Endress+Hauser Mycom S CPM 153 Operating Instructions Manual page 54

6 Start-up
Batch or inline process
arrangement:
54
With active control, the batch and inline processes are different in their relationship to
the medium current:
Pure batch process: the batch container is filled with the medium. During the subse-
quent batch process, no additional medium is fed in. The change in pH value is deter-
mined only by the controller. To be able to compensate for possible so-called
"overshoots", use a two-sided controller. For as long as the actual value is within the neu-
tral zone, no additional dosing agent is added.
Pure inline process: Here, the control works with the medium flowing past. The pH value
of the medium in the inflow may be subject to strong deviations for which the controller
should compensate. The volume of medium which has already flowed past can no lon-
ger be influenced by the controller. For as long as the actual value corresponds to the
set point, the set value has a constant value.
The Mycom controller takes this differing behaviour into account. It is the internal hand-
ling of the integral part of the PI or PID controller which is different for these settings.
In practice, the most common option is the semi-batch process. Depending on the ratio
of inflow to tank size, this process shows the behaviour of an inline or a batch process.
Look-ahead pH measurement
To be able to resolve the general problems of a purely inline process, the CPM 153 is
able to "look into the future" using a second pH electrode and a flowmeter. This means
that the controller can react to strong variations in the inflow at an early stage.
Controlling the actuators
The CPM 153 has four different methods for controlling the actuators (see above).
1. PWM (Pulse-width modulation, "pulse-length controller")
With PLM, one side (i.e. acid or alkali) of the internal, continous actuating variable is
output to a relay as a rhythmic signal.
The larger the calculated actuating variable, the longer the appropriate contact remains
picked up (i.e. the longer the switch-on period t
length freely between 1 and 999.9 seconds. The minimum switch-on period is 0.4
seconds. Pulse-length-modulated outputs are intended to control solenoid valves, for
example.
A two-sided process requires two PLM relays or one PLM and a three-point step cont-
roller (see below). One PLM relay on its own can only output
• a actuating variable of -100% ... 0% or
• of 0% .... +100%.
To avoid pulses which are too short, enter a minimum switch-on period. Pulses which
are too short are not given to the relay/or the actuators. This benefits the actuator.
2. PFM (PFM; "pulse-frequency controller")
As with PWM, PFM is output as a rhythmic signal by the relay.
The greater the calculated manipulated variable, the higher the frequency of the related
contact. The maximum settable frequency 1/T is 120 min
a constant factor of the entered frequency (s. Fig. 18). Pulse-frequency-modulated out-
puts are intended to control magnetic dosing pumps, for example.
Here too, two PFM relays are required for a two-sided process.
Mycom S CPM 153
; s. Fig. 18) is. You can set the period
ON
-1
. The switch-on period t
Endress+Hauser
is
ON