Which Motor for a Motion Stage, and How to Read the Numbers?

A motion stage needs the carriage to travel a distance along one axis and stop near the command. The motor provides the force; the screw, ball screw, or belt that turns rotation into linear motion is transmission, not another motor; a linear motor applies force directly to the carriage with no screw in between.

This article answers one question: which motor for a motion stage, and how to read the numbers? The answer: first separate five quantities (command resolution, feedback resolution, minimum increment MIM, bidirectional repeatability, positioning accuracy), then choose the motor family (stepper, brushed DC, brushless DC, linear motor, piezo, voice coil); the motor’s no-load angular spec, the whole stage’s positioning spec, and the workpiece dimensional tolerance are three different quantities, and each stage catalog’s test and load conditions must be read series by series.

Figure 1 | Generic ball-screw configuration (not a specific product cross-section). The nut is fixed to the carriage and does not rotate with the screw; the optional encoder measures the motor shaft and cannot read the lead error, backlash, or elastic deformation behind the screw. Click for full size.

1. Five quantities: resolution, minimum increment, and positioning accuracy

Newport’s UTS user manual separates MIM from resolution and defines accuracy as the positioning error after linear-error compensation — it cannot be read directly as uncompensated absolute measurement accuracy^[10]. This article keeps the vendors’ ± notation: 0.08 μm peak-to-peak can be written ±0.04 μm; do not multiply or divide again.

Steppers often run open loop, estimating position from issued pulse counts; when overload or excessive acceleration causes missed steps, the estimate no longer represents the actual position. An added encoder may be monitor-only or part of a closed loop — it depends on the driver and wiring.

Figure 2 | Six drive principles (tooth counts, phase counts, and dimensions are schematic). The piezo stack is a direct-elongation actuator, not a friction-stepping piezo motor. Click for full size.

2. Steppers and ball screws: step angle and lead conversion

Oriental Motor’s 2-phase PKP/PK has base step angles of 1.8° (standard) / 0.9° (high resolution); the 5-phase PKP/PK is 0.72° / 0.36°^[1][2]. Stopping accuracy (catalog): the 5-phase 0.72° type is ±0.05°; the US site lists ±0.034° (no load) for the 0.36° type; the RKⅡ combination’s ±0.05° is noted as no-load^[3][5]. You cannot write ±0.05° for all high-resolution types — model restrictions must be checked one by one.

Direct-coupled, no gearbox, ignoring screw and coupling error first, the angle conversion is Δx = lead L × Δθ / 360° (millimeter lead × 1000 gives micrometers):

The RKⅡ page’s 10 mm lead example converts ±0.05° to ±1.4 μm^[3] — this only estimates the motor angular error’s contribution to linear position, not the guaranteed accuracy of the whole screw stage. With the 0.36° PKP’s ±0.034°, the same lead converts to about ±0.94 μm.

A ball screw must be specified by vendor, shaft diameter, lead, and accuracy grade. HIWIN external-circulation lists shaft 8–100 mm, lead 2.5–60 mm (not every shaft has every lead); in-stock forged C7 is marked 50 μm/300 mm^[7][8]. THK’s lead accuracy per JIS B 1192 (ISO 3408): C3 positioning example (effective thread ≤100 mm) representative running-distance error 8 μm, 8 μm variation per 300 mm; C5 at the same length 18 μm; C7 forged example ±50 μm/300 mm; C10 ±210 μm/300 mm^[6]. Shortening the lead only shrinks the motor angular error’s conversion — it does not eliminate the screw’s own lead error.

Figure 3 | Generic coreless configuration with fixed magnet track and moving coils (not an XMS product cross-section). The scale and read head measure the carriage’s displacement relative to the base. Click for full size.

3. DC servo and linear motors

Newport’s UTS series offers two drives in the same size for comparison^[9]:

The UTS-CC’s rotary encoder is mounted directly on the screw (the motor drives the screw through an 11:4 belt reduction); the UTS-PP drives the screw through a coupling and positions by commanded steps^[9]. The UTS-CC’s maximum speed drops to 20 mm/s when the axial load exceeds 1 kg. This DC version is faster with tighter bidirectional repeatability — but you cannot generalize that every DC motor beats every stepper. The UTS manual lists a 20 ±1 °C test environment, and meeting the specs also requires a 5 μm-flat mounting plane^[10].

A linear motor is a motor unrolled along the travel: a magnet array and coils move linearly against each other, force is proportional to current, and the carriage skips the screw. Newport’s guaranteed values^[11][12][13]:

Conditions cannot be omitted: the XM’s 1 nm MIM refers to the XPS-DRV11 drive configuration; the manual requires a controlled environment and a 5 μm-flat mounting surface^[11]; the IMS-LM’s 1000 mm/s is the no-load, XPS-EDBL-driver condition — switching to XPS-DRV11 caps it at 500 mm/s^[13]. Coreless movers avoid cogging from iron teeth; cored types must handle attraction and cogging effects. Direct drive removes the screw error, but guide-rail friction, scale error, Abbe error, mounting deformation, and temperature still need evaluation.

4. Piezo, voice coil, and selection

Piezo stacks use the ceramic’s direct elongation with flexure guidance; the deformation of multilayer ceramics adds mechanically; travel is limited by stack length and mechanism amplification^[14]. Piezo motors convert periodic deformation into continuous motion through stepping, inertia, or ultrasonic friction — the stepping type’s total travel is set by the drive-rod length, not by a single ceramic deformation, and it holds position without power^[15]. Voice coils take force from a coil in an air gap; in the linear range F ≈ Kf·I; useful travel is set by the magnetic circuit and coil geometry, and holding a sustained load usually requires power^[15].

Select by travel, load, and the band you must hold (still per series specs):

• Pulse positioning, simple structure: stepper. Check step angle, torque-speed curve, and missed-step risk first, then decide whether closed loop is needed.
• Same size, faster, tighter bidirectional repeatability, shaft-encoder closed loop: brushed DC (UTS: 40 mm/s, ±1.75 μm vs. stepper 20 mm/s, ±3.0 μm).
• Electronic commutation, no brush maintenance, still on a screw: brushless DC. Torque-speed and micron numbers per that series.
• Higher speed, no screw transmission, linear scale reading the carriage: linear motor. Short-travel coreless XMS guaranteed ±0.75 μm; long-travel cored IMS-LM several micrometers.
• Small-range fast fine adjustment: piezo stack with flexure guidance; longer travel with power-off holding: stepping piezo motor; smooth force control and fast focusing: voice coil.

When selecting a stage, the acceptance spec should list bidirectional repeatability and guaranteed positioning accuracy with the travel noted; minimum increment and encoder counts describe control and sensing capability, while part dimensional tolerance is accepted against the drawing and measurement results. List the full part number, travel, load direction/center of gravity, speed and acceleration, driver, feedback position, test temperature, and compensation status together to get comparable accuracy conditions — the maximum in each column does not mean they are all achievable at once.

Back to the opening question: choosing a stage motor starts with the band you must hold (micrometer class on screw stages, sub-micrometer on linear motors or piezo), then read the catalog’s guaranteed values and test conditions. Scope: the stepper numbers here are from Oriental Motor, the servo and linear-stage examples from Newport — all catalog values for specific models. Unresolved limits: a finer motor does not automatically upgrade the workpiece — a 0.36° full step at a 10 mm lead is 10 μm, and the ±0.05° conversion of ±1.39 μm is one error component only; stage acceptance reads the whole-stage spec, workpiece acceptance reads the drawing and measurement results (see “What Do the 10 μm and 1 μm on a Drawing Actually Measure?”).