When taking a shower last week, I did some serious thinking to an idea I have since a long time: automating the shower.
Two values would be regulated: pressure and temperature. Cold water would control pressure and hot water would control temperature.
The system would remember settings for each user, and would provide default values for strangers.
Buttons in the shower or on the wall next to it would allow the user to change settings manually and save those for future use.
A pressure control loop of the mixing chamber controls the cold water valve, with feed-forward via the cold water intake pressure. A temperature control loop of the mixing chamber contrls the hot water valve, with feed-forward via the hot water intake pressure.
Pressure is controlled with cold water: cold water valve is opened to generate a given pressure in the mixing chamber. If the cold water is used somewhere else in the house, the pressure drops in the cold water network and the feed-forward loop would compensate by opening the valve further.
Temperature is controlled by the hot water valve. That valve is opened to create a mix of water of a given temperature. A drop in the hot water network would be compensated via feed-forward of the pressure in the network. The hot water valve would open further to prevent the mixing chamber to drop in temperature.
Opening the hot water valve would increase the pressure in the mixing chamber. The pressure loop will then close the cold water valve to maintain the pressure. Less cold water entering the mixing chamber would warm the water and the hot water valve will have to close. Closing that valve will decrease the pressure and the cold water valve will open. This will lower the temperature… To prevent oscillation, the loops will have to be carefully tuned to prevent oscillation to build-up.
The valves could be made of RC servo mechanically linked to ball valves. When not in use, the valves are closed and power is removed so no movement is possible and electric consumption is negligeable.
Finally, the whole automatic control will be easy to bypass using isolation valves to switch which control method (original fosset or automated) is used