Process control pneumatics
195
Gain -
PV
Error
Output (a) Block diagram
~
~
Ti
inal step I doubles in T i
(b) Step response
PV - - - a ' v v v
Integral bellows
, . . . . - - - - - - .
sP
( Feedback " " bellows
time control
/
I"~
I
-F supply
1t
|
Output (c) Construction
Figure 7.8
Proportional plus integral (P + I) controller
Pneumatic three term control can be achieved with the arrangement of Figure 7.9, where the action of the feedback bellows has been delayed. The three user adjustable terms in expression 7.3 (gain K, integral time Ti, derivative time Td) are set by beam pivot point and two bleed valves to give the best plant response. These controls do, however, interact to some e x t e n t - a failing not shared by electronic controllers. Figure 7.10 represents the typical front panel of a controller. Values of SR PV and controller output are displayed and the operator can select between automatic and manual operation. The desired value (SP) can be adjusted in auto or the controller output set directly in manual. The operator does not have access to K, T i, T a setting controls; these are adjusted by the maintenance technician.