Honeywell VRX180 - Manual (Page 83)

Programming and Operating Concepts
Video Recorder – User Manual 69
AI1
REACTION
VESSEL
THERMOCOUPLE
AI1 OV
LP1
PV LP1 OV
TYPE = CAS_P
SP1 = 1234.5
FB
AO1 IN
4 TO 20 mA
LP2
SP2
LP2 OV
TYPE = CAS_S
FB
PV
AI2
OIL
THERMOCOUPLE
AI2 OV
LP2 BC AO1 BC
NOTE: 1) SP1 is desired reaction vessel temperature.
2) SP2 is the remote setpoint input of LP2.
Figure 3-22 Function Block Diagram Of The Cascade Control Strategy
Recall that based on the instrument’s model number, up to eight control loops (LP1 through LP8) are
potentially available for use within the instrument. All control loops in this product may be programmed
to operate using up to two user defined set point parameters, designated by SP1 and SP2. Should you
implement a control loop using one or both setpoints? That depends on what is necessary to meet the
requirements of the specific application being dealt with. When in the on line mode and viewing a control
loop’s dedicated on line display, the working set point of the live control loop can be switched between
SP1 or SP2 by simply pushing the “SP” key on the instrument’s front door. Note that while both set point
parameters may be programmed to have straight numeric values, only SP2 may be defined as a remote
set point. That is, SP2 may be set up so that its value is determined by the output value of another
function block, such as a setpoint profile. In the cascade control strategy demonstrated in Figure 3-22,
SP2’s remote set point functionality is exploited by the LP2 secondary cascade loop. When this control
configuration is made operational, LP2’s working set point, SP2, will have a value determined by LP1
OV.
In Figure 3-22, the process values of each loop are the output values of the AI1 and AI2 analog input
function blocks. AI1 will produce temperature measurements of the reaction chamber and provide them
to the process variable input of LP1, while measurements of the oil temperature in the jacket tank will be
furnished to LP2’s PV input by AI2. Because LP1 OV will provide LP2 with its operating set point, LP1’s
output range will be defined in engineering units of temperature instead of the usual 0 to 100%. LP2’s
output range is 0 to 100%, in anticipation of using it to drive the AO1 function block’s 4 to 20 mA signal.
Note that the range covered by LP1 OV will have to be consistent with the operating temperature range
of the oil. For example, if it is determined that the oil temperature will be manipulated between 75 and
500 ºF, the low and high limits assumed by LP1 OV (and, for that matter, SP2) will equal 75 and 500,
respectively. Finally, LP2 BC and AO1 BC are the two back-calculated feedback paths shown. As is
true for the operation of all back-calculated feedback paths, both LP2 BC and AO1 BC work together to
acknowledge the cascaded control loops that the appropriate actions have taken place in response to
both loops’ output values.
The method used to coordinate the tuning of the cascaded loops is particularly interesting. Using the
diagram of Figure 3-22, the first priority is to tune the secondary cascade loop of LP2. With LP1 kept in
manual mode, tuning may begin by first placing LP2 in manual mode and then manipulating LP1’s
output. This will allow the generation of an LP2 set point that will induce a process upset when the
secondary loop is placed back in automatic mode. Only after LP2 has been tuned can LP1 be tuned.
When tuning LP1, LP2 will be kept in automatic mode throughout the entire time LP1 is exercised. Since
the tuning of LP2 will have already been established, tuning LP1 may be approached by first mentally
“blocking out” the secondary control loop’s existence and visualizing LP1’s output as connected to a sort
of virtual analog output function block. In this light, tuning the overall cascade control configuration
becomes the considerably simpler matter of tuning a single control loop.