How does a 4-20 mA signal drive a pneumatic valve with a positioner and what is the effect?

How does a 4-20 mA signal drive a pneumatic valve with a positioner and what is the effect?

• A. The 4-20 mA signal is converted by the positioner into proportional pneumatic pressure to move the valve; feedback helps achieve commanded position.
• B. The 4-20 mA directly moves the valve stem.
• C. The positioner converts signal to digital bits.
• D. The loop uses voltage instead of current to drive the valve.

Answer: (A)

The key idea is that a 4-20 mA control signal is used to command a pneumatic valve through a positioner. The positioner reads the current and converts it into a proportional pneumatic pressure that drives the valve actuator. The valve’s actual position is fed back to the positioner, which compares it to the commanded setpoint and adjusts the air pressure to bring the valve to the desired position. This creates a closed-loop, giving precise, stable, and repeatable valve movement and reducing effects of friction and other nonidealities. Typically, 4 mA corresponds to 0% opening and 20 mA to 100% opening, and using current helps the loop stay robust over long distances and in noisy environments. The other options aren’t right because the current signal doesn’t directly move the stem, the positioner isn’t simply converting to digital bits in this setup, and the loop is driven by current rather than voltage.

The key idea is that a 4-20 mA control signal is used to command a pneumatic valve through a positioner. The positioner reads the current and converts it into a proportional pneumatic pressure that drives the valve actuator. The valve’s actual position is fed back to the positioner, which compares it to the commanded setpoint and adjusts the air pressure to bring the valve to the desired position. This creates a closed-loop, giving precise, stable, and repeatable valve movement and reducing effects of friction and other nonidealities. Typically, 4 mA corresponds to 0% opening and 20 mA to 100% opening, and using current helps the loop stay robust over long distances and in noisy environments. The other options aren’t right because the current signal doesn’t directly move the stem, the positioner isn’t simply converting to digital bits in this setup, and the loop is driven by current rather than voltage.