AMALGAMATED INSTRUMENT LD4-LN, Operation And Instruction Manual

Page: 31 / 46

C_Po
(proportional offset %)
The proportional offset is initially used to set the output value when operating the instrument as a
proportional only controller. The proportional offset determines what % of the proportional control output will
be given when the process value reaches the setpoint value. If set to
0.0
then there will be zero output (e.g.
4mA for a 4-20mA output) when the process value reaches the setpoint value. If set to
50.0
then there will be
a 50% output (e.g. 12mA for a 4-20mA output) when the process reaches the setpoint value. If set to
100.0
then there will be a 100% output (e.g. 20mA for a 4-20mA output) when the process reaches the setpoint
value.
If using proportional only control then when stable control is established there may be a difference
between the process and the setpoint values. By altering the proportional offset value the difference may be
minimised.
Proportional only control examples.
For a 4-20mA control output (0% = 4mA & 100% = 20mA) the setpoint is 7.0, the span is 2.0, the
proportional gain is 1.000 and the offset is 0.0. If the reading on the display is 6.8 then the error is 10% (i.e.
10% of the span figure).
Proportional control output Error Proportional gain O
= × +
ffset
Proportional control output or mA
= × + =
10 1 0 10 5 6 % % % .
If the proportional gain were to be changed to 2.000 then:
Proportional control output or mA
= × + =
10 2 0 20 7 2 % % % .
If the proportional gain were to be changed to 0.500 then:
Proportional control output or mA
= × + =
10 0 5 0 5 48 % . % % .
If the offset were now to be changed to 50.0 (50%) then:
Proportional control output or mA
= × + =
10 0 5 50 55 128 % . % % .
7.2 Integral control output
The integral control output can be found from:
Integral control output
error Ig time secs
previ
=
× ×
+
( .)
60
ous integral control output
Where Ig is the integral gain set via the
C. IG
function.
C. Ig
(integral gain)
The integral control action will attempt to correct any offset which the proportional control action is
unable to correct (e.g. errors due to a changing load). When the integral gain is correctly adjusted the control
output is ramped up or down to maintain control by keeping the process variable at the same value as the
control setpoint. An integral gain which is too large will cause a rapid response to any error but can also lead
Page 31 of 46 LD4LNMAN-1.0-1
OUTPUT
SETPOINT
SETPOINT
OUTPUT
C. PG 0.0
=
C. PG 50.0
=
20mA
20mA
12mA
4mA
4mA