AT-2471 -- Stability of an Air-Temperature Control Loop

On certain types of air conditioning systems, an increase in the gain of the controller willreduce the energy requirements of the system. In those systems, it is advantageous to increase thegain, thus reducing the throttling range, but a value of the gain that is too high will cause the loop to become unstable.

This investigation explored the borderline of stability and instability of a pneumaticair-temperature control loop, and developed an analytical model to predict the dynamic performanceof the control loop. The model of the coil that proved to be adequate was a simpleone that treated the coil as a single block of metal. The model of the valve required inclusionof the effects of hysteresis, and the model of the controller was non-linear in that itrequired a different time constant for decrease of the control pressure in comparison to anincrease in control pressure.