Many houses use electric hobs, these are quite inefficient and waste a lot of energy by over-heating and then stay hot for a long time after being switched off. They are present in 31% (Gov Report 9: Domestic appliances, cooking &
cooling equipment) of homes all over the uk and so a small energy saving would equate to big net savings.
On of the inherent inefficiency of the hob when it is being carefully used, a large amount of energy is wasted by users who are not using their hobs properly, such as boiling over or maintaining a temp that is above what is needed.
To produce a device capable of monitoring and controlling the temperature of a hob to ensure that it is operating at its most efficiently. As well as the sustainability benefits, it should enhance the users cooking experience and be easy to use.
The average house uses 1.5kwh per day cooking.
How much energy does it take to boil 1 litre of water on the hob?
Our experiments showed that bringing a pan of water to the boil over 10 minutes used 0.2Kwh.
Additionally we found that after bringing water to a boil, we could continue to boil water for 4 minutes after switching it off due to the retained heat in the ring.
How will our project help users?
Evidence shows that telling people alone is not enough to make them change their habits, that is why our project will aim to automatically improve energy usage without the user having to do anything. Additionally it should make the cooking process easier which will encourage them to use it.
-Arduino R3 Uno
-DS18B20 waterproof temperature sensor
-Max6675 breakout board
-K type thermocouple
-i2c board for LCD
-240v relay (5v input)
Using sensors to plot temperature
Our first step was using both our sensors to read the temperature of the hob and the water.
Both sensors use their own libraries to convert data to a useable format, you need to use the correct functions to call the data from the sensors in order to use it.
Once we had both sensors working together we could easily record the temps on the serial plotter like this.
Relay control of the hob
This hob is very simple, having essentially only on/off switches. This meant we could control it using a relay inline in the positive feed.
Simple on or off relay control is as simple as writing pins high or low, but for more accurate control “time proportioning control” is used, this is where a window of maximum on time is created, then the relay can be turned on for a proportional of that window equivalent to the desired output.
ie if 50% power is required relay is on for 1/2 the window.
This system is very easy to set up and can be good for use with relays due to the fact it doesn’t produce excessive switching.
The bulk of this project has been spent on understanding and applying control to the hob. We started with a very simple “bang-bang” control, however this proved completely unusable due to the large thermal lag in the system.
We needed something better, hence we started investigating PID control.
This highly tuneable method is an industry standard for maintaining tight control over closed-loop systems. It uses 3 terms to compute the output needed to get a measured variable to a set value and keep it there, the first is proportion, which means increasing the output in proportion to the gap (error) between the measued and desired variables. It also takes account of the previous errors in the integral term and uses these to try to address consistent errors, this makes it adaptive to change in the system. The final term is the differential, which tries to predict what the next error will be based on how the error is changing, this can be used to make the system react quickly to upsets.
Writing an entire controller from scratch would be time consuming, luckily there is an arduino library available for the job. However implementing the library is not the easiest task, it requires careful tuning and a good knowledge of the underlying system.
After several hours attempting to tune it, the controller was only able to be kept within 15* of the setpoint. However this could be improved with additional tuning time.
The user inteface
A key part of the project has been trying to add benefit to the user experience of cooking on the hob. Inspiration came from the simple interface on a microwave, where an lcd displays time and input is given by buttons.
Bringing it all together
The final version contains 2 sequential PID controllers, setup to feed into one another. Using this method, the stabilisation temperature of the water was improved significantly, now being stable to within a few degrees.
updated code, 23/04/2018:
In conclusion, this project has demonstrated the use of PID control to reduce the energy consumption of a hob.
With some careful tuning it could have a very good impact on reducing as well as benefiting the cooking experience.