Limit Module
The Limit module is used to perform simple tests to see if an asset is either Inside or Outside the expected operating limits of the machine.
It is very similar to the Control module, except for the fact that the Control module takes a single target and a span, while the Limit module takes an upper threshold and and lower threshold instead.
The analytic returns a list of time-frames, and the assets status (Inside or Outside) during those time-frames.
Starting Out
When you first create a new Limit analytic, you'll see the graph below…
The node in the middle is the Control node, which takes the measured value and both the maximum and minimum values, and determines if the the max or min has been exceeded.
On the right is the Control Output node - if the value going into this is true, the analytic considers you outside the limits. If it's false, you're inside them.
Why Two Nodes?
There are a number of reasons why systems can deliberately drift outside the expected range, such as when the system is changing speeds, running manually, or as part of starting up and/or shutting down.
The gap between the 'control' node and the 'Out Of Control' node is where you can add any of this additional logic.
You can also add Timers to your graph if you want to delay things. Ensuring that it stays out of range for more than 5 seconds can help deal with sudden spikes, inrushes or sensor issues.