Both machines may specify very small position errors, but they do not ask the motor to do the same job. In the first machine, settling time may control throughput. In the second, velocity ripple during the scan may directly affect image quality.
HansMotor's AOI motion system guide makes this distinction explicit because step and settle motion and scan on the fly motion lead to different acceptance criteria. That is a better starting point than asking which motor type is generally more accurate.
Why Ironless Motors Move Smoothly
An ironless linear motor has no laminated iron teeth in the moving coil assembly. Without tooth and magnet interaction, it eliminates cogging associated with slotted iron core construction. It also avoids the large normal magnetic attraction force found in iron core motors.
These characteristics are valuable when the axis needs smooth low speed motion, stable scanning or low disturbance forces. The moving coil can also be light, which helps high acceleration and short settle times when the rest of the stage is equally stiff.
Kollmorgen's direct drive linear motor selection guide describes ironless designs as having zero cogging and no attractive force between the coil assembly and magnet way, which is why they are often selected for very smooth constant velocity motion.
One important qualification is that zero cogging does not mean zero force ripple. Commutation error, current measurement error, magnet harmonics, encoder error and servo behavior can still create disturbances. An ironless motor removes one major mechanical source of periodic force, but it does not remove every source of motion error.
Why Iron Core Motors Remain Attractive
Iron core linear motors use a laminated steel structure to concentrate magnetic flux. That usually gives higher force density and strong continuous force capability within a compact motor envelope.
This makes iron core designs attractive for machine tools, high payload automation, large gantries and process axes that need substantial sustained force. The tradeoff is normal magnetic attraction and some level of cogging or force ripple, even though modern motor designs and control techniques can reduce those effects significantly.
The normal attraction does not move the payload in the travel direction, but the guideway has to carry it continuously. The bearings, preload and supporting structure therefore need to be sized with the motor rather than after it.
Positioning Accuracy Alone Does Not Decide
It is common to assume that a high accuracy stage must use an ironless motor. That is not generally true. Closed loop positioning accuracy depends on the feedback system, mechanical geometry, thermal stability, calibration and control architecture as well as the motor.
An iron core motor can be used in a high accuracy positioning system if its disturbance forces are small enough for the process and the servo system can control them. Conversely, an ironless motor with a poor encoder, flexible structure or unstable thermal environment will not become accurate simply because the coil contains no iron.
The better question is whether motor generated disturbance is significant in the process error budget.
Scanning Changes the Decision
Continuous scanning makes smooth force production more valuable. In a line scan system, interferometric measurement or other synchronized acquisition process, small velocity errors can alter effective sample spacing or create image artifacts.
Now the absence of cogging has direct process value because one source of periodic force disturbance is removed mechanically rather than corrected later through the servo loop. This is one of the strongest reasons to consider an ironless linear motor for inspection, metrology and optical scanning.
Step and Settle Changes It Again
A step and settle machine has a different optimization target. The axis accelerates, decelerates and then waits until residual motion falls inside a defined window. Low moving mass and smooth force generation help, but structural stiffness, payload geometry, encoder noise and servo bandwidth can matter just as much.
A larger motor does not automatically shorten the cycle. Added motor mass can lower a mechanical resonance, and aggressive acceleration can excite the base or payload strongly enough to increase settling time.
This is why a complete linear motor stage should be evaluated as a system rather than by motor force alone.
Thermal Requirements Can Reverse the Choice
Ironless motors are attractive dynamically, but high continuous force in a small envelope can move the decision toward an iron core design. If the process applies sustained force or the axis runs a very high duty cycle, force density and thermal path may become more important than the absence of cogging.
Cooling can shift the boundary again. Water cooled iron core motors can provide high continuous force while reducing the amount of heat that reaches the surrounding machine structure. The right motor type is therefore also a thermal packaging decision.
Magnetic Attraction Can Become a Mechanical Cost
The magnetic attraction of an iron core motor is sometimes ignored because it does not appear in the thrust requirement. The guideway still carries it. That can influence rail size, preload, friction, structural stiffness and bearing life.
On a lightweight precision stage, the surrounding mechanical structure may have to become heavier to carry the motor's normal force. This is why motor force density and system force density are not always the same thing.
Choose Around the Measurement
The most useful selection question is what error the process actually sees. If the machine measures while moving, velocity ripple and smoothness may dominate. If it measures only after stopping, settle time and in position stability matter more. If the axis carries a heavy load or applies continuous process force, force density and thermal capacity can dominate.
There is also no requirement that every axis in a machine use the same motor topology. A high force transport axis and a precision scanning axis can have completely different optimization targets.
Do not choose an ironless linear motor because the word "precision" appears in the machine specification. Choose it when the motion profile, error budget and mechanical structure show that its low cogging, low moving mass and lack of magnetic attraction create measurable system value.
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