Yokogawa ISC450 4-Wire Analyzer for Inductive Conductivity - Manual (Page 32)

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IM 12D06D05-01E
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
associated to the output. Available process
parameters depend on selected “sensor type”
and “measurement setup”.
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 configu-
ration of an output curve by 21 steps
(5% intervals). In the main menu
concentration can be selected 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 exces-
sive overshoot and oscillation. Too much gain
may result in an unstable- or oscillating proc-
ess. 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.
SP
PV e
+- ++ ++
+-
e
Range
ºe dt
1
T
i
T
d
dPV
dt
z
Process
Controller
Actuator
Process
Figure 5-1. Control Diagram
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.