Actuators, positioners and I/P converters

Why it matters

The controller's 4–20 mA output is only a number until something turns it into valve movement. The I/P converter, the actuator and the positioner do that job, and they decide whether the valve goes where it is told, how fast, with how much force, and what it does when air or power fails. Many "badly tuned" loops are really sticking valves or undersized actuators.

Key ideas

Signal chain. Controller output (4–20 mA) → I/P converter (or a smart positioner with built-in I/P) → pneumatic signal, conventionally 20–100 kPa (0.2–1.0 bar, 3–15 psig) → actuator → valve stem. A separate instrument-air supply, typically about 140 kPa gauge for signal devices and higher for positioner-fed actuators, provides the power.

I/P converter. Converts a current into a proportional pneumatic pressure. A coil in a magnetic field moves a flapper against a nozzle (force balance); the nozzle back-pressure is amplified by a pneumatic relay. Output is linear: 4 mA → 20 kPa, 20 mA → 100 kPa. The live zero (4 mA, 20 kPa) lets a broken wire or lost air be distinguished from a genuine zero signal.

Pneumatic spring-diaphragm actuator. Air pressure on a flexible diaphragm of effective area A pushes against a spring. At equilibrium P·A = k·x + F₀, so stem travel is proportional to pressure over the "spring range" (bench set), e.g. 20–100 kPa for full stroke.

• Direct-acting actuator: air pressure pushes the stem down (extends).
• Reverse-acting actuator: air pushes the stem up (retracts).
• Combined with a push-down-to-close or push-down-to-open valve, this gives air-to-open (fail-closed) or air-to-close (fail-open) assemblies. The spring alone sets the failure position, which is the main safety advantage of pneumatics.

Other actuators.

• Piston (cylinder) actuators — higher pressures and forces, long strokes, used on large rotary valves; double-acting types need a spring or accumulator for fail-safe.
• Electric actuators — motor and gearbox; precise, no air supply needed, good for remote sites; slower, fail in place unless fitted with a spring or battery, and need hazardous-area certification.
• Hydraulic and electro-hydraulic — very high force and stiffness (turbine governor and large damper drives).

Positioner. A local feedback controller on the valve: it compares the demanded position (signal) with the measured stem position and adjusts actuator air until they agree. It

• overcomes packing friction, stiction and unbalanced fluid forces;
• supplies full supply pressure to the actuator, so the actuator can develop more force than the 20–100 kPa signal alone allows;
• speeds up large actuators by passing more air;
• allows split ranging, reversed action and characterisation by recalibration;
• in smart (HART or fieldbus) versions, gives valve signatures, travel diagnostics and partial-stroke testing. A positioner adds a fast inner loop; on very fast loops (some liquid-flow and pressure loops) an old-style pneumatic positioner could interact with the process loop, but modern digital positioners are fast enough to be used almost everywhere.

Accessories. Volume boosters (faster stroking), solenoid valves (trip the valve to its safe position), limit switches and position transmitters (feedback to the control room), air filter-regulators, and handwheels for manual operation.

Formulas

P_out = P_min + (P_max − P_min)·(I − 4)/16

• I: input current (mA, 4–20); P_out, P_min, P_max: output pressure and its range limits (kPa). Linear I/P conversion.

F = P·A

• F: force on the diaphragm (N); P: gauge pressure (Pa); A: effective diaphragm area (m²).

x = (P − P_start)·A / k = stroke·(P − P_start)/(P_end − P_start)

• x: stem travel (m); k: spring rate (N/m); P_start, P_end: spring range (Pa). Spring-diaphragm actuator in equilibrium, friction neglected.

k = (P_end − P_start)·A / stroke

• Spring rate that gives full stroke over the spring range.

F_seat = (P_supply − P_end)·A

• Extra seating force available at the end of travel when a positioner applies full supply pressure P_supply (air-to-close, direct actuator).

Worked examples

Example 1 (standard) — signal chain. An I/P converter maps 4–20 mA to 20–100 kPa and drives an air-to-open valve with a 40 mm stroke and a 20–100 kPa spring range (no positioner, friction neglected). Find the pressure and valve position for a controller output of 7 mA, and the pressure at 12 mA.

1. P = 20 + 80 × (7 − 4)/16 = 20 + 80 × 0.1875 = 35 kPa.
2. Travel x = 40 × (35 − 20)/(100 − 20) = 40 × 0.1875 = 7.5 mm, i.e. 18.75 % open.
3. At 12 mA: P = 20 + 80 × 8/16 = 60 kPa (50 % of span).

Example 2 (GATE level) — does the actuator close the valve? A direct-acting, air-to-close spring-diaphragm actuator has effective area 0.06 m², spring range 20–100 kPa and stroke 30 mm. The unbalanced plug closes against a 6 bar (600 kPa) differential on a 50 mm diameter seat. (a) Find the spring rate. (b) Find the fluid force on the plug at shut-off. (c) Can the valve shut off with the 20–100 kPa signal alone? (d) What seating force is available with a positioner on a 240 kPa supply?

1. (a) k = (P_end − P_start)·A/stroke = 80 000 × 0.06/0.030 = 1.6 × 10⁵ N/m.
2. (b) Seat area = π × 0.025² = 1.963 × 10⁻³ m². F_fluid = 600 000 × 1.963 × 10⁻³ = 1178 N.
3. (c) At 100 kPa the diaphragm force just balances the spring at full stroke, leaving zero extra force. Holding the plug shut needs an additional 1178/0.06 = 19 600 Pa ≈ 19.6 kPa, i.e. about 120 kPa, which a 20–100 kPa signal cannot supply. The valve cannot shut off without a positioner (and packing friction makes it worse).
4. (d) F_seat = (240 − 100) × 10³ × 0.06 = 8400 N, comfortably more than 1178 N plus friction.

Common mistakes

• Using the pneumatic signal range as if it starts at zero; 4 mA corresponds to 20 kPa, not 0.
• Confusing actuator action (direct or reverse) with valve action (air-to-open or air-to-close); the failure position comes from the combination.
• Assuming the valve position equals the signal without a positioner; friction and fluid forces cause dead band and offset.
• Forgetting fluid forces on the plug when sizing an actuator, especially for tight shut-off.
• Fitting an electric actuator where a fail-safe position is required without a spring-return or backup power.

For GATE IN

Practise: linear mapping between 4–20 mA, 20–100 kPa and % output (including live zero); force and stroke of a spring-diaphragm actuator; identifying fail-open versus fail-closed combinations; and the role of a positioner as an inner feedback loop. Signal-conversion numericals are quick marks if you keep the zero offset straight.

Quick check

1. An I/P converter (4–20 mA → 20–100 kPa) receives 16 mA. What is its output?
2. A diaphragm of area 0.05 m² sees 80 kPa gauge. What force does it produce?
3. An air-to-close valve loses its air supply. Where does it go?
4. Name two jobs a positioner does besides improving accuracy.

Answers: 1. 80 kPa. 2. 4000 N. 3. Fully open (fail-open). 4. Any two of: provides more force and speed by using full supply pressure, allows split ranging or reversed action, gives diagnostics and partial-stroke testing.