modeling of a bulk heater

Mahmoud

Dear all. nowadays I control a temperature of a heater, the heater is consisting of a Bulk Aluminium and at its bottom there is the Coil Heater. When I apply the current to the Coil heater the temperature begins increasing when the power is switched of the temperature continue increasing till it reaches a maxima. then temperature decreases, while it decreases if the power is switched again it continu decreasing till reach a minima. and so on. Is this a form of inertia in the heater? I have developped many PID controllers that control Motor Speed, Flow, Pressure using MATLAB and VISSIM, i did that because i was knowing the modeling ( Transfer Function ) of the system i want to control, but now I could n't do that because i do n't know the Transfer Function of such heater...

How to model this heater?

Is there any way to model it?

Plz guide me in my PROBLEM.

thanx in advance.. M.Shendy