I have created a mock up of a humidity chamber which i will soon be integrating into a temperature chamber for undergraduate project. I am confused on how i can control the system. I have currently a humidifier with a relay built in, this relay is going to switch on and off from a PWM signal generated in a controller. Also within the chamber i have a mini fan to mix the steam and a analogue voltage output humidity sensor which i can use to get feedback from the system. At the moment all of these are linked up to a computer but i need to design the controller. I was thinking PID. Im having trouble modelling the system to work everything out. The plan was to introduce a step response in terms of duty cycle so say 20% and then check the corresponding voltage output obtained from the sensor. Im having trouble working out how i can find the transfer function of the system and generally modelling the system for control analysis. How can a step response in terms of % create a transfer function with an output in volts. The humidifier runs at 6 volts so its either switched on (6 volts) or off (0 Volts) Once iv done this i need to design the controller Any advice would be great thanks.