ratio feedforward hvac

[Sheppardp]

Hi all,

I am a newby to Labview running the 2010 Evaluation with Control Design and Simulation Module and PID Module.

My field is HVAC controls and I am trying to design a simulation of a control loop that implements a very simple ratio function known in the industry as outside air compensation. Here the temperature of the outside air influences the setpoint for the heating water in such a way that when the outside air temperture is say 20 degrees C the setpoint needs to be say 40 Degrees C and when the outside air temperature is say -15 Degrees C the setpoint for the heating water is 90 Degrees C. This is generally known as a heating curve and is a function every DDC (Direct Digital Control) controller in the HVAC market implements as a standard. See Picture below.

It mus be my lack of experience with LV but I cannot seem to find a function that implements this. I searched the forums and the help files, but no luck. The PID setpoint profile function provides a ramp but based on time, the PID lead Lag function also does not seem to fit. Should I use an array as an input to the PID controller setpoint or is there a better method?

Any help would be greatly appreciated.

Peter