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System Monitoring
7.16 MONITOR EXAMPLE
Now let's look at an application of the monitor program. In the darkroom problem of section 6 the strip
chart program was used to monitor the temperature of a darkroom, and of a controlled-temperature bath
within the darkroom. Let's expand upon this example.
The MIMI TOR program allows you not only to monitor a system, but to control the system using limit
tests. Consider a production darkroom with a controlled-temperature bath that must be maintained
within one degree of 25°C. As in the example of section 6, two type T thermocouples are used to monitor
the temperatures. The room-temperature thermocouple is connected to channel 3 of the HP 3421A Data
Acquisition/Control Unit, and the bath-temperature thermocouple is connected to channel 4. In addition,
the bath heater is controlled by a relay, which in turn is controlled by actuator channel O of the
HP 3421 A. The HP-71 is connected to the HP 3421A, and to an 80-column video interface and video
monitor, as shown in figure 7-2:
MONITOR
onrTo
RELAY LH)
C3
BATH
BATH
HEATER
TEMP
VIDEO INTERFACE
CONTROLLED TEMPERATURE BATH
ROOM
TEMP
1
0
3
4c
~
3
a
00
00
HP-71
HP3421
Figure 7-2. Temperature Monitoring System With Actuator Control
In our example the MCMITOR program continuously displays the room temperature and bath
temperature on the video monitor.
In addition, two sets of limit tests are specified: the lower and upper
control limits, and the lower and upper alarm limits.
The lower and upper control limits are set at 24.5
and 25.5°C. When the temperature falls below 24.5°C, the actuator channel is closed, turning on the
heater by means of a relay.
When the temperature rises above 25. 5°C, the actuator channel is opened,
turning the heater off.
The lower and upper alarm limits are set at 24 and 26 °C.
If the control system
fails, one of the alarm limits will be reached and the HP-71 will beep.
In our example, the MIMI TOE program performs one other function. Temperature readings are stored
in a file in RAM every 15 minutes.
7-25
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