Справочник Пользователя для National Instruments FP-3000
Chapter 3
Example Applications
©
National Instruments Corporation
3-15
4.
Read the
MODE_BLK
parameter. The
ACTUAL
mode should read
IMan
,
for Initialization Manual. This means the PID is not able to enter
Auto
because the AO block is not in Cascade mode.
5.
Go to the AO block, and look at its
MODE_BLK
.
6.
Set the
TARGET
to
Cas
and
Auto
(check both the Cascade and Auto
boxes). This tells the AO to operate in Cascade if possible, and
otherwise, to fall back to Auto.
otherwise, to fall back to Auto.
7.
Re-read the
MODE_BLK
parameter.
ACTUAL
should be
Cas
. If this does
not happen, refer to Appendix C,
, for more
information.
Now, your loop should be running under automatic control. Verify this by
reading the
reading the
ACTUAL
mode of the PID block. If it is
Auto
, the PID is trying
to control the temperature. You can change the desired temperature by
changing the Set Point (SP) parameter of the PID. Remember that the units
of SP are the same as
changing the Set Point (SP) parameter of the PID. Remember that the units
of SP are the same as
PV_SCALE
for the PID, which in our example is
degrees Celsius.
Tune the PID
Adjust the PID tuning constants to match the dynamics of your temperature
process. A general description of how to tune a PID is beyond the scope of
this document. However, the parameters to change in the PID block are
process. A general description of how to tune a PID is beyond the scope of
this document. However, the parameters to change in the PID block are
GAIN
,
RESET
, and
RATE
, and for temperature control, which is often fairly
slow, the
RATE
parameter need not be used. You can adjust these constants,
change the PID Set Point, and watch how the temperature changes
over time.
over time.
Alarming
In the above example, it might be convenient to have FP-3000 generate an
alarm whenever the temperature goes above 40° C. This behavior can be
configured from the PID block or the AI block. This example uses the PID
block.
alarm whenever the temperature goes above 40° C. This behavior can be
configured from the PID block or the AI block. This example uses the PID
block.
To set a high limit alarm, follow these steps:
1.
Open the PID block and locate the
HI_PRI
parameter. This is the
priority of the high limit alarm.
2.
Set the
HI_PRI
parameter to
2
. Fieldbus alarms can range in priority
from 0-15, with 0 being disabled and 1 meaning that the alarm is
detected but not reported. All other priorities cause the alarm to be
reported.
detected but not reported. All other priorities cause the alarm to be
reported.