A motion stage is the mechanical system that moves a workpiece or sensor to a precise position. When selecting one, you face a wall of terms: screw, linear motor, piezo, hexapod, stepper, DC servo — each refers to something different, and mixing them up leads to a wrong choice. This article answers one question: what kinds of motion platforms exist, and how do you choose?
The answer splits into two orthogonal classification dimensions. Drive principle decides how one axis produces motion (screw-driven, linear motor, piezo, voice coil); axis configuration decides how many axes and how they combine (single-axis, orthogonal multi-axis, six-DOF hexapod). Selection is actually driven by four requirements: accuracy class, travel, speed, and load — the drive principle follows these four, not the other way around.
This article gives the classification map, how to read the five key spec quantities, and the selection decision. Later articles in this series go deep into each type: screw-driven, linear motor, piezo, voice coil, and multi-axis/hexapod.
1. Two Classification Dimensions: Drive Principle × Axis Configuration
A motion platform can be classified along two independent dimensions:
- Drive principle: how one axis produces linear motion. It sets the accuracy ceiling, speed, and load capacity.
- Axis configuration: how many axes and how they combine. It sets the degrees of freedom and the application form.
The two dimensions are not mutually exclusive. A screw-driven mechanism can be a single axis or one axis in a multi-axis system; a linear motor can be single-axis or one of the six actuators in a hexapod. So “screw-driven” and “multi-axis” describe different things, and you decide them separately when selecting.
Figure 1 | The two-dimension taxonomy map. Any stage = one drive principle + one axis configuration. Click for full size.
2. Four Single-Axis Drive Types
| Type | Principle | Typical accuracy class | Typical travel | One-line trait |
|---|---|---|---|---|
| Screw-driven | Motor rotation converted to linear by a screw | Micron class | mm to m | Most common, low cost, has screw lead error |
| Linear motor | Direct drive, no screw | Sub-micron to nanometer | mm to m | No screw error, high acceleration |
| Piezo | Piezoelectric ceramic elongation | Nanometer class | Nanometer to mm | Nanometer resolution, self-holding when powered off |
| Voice coil | Electromagnetic force F≈Kf·I | Micron class | Millimeter class | Fast, smooth, no backlash |
The accuracy classes are typical ranges, not guaranteed values; specific numbers are in each dedicated article and the datasheets. Each of the four types has its own article covering principle and numbers.
3. Axis Configuration: Single-Axis, Multi-Axis, Hexapod
- Single-axis: one axis (X, Y, or Z). The basic form — one actuator moving along one direction.
- Orthogonal multi-axis: single-axis stages combined into orthogonal multi-axis (XY, XYZ, XYθ). Each added axis requires re-evaluating error accumulation and Abbe effects.
- Hexapod (Stewart platform): six actuators support a platform in a Stewart geometry, providing six degrees of freedom (three translations + three rotations). Used where position and orientation must be controlled precisely at the same time, e.g., optical alignment and metrology.
Axis configuration does not change the single-axis drive principle — an XY stage can be screw-driven or linear-motor-driven. It decides “how many axes, how combined,” not “how one axis moves.”
4. How to Read the Five Quantities
When reading a platform datasheet, keep five quantities separate; do not substitute one for another:
| Quantity | Definition | Common misreading |
|---|---|---|
| Command resolution | How small a position command the controller can issue | For a stepper, computed from step angle and subdivision |
| Feedback resolution | How small a position change the sensor can resolve | Not equal to command resolution or measurement accuracy |
| Minimum incremental movement (MIM) | The smallest displacement the platform can consistently and reliably complete | An actually repeatable displacement, not a resolution |
| Bidirectional repeatability | How scattered the landing points are when approaching the same point from two directions | Describes repeatability, not absolute position |
| Positioning accuracy | The difference between commanded and measured position | Confirm whether it is compensated, and the peak-to-peak or ± definition |
Newport’s UTS manual separates MIM from resolution and defines accuracy as the positioning error after linear-error compensation — it cannot be taken directly as uncorrected absolute measurement accuracy[1].
5. Selection Decision: Accuracy + Travel + Speed + Load
Start the selection from the accuracy class; it narrows the drive type to one or two:
- Accuracy class: micron → screw-driven; sub-micron to nanometer → linear motor; nanometer → piezo.
- Travel: long travel (greater than 100 mm) → screw or linear motor; short travel (less than 10 mm) → piezo or voice coil.
- Speed and acceleration: high-dynamics requirement → linear motor or voice coil; low-dynamics → screw.
- Load: heavy load → screw or core-type linear motor; light load → piezo or voice coil.
The flow is: fix the accuracy class to narrow the drive type, then confirm with travel, speed, and load, and finally decide the axis configuration (single-axis, multi-axis, or hexapod).
Figure 2 | The selection decision flow. Fix the accuracy class to narrow the drive type, confirm step by step, then decide the axis configuration. Click for full size.
Example: A semiconductor inspection application requires 1 μm accuracy, 200 mm travel, moderate speed, and light load — the accuracy class points to screw-driven, and the travel and speed confirm a C5-grade ball screw with a closed loop, with an XY multi-axis configuration. If the same application instead required 10 nm accuracy and 5 mm travel, it would switch to a piezo stage.
Common Misconceptions
- “Screw = low accuracy”: Wrong. A good ball screw (C3/C5 grade) with a closed loop can reach sub-micron; the screw’s limits are lead error and backlash, not a fundamental inability.
- “Linear motor is always better than screw”: Wrong. Linear motors cost more, the mover mass affects acceleration, and the magnetic field needs shielding; for micron-class applications a screw is more economical.
- “A hexapod is a six-axis platform”: A hexapod uses six actuators to provide six degrees of freedom, not six orthogonal axes; its kinematics differ from orthogonal multi-axis.
Self-Check for the Reader
- Can you state that drive principle and axis configuration are two independent dimensions?
- Given a micron-class, 100 mm travel, low-dynamics application, which drive type would you choose?
- Of the five quantities, which two describe control and sensing capability, and which three describe actual accuracy?
Back to the opening question: what kinds of motion platforms exist, and how do you choose? The answer is two classification dimensions (drive principle × axis configuration) plus four selection requirements (accuracy, travel, speed, load). Fix the accuracy class to narrow the drive type, confirm with travel, speed, and load, then look at axis configuration. Later articles in this series go deep into the principle, numbers, and datasheet conditions of each type.
Applicable conditions and limits: the accuracy classes here are typical ranges, not guaranteed values; specific numbers must be checked in each model’s datasheet with the test conditions (temperature, load, whether compensated). The hexapod kinematics and decoupling, and the datasheet numbers of each drive type, are covered in the dedicated articles.