Manipulating Devices & Actuators in Process Instrumentation

The function of the manipulating device is to influence the process variable. Its main purpose is to regulate a mass or energy flow. Mass flows can be either in gaseous or liquid state e.g. steam, natural gas, fuel oil, etc.

The energy flows typically take the form of electrical energy. The energy supply can be varied discontinuously through contacts, relays or contactors or continuously by means of variable transformers, variable resistors or thyristor units.

Examples of Manipulators and Actuators commonly used in Instrumentation and Control Systems have been discussed below.

Mass Flow Manipulators

Manipulators for mass flow include: dosing pump, valve, flap and slide. They are illustrated in the Figure below:

Energy Flow, Discontinuous Manipulators

Here the manipulators may include: Contact, Relay/contactor or a Thyristor.

Continuous Variable Manipulators

Examples of the manipulators that are used here are: Variable resistance and variable transformer.

Actuators

The manipulating device is often operated by an actuator where the controller cannot operate it directly, for example, if it cannot provide enough power or where the output of the controller is in the wrong energy form for driving the manipulator. The controller then operates either a mechanical-pneumatic or electrically powered driver. For instance, the relays built into switching controllers can typically handle currents up to 5A; external contactors or solid state relays are then used to control the higher power required by the process.

Electric Actuators

These may include: Electromagnet, magnet, 3-phase motor drive, single-phase motor drive as illustrated in the figure below:

Related: Electric Solenoid Actuator – Features & Operation

Pneumatic/Hydraulic Actuators

Examples include: Diaphragm (spring loaded), Diaphragm (double-acting), piston spring-loaded, etc.

Table 1.0 an overview of controllers and their various manipulators/drivers

Controller Type	Operated Manipulators/Drivers
Continuous controllers	Thyristor unit Adjustable resistor Speed-controlled motors Valves, flaps, slides
2-State controllers	Contact Relay, contactor, solenoid valve Solid-state relay
3-state controllers	Relays
Modulating controllers	Actuating motors (AC, DC, 3-phase, etc.)

Also Read: Pneumatic Actuators