A reverse acting actuator operates in opposition to a direct acting actuator. It functions such that an increase in the process variable leads to a decrease in controller output. For instance, in a pneumatic sliding stem valve, an increase in pressure lifts the diaphragm and the stem, causing the valve to close if it's a "push-down-to-open" (PDTO) valve. The fail-open or fail-close configuration is determined by the placement of the spring and diaphragm, with the spring's action counteracting the loss of air pressure.
