When selecting, commissioning, or repairing electric actuators, the jargon in the manuals often conflicts. This page organizes the 54 most-used terms by theme; definitions follow public industry terminology references and national-standard usage. Related reading: The Difference Between Electric Actuators, MOVs, and Electric Devices; The Complete Installation & Commissioning Guide.
1. Actuator: a device providing linear or rotary motion, driven by a power source and working under a control signal; its basic function is to drive a valve fully open or fully closed, while modulating types can hold any intermediate position.
2. Control valve: the terminal element of a process control system, composed of a valve body assembly and an actuator, changing the process flow rate.
3. Linear-stroke actuator: outputs linear displacement (thrust), for globe control valves and gate-type valves.
4. Quarter-turn actuator: outputs a 90° (up to 360°) rotation, for butterflies, balls, and dampers.
5. Multi-turn actuator: outputs multiple revolutions for rising-stem valves such as gates and globes.
6. Part-turn actuator: outputs up to 360° rotation (typically 90°), usually a multi-turn base plus a secondary reduction stage.
7. Integrated / split type: control unit built into the actuator body, or the servo amplifier located separately.
8. Actuator assembly: the actuator plus all related accessories forming a complete operating unit.
9. Accessories: devices mounted on the actuator to complete it — positioners, solenoid valves, limit switches.
10. Dead band: the range an input can change direction without producing an observable output change, expressed as percent of input span.
11. Backlash: temporary interruption between input and output caused by loose or worn mechanical linkage when the input reverses.
12. Hysteresis: the difference between up-stroke and down-stroke measurements at the same position.
13. Basic error: the difference between measured and theoretical characteristic, in percent of span (e.g., ≤±1.0%).
14. Dead time: the interval between the instant the input changes and the instant the output begins to respond.
15. Rise time: in step response, the interval from the output first reaching a defined low percentage (e.g., 10%) of final value to first reaching a defined high percentage (e.g., 90%).
16. Settling time: the interval from the input step until the output enters and remains within a defined tolerance band (e.g., ±2%) of its final value.
17. Overshoot: the maximum transient deviation of the output beyond its final steady value, as a percentage of the difference between the steady values.
18. Frequency response: how output amplitude and phase vary with input frequency under a sinusoidal input — a marker of actuator sensitivity.
19. Output torque: the rotational moment delivered by the actuator, in N·m.
20. Output thrust: the linear force delivered by a linear-stroke actuator, in kN.
21. Torque lever: the component (e.g., in 2SA3-type units) transmitting output torque to the torque-detection mechanism.
22. Travel cut-off: automatic stop upon reaching the set open/close limit position.
23. Torque cut-off: automatic stop when output torque reaches the set value, protecting valve and actuator.
24. Mechanical limit: a device restricting output rotation or travel mechanically.
25. Hand/auto changeover: the clutch mechanism between hand-jacking and electric operation.
26. Worm-gear self-locking: the reverse self-locking property of worm gearing — with power lost, media pressure cannot back-drive the valve.
27. Air-to-close (FC): on loss of air, the plug moves to closed.
28. Air-to-open (FO): on loss of air, the plug moves to open.
29. Loss-of-air safe / lock-up (FL): on loss of the power supply, the valve holds its last position — also a process-protecting safe state.
30. Fail-safe: the design principle that on energy or signal loss the valve moves to the safest position for the process.
31. Double-acting actuator: an actuator providing power in either direction (no spring return).
32. Spring setting range: the adjustment range of the actuator spring, used to balance the actual process force.
33. 4–20 mA: the standard analog command/feedback signal — 4 mA full close, 20 mA full open (0–100% travel).
34. DO / AO: discrete / analog outputs — the two command forms from the DCS.
35. Closed control loop: an arrangement where process-variable information is continuously fed back to the controller setpoint for automatic correction.
36. Positioner: an accessory receiving the modulating command and precisely positioning the actuator.
37. Solenoid valve: an electrical accessory switching the air path of pneumatic actuators.
38. Limit switch: a contact accessory indicating valve open/close position.
39. Non-intrusive: commissioning and setting without removing covers (via infrared setter or local menus).
40. Infrared setter: a handheld tool for commissioning, operating, and browsing parameters over an infrared link.
41. Hall sensor (single-turn absolute): a contactless sensor for precise positioning and travel detection.
42. Phase-sequence adaptive: automatic correction of three-phase supply phase order, with freely settable rotation direction.
43. Loss-of-phase protection: stops and alarms on supply or output phase loss.
44. Fault self-diagnostics: automatic detection, logging, and alarm of internal faults.
45. Protection class (IP): enclosure dust/water ingress rating, e.g., IP67, IP68.
46. Ex marking: flameproof/increased-safety type with gas group and temperature class, e.g., Ex dⅡBT4.
47. Duty classes S2/S4/S5: motor duty — short-time (S2-10 min), intermittent periodic, and intermittent with electric braking (S5-10%, 630 starts/hour).
48. SIL 2/3: safety integrity levels per IEC 61508/61511 functional-safety certification.
49. Class 1E and K1/K2/K3: nuclear electrical-equipment qualification levels and nuclear-safety classes (inside/outside containment, function during accidents).
50. Effective area: the diaphragm area effectively producing output force in a diaphragm actuator.
51. 2SA3 / 2SA8: model codes of the Siemens-technology lineage — 2SA30/2SA35 multi-turn on/off and modulating types, 2SA8 the intelligent multi-turn series.
52. IQ / IQT / IQM: the Rotork intelligent actuator family (IQ precision, IQT part-turn, IQM enhanced modulating).
53. FCBB / FCBA: BIFFI quarter-turn electric actuator models (the brand has been consolidated into Emerson's Bettis portfolio).
54. Machine code / pass-code: the access protection for entering parameter and maintenance menus of intelligent actuators.
