Yokogawa EXAxt PH450 4-Wire Analyzer for pH and ORP - Manual (Page 36)

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IM 12B07C05-01E
5-7. 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 se-
lected “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
configuration 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
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
SP
PV e
+- ++ ++
+-
e
Range
∫e dt
1
T
i
T
d
dPV
dt
z
Process
Controller
Actuator
Process
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 rising time and settling time. It is
difficult to realize in practice because differen-
tiation leads to “noisy” signals.