FOXBORO Positioner SRI986 Series - BIDS7FAANA-1

The SRI986 works on a force comparison principle to control a valve actuator. It converts an electrical input signal (0/4-20 mA) into a pneumatic output signal that moves a valve to a specific position. The process involves balancing the torque from the electrical signal with a feedback torque from the valve's position, using a force-balance mechanism with a nozzle-flapper-relay system to achieve the desired position.

Step-by-step breakdown:

1. Input signal: A current signal flows through a moving coil  in a permanent magnet.
2. Torque generation: This current creates a force on the coil, which exerts a torque on a balance arm.
3. Nozzle and back pressure: The balance arm's position determines the distance between a nozzle  and a cone. Moving them changes the back pressure in the nozzle chamber.
4. Pneumatic feedback: A Venturiconverts this back pressure into a new pneumatic pressure signal that is fed to a compensator  and an input diaphragm .
5. Equilibrium: The system seeks an equilibrium where the torque from the moving coil is equal to the torque reaction from the compensator.
6. Diaphragm and flapper: The movement of the input diaphragm  is transferred to a flapper lever which adjusts the distance between a flapper and another nozzle .
7. Amplifier and output: Changes in the nozzle back pressure act on an amplifier , which adjusts its output pressure .
8. Actuator movement: This output pressure  is sent to a pneumatic actuator, causing it to move the valve stem.
9. Position feedback: The movement of the actuator stem provides a physical feedback to the system, which is used to adjust the flapper and nozzle, completing the loop and moving the valve to the correct position. 

The core mechanics

1. Input signal conversion: The electrical input signal is sent to a moving coil situated within the magnetic field of a permanent magnet.
2. Force and torque generation: The electric current in the moving coil produces a force that creates a torque on a balance arm.
3. Pressure control: The movement of the balance arm controls the distance between a nozzle and a cone. This action alters the back pressure at the nozzle.
4. Pressure amplification: A venturi tube receives the changed back pressure and amplifies it, converting it into an output pressure signal for the actuator.
5. Actuator movement: This amplified air pressure is sent to an amplifier, which delivers the final output pressure to the pneumatic actuator. This causes the valve to move.
6. Position feedback: A mechanical feedback lever connects the valve's stem or shaft back to the positioner. This lever counteracts the initial torque from the moving coil, establishing a state of equilibrium. This feedback mechanism is how the positioner ensures the valve reaches and holds the correct position, compensating for external forces like friction.

Operation for single and double-acting actuators

• Single-acting positioner: In this configuration, a spring on the actuator provides the return force. The SRI986's single output pressure drives the actuator in one direction against the spring.
• Double-acting positioner: For actuators without a return spring, the SRI986 uses a double-acting amplifier to provide two opposing output pressures, controlling the actuator in both directions.

 Benefits of the operating principle

The force-comparison and feedback loop design of the SRI986 offers several key advantages for process control:

Precision and stability: The continuous comparison of the desired position (input signal) with the actual position (feedback) allows the positioner to make immediate, small corrections. This ensures a highly accurate and stable valve position, even with varying process conditions.

Faster response time: By providing a high-capacity air output, the positioner can move the actuator quickly, significantly reducing positioning time.

Overcoming friction: The robust output pressure generated by the positioner helps overcome static and dynamic friction within the valve, which can cause sticking or slow responses in simpler control systems.

 Full Datasheet

Positioner Transmitter Feature  

• Manufacturer: SRI986
• Product No. :  BIDS7FAANA-1
• SRI986- Electro-Pneumatic Positioner
• Version: B - Single Acting
• Input: I - Signal Range 4 - 20 mA
• Mode of Action: D - Direct Version: Increasing Input increases Output
• Built-In Limit Switch/Position Transmitter: S - Without
• Cable Entry: 7 - M20 x 1.5 With One Plastic Cable Gland, Color Gray
• Electrical Certification: (Only Standard Device): FAA - FM Approved For Intrinsic Safety Cl. I, Div. 1, Groups A,B,C,D Hazardous Locations Indoors
• Attachment Kit: N - Order as Auxiliary
• Manifold: A -  Order as Auxiliary
• Options: 1 - LLoyd's Register of Shipping

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

Basic Ordering Information