Feed Forward (Ff) Configuration; Sampling And/Or Flow Pacing Controller - ABB AW400 Series Instruction Manual

These parameters represent the absolute high and low limits allowed for the controller output. The output
limits have to be set if required by the process. Default factory setting are: High Limit 100%, Low Limit 0%.
5.2.2.2

Feed Forward (FF) Configuration

The instrument will activate this function only if it detects the presence of a 4-20 mA input signal connected to
Channel 2.
If flow rate is variable, proper control action has to take into consideration this variable, and this strategy is
achieved with the Feed Forward action, which is calculated from a flow rate measure and it acts directly on
the controller's output, anticipating the correction eventually generated by the controller's PID algorithm.
Flow rate measure is a 4-20 mA analog signal coming from an external flowmeter connected to channel 2.
AW400 gives the possibility to calculate output limits as a function of the instantaneous flow rate. If this
functionality is active the standard High and Low limits on the controller's output are automatically disabled.
This functionality is useful when control action is aimed at the dosage of a reagent, as it prevents excess
dosages in the case of low flow rate. High limit is calculated by the multiplication of FFH (Feed Forward High
factor) and the instantaneous flow rate, low limit is calculated by the multiplication of FFL (Feed Forward Low
factor) and the instantaneous flow rate.
Example:
Flow rate is 20 %; FFH is 1.5 and FFL is 0.5. Limits are 30% and 10% of the controller's output range.
Absolute maximum limit of the control output is set with the parameter Abs. Max.
The Gain is multiplication factor of FF term.
5.2.2.3

Sampling and/or Flow Pacing Controller

Process with dead time, which is often the case for Chlorine dosage, can be best controlled with this strategy.
Therefore this menu is made available automatically when the instrument detects that Chlorine input Channel
has been selected, and it can be activated or deactivated at choice by the user.
Sampling control updates the PID function on a periodic basis (cycle) for a specified length of time (active
time). The sampling rate is determined by time (time sampling) or the amount of product measured (adaptive
sampling), the latter solution possible only when an external flowmeter is connected to Channel 2. To select "
time sampling" set TATP parameter to "NO", to select "volume sampling (adaptive)" set TATP parameter to
"YES". When volume sampling is selected, the parameter "K" becomes available: this is a factor
corresponding to the flowmeter full scale value expressed in cubic meters (m
3
and 17000 m .
The CYCLE parameters represent the complete control cycle and it has to be set in minutes (for time
3
sampling) or m (for volume sampling), depending on TATP setting.
Active time parameter represent the period during which the PID parameters are updated in the cycle, and it
must be set in minutes (for time sampling) or m
Feed Forward action can be activated with this strategy, and it is operating regardless of the cycle phase, that
is also when the controller is not in the active phase.
3
(for volume sampling), depending on TATP setting.
3
) , and it can be set between 0
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