Watlow Controller Series D8, Руководство пользователя

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D8 Series User’s Guide
Chapter 4: Operation and Setup
82
As the temperature in the middle of the tank (loop 1) drops, the output goes up proportionally and
the set point of loop 2 goes up proportionally. Thus heat is added to the system at the element even
though the temperature near the element may have been at the desired temperature.
With proportional control, when loop 1 is at set point, its output is 0 percent, and the set point of
loop 2 is equal to the desired temperature 150º F. If the temperature of loop 1 drops to 149º F,
the deviation results in a proportional output of 10 percent. This results in an increase to the
set point for loop 2 equal to 10 percent of the set point range. In this case the range is 40º F
(190º F - 150º F = 40º F), and 10 percent of 40º F is 4º F.
So when the temperature at loop 1 drops 1º F, the set point of loop 2 increases by 4º F until the
output of loop 1 is 100 percent and the set point of loop 2 is 190º F. At this point, further decreases
of the loop 1 process variable have no additional affect on loop 2. Figure 4.12 illustrates this
relationship.
Set Point of the Secondary Loop
(Engine
ering Units)
Process Variable of Primary Loop (ºF)
Heat Output of Primary Loop
(Percent of Full Scale)
0%
50% 100%
150º F 145º F
140º F
190º F
170º F
150º F
Figure 4.12 – Relationship of Secondary Loop
Setting Up Ratio Control
Ratio control allows the process variable of one loop (master loop), multiplied by a ratio, to be the set
point of another loop (ratio loop). You can assign any process variable to determine the set point of a
ratio loop.
By adjusting the ratio control parameters, you can adjust the influence that the master loop process
variable has on the set point of the ratio loop.