Valve positioners and position transmitters are both devices used in control systems to monitor and control the position of valves, particularly in industrial applications. While they serve similar functions, there are some key differences between the two:

Valve Positioner:

1. Function:

   • A valve positioner is a device that is directly attached to a control valve actuator. Its primary function is to precisely control the position of the valve plug or disc based on an input signal from the control system.
   • Valve positioners ensure that the valve reaches the desired position accurately and maintains that position even in the presence of external factors such as variations in supply pressure, friction, or load changes.

2. Operation:

   • Valve positioners receive a control signal from the control system, typically in the form of a pneumatic or electronic signal. Based on this signal, the positioner adjusts the air pressure supplied to the actuator, thereby positioning the valve accordingly.
   • Valve positioners may incorporate feedback mechanisms such as limit switches or position sensors to provide feedback on the actual valve position to the control system.

3. Features:

   • Valve positioners often include features such as calibration adjustments, zero and span adjustments, response time adjustments, and diagnostic capabilities to ensure optimal valve performance and reliability.
   • They may also include additional functionality such as split-range operation for controlling multiple valves simultaneously.

Position Transmitter:

1. Function:

   • A position transmitter, also known as a valve position transmitter or valve position indicator, is a device used to monitor and transmit the position of a valve to the control system.
   • Its primary function is to provide feedback to the control system regarding the actual position of the valve, allowing the control system to verify the position of the valve and make adjustments as necessary.

2. Operation:

   • Position transmitters typically consist of a sensor or mechanism that is mechanically coupled to the valve stem or actuator. As the valve moves, the position transmitter detects the movement and converts it into an electronic signal.
   • This electronic signal is then transmitted to the control system, where it is used to monitor the valve position and provide feedback for control purposes.

3. Features:

   • Position transmitters may include features such as analog or digital output signals, position indication displays, and diagnostic capabilities to ensure accurate and reliable transmission of valve position information to the control system.
   • They are often used in conjunction with valve positioners to provide feedback on the actual position of the valve and verify that it is in the desired position as commanded by the control system.

In summary, while both valve positioners and position transmitters are used to monitor and control the position of valves in industrial applications, valve positioners actively control the position of the valve based on input signals, while position transmitters provide feedback on the actual position of the valve to the control system for monitoring and verification purposes.