User guide

24
IM 12D08N05-01E
5-6. mA output setup
The general procedure is to first define the
function (control, output, simulate, off) of the
output and second the process parameter as-
sociated to the output.
Available process parameters depend on the
selected “main parameter” and “measure”.
Off :Whenanoutputissetoffthe
output is not used and will give an
output of 4 mA
Control : A selection of P- PI- or PID control
Manual :Staticoutputrequiredtomaintain
reset equilibrium state with setpoint
Direction : Direct
If the process variable is too high
relativetotheSP,theoutputofthe
controller is increased
(direct action).
: Reverse
If the process variable is too high
relativetotheSP,theoutputofthe
controller is decreased
(reverse action).
Output :Linearornonlineartableoutput.
The table function allows the confi-
guration of an output curve by 21
steps(5%intervals).Inthemain
menu concentration can be selec-
ted to set the concentration range.
Simulate :Percentageofoutputspan.
Normal span of outputs are limited
from 3.8 to 20.5 mA
Failsafe :ContactS4isprogrammedasafail-
safe contact.
BurnLoworHighwillgiveanoutputof3.6
resp.21mAincaseofFailsituation.
Note! When leaving Commissioning, Hold
remains active until switched off
manually. This is to avoid inappropriate
actions while setting up the
measurement
Proportional control
Proportional Control action produces an output
signal that is proportional to the difference
betweentheSetpointandthePV(deviationor
error). Proportional control amplifies the error to
motivate the process value towards the desired
setpoint. The output signal is represented as a
percentageofoutput(0-100%).
Proportional control will reduce but not
eliminate the steady state error. Therefore,
proportional Control action includes a Manual
Reset. The manual reset (percentage of output)
is used to eliminate the steady state error.
Note! Any changes (disturbances) in the
process will re-introduce a steady
state error. Proportional control can
also produce excessive overshoot and
oscillation. Too much gain may result
in an unstable- or oscillating process.
Too little gain results in a sustained
steady state error. Gain = 1/Range.
[PV units]
Integral Control
Integral control is used to eliminate the steady
state error and any future process changes.
It will accumulate setpoint and process (load)
changes by continuing to adjust the output
untiltheerroriseliminated.Smallvaluesof
integral term (I-time in seconds) provide quick
compensation,butincreaseovershoot.Usually,
the integral term is set to a maximum value that
provides a compromise between the three sys-
tem characteristics of: overshoot, settling time,
and the time necessary to cancel the effects of
static loading (process changes). The integral
term is provided with an anti windup function.
When the output of PI portion of the controller
isoutsidethecontrolrange(lessthan-5%or
greaterthan105%),theI-partisfrozen.
Derivative control
The control acts on the slope (rate of change)
of the process value, thereby minimizing
overshoot. It provides “rate” feedback, resulting
in more damping. High derivative gains can
increase the rizing time and settling time. It is
difficult to realize in practice because differen-
tiation leads to “noisy” signals.
SP
PV
e
+
-
+
+
+
+
+
-
e
Range
∫e dt
1
T
i
T
d
dPV
dt
z
Process
Controller
Actuator
Process
Figure 5-1. Control diagram