The previous two examples, “ta>25” and “ta<15”, are step function formulae. To create a control action across a range of values – a proportional band – a value must be entered in the box headed “Subject to a proportional band of”. This is best described by example. To specify a control action that starts to open a window at 23°C and modulates the degree of opening linearly until it is fully open at 27°C, proceed as follows:
Select “Room air temperature (°C)” from the first drop down list, “is greater than” from the second drop down list, then choose “<numerical value>” and type the value “25” into the third drop down list (specifying the centre of the proportional band) and the value 4 into the “Subject to a proportional band of width” box.
Click on the Create formula button and the formula “gt(ta,25,4)” will appear in the Formula profile box. The gt() function is a ‘fuzzy’ version of the “greater than” step function, its third argument indicating the proportional bandwidth. The dialog will then look like this:
By clicking on the Graph button, you can display a graph of the control characteristic:
This indicates the range over which the action controlled by the profile will be applied. Here you can see the upward ramp function:
By choosing “is less than” a downward ramp function can be created:
Proportional bands give a smoother type of control than step functions and are usually a better representation of real controls.
