Stepper-driven linear actuator modules hold 3–5% of one step as noncumulative step-angle accuracy in typical industrial builds, an open-loop positioning band most OEMs will spec without an external encoder [S1].
Servo-driven linear actuator assemblies with integrated linear encoders reach 0.05 µm minimum incremental motion at 4 nm sensor resolution, with catalog velocities from 0.75 to 1.5 mm/sec and push/pull forces of 40–70 N, an order of magnitude tighter than open-loop stepper [S2].
Stepper Open-Loop Behaviour: How the Accuracy Band Forms
Stepper motors move in discrete pulses from a motion controller or PLC, so position is set by counting pulses rather than reading back the carriage [S6]. Tolomatic's published tolerance window, 3 to 5 percent of one step noncumulative, is the per-step electrical position error after a move; the figure is noncumulative, so a 200-step move does not stack 200 individual errors [S1].
The mechanical side matters as much as the electrical side. A 1.8° stepper on a 5 mm lead screw with 1.8°/step produces roughly 25 µm per step at the carriage; a 0.9°/step motor on a 2 mm lead screw drops the same number to 5 µm per step [S6]. Holding torque is the limiter: once external load approaches roughly 60–70% of rated holding torque, the rotor can lose synchronisation and the move ends short of commanded position, with no error flag unless a separate encoder is added.
Open-loop stepper is also the cheaper path. Both motor and drive are less expensive than the equivalent servo pair, no feedback cable is required, and commissioning is mostly a current/tune exercise. For point-to-point moves under about 1 m of travel with moderate loads and cycle times that do not demand aggressive acceleration, this is the configuration most integrators will reach for first [S4] (2023-01).
Servo Closed-Loop Behaviour: Where the Extra Micron Comes From
Servo systems use closed-loop control, constantly adjusting to maintain precise position, speed, and torque against load disturbances [S5]. The improvement over open-loop stepper is not the motor itself, it is the feedback loop and where the feedback device is mounted [S3].
PI's published precision actuator families show what closed-loop with a linear encoder actually buys: 0.05 µm minimum incremental motion at 4 nm sensor resolution, 0.75 to 1.2 mm/sec max velocity, 70 N push/pull, with 10, 25, and 50 mm travel options on recirculating ball-screw frames [S2]. A recirculating ball screw reduces sliding friction to rolling friction, so the same motor can hold higher duty cycles and velocities with less heat; the trade is lower power-off holding force, so a fail-safe brake or back-driven check is sometimes required [S2].
Feedback device location is the under-appreciated variable. Kyntronics' engineering note points out that motor-mounted rotary encoders measure the screw, not the nut; backlash, screw windup, and thermal growth sit between the motor and the actual carriage, so the reported position can be off by tens of microns even on a 'closed-loop' system [S3]. Rod-end mounted feedback reads the carriage directly and is the configuration to specify when sub-10 µm repeatability is on the print.
Decision Matrix: Stepper vs Servo on Four Spec Criteria
Lining the two options up against the four criteria that drive most selection meetings:
<b>Positioning accuracy (one-step, no load):</b> Stepper 3–5% of one step noncumulative [S1]; servo with linear encoder 0.05 µm minimum incremental motion, 4 nm sensor resolution [S2]. The servo figure is roughly two orders of magnitude tighter, but only when the feedback is mounted at the rod end and not at the motor [S3].
<b>Repeatability under load disturbance:</b> Stepper is open-loop, so a load transient that pushes the rotor out of synchronisation ends the move early with no error signal [S6]. Servo with closed-loop current and position loops corrects within the controller bandwidth, typically 1–2 kHz current loop on a modern digital drive, so a torque disturbance shows up as a transient position error that the loop nulls out within a few milliseconds [S5].
<b>Speed and acceleration:</b> Stepper loses torque above roughly 1,000 rpm, and usable acceleration is limited by the risk of stalls; servo maintains torque across the speed range up to the drive bus limit, so cycle times on indexing moves drop measurably when the move exceeds a few hundred milliseconds of stepper time [S4] (2023-01).
<b>System cost and integration effort:</b> Stepper is the lower-cost path: cheaper motor, cheaper drive, no feedback cable, simpler wiring. Servo adds the drive, the encoder, and typically a tuning step, but it removes the brake, the oversized safety margin on the motor, and the risk of a mid-cycle stall that jams the machine [S4] (2023-01).
Where Each Architecture Fits, and Where It Doesn't
Stepper is the right answer when the application is open-loop-friendly: light to moderate load, modest acceleration, predictable moves, no requirement for sub-10 µm repeatability, and budget pressure. A typical fit is indexing a packaging gate, a label dispenser, or a pick-and-place slide under about 1 m of travel [S4] (2023-01).
Servo is the right answer when the print calls for sub-10 µm repeatability, dynamic correction under varying load, higher speeds over longer travels, or any safety-relevant motion where a missed step would create a hazard. Common fits are semiconductor handling, precision dispensing, and machine-tool axis retrofit where a pneumatic cylinder is being replaced by an electric actuator for repeatability and data logging [S1].
Neither architecture is the right answer for everything. Stepper is wrong for high-inertia loads that swing close to the motor's pull-out curve, and the failure mode there is silent, no alarm, just a part out of position. Servo is wrong for a simple two-position clamp where the cost and tuning effort cannot be justified; a solenoid or pneumatic cylinder is the better fit. The middle ground, closed-loop stepper with an external encoder, exists in the market and sits between the two on both performance and cost, but it is a stepper drive, not a servo, and the loop bandwidth and tuning tools are not equivalent.
Standards, Sourcing, and Spec Discipline
There is no single IEC or ISO standard that dictates stepper-versus-servo selection for linear actuators; the choice is driven by application specs (resolution, repeatability, speed, force, duty cycle) and the machine's functional safety requirements under ISO 13849 or IEC 61508, which govern the safety integrity of the drive, not the motor type. [S2]
On the sourcing side, the same vendor often sells both architectures, so cross-checking datasheets is the fastest way to keep the comparison honest. Tolomatic, PI, and Kyntronics all publish both stepper and servo actuator lines with stated resolution, repeatability, encoder type, and feedback location [S1][S2][S3]. When the print calls for a specific repeatability number, confirm whether the datasheet value is at the rod end or at the motor shaft; the difference is real and routinely missed on first-pass reviews [S3].
The drive side matters as much as the motor side. A modern digital servo drive running field-oriented control at 8–16 kHz current loop will hold a tighter dynamic position than an older analogue drive on the same motor, and a microstepping stepper drive running 256 microsteps/step is a different machine than a full-step drive on the same motor. Specifying the drive behaviour is part of specifying the actuator.
Verdict and What to Track Next
For point-to-point moves under 1 m with moderate load and cost pressure, start with stepper and budget for 3–5% of one step noncumulative as the achievable accuracy band [S1]. For sub-10 µm repeatability, dynamic load rejection, or higher speeds, specify servo with a rod-end linear encoder and budget for the closed-loop tuning step [S3].
Two signals to watch: the roll-out of integrated closed-loop stepper modules with on-board encoders, which is closing the cost gap for mid-tier applications; and the migration of digital servo drives into smaller form factors, which is pulling the servo price band down on smaller actuators. Either shift can re-rank the decision matrix on a 2027 retrofit.
Related analysis: Heat alarm vs heat detector: sensing element, setpoints, and where each fits.