Thermostatic Faucet

2016-04-13 | By Sergey Sokol

Overview 

When I lived in Japan, my apartment had an electronic interface for the shower that made the hot water heater output exactly the water temperature I wanted. I was a big fan of this ease of use and thought that it could be applied to sink faucets as well. Upon returning to the United States, I realized that while I could use instant water heaters, the gas or electrical infrastructure simply wasn't there to retrofit these installations into current construction, especially the bathrooms. Also, in the few cases the infrastructure was available, the interfaces had been clunky, with definite room for improvement. As I was mulling this over, I realized valves could be installed on the cold and hot water lines at the point of usage and electronically controlled to automatically provide consistent water temperatures. This setup could be used in new or old construction without any infrastructure changes, would increase safety as the temperature would never go above the desired point, and could even save water by optimizing the process of replacing the cold stagnant water in hot water lines.

Approach

In order to reduce the difficulty of a retrofit, the design must be simple to install. As there will be both daily users and guests, the interface must be intuitive and able to accommodate both regular users and first time users. The interface for a shower or bath is much more simple than the interface required for bathroom or kitchen sinks. For the faucet, the valves and control logic will be installed at the point of use, going between the copper tubing and the braided metal hose. With this setup, the end user will have the flexibility of using their current faucet or obtaining a specific faucet for this without controls. Plugging into the outlet under the sink that is generally provided for garbage disposals will provide power. The controller itself will be a simple microcontroller-based PID controller, with a single thermocouple measuring output water temperature and two motors turning the valves. For the prototype, the interface will be a simple potentiometer to control both flow and temperature. In future incarnations I anticipate incorporating a flowmeter to give greater control, but at the moment, the flow will be roughly estimated with the main focus on maintaining the proper temperature.

Challenges and Design Considerations

The biggest challenge I anticipate is powering the controller for the shower. Under sink plugs are very common but finding a similar power source has been, and unfortunately continues to be, a nearly insurmountable challenge. A different yet related problem is what to do when a power outage occurs. Being able to use the system when the power is off and also turning the water off if the power cuts out when the water is already on are both concerns.

Another large, though more secondary, challenge is to keep the costs minimal. Thermostatic faucets are not an original idea. However, even those products designed specifically for new construction costs thousands of dollars for a single, simple faucet. To be competitive with these, a significantly lower price point needs to be met. Though it may seem counter-intuitive, the easiest portion of this project is likely the actual PID control algorithm. It is a fairly simple control problem that has been solved, perfected, and well documented over the years.

End Result

The implementation to create a proof of concept went much as anticipated. The interface between the motors and the valves were a bit of a challenge, but with the help of a mechanical engineer, a crude but effective rig was setup. We selected multi-turn valves to make certain that the motors would be able to turn them effectively. While this is effective in making sure the motors are never under any undue strain, it made the response time a bit sluggish. Also, while the valves are fully opened after four turns, the water comes out at nearly full flow after one turn. As anticipated, the flow had to be adjusted manually, setting the parameters in the software to consider one turn as one hundred percent flow.

The PID parameters needed tweaking, but it was a fairly simple matter of adjusting those parameters to get a critically damped response. The response time, while slower than ideal, was acceptable. The concern at first was there would need to be another thermocouple on the hot water input, but the PID worked well as a single input, dual output system. There also needs to be some sort of feedback to indicate the full closure of the valves to both ensure that the valve is entirely closed and also to limit the time the motor is stalling while attempting to close the valve. This will likely be done by using a half effect sensor to detect when the current consumed by the motor increases.

Finding a power source for the shower unit has thus far been unsuccessful, a concerning stumbling block. Also, the current prototype is still so far from manufacturability that the second challenge of being cost effective is not even a goal at the moment.

Moving Forward

The basic concept of a thermostatic faucet is not only possible, as shown previously by large manufacturers, but also within the realm of anyone with reasonable electronic and mechanical ability. To move beyond a proof of concept, the device needs a considerable amount of work. The main items identified are:

1) Solution to power concerns
2) Flow feedback
3) Position and current sensing feedback of motors
4) More elegant mechanical interface between motors and valves

Other than the power, all other concerns can be overcome with time and effort, leaving power the only potential problem that cannot be overcome with previous construction. At this time, it is difficult to decide whether the project is worth the time and expense of pursuit and may be temporarily shelved until a satisfactory solution is found for the power.

Code - main.c: