Electric placemats

My fiancee suggested a project for me - make placemats that would keep plates warm throughout dinner, and therefore keep the meal palatable (she really doesn't like cold food).

I see two ways to do this - induction heating, and a ceramic thermal mass with heating. Leaving aside the issues common to both, ie how much power would be needed, how to control the temperature, and running power to them on the kitchen table (or whether batteries are even feasible), let's examine the options.

Induction heating

There are some fancy stoves that do this. From memory, it basically electromagnetically induces a current in a metal body with AC, which heats up from all the eddy currents. This requires pots and pans which are suitable for induction, but on the other hand, it can be more safe because you can put a plastic container or your hand on the stove and nothing gets hot, as long as it isn't already hot from heating something before.
However, there are some downsides. Few plates are made of metal, so you'd need special plates, or rather a plate and platemat set, for every person eating. Cutlery is also metal, so it would probably heat up too - you wouldn't want to leave your cutlery on the plate for a few minutes, then pick it up again burning hot! Finally, you have to run AC to your placemats, which means thinner cables than DC, but more dangerous current to have around drinks and knives.


Heated ceramic

I imagine a ceramic insulator/thermal mass around nichrome wire, or tungsten heating element such as used in electric frypans. The base would need to be a good thermal insulator, possibly wood, to stop it from burning the table, which might make it rather thick. You could use AC or DC power. You can use any plates that can stand being warmed.


Let's summarise:

Induction heating - pros:
Safer heating mechanism
Heats faster
Thinner cables
Thinner placemat?

Induction heating - cons:
Requires plate + placemat set
Cutlery is heated too
More dangerous AC

Heated ceramic - pros:
Can use any suitable plates
Choice of AC or DC (DC would have thicker cables, but is safer)


Heated ceramic - cons:
Probably thicker placemats due to insulation
Takes more time to cool off
Heavier?


I'm not sure which of these approaches would use more power, or be more costly to produce. Then there is heating control, but if I solve my electric frypan heating problem first, I can use that solution.

I'm going to ask Hackerspace Brisbane for their thoughts. 

A better electric frypan heater control

Cooking is based on applying heat to food, ideally in a consistent, controlled way. Electric frypans suck at consistent heating. This is because they use a cheap thermistor, and dump all their power into the element until it reaches the set temperature, and then *turn off* completely, so your steak stops sizzling, until the temp drops enough and it turns the element back on again.

For that matter, some electric stoves also suffer the same problem, but because of their thermal mass the temperature doesn't drop so much.

Gas stoves are highly regarded by cooks, because you can finely control the amount of heating, and it doesn't turn itself on and off randomly at inconvenient times! But gas stoves are more complicated, expensive, require maintenance (replacing gas bottles), and some people are scared of using gas.

Why should electric heating work so badly?? Don't we control electricity? Isn't it supposed to make things more flexible?

Well there is a simple, well-known way to control electric power for motors, heating, etc: Pulse Width Modulation. I don't know why frypan makers don't use it, except that it's a bit more complicated and maybe more expensive. But you vary the % of time you are heating, and you can do it very fast. It changes the control mechanism from desired temperature to % of power to output. And you can do this pretty easily with a 555 timer IC, make the dial control the pulse width, and pass the AC current with a bigass MOSFET, SCR, or relay - solid state if possible, or there will be lots of clicking.

My friend Dylan points out that changing the dial to increase the power just means the % is increased, which is different from the existing functionality where it will dump all power until it reaches that temperature. In short, this means it will take a lot longer to reach the desired increase in temp (or a drop). I'm not sure how to approach this problem. Perhaps a hybrid thermistor and PWM solution, where some fuzzy logic figures out what PWM value to use to maintain the temperature, and nudges it up and down if the temp changes. That immediately brings to mind the idea of using a microcontroller like AVR (eg Arduino), instead of 555 Timer and so on, but I'd like to avoid that, at least in the final product. Still, it could be an interesting project!