Proportional (P) Control Action
When measured value becomes higher than the set value (deviation), the upper end of proportional lever shifts to the right. Thus the flapper approaches to the nozzle and the back pressure of the nozzle, i.e., the pressure charged upon the pilot relay, increases. Consequently valve in pilot relay open and supply pressure flows into control side to increase the pressure. At the same time, this pressure is charged upon proportional bellows and lifts up proportional lever and thus flapper is detached from the nozzle and control pressure is set in proportion to such deflection. All of the above actions occur simultaneously in the actual operation. When both pointers overlaps (deviation is zero) control pressure becomes 60 kPa (20-100kPa).
Proportional-Integral (PI) Action
Assuming that the controller is acting properly and measured value and set value are in equilibrium, (or deviation is zero), and the same pressure as control pressure is sealed in the proportional bellows and reset bellows. If measured value becomes too high as in the above case, P action immediately takes place and control pressure increases. Thus control pressure flows into the reset bellows through reset throttle valve.
As the pressure inside reset bellows increases, proportional lever comes down and flapper approaches to the nozzle and back pressure increases. Consequently pilot relay valve opens to increase the control pressure and the increasing pressure inside proportional bellows lifts up the proportional lever and causes the nozzle to detach from the nozzle again. This resetting effect continues until control pressure increases to such extend that the control valve opening enables the reversion of the measured value to the set value (until deviation becomes zero). Pressure of proportional bellows and reset bellows thus balances and the original balance condition is established.
Proportional-Integral-Derivative (PID) Action
Rate throttle valve and bellows chamber are connected in parallel between the pilot relay and proportional bellows. In the aforesaid P and PI action, the inside pressure of proportional bellows is in proportion to the amount of deviation. Therefore, when the measured value changes, control pressure flows in or out with the speed corresponding to such changes, so that the pressure inside the proportional bellows will synchronize with the change of measured value. Since the pressure reduction taking place as it passes through the rate throttle valve is in proportion to the speed of fluctuation of measured value, pilot relay output, i.e., control pressure also become larger or smaller than the internal pressure of proportional bellows to the extent of the differential pressure at the throttle valve.
Rate bellows chamber is provided to transmit control pressure to the proportional bellows utilizing the volume change of rate bellows caused by its elasticity and to give stability to the system.
Therefore, when rate action is utilized, control valve opening can be adjusted more quickly and it certainly gives convenience especially to the process where time lag is great.