The pneumatic relay operates on the principle of a nozzle-flapper arrangement. Air supply pressure acts on a diaphragm located below a spring. A rod and plug, connected to the diaphragm, control the flow of output air through a nozzle. A flapper is positioned near the nozzle.
Operation:
- An input signal (which can be a change in pressure or displacement of the flapper) affects the flapper's position.
- Flapper movement changes the distance between the flapper and the nozzle. A decrease in distance (flapper closer to the nozzle) restricts the output airflow. Conversely, an increase in distance increases output airflow. This is the direct action of the relay.
- Changes in the output air flow alter the back pressure at the nozzle.
- Increased nozzle back pressure pushes the diaphragm downwards, compressing the spring and further reducing the output airflow. Decreased nozzle back pressure allows the spring to push the diaphragm upwards, increasing output airflow.
- A portion of the output air is fed back through a line connected to a bellows and a feedback-adjusting spring (as shown in the sketch). This feedback pressure acts against the diaphragm, opposing the effect of the input signal. The bellows and spring arrangement allow the system to fine-tune the feedback strength. This negative feedback stabilises the system and increases the control range, preventing excessive overshoot or oscillation. The feedback mechanism subtracts from the effective input pressure, acting as a negative feedback loop.