STI Elctro-Pneumatic Positioner,  SA/CL Series 

The SA/CL positioner works on the balanced-forces principle, using an internal spool valve to control air distribution to a linear actuator. A pneumatic control signal acts on a diaphragm, which moves a spool valve connected to a feedback spring. This creates a stable position for the actuator stem only when the force from the pneumatic signal exactly balances the force of the feedback spring. For electro-pneumatic versions, an I/P converter converts the 4-20 mA electric signal into a pneumatic signal to pilot the positioner.

Working principle

• Control signal input: A pneumatic (3-15 psi) or electrical (4-20 mA) signal from a controller is sent to the positioner.
• Diaphragm and spool valve: The control signal pushes against a diaphragm, which in turn moves a spool valve via a rod.
• Force balance: The force on the diaphragm from the control signal is opposed by a feedback spring.
• Spool valve movement: The spool valve directs air to either side of the actuator's piston.
• Actuator movement: The actuator stem moves in response to the air pressure, and this movement is transmitted back to the feedback spring via a cam and lever.
• Stability: The actuator reaches a stable position when the force from the control signal is equal to the force from the feedback spring, which causes the spool valve to return to its neutral position, stopping air flow.

Key features

• Balanced-forces principle: The positioner maintains a stable position by balancing the control signal force against the feedback spring force.
• Spool valve: An internal spool valve distributes air to the actuator chambers.
• Double or single acting: It can be configured for double-acting (air on both sides of the piston) or single-acting (air on one side) actuators.
• Analogue positioner: It is available in both pneumatic and electro-pneumatic versions.
• I/P converter: Electro-pneumatic versions include an integrated I/P converter to translate the electrical signal into a pneumatic one.

Working Principle

The IMI STI SA/CL positioner works on the balanced-forces (or force balance) principle to ensure the actuator stem position is directly proportional to the control signal.

1. Input Signal Reception:

• Pneumatic Version: The positioner receives a pneumatic control signal (typically 3-15 psi) from a controller. This pressure acts on a diaphragm inside the positioner.
• Electro-pneumatic (SA/CL-EP) Version: An I/P (current-to-pressure) converter is installed, which transforms an electrical signal (typically 4-20 mA) into a proportional pneumatic signal that then pilots the pneumatic positioner stage.

2. Force Balance Mechanism:

• The force exerted by the input signal on the diaphragm is balanced against the force of a feedback spring.
• The feedback spring's force is determined by the position of the actuator stem, which is transmitted to the spring via a lever and a cam system.

3. Air Distribution (Spool Valve Operation):

• A double-acting spool valve is connected to the diaphragm and the feedback mechanism.
• If the forces are unbalanced (meaning the actual stem position does not match the desired position dictated by the input signal), the spool valve displaces.
• This displacement directs compressed air from the supply into one chamber of the pneumatic actuator (and exhausts air from the other chamber for double-acting models) to move the valve stem.

4. Position Feedback and Equilibrium:

• As the actuator stem moves, the feedback mechanism (cam and spring) adjusts the opposing force until it exactly balances the force generated by the input signal.
• When the forces balance, the spool valve returns to its neutral position, the air flow to the actuator chambers stops, and the actuator stem is held in a stable position corresponding to the input signal.

This continuous feedback loop ensures a linear and accurate relationship between the control signal and the actual valve position, compensating for external factors like friction or differential pressure changes across the valve.

 Full Datasheet