User manual

IEC61131 User and Reference Manual
April 22, 2008
325
Inputs
Type
Description
deadband
Real
The setpoint deadband is used by the PID algorithm to
determine if the process requires control outputs. If the
absolute value of the error is less than the deadband, then the
function block skips execution of the control algorithm. This
permits faster execution when the error is within a certain
acceptable range or deadband. Valid range is any value
greater than 0.
full
Real
The full input is used in limiting the maximum output value of
the pida function block. If the PID algorithm calculates an out
quantity that is greater than the value stored in full the out
quantity is set equal to the value stored in full. The full input
should always be greater than the zero input.
zero
Real
The zero input is used in limiting the minimum output value of
the pida function block. If the PID algorithm calculates an out
quantity that is less than the value stored in zero, the out
quantity is set equal to the value stored in zero. The zero
input should always be less than the full input.
cycleTime
Time
The value of the PID algorithm execution period measured in
seconds. Any value greater than or equal to 0.001 seconds (1
ms) may be specified. If the cycleTime specified is less than
the scan time of the ISaGRAF application program, the scan
time of the application becomes the PID cycleTime.
auto
Boolean
When set to TRUE to the out value is calculated using the
PID algorithm. When set to FALSE the out value is set to the
value of outManual.
outManual
Real
The value that the out is set to when the pida function block is
in the manual mode.
Outputs
Type
Description
out
Real
PID algorithm output.
PID Velocity Algorithm
The PIDA function uses the velocity form of the PID algorithm. The velocity form calculates
the change in the output and adds it to the previous output.
 







 2111
2
nnnn
i
nnnn
ppp
T
R
e
T
T
eeKmm
where
nnn
pse 
and: e = error
s = setpoint
p = process value
K = gain
T = execution period
T
i
= integral or reset time
R = rate gain
m = output