CTEK Automation Control Application APN007 - Manual (Page 32)

Application Note – Automation Control Application
APN007 25 July, 2014
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boundaries of an operating range that would otherwise be in conflict with level sensitive
thresholds. The diagram and tutorial following demonstrates this application.
The last key concept is the order in which thresholds are evaluated. This is critical because
thresholds MUST be defined on the input screen in the order that they will be evaluated. In MOST
cases this means that the smallest value to be considered must be on the first threshold line of
the input edit screen and that the largest value to be considered must be on the last line of the
input edit screen. Values between the largest and the smallest should be entered in increasing
value from the top.
Thresholds – An Example
Figure 18 shows a simple example of an application designed to keep a tank level between 2 and
8 feet. As the tank level rises past eight feet pump #1 and the level light come on. If the tank level
continues to rise past fifteen feet pump #2 is started. At the eighteen foot level an alarm is issued.
As the tank is pumped down a below eight feet the level light is turned off and at the two foot level
both pumps are turned off.
The eight foot decreasing level edge triggered threshold demonstrates one application of edge
triggering. The requirement is to both turn the light on and turn it off at the eight foot level. Clearly
these two things would be in conflict if level sensitive thresholds were used. However, because
the light is on at any level above 8 feet and turned off on any transition below eight feet this
requirement can be met.
Figure 28 - Threshold and Direction