ABB 266 Operating Instruction page 42

266 Models - FOUNDATION Fieldbus
The Bypass Enable option is necessary because not all cascade control schemes will be stable if BYPASS is true. BYPASS can only
be changed when the block mode is Man or O/S. While it is set, the value of SP, in percent of range, is passed directly to the target
output, and the value of OUT is used for BKCAL_OUT. When the mode is changed to Cas, the upstream block is requested to
initialize to the value of OUT. When a block is in Cas mode, then on the transition out of bypass, the upstream block
is requested to initialize to the PV value, regardless of the "Use PV for BKCAL_OUT" option.
GAIN, RESET, and RATE are the tuning constants for the P, I, and D terms, respectively. Gain is a dimensionless number. RESET and
RATE are time constants expressed in seconds. There are existing controllers that are tuned by the inverse value of some or all of
them, such as proportional band and repeats per minute. The human interface to these parameters should be able to display the
user's preference.
The Direct Acting control option, if true, causes the output to increase when the PV exceeds the SP. If false, the output will decrease
when the PV exceeds the SP. It will make the difference between positive and negative feedback, so it must be set properly, and
never changed while in an automatic mode. The setting of the option must also be used in calculating the limit state for BKCAL_OUT.
The output supports the feed forward algorithm. The FF_VAL input brings in an external value which is proportional to some
disturbance in the control loop. The value is converted to percent of output span using the values of parameter FF_SCALE. This
value is multiplied by the FF_GAIN and added to the target output of the PID algorithm. If the status of FF_VAL is Bad, the last usable
value will be used, because this prevents bumping the output. When the status returns to good, the block will adjust its integral term
to maintain the previous output. The output supports the track algorithm.
There is an option to use either the SP value after limiting or the PV value for the BKCAL_OUT value.
Equations
The algorithm applied is as in the following formula:
[
OUT = GAIN (BETA · SP – PV) +
Where the standard variables are:
GAIN: Proportional Gain Value
RESET: Integral action Time constant in seconds
s: Laplace operator
RATE: Derivative action time constant in seconds
FF_VAL: Feed-forward contribution from the feed-forward input
SP: Setpoint
PV: Process Variable
Configuration hints
The minimum configuration for having the PID working and/or moving out from the OOS needs at least the following settings:
– OUT_HI_LIM > OUT_LO_LIM
– SP_HI_LIM > SP_LO_LIM
– BYPASS = OFF
– SHED_OPT = Normal Shed Normal Return
– GAIN > 0
42 OI/266/FF/ADD-EN Rev. B | 2600T Series Pressure transmitters
1
(SP – PV) +
RESET · s T1_RATE · s + 1
RATE · s
(GAMMA · SP – PV)
Where the the enhanced variables are:
T1_RATE: Derivative 1st order filter
BETA: Setpoint weight proportional part [0...1]
GAMMA: Setpoint weight derivative part [0...1]
+ FF_VAL