Calculation example: controlling when a pump runs

In this example of using the Calculation feature, we want to control when a pump shuts down and starts up again. The pump we want to control is called Pump_3 and it pumps a liquid into a tank called Tank_FR2 as indicated by the illustration below.

We want to prevent the pump from running when the level of the tank is above a certain level. If the level in the tank falls below the level defined, then the pump should start up again. Our pump has two states, OFF (0) and ON (1).

$images\DEF_Calc_Pump_3_Tank_FR3.gif$

In the Definition module, double click on Pump_3 and go to the Calculation tab.
Select the Command atom in the Atom list. (We don’t want to use the State atom, which can be overwritten by the PLC when informing IGSS of the present state of the pump.)
Under the Execution trigger select the option On object changes in expression, which ensures execution when changes occur on the object in question.
In the field under Expression, key in the following parameters: IIF(Value('Tank_FR2')>4500.75, 0,1)

The expression says that if the level of the tank, Tank_FR2, is greater than 4500.75 m3, then the command 0 is to be sent to the pump, Pump_3 and remains in force until the level falls below 4500.75 in which case the pump is sent the command 1.

Make sure to click the button Test Expression… to ensure correct syntax.
Key in a comment about how the expression is being used for controlling purposes.

$images\DEF_Calc_Pump_3_Tank_FR2calcparameters.gif$
Set the pump’s state to 1 and save and install the configuration.
Activate the IGSS Starter and test if the expression is executed by changing the process value of the tank to a level above 4500.75 m3. Watch for the state change to 0.

If it's necessary to record the workings of the Calculation function, then make sure All changes and Log outgoing commands is selected on the object's Data Management Definitions tab.

For a complete list of the object functions available for Calculation, please click here.

For a complete list of the arithmetical functions and operators available for Calculation, please click here.