Valve Positioners: Function & Types

Basic Features of a Positioner

Pneumatically operated valves depend on a positioner to take an input signal from the process controller and convert it to valve travel. In other words, a positioner is a motion control device made to actively compare the stem position against the control signal, adjusting the pressure to the actuator diaphragm or piston until the correct stem position is achieved. A positioner can be considered as a simple closed loop/feedback control system where the valve’s stem position is the process variable (PV), the command signal to the positioner is the setpoint (SP), and the positioner’s signal to the valve actuator is the manipulated variable (MV) or output. Therefore, when a process controller sends a command signal to valve equipped with a positioner, the positioner receives the signal and applies more or less air pressure to the actuator as required in order to reach the desired stem position.

Types of Positioners

Positioners are available in three configurations:

1. Pneumatic positioners
2. Analog Current/Pressure (I/P) Positioner
3. Digital Valve Controller

Pneumatic Positioner

A pneumatic signal (normally 3-15 psig) is supplied to the positioner. The positioner translates this to a required valve position and supplies the valve actuator with the required air pressure to move the valve to the correct position.

Analog Current/Pressure (I/P) Positioner

This positioner performs the same function as the above type, but utilizes electrical current (usually 4-20 mA) instead of air as the input signal. The input electrical signal is converted to the standard pneumatic signal (3-15 psig).

Digital Valve Controller

Though this positioner works very much as the analog I/P positioner aforementioned, it differs in that the electronic signal conversion is digital rather than analog. The digital capabilities can be classified into three categories:

• Digital non-communicating – a current signal 4-20 mA is supplied to the positioner, which both powers the electronics and controls the output.
• HART – this is similar to the digital non-communicating but is also capable of two-way digital communication over the same wires used for the analog signal.
• Fieldbus – this type receives digitally based signals and positions the valve using digital electronics circuitry coupled to mechanical components. It is an all-digital control signal re-equipped with a digital controller. Most importantly, it allows two-way communication for process, valve, and instrument diagnostics.

End users prefer digital valve controllers because they offer the following advantages:

• They employ diagnostics to maintain loop performance levels. Through the distributed control systems (DCS), PC software tools, or handheld communicators, users can diagnose the health of a valve while it is in line.
• You can diagnose the health of a valve remotely via HART or Foundation fieldbus.
• On-line diagnostics enable predictive maintenance without disrupting the process.
• Reduced cost of loop commissioning, including installation and calibration.
• Enhanced process control accuracy that reduces process variability.
• Automatic calibration and configuration hence saving time as compared to conventional zero and spanning technique.

Related articles:

• The Parts and Functions of a Valve
• Key Factors to Consider in Selection of Valves
• Pneumatic Actuators
• Pressure Control Valve Operation
• Pneumatic System Components: Types & Functions
• Directional Control Valves: Function & Principle of Operation