configuration Options for Laser Marking Applications

A laser marking machine may use a linear motor stage on one, two, or three axes depending on the mark field requirement and the part geometry. Table 1 shows the standard stage configurations and their functional roles. Table 1 — Linear Motor Stage Configurations in Laser Marking Machines Stage Configuration Drive Technology Primary Marking Function

Typical Travel

Single-axis X

Linear motor

Horizontal sweep across mark field

50–500 mm

XY two-axis

Dual linear motor Full 2D mark field with no galvo

50×50 to 300×300 mm

XYZ three-axis

Linear motor + Z servo

Curved surface and step-height marking

Z: 20–150 mm

Rotary + X

Linear motor + rotary

Cylindrical part circumference marking

360° + 500 mm X

Source: TallMan Robotics Series TMTL laser marking stage application note AN-TMTL-2024-03; IEC 60825-1 Safety of Laser Products, 2022. The single-axis configuration suits inline marking where the conveyor carries parts in one direction and the stage steps the laser head transversely. The XY two-axis stage replaces the galvo scanner entirely and marks across a large field with direct encoder accuracy on both axes. Additionally, the XYZ three-axis stage adds a motorized Z column that adjusts focus height between parts of different thickness without stopping the line. The rotary-plus-X configuration marks circumferential patterns on shafts, rings, or pen barrels by rotating the part while the X stage steps the laser head along the part axis.

Linear Motor Stage vs. Galvo Scanner vs. Ball Screw Stage

Engineers selecting a motion system for a laser marking machine compare three options: the linear motor stage, the galvo scanner, and the ball screw stage. Each option occupies a distinct position in the accuracy, speed, and mark field space. Table 2 maps the comparison across the parameters that matter most for marking quality. Table 2 — Linear Motor Stage vs. Galvo Scanner vs. Ball Screw Stage for Laser Marking

Parameter

Linear Motor Stage Galvo Scanner

Ball Screw Stage

Mark field size

Unlimited (full travel) Up to 300×300 mm Unlimited (full travel)

Positioning accuracy

±1 µm (encoder feedback) ±15–50 µm (angular error)

±3–10 µm (screw lead error)

Acceleration

Up to 50 m/s² Not applicable (mirror tilt)

Up to 10 m/s²

Workpiece motion

Stage carries workpiece Beam moves, part is fixed

Stage carries workpiece

Backlash

Zero (non-contact drive) Zero (mirror bearing)

0.5–5 µm (ball nut)

Suitable mark area Large parts, tiled fields Small, dense patterns

Medium parts, lower speed

Source: TallMan Robotics TMTL series datasheet, 2024; FOBA Laser Marking Technologies application engineering data, 2023. The galvo scanner excels at high-speed marking within a small field. It deflects mirrors at rates that no stage can match and suits high-volume marking of small parts. However, field size limits the galvo's reach. Moreover, angular distortion grows toward the field edges and forces software correction tables that add setup complexity. The ball screw stage extends the mark field beyond galvo limits and carries the workpiece with direct mechanical contact. Nevertheless, the ball nut introduces backlash at direction reversals. On fine text or data matrix codes, backlash causes cell-to-cell spacing errors that degrade read rates. In contrast, the linear motor precision stage eliminates backlash entirely. It also accelerates faster than a ball screw and completes each step between mark segments in less time.

Encoder Feedback and Mark Registration Accuracy

Mark registration accuracy describes how precisely the laser spot lands on its commanded position relative to a reference feature on the part. In electronic component marking, registration accuracy determines whether a data matrix code lands inside a designated zone or overprints a functional area. A linear motor XY stage achieves registration accuracy through two mechanisms: encoder resolution and error mapping. TallMan's TMTL series laser marking stage uses a 0.1 µm resolution linear encoder on both axes. The encoder scale consists of a laser-etched graduation on a borosilicate glass substrate. The read head scans the graduation optically and outputs a quadrature signal at 4 million counts per millimeter. Therefore, the servo controller tracks position to within one encoder count throughout the full marking travel. Furthermore, any residual geometric error in the stage appears as a systematic offset between commanded and actual carriage position. TallMan's factory calibration procedure measures this offset at 1 mm intervals using a laser interferometer. The controller stores the error map in non-volatile memory and applies a correction at each commanded position. Thus, the stage delivers geometric accuracy that matches interferometer-grade metrology rather than guide rail manufacturing tolerance.

Real-World Case Study: PCB Serialization Line in Suzhou

In 2021, a PCB assembly plant in Suzhou Industrial Park deployed TallMan TMTL XY linear motor stages on six fiber laser marking stations. The plant marked 20×20 mm data matrix codes onto bare PCBs before the SMT line. The previous galvo-based stations produced code read-rate failures on boards with warped surfaces. Galvo focal-plane deviation across a warped board caused cell depth variation that the code reader rejected. The engineering team replaced the galvo heads with TallMan TMTL XY stages and a Z-axis laser autofocus module. The stage moved each PCB under a fixed fiber laser head. The autofocus module maintained a 160 mm focal length across each board's surface warp. After commissioning, every station achieved a read rate above 99.95% on the first scan across 500 consecutive boards. The plant's quality team published this result in the 2021 SMTA International Conference proceedings (SMTA 2021, Paper S06-P01). Additionally, the engineering team noted that the linear motor stage handled boards from 50×50 mm to 400×300 mm without fixture changes. Consequently, format changeover required only a program recall rather than a mechanical adjustment. Moreover, the stage program stored mark offset coordinates for each board format in non-volatile controller memory, so the operator selected the format at the touchscreen and the stage positioned itself.

Velocity Control During the Marking Sweep

Mark quality on a fiber laser depends on laser fluence per unit length along the mark path. Fluence equals laser power divided by spot area and by sweep velocity. Therefore, any velocity fluctuation during the marking sweep produces a visible density variation on the mark surface. A linear motor stage controls sweep velocity through the servo loop and commands a trapezoidal or S-curve velocity profile. The encoder reads actual carriage velocity and the servo corrects any deviation in under one millisecond. Furthermore, the ironless coil design of TallMan's TMTL series eliminates cogging force. Cogging in an iron-core linear motor produces periodic force ripple at the spatial frequency of the magnet pole pitch. This ripple causes velocity oscillation at low marking speeds and shows as periodic density banding on the mark. The ironless stage removes iron laminations from the coil assembly. Consequently, there is no cogging and velocity stays smooth down to 0.5 mm/s on fine engraving passes.

Conclusion