ABB CM30 User Manual page 41

ControlMaster CM30 and CM50
Universal process controllers,
...Control / ...Loop 1 (2) Control
Control Type
PID
pPI
On/Off
IM/CM/ED–EN Rev. B
1 1
/ and / DIN
4 2
Selects the basic type of controller required.
Standard proportional, integral and derivative control.
pPI controller:
SP
pPI Controller
PV
I
P
pPI (predictive PI control) is a deadtime-compensating control for use when the process
deadtime is longer than twice the dominant process time constant. Unlike other deadtime
compensation algorithms, pPI does not require a process model to be specified; it creates
its own process model from proportional, integral and deadtime settings. If the deadtime is
varying, the pPI controller can be combined with gain scheduling. The Autotune function is
not used when pPI control is configured.
The pPI controller is easy to tune, first by making a step response test on the process and
then setting the controller parameters by observation. After tuning, the pPI controller can be
adjusted manually. In most cases a decrease in proportional band or a decrease in integral
time gives faster control but poorer damping.
From a steady state condition, make a step change in the output from the neutral value (u0)
to a new value (u1). Observe the process value (y) and make the following calculations:
By determining the 4 key parameters: L, T,  and u from a step response, the 3 process
parameters: static gain K
p
according to the following expressions:
Knowing these, the pPI's parameters can be calculated easily:
Proportional band = 100 x Kp
Integral time = Tp
Derivative (deadtime) = Lp
Notes.
y0 and y1 are expressed in % of engineering range.
When pPI is selected, the derivative time is used to set the deadtime.
A simple 2-state control.
OP
Deadtime
, dominant time constant T
7 Advanced Level
Process
Model
Model Without
Deadtime
and deadtime L can be determined
p p
39