A Very Simple Instance of Control Logic

My latest idea for a blog post might get some readers interested, and others annoyed!

I'm going to describe how I think a humble (old-time) clothes-dryer works, and then I'm going to Google that question, and see how far off I am!

The whole idea of control systems---temperature control systems, for example---is very clever, and over several decades, engineers who design these systems have got them refined really nicely, and it's a lot of fun to learn about.  (While we're doing it, I can show you how this sort of thing is represented using very traditional computer code.  Computer code is a lot of fun, and little kids are beginning to learn it, at school, or at home.)

Your Clothes-Dryer

(1) You put your damp clothes in.  Avoid putting actually wet clothes; unless you have items that are very delicate indeed, it is a good idea to run your clothes through the spin-dryer a second time, especially if you have jeans, or towels, or other really heavy items.  There are two clocks.  One is the timer you set, for how long you want your clothes dried.  The other has to do with a little internal timer, just so that each step goes for a minute, or whatever.
You turn the Dryer ON.

(2) The temperature starts to rise.  The Dryer waits a measured amount of time.

(3) Here we have a branching step.  The Dryer checks the temperature.

(4a) IF the temperature is below the cutoff temperature, the Dryer heats again, for a measured amount of time.

(4b) On the other hand, IF the temperature is at or above the cutoff, the Dryer stops heating, and waits a measured amount of time, and runs its little timer thing (clock).  If there's no time left on the clock, the Dryer powers off.

(5) Dryer checks the temperature again.

(6a) IF temperature is below the cutoff, Dryer heats again, for a measured amount of time, but does not run the clock.

(6b) If temperature is at, or above, the cutoff, Dryer stops heating, and runs the clock.  If there's no time left, Dryer powers off.

So that's essentially it!  As you can see, steps 3 and 5 are actually identical, and steps (4a) and (4b) are identical with (6a) and (6b).  Quite honestly, most people who have a dryer have probably guessed that this is the program of the thing.

The important detail to remember is that when the heating cycle is going on, the clock is paused.  The clock only runs when the clothes are not being heated.

Explanation
What we really want to know is how dry the clothes are.  But dryness is difficult to measure directly.  Here, the dryer does something very clever; it depends on the idea of latent heat.  Water needs some energy (heat) to evaporate; while it is being evaporated, its temperature does not rise.

When the clothes are heated, if the clothes do not get hotter, this means that there is water, and more heating will be good, the clothes will not burn.

When the clothes are heated, if the clothes do get hotter, it's time to stop, or the clothes will get scorched.

To summarize, the dryer keeps heating the clothes (for measured periods of time) only as long as the temperature drops at the end of the cycle.  When the temperature rises at the end of a cycle, the heat is switched off, and the timer starts counting down.  If you've set the 'Dryness Level' to be very dry, the Dryer keeps returning to the heating cycle, and sees whether a little heating is received without overheating the clothes.  This can easily happen; the surface moisture of a towel, or a pair of jeans can dry off, but the moisture in the inner layers with diffuse out, and the item will behave again like a damp fabric.  So given that your Dryer is not going to use X-rays of some chemical magic to analyze your clothes, it's up to you to set the Dryer to go for longer if you have heavy, dense fabrics, and to go for less time, and use lower cutoff temperatures, if you have delicate fabrics that should dry quickly.

Now: I have to Google this question: how does the control on my dryer work?

Back From Google

Well, that was disappointing; all the articles were packed with information such as 'How the temperature limiter switch works', and 'What to do if your dryer won't start.'  A lot of the self-help information on the Web has been put there by people who are trouble-shooters, and not those who are merely curious; or people who think that the mechanism is so obvious that it doesn't need further elaboration!

The diagram at right, called a flow-of-control chart, or just a flowchart, shows visually how the control branches when the various sensors (actually, just a temperature sensor, and of course, the clock(s)) make the logical system ask itself questions.  In this version, the control starts off with a couple of checks, one just for safety, and the other to simplify one of the loops.

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