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Limit Programs
log CLOSE 1 @ WRIT 2 ® OFEM 1
11a IF I<3 THEW HAIT .2
128 HET I ® FRINT "ALARM "3 TIME#$
126 EHD SUE
This limit program examines the measurement to decide which action to take. If the measurement is less
than 20, two actuators are closed.
If the measurement is greater than 45.66, the program switches an
alarm horn on and off five times by controlling an actuator, then prints a message. If the measurement is
between 20 and 45.66 the program opens two actuators.
E.3 TRANSITION EVENTS
Transition events occur only when a measurement crosses a specified boundary.
Some history must be
available in order to determine that a measurement is out of bounds for the first time, defining the point
at which the transition occurs.
To build this history, a limit program can use a data file to store the last reading that triggered a limit
action. Suppose a limit action is desired which prints a message when a measurement rises above a critical
point of 10 volts.
In this example the limit program is part of a data logging application which scans a
group every two minutes.
1a SUE TEAMSLIMOR
SB ON ERECE GOTO 98
= AE=IGH #1 TO HISTORY :MAIH
48 READ #13H
-E RESTORE #1
sl IF R18 AMD H:18 THEM 128
dE PRINT "Transition occured:
"3s TIMEF
SE GOTO 128
20 OFF EREROE
Tae CEEATE OATH HISTORY, 1,2
11ey A=SIGH #1 TO HISTORY
128 FEIMT #13F
Iz A=SIGH #1 TO *
148 EMD SUE
Lines 20 and 100 to 110 are used to create the file HI'= TCR" if it is missing. Line 60 determines if the
transition has occurred, and line 70 performs the limit action if the transition has occurred. Line 120
stores the current reading.
Notice that this system does not work very well unless the limit test is set up correctly in the setup file.
For a transition to be observed, the history file must contain a reading below the critical point.
This
suggests that a "high" limit test should call the limit program for values that occur prior to the critical
point.
In the example above, the limit test should be at nine volts.
This would permit values between
nine volts and the critical point (10 volts) to be stored in the history file. As the voltage rises, the history
file begins to record voltages until the transition is seen.
Another issue of concern with transition limits is the effect of spikes in the signal data.
Using our
example again, suppose the system is yielding readings of about five volts.
A spike going directly to 12
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