I'm a physics student so don't do much in the way of electrical engineering, pardon my ignorance. However I'm looking at using a PID controller with a resistance thermometer sensor and heating element plant, with a reference point of some resistance on the thermometer. That is heat up a device to a particular temperature. However when considering generally whats going on here, I get a bit confused. That is considering the stability of the system, the PID controller seems to be 2nd order, surely there is no unstable point for a 2nd order transfer function? I know I can simply use the Ziegler–Nichols method to come up with tuning parameters, what I don't get is how there is a point at which the system starts to become unstable? Separately does anyone know of any digital tuning scripts that would work with labview, well free ones anyway? (Ignore this really, it's a long shot to a separate issue).