This article discusses the advantages of fieldbus technology for electric actuators, proposes design principles and key points, and notes practical precautions.
Keywords: fieldbus; electric actuator; application
A fieldbus is a digital, serial, bidirectional, multidata-point bus between field devices and control equipment. Following unified international protocol standards, it is open, interoperable and compatible, decentralising automation functions and simplifying systems, with remote diagnosis and commissioning — raising the safety and reliability of control systems.
The electric actuator is a key field drive in process control, with travel control, torque protection, manual/electric changeover, and remote/local control. It converts the electrical command from supervisory equipment (controller, PLC, DCS) into displacement, driving valves, dampers etc. to the required position to regulate flow, air, temperature and pressure. Actuators usually work with analogue or pulse signals, point-to-point wired to the system.
Three fieldbus types suit process automation: FF (Foundation Fieldbus), PROFIBUS and WorldFIP, plus others such as MODBUS and PROCESS-NET. PROFIBUS has three compatible parts: DP (a fast, low-cost link for device-level control and distributed I/O), PA (designed for process automation, connecting transmitters and actuators on one intrinsic-safe, bus-powered line) and FMS. WorldFIP (World Factory Instrument Protocol) is widespread in instrumentation. FF arose from the ISP protocol led by Fisher-Rosemount with 80 companies and the WorldFIP protocol led by Honeywell with 150 European companies.
Given actuator construction and performance requirements, fieldbus can be used on actuators.
(1) Less auxiliary field equipment. Conventional wiring is laborious: at least 7–14 cores per device to carry parameters to the control racks, plus a local converter cabinet (motor-control cabinet) for intermediate or local control. With bus communication even redundant wiring is just two cables/four cores — no isolation, conversion or I/O modules — cutting signal cabling, cable trays and distribution cabinets, lowering cost and easing site wiring.
(2) High reliability. Bus actuators output digital signals: unlike analogue outputs, signal-conversion losses vanish, interference immunity rises, and signal quality is stable and reliable. Far less wiring and far fewer intermediate devices also mean fewer human errors and system faults.
(3) Easy operation and maintenance. Two-way communication lets operators calibrate, set parameters and configure field actuators from the control room, monitor online, diagnose faults, predict failures and perform condition-based maintenance — no site trips. Constantly monitored, the actuators are easy to use and maintain.
(4) Remote programming, operation and diagnosis. For stubborn field problems, the communication interface allows remote expert support over the network — no manufacturer site visit.
(1) General principle: the actuator's fieldbus must match the system's bus type — PROFIBUS control requires PROFIBUS actuators; a PROCESS-NET system requires compatible actuators. Different buses, different standards, mutually incompatible: only devices following the same bus requirements can communicate. The bus is a component of the whole actuator and must integrate with it without conflict. Bus-capable actuators are more advanced and convenient than non-bus ones.
(2) Key points: the function-control board, communication-interface board and system software. The function-control board is the core — its design decides how well bus features work. Different standards need different boards; for PROFIBUS the key part is the ASIC SPC-3 (a dedicated IC).
The communication-interface board is another key: each bus needs its own communication card — PROFIBUS needs a PROFIBUS-capable card; a MODBUS card implies a MODBUS bus. Users plug in the required card and the actuator joins PLC/DCS equipment supporting that protocol in different bus systems.
Besides interface boards there are the respective ports: PROFIBUS uses RS-485; PROCESS-NET uses RS-485 and RS-232. Dedicated software function blocks handle the man-machine dialogue: Siemens uses SIPPOS5 software from the SIMATIC S7-400 family on its actuators; SDA's AS actuators use SMARTMANAGER.
(1) Master-unit-to-device connection. Under bus control all actuators in a loop are daisy-chained on one screened twisted pair carrying all control and feedback information — wholly replacing the traditional 14-wire scheme. Adding an actuator needs just one twisted pair and one serial port on the supervisory PLC/DCS — no individual multicore cables, no mass of PLC I/O.
(2) Bus protection. Line failure is fatal to conventional bus control, so design strong redundancy: two screened twisted pairs, one in and one out for single-channel operation, the two wires in each cable mutually redundant for dual-channel operation. On a line fault — break, short, earth — the protection must start promptly to keep the greater system running. When an actuator or its driven equipment misbehaves, the system must identify it, report its parameters to the master, or detach the node to preserve stability. Self-diagnostics minimise repair time, raise repair quality and cut cost; for problems unsolvable on site, the bus interface supports remote technical assistance.
(3) Power quality. Bus systems are demanding on the environment; maintain supply quality and reliable bus power.
As process-control demands keep rising, actuators for process control must keep adapting. Only by meeting the requirements of fieldbus control systems (FCS) can the electric actuator fully deploy its advantages.
