Top24,
If you are just trying to simulate the effects of a control system without using any hardware, then that may be straightforward or extremely complex, depending on your system model.
Start by drawing a block diagram on paper to describe your system. Show your heaters, temperature sensors, and the power control device. At each point in the system write down what the relevant variable is. At the heater output the variable is the temperature. At the output of the temperature sensor the variable is probably a voltage proportional to the temperature. The power control device you mentioned earlier has current (4-20 mA) input and power as the output. Your control system will have the temperature sensor output voltage and the setpoint (probably just a number) as inputs and control current (4-20 mA) as output.
Next write equations for each block relating output to input. In the heaters for instance there will be a time delay due to thermal conductivity and heat capacity as well as the power to temperature transfer function. When you have the equations for all the blocks, combine them into a system of equations. This system of equations is your simulation of the system. Implement the equations in LV. Run it with various values to see how things behave.
You may write the equations initially as differential equations but it may be better for simulation to convert them to difference equations. Then each iteration of your loop represents one timestep.
Lynn
