Future Design MCTB 4.3, Руководство пользователя

Страница: 82 / 240

MCTB
82
If the outer loop is configured for heat/cool operation, it will have an output of -100 to 100%
where -100% is full cooling and 100% is full heating. If the lower and upper limits entered in
the runtime on the Cascade screen are 0 and 50, an output of -100 to 100% on the outer loop
will generate a set point of 0 to 50 on the inner loop. So, an output of 0% (no heating or cooling)
would be a set point of 25 on the inner loop.
Note: This mode is recommended for heat only or cool only applications where the inner loop
must exceed the outer loop set point in order to account for losses in the system in order
to get the outer loop to reach set point.
To utilize this mode of operation, the control outputs of the inner loop must be wired to
the final control elements. The outer loop is not used to directly control the process. It
is used to generate the set point for the inner loop.
Deviation This mode allows the lower and upper set point range of the inner loop to be defined as an
offset from the outer loop set point. When selected, entry fields (on the Cascade screen in the
runtime application) are provided and used to set the lower and upper deviation from the outer
loop set point. The lower and upper range of the inner loop set point is calculated by adding
and/or subtracting these values as offsets from the outer loop set point. As the outer loop set
point changes, the window defined by the lower and upper limits automatically changes with it.
For example, if the outer loop is configured for heat/cool operation, it will have an output of -
100 to 100% where -100% is full cooling and 100% is full heating. The cooling output of 0 to -
100% will apply an offset of 0 up to the lower limit value and the heating output of 0 to 100%
will apply an offset of 0 up to the upper limit value. With an output of 0% on the outer loop and
an outer loop set point of 25, the inner loop set point would also be 25 (no offset).
However, if the lower limit value is -10 for example, the inner loop set point will vary from 25 to
15 as the outer loop cooling output goes from 0 to -100%. Likewise, if the upper limit value is
10, the inner loop set point will vary from 25 to 35 as the outer loop heating output goes from
0 to 100%. As the outer loop set point is changed, to 80 for example, the inner loop set point
window then varies with it and becomes 70 to 90. This limits the range of the inner loop set
point to a small band around the outer loop set point.
Note: This mode is recommended for heat/cool applications to limit the difference between the
outer and inner loop process values to minimize the amount of overshoot/undershoot
that could occur once set point is reached.
To utilize this mode of operation, the control outputs of the inner loop must be wired to
the final control elements. The outer loop is not used to directly control the process. It
is used to generate the set point for the inner loop.
Ratio This mode of operation allows the inner loop set point to be defined according to the outer
loop’s process value. When selected, a single entry field (on the Cascade screen in the runtime
application) is provided and used to enter a ratio for the inner loop. The value entered is
multiplied to the outer loop’s process value and the result is used as the inner loop set point.
For example, if the ratio is set to 0.2, with an outer loop process value of 0, the inner loop set
point will also be 0. As the process value of the outer loop increases, the inner loop set point
will also increase proportionally to the outer loop’s process value. So, with an outer loop
process value of 10, the inner loop set point would be 2 (10 multiplied by 0.2).
Note: This mode is typically used in mixing applications (flow) where the outer loop is directly
controlling flow of water for example, and the inner loop would be controlling the flow of
an additive that is required to be mixed at a certain percentage.