Source: World Instrument & Automation, December 1997
Since the mid-1980s, having successfully developed high-performance, high-pressure, anti-cavitation, low-noise, wear- and corrosion-resistant special products, foreign actuators entered a new technological contest to meet control systems drawing on computer networking.
Field instruments, transmitters and actuators under the traditional 4–20 mA analogue signal face a new challenge — adapting to the fieldbus control model with two-way digital communication. Actuators must therefore fundamentally change their control model and functions: digital, intelligent, communicating.
The major foreign control-valve and actuator makers began digital/smart product development in the mid-1980s: earliest was VALTEK (USA), followed by ROTORK (UK), Siemens, SAMSON and SENSYCON (Germany) and FISHER (USA). Siemens' SIPART PS smart positioner and ROTORK's IQ actuator were first to market. China has used small quantities in power and petrochemical projects since 1995 with good results and a trend to wider adoption.
Fieldbus is a fully digital, multivariable, bidirectional, multi-drop communication system between field instruments and control systems, replacing the 4–20 mA analogue signal — making acquisition, transmission and data conversion more efficient and convenient. As a fully open bus standard it concerns every major controls company; yet because of direct interests in compatibility with existing technology, no unified fieldbus standard has formed — though the development of control technology makes one imperative.
HART is a transitional technology born of that standoff, developed by Rosemount: Highway Addressable Remote Transducer. Using FSK (frequency shift keying) it superimposes a frequency signal on the analogue signal (Fig. 1), letting analogue and digital two-way communication proceed simultaneously without interference; it can also communicate fully digitally over one twisted pair supporting multidrop networks of up to 15 devices — today the most influential smart-instrument protocol. Fisher-Rosemount also offers a handheld communicator for HART instruments (communication and hand operation); its typical integration with DCS is shown in Fig. 2.
With simple, reliable, intrinsically safe and easily maintained construction, pneumatically actuated control valves have remained the mainstream. Even as electric control instruments replaced pneumatic ones from the 1970s, pneumatic valves persisted via electric-pneumatic conversion, continually improving. But that conversion technology — force/position balance on the nozzle-flapper structure — could never break beyond analogue control into digital, microprocessor-based intelligence. The key to smart pneumatic valves is therefore fundamentally redesigning the positioner from classic mechanics to electronics.
Of the options in Fig. 3, scheme d is the most effective: it preserves the positioner's useful functions, improves valve performance, and technologically accommodates all actuator and valve types. Developing a few typical variants by explosion-proof or not, long or short travel, linear or quarter-turn upgrades existing pneumatic valves into a new smart generation; further simplification of the smart positioner and valve redesign (e.g. a plug profile optimised for erosion resistance and low flow resistance, with flow characteristics realised by the positioner's intelligence) improve structural stability, vibration resistance, response speed, seat sealing pressure and torque protection.
Siemens' SIPART PS smart positioner is a successful implementation of exactly this thinking (Fig. 4). Its key breakthroughs:
Functional modularisation gives the product multiple features:
Electric actuators are also widely used, especially in power and metallurgy. Being mains-powered, digitalisation is easier than for pneumatic valves. As early as the 1980s products existed — microprocessor servo amplifiers, high-precision stepper actuators — but they only added control functions without bus networking.
ROTORK's IQ intelligent actuator achieved that goal with multiple advanced technologies (Fig. 5). Its key breakthroughs and features:
As automation technology advances, demands for efficient, reliable, safe systems grow, and actuators matter ever more. Far-sighted people at home and abroad watch actuator development closely, investing more people and money to accelerate it. Given actuators' special place in control systems, intelligence and communication are necessary — but more important is developing more control valves for every field demand: high temperature, high pressure, corrosion resistance, radiation resistance, fire safety, tight shut-off, wear and erosion resistance, low noise — improving their structures toward instrumentisation, generalisation, multifunction and modularisation; raising reliability, life and operability; and fully valuing actuator selection and application technology. Internationally, IEC has begun standards for flow-characteristic selection, while noise and flow-coefficient calculation standards, accepted after years of effort and implemented in software, form the common basis of international exchange and trade. To speed product development, test technology has advanced too — test facilities and techniques are now a key competitive weapon and a prerequisite for verification standards. American firms have long led control-valve technology precisely because of their excellent development and test facilities.
In sum, actuator technology faces challenges from every new technology. As a basic element, today's new techniques, processes and materials also provide better conditions and foundations for its progress. The actuator industry has decades of accumulated technology and a good environment. The author believes the 1990s will be another flourishing period for actuator technology. (End)
