Refrigerator control

radolin · 2006-02-28T08:58:23+00:00

Hi gyus,I have an old little heater powered refridgerator whose thermostatburnt out. I couldn't find a replacement, so I figured I'd put togethera microcontroller, a temp sensor and one relay instead.I decided to just copy the thermostat hysteresis way of controllingthings, so let's say I want to keep 4 degrees celsius inside, I turnthe heater on when it's 5 degrees, and off when it's 3.All looked so simple :). But in practice when I installed the newelectronic controller it turned out the system has huge lag/inertia.For example if it's 5 degrees and I start the heater the temp insidewill rise up to 6-7 before it starts to fall down, obviously the heaterneeds time to warm up. And when I turn it off at 3 degrees the hatercompartment is so hot, that after the heater is off it continues tocool down to 0 degrees celsius inside (my youghurt freezes).I have searched around and the obvious solution for such problems isPID regulation. The problem is I don't want to make a complicatedcircuit for variable control of the heater.Can you suggest an algorithm to compensate for the lag/intertia butstill keep my simple on/off control of the heater?I have plenty of power to do calculations, 1024 bytes of RAM and 15kbFLASH on the MCU, I know it's an overkill for this task but that's whatI had lieing around. Let's hope with your help I will put this powerto some useful task :).Regards, Rado

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Tim Wescott 20 years ago

See

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and other articles on, or linked from, my website. Sometime this spring my book,

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, should be coming out. I hope it will be helpful (it does go into the math, but I tried to make it as painless as possible).

There's PWM and there's PWM. To effectively PWM a driver you need to switch it slower than the time constant of your plant. In this case your plant's time constant is measured in minutes, if not 10's of minutes, so having a PWM period of a minute (or two, or five) is no problem at all. The two biggest concerns that you should have if you choose to PWM are wear on the relay (which you could fix with a solid state relay) and any efficiency lost in the cooling process when the heater is heating up but not yet making the cooler percolate.

If you use my suggestion of switch then wait a while you will be guaranteed switching times that are no shorter than the wait, and will probably be longer because the temperature will change.

1LSB accurate or repeatable?

You may get better resolution (_not_ accuracy) by measuring significantly faster and averaging. Averaging over a power of two interval is easy, because you just add up the readings and right shift the result. If the last ADC bit is really random you'll pick up quite a bit of resolution this way.

I'd suggest 32 times a second -- take 16 samples, add them up. Don't shift them down for internal use -- use the extra 4 bits of noisy data for the resolution they'll give you. Do shift them down for temperature display, however.

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Jerry Avins 20 years ago

...\

My experience with an absorption refrigerator rated 90 to 135 volts was that cooling capacity maxed out at 120-125 volts and dropped a bit at

135. (Too much froth, not enough liquid?) 90 volts cooled at about the same rate as 135, and it didn't cool at all at 85 volts. What kind of PWM can be done with that? Just think of the thermostat as a long-period pulse-width modulator if that scratches your itch. :-)

A sticky switch is hysteresis of a sort.

We're on the same track here.

...

Jerry

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Tim Wescott 20 years ago

Find out how long it needs to be on to be efficient and make sure you keep it on that long.

Yes, the thermostat acts as a long-period pulse-width modulator, but the period is long enough that the plant temperature discernibly changes. The one and only one PWM'd thermal control system that I am familiar with had a period of about 10 seconds. It was well within the time constant of the 600 cu-ft thermal chamber that it controlled, but outside of the time constant of the LN2 valve (at least once the gaseous N2 was purged and the liquid actually made it to the chamber).

Keeping in mind that I don't think a PID controller with a PWM'd actuator is necessary, I doubt that a 10 second period would be right in this case: I think the percolator would be wasting a lot of energy on thermal cycling for not much cooling. If you _were_ going to use PWM you'd have to figure out how long the percolator needs to warm up (and hence how much energy you waste), decide how much energy you wanted to waste doing that vs. actual cooling, then set your minimum PWM interval accordingly.

From my perspective this is mostly moot: I would just use the thermostat, and let the fridge temperature vary a bit.

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sQuick 20 years ago

what an interesting thread this has been......

Never thought that simple on/off cooling control could make such intersting read.

sQuick..

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radolin 20 years ago

Yes, indeed!

I'm really impressed with the replies to my first post in the sci.engr.control group.

Thanks everyone for their time and expertise.

I'll post the results when there's some progress, or ask again if I reach some roadblock :).

Greetings, Rado

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Andy McC 20 years ago

ITYM 'faster'. I don't mean this as a jibe, just clarification.

While there is a consensus that, for a domestic fridge, PID, modelling, etc are unnecessary, I think that as a practical learning exercise it has a lot to offer. Next stop fuzzy logic?

Further thoughts:

Use the door operated inerior light switch as an input to: maybe switch off while open, switch on for longer once closed (shift the setpoint, or feedforward an integrated term), sound an alarm if open too long. Other alarm conditions can also be incorporated.

With the thing connected to a PC we have the potential for a 'web-fridge'. You dont have a barcode scanner to hand do you, Rado?

This has been a fun thread and goes to show that 1 problem + n control engineers = n + m solutions.

Cheers, Andy

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Jerry Avins 20 years ago

I assumed he did too.

PWM is just a way to exercise proportional control. An absorption refrigerator is not amenable to proportional control. If enough heat is supplied, it cools; otherwise not. The rate of cooling is nearly independent of the supplied heat within the operating range.

...

Jerry

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Jerry Avins 20 years ago

By all means, do. You generated a lot of interest. Good luck!

Jerry

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Tim Wescott 20 years ago

Sigh. Thanks.

One of the best pieces of advice that I was never given (it was related to me by a software engineer who really didn't do control) deals with figuring out whether your controller should have an inverting amplifier or not. The advise is "Count all the sign inversions in the system, then add in the one you missed." I guess this is the one I missed...

I have a knee jerk negative reaction to that term, because "fuzzy logic" is one of those things that makes the less talented get all starry-eyed. From what I can tell it's a useful technique but it's not a panacea -- it's just a structured, formal way to apply common sense to difficult nonlinear control problems. If you can do the control yourself, but can't figure out how to build a controller for it, then fuzzy logic may help you get there.

What it _can't_ do for you is verify that you've built a controller that will actually work, and if you're using it to control a nonlinear system, or if your controller has introduced significant nonlinearities to an otherwise linear system, then you still have to analyze the resulting system to make sure that it will suit. Since the essence of a nonlinear system is that it's behavior is not global this means that you have to either subject it to difficult and messy stability analysis, or you have to run about a gazillion simulations on it from all sorts of screwy starting points.

To date I've always been able to find a common sense solution without using fuzzy logic just by telling myself "well, if I can't do this directly I can always learn fuzzy logic!".

Using the door switch for additional information can't be a bad thing.

I think that's n * m, with m unbounded.

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Andy McC 20 years ago

[some snipping, without, I hope, changing the meaning]

This is pretty much my take too.

Hmm, yes, I failed to qualify 'm', my bad.

A manager from my past once said words to the effect that the more difficult a system is to control, the more difficult it is to control the engineer who can control it.

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