With the rapid advance of microelectronics, computing, communication networks and mechatronics, the intelligent electric actuator appeared internationally after the 1990s — a new smart final control element with a microprocessor. Intelligence has since become the general trend of actuator development, making it essential to follow the state of the art.
An industrial process control loop consists of detector, controller, actuator and controlled object. The actuator is always indispensable, itself comprising an actuating mechanism and a regulating mechanism (usually the control valve), plus optional positioner and handwheel accessories.
Actuators are vital to process automation — often called its "hands and feet". Powered by electricity, the electric actuator receives the controller's standard signal (analogue or digital) and converts it into the corresponding mechanical displacement (rotation, linear or multi-turn) to change the manipulated variable (control valve, damper, louver opening), automatically regulating the controlled parameters — temperature, pressure, flow, level — so the process runs as required. Electric actuators therefore bear strongly on loop safety, reliability and control quality.
Against pneumatic and hydraulic actuators, electric ones have three main advantages:
Electric actuators divide into quarter-turn and linear types. The DKJ quarter-turn actuator is the execution unit of the DDZ-II instrument family and the classic traditional product: a servo amplifier plus an actuating mechanism converting the standard current signal from controllers, transmitters or manual stations into mechanical displacement.
Though widely applied across industries, DKJ actuators have shortcomings:
In recent years electric actuation has developed rapidly, with vendors at home and abroad launching intelligent actuators with fieldbus communication. The new intelligent actuator merges servo amplifier and mechanism using microcomputer and fieldbus technology, realising two-way communication, PID control, online auto-calibration, self-tuning and self-diagnosis, plus travel protection, torque protection, motor overheat protection, power-loss signal protection, local position indication and fault alarms. It operates locally or remotely, manually, and bumps between manual and automatic without disturbance.
Novel components abound: the servo amplifier uses a microprocessor — all control functions programmable — with a digital interface providing HART or fieldbus communication as a node of the FCS; some amplifiers add variable-frequency technology for better servo-motor control. Gearboxes use novel transmissions: smooth, efficient, anti-slip, low friction. Position transmitters adopt new methods — Hall sensors sensing stem travel or rotation contactlessly; special potentiometers with ball bearings and resistive films of conductive composites; or magnetoresistive non-contact angle sensors.
Intelligent actuators usually have an LCD and manual buttons for status display, configuration input and hand operation.
Compared with traditional actuators the intelligent type offers:
In sum: mechatronic integrated construction replaces combined construction; intelligent control replaces pure electronics; communication-capable replaces non-communicating; digital control replaces analogue; infrared non-contact commissioning replaces contact manual tuning.
Connected to a fieldbus, the intelligent actuator becomes a field instrument of the FCS with control, computation and communication functions besides actuation. China's new generation of fieldbus-compliant intelligent actuators is thus the inevitable direction of development; successful research and application lays a good foundation for further progress and opens broad market prospects.
