FOXBORO Positioner SRD998 Series - HAD0-B0S-1SA07-A1

The SRD998 intelligent positioner works by receiving a control signal (typically a 4-20 mA HART signal) and converting it into pneumatic pressure to control a valve actuator. It compares the desired valve position (setpoint) to the actual position, measures the difference, and uses a digital controller and an electro-pneumatic (I/P) module to adjust the air pressure to the actuator, moving the valve until it reaches the setpoint. This creates a closed-loop control system, allowing for precise control of both linear and rotary actuators.

Working principle breakdown

• Control input: The positioner takes an electrical input signal (e.g., 4-20 mA) from a control system or controller.
• Position feedback: A mechanism, such as a lever, measures the actual position of the valve actuator and sends this information to the positioner's internal electronics.
• Signal conversion: The positioner's analog-to-digital (A/D) converter transforms the input signal and the feedback position into a digital format for the digital controller.
• Digital control: The digital controller compares the setpoint signal with the actual position feedback. It then calculates the required adjustment.
• Pneumatic output: The controller's output drives an electro-pneumatic (I/P) converter, which precisely regulates the air pressure to the actuator via an integrated pneumatic amplifier.
• Actuator movement: This pneumatic pressure moves the valve until its actual position matches the desired setpoint.
• Self-calibration: Upon startup, the SRD998 performs an "auto-start" function to calibrate itself to the specific actuator it is connected to.

  Core operating principles

• Command signal: The positioner receives a 4–20 mA electrical current signal from a control system, which represents the desired valve position (e.g., 4 mA is 0% open, 20 mA is 100% open). A digital controller inside the SRD998 processes this signal.
• Position feedback: A position sensor, typically a potentiometer, is mechanically linked to the valve stem or shaft. This sensor provides continuous feedback on the actuator's actual position.
• Electro-pneumatic conversion: The digital controller drives an electro-pneumatic converter, also known as an I/P (current-to-pressure) module, based on the control signal. This module converts the electrical signal into a corresponding pneumatic (air pressure) output. Foxboro's Flapper-Nozzle technology is central to this conversion process, ensuring stable and reliable control.
• Pneumatic amplification: The output from the I/P module controls a pre-amplifier and a pneumatic power amplifier. This amplifies the air pressure to a level sufficient to drive the valve's actuator.
• Actuator movement: The amplified air pressure moves the valve actuator, which in turn adjusts the position of the valve. The SRD998 can be configured for single-acting actuators (using one air output) or double-acting actuators (using two air outputs).
• Digital control loop: This entire process operates as a high-speed, closed-loop control system. The digital controller constantly compares the electrical input (setpoint) with the electrical feedback from the position sensor. It then issues precise commands to the I/P converter to eliminate any error and hold the valve in the exact setpoint position.

Key intelligent features

• Automatic startup (Autostart): A key feature of the SRD998 is its ability to automatically calibrate itself. During autostart, it identifies the mechanical end positions and other actuator parameters, and then optimizes its own control settings for the specific valve.
• Advanced diagnostics: The positioner includes extensive self-diagnostic capabilities. It monitors internal functions and valve performance, detecting potential problems early.
• HART communication: The SRD998 uses the HART protocol, which superimposes digital information on the 4–20 mA analog signal. This allows for remote configuration, monitoring, and diagnostics from a central control system.
• User interface: A local graphical LCD and rotary selector allow for easy local configuration and troubleshooting.

 Full Datasheet

Positioner Transmitter Feature  

• Manufacturer: SRD998
• Product No. :  HAD0-B0S-1SA07-A1
• SRD998 - Intelligent Positioner
• Communication: H - HART
• Diagnostics Performance: AD - Advanced Diagnostics
• Electrical In-Out Option: 0 - No In-Out Option
• Version: B - Single Acting
• Pneumatic Performance: 0 - Standard Pneumatic (6 bar max air supply)
• Fail in case of loss of power: S - Fail safe
• Housing: 1 -  Aluminum
• Mounting: S - Standard
• Pneumatic Connection: A - 1/4 NPT
• Gauges: 0 - Without gauge
• Electrical Connection: 7 - M20x1.5
• Electrical Connection: A1- ATEX/IECEx: II 2G Ex ia IIC T6...T4 Gb; II 1D Ex ia IIIC (c) or IIIB (d) T100°C Da

There are several Model of positioner, for more details read the PDF below 

Basic Ordering Information