PID with constraints

Hi All,

I have implemented a PID temperature controller in my model that controls the voltage applied to a resistive heater to achieve a set temperature. However the PID algorithm achieves this setpoint by applying voltages outside of whats practical for my system to achieve the shortest rise time.

I want to know how to constrain my voltage applied so that it only solves for and applies voltages within practical limits say V_ctrl<20. I tried using a global constraint in the ODE interface but it does not constrain the voltage.

Thanks in advance for your help.
Alex