Hitachi CONVENTION 14 Instruction Manual page 208

PID Loop Configuration
The inverter's PID loop algorithm is configurable for various applications.
PID Output Limit - The PID loop controller has a built-in output limit function. This
function monitors the difference between the PID setpoint and the loop output (inverter
output frequency), measured as a percentage of the full scale range of each. The limit is
specified by parameter A078.
When the difference |(Setpoint – loop output)| is smaller than or equal to the A078
limit value, the loop controller operates in its normal linear range.
When the difference |(Setpoint – loop output)| is larger than the A078 limit value,
the loop controller changes the output frequency as needed so that the difference
does not exceed the limit.
The diagram below shows PID setpoint changes and the related output frequency
behavior when a limit value in A078 exists.
Output limit
PID Setpoint
Output freq.
Output limit
Error Inversion - In typical heating loops or ventilation loops, an increase in energy into
the process results in an
cooling loops, an increase in energy into the process results in a
case, the Loop Error = –(SP – PV). Use A077 to configure the error term.
A077 =00
SP
+
PV
Other PID-related topics:
"PID Control" on page 3–22
"PID ON/OFF and PID Clear" on page 4–26
"Output Deviation for PID Control" on page 4–43
"PID Second Stage Output" on page 4–47
%
A078
A078
increasing PV. In this case, the Loop Error = (SP – PV). For
Error Freq.
PID
calculation
-
PV from process with
positive correlation
Limit imposed
on output
Limit imposed
on output
decreasing
A077 =01
SP
Error
-
calculation
+
PV from process with
PV
negative correlation
4 − 59
4 59
t
PV. In this
Freq.
PID