Honeywell ST700 User Manual page 69

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 is used as this prevents
bumping the output. When the status returns to Good, the block adjusts its integral term to
maintain the previous output. The output supports the track algorithm. The block provides
an option to use either the SP value after limiting or the PV value for the BKCAL_OUT
value.
PID Control block is an algorithm that produces an output signal in response to the
measured variable and the setpoint. The PID block allows you to choose either a standard
PID control equation (Ideal) or a robust PID equation defined by Honeywell. This selection
is defined in the PID_FORM parameter.
The output has three terms, namely Proportional, Integral, and Derivative. The output is
adjusted by tuning constants. There are three tuning constants in the ideal PID equation.
The robust PID uses four tuning constants:
1. GAIN is the tuning constant of the Proportional term.
2. RESET is the tuning constant of the Integral.
3. RATE is the tuning constant of the Derivative. RATE is usually modified by a lag,
which is set at some fixed ratio higher than the rate time, to create a rate gain. There is
no lag with the rate in this implementation.
4. OUT_LAG is the fourth tuning constant used in the robust PID; it adds roll off to the
output response. The action is similar to PID with rate gain.
PID Ideal and PID Robust
The ideal equation is a parallel or non-interacting implementation of PID control using
three tuning constants. It automatically fixes OUT_LAG to 16 times the RATE time
constant. This produces response characteristics equivalent to the algorithms used in TPS
products.
The robust equation is the same parallel implementation of ideal PID control but allows the
engineer to set the OUT_LAG and effectively change the rate gain.
ALGO_TYPE is a configuration parameter that contains one of three selected algorithm
types, A, B, or C.
Where:
•
A - RATE, GAIN and RESET all act on the error between set point and measured
variable.
•
B - RATE acts on the measured variable only, GAIN and RESET use the error.
•
C - RATE and GAIN act on the measured variable only, and RESET uses the error.
PID Tuning Parameters
Table 13 lists the valid ranges for the tuning parameters for the PID block. Note that
OUT_LAG parameter is not configurable when Ideal PID is selected (PID_FORM = 1)
and can be configured when Robust PID is selected (PID_FORM = 2).
Revision 2.0 FOUNDATION Fieldbus ST 700 Pressure Transmitter User's Guide Page 55