Off-the-shelf temperature controller modules

I'm looking for a small embedded controller module which can interface directly to a thermocouple (K-type, I think) and provide about 16 lines of digital I/O to drive control switches and LED displays. We're currently using DIN-cased PID controllers but they're overkill for this project (pricewise as well as capability and complexity) and designing a dedicated control unit may be to our advantage, if we can keep it simple enough to be cheap.

We figure PIC, 8051 or similar will be smart enough to do the job. Can anyone recommend something off-the-shelf that would suit us?

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 To reply, my gmail address is nojay1              Robert Sneddon
Reply to
Robert Sneddon
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Looking only at the thermocouple...

You can use pretty much any of the Maxim "battery monitor" ICs to monitor a thermocouple. The key is that the battery monitors have an ADC that's designed for current sensing, and has a very small (like uV) resolution. The chip also has a temperature sensor for "cold side" sensing, making it ideal for a cheap thermocouple interface. Parallax sells a kit using a DS2760, for example, which I'm using to monitor my woodstove from the furnace computer. If you stick to the

1-wire interface, it only uses up one GPIO on whatever MCU you choose to use.

DS2760 - $3 R8C/26 - $5

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Reply to
DJ Delorie

Regarding the thermocouple input: TI has a decent app-note on using 24bit delta sigma and PGA for reading thermocouples, (example uses TI MSC1201). See the following URL:

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The MSC1201 is a 8051 MCU and has 16 digital i/o pins

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Another options is Silicon Labs also has a family of high precision 8051 products. I've used their C8051F350 product and it was easy to use. This one has 8 analog inputs multiplexed through a PGA to a 24bit delta sigma ADC.

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Regarding the LCD, if it is a low quantity application and you want an easy approach, you can purchase an off-the-self LCD with a serial interface. Then you can just send it a serial string. I've used the LCD products from Scott Edwards in the past, specifically their VFD product.. Easy to use.

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Reply to
Freelance Embedded Systems Engineer

The actual model of CPU isn't that important other than it should have about 16 I/O pins. There's no serious computational load required for this task.

I want to use 7-segment LEDs as they will have to be readable from a metre or two in variable light conditions.

I was hoping to find:

a) (long shot) a single-chip MCU with integral thermocouple interfacing that would require minimal design overhead (i.e. just connect the thermocouple wires to two of the MCU's pins).

or b) a pre-built single-board computer (SBC) with on-board circuitry for interfacing a K-type thermocouple.

Low cost is important in this project; our current proof-of-concept prototype cost us about 300 bucks in parts and is too big physically to do the job we want (liquid bath temperature control in the range 35 deg C - 85 deg C). It's also too complex to use in the real world. We'd like to bring this down below 50 bucks for the controller and sensor (which must be submersible) and simplify the front-end controls a lot.

We're not fixated on thermocouples but we do need +/- 0.5 deg C accuracy without drift over time. The info on the Dallas DS1620 suggests it could do the job as a sensor/digitiser but we'd have to encapsulate it to make it submersible in fluid (water, mineral oil etc.). Thermocouples are available off-the-shelf already packaged and proven for this sort of application.

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 To reply, my gmail address is nojay1              Robert Sneddon
Reply to
Robert Sneddon

A few things I would ask in a project like this.

1) How many are we talking about here ? 1, 10, 100 units

2) What is the life for these units ? Should last 10 years, 50 years ?

3) What are the other 15 port pins used for ? I am sure one pin controls the power to the heater, but what are the rest for ?

donald

PS: There are no single chip MCUs with a built in thermocouple input. But there is a app note from TI that can help you: slaa216.pdf.

Reply to
donald

Oh ya, $50 may be parts cost, but I doubt you'll see a $50 complete unit.

donald

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donald

There are ones designed to measure current shunts

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I suspect it'll be out of the OP's price range. I seem to remember there being a TI MSP with a PGA as well.

Robert

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Reply to
Robert Adsett

Unlikely.

The temperature range you quote is the so-called "organic" range, and you may find it easier to use an RTD (aka PT100) sensor. These sensors

*may* be a tad pricier than a type K t/c, but they are easier to read, either in 3-wire or 4-wire configurations, and you don't need a cold junction (but you do need a stable current source). Also the accuracy you're after may well be easier with an RTD.

Re cost: I've designed temperature controllers (albeit for high-volume production) with way tighter budgets than that ;). Have you considered some of the 1/16th DIN controllers from e.g. CAL or West Instruments? Also, have you considered how you'll amortise the development costs? A PID algorithm is not rocket science, but there are a number of wrinkles (e.g. integral wind-up etc etc) that newcomers tend to have to learn the hard way...

Steve

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Steve at fivetrees

In message , Steve at fivetrees writes

We thought about Pt100 sensors but they're intrinsically more expensive than "simple" thermocouples -- we may have to supply sets of different sensors built for different physical layouts within the bath. Worst case we roll our own sensors using regular twin-core K-series thermocouple wire, insulate them and encapsulate them in thin-walled copper or stainless tubing to prevent water ingress and corrosion. Other possibilities include thermistors but they have their own problems like individual calibration, drift, insensitivity etc. Nothing we can't fix but it's more work again.

We're using a DIN PID module at the moment in the prototype, but unless you know of anyone selling such modules under fifty bucks we're screwed. Best price we've been quoted for small quantities is about a hundred bucks each, and more typically they're about 150-200 bucks. They're also over-complex for naive users to configure, with too many things that can be programmed wrong for the job it's meant to do.

Yah, one reason we prototyped using an off-the-shelf PID to start with. There's a secondary R&D project running with the existing prototype which we're also planning to make some money out of but we're looking to sell units to end-users as well. For those end-users PID *is* rocket science, just about. I've already had to reprogram the PID once after my assistant managed to disable the PID autolearn function which we were relying on to deal with variable masses of water in the bath. This was after he managed to lose the programming instructions which were written in Janglish anyway.

The idea is to roll a neutral set of characteristics for the bath controller to preventing ringing, hunting and overshoot -- with no active cooling system for the bath it takes a long time for the temp to recover from an overshoot, one reason we're considering mineral oil or something with a lot less thermal capacity than water. If it takes longer to reach the target temperature but doesn't overshoot that doesn't matter in user terms. Overshoot in operation is *the* major worry as that will cause Bad Things to happen.

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 To reply, my gmail address is nojay1              Robert Sneddon
Reply to
Robert Sneddon

Quick reply: if you're worried about overshoot, but not about time to reach target, you might want to consider reducing (or at least tailoring) the heater power such that significant overshoot is hard to do. Ideally, you want to be capable of delivering just enough power to overcome losses, plus a reasonable margin, under worst case conditions. Alternatively, some controllers have an output power limit, which can achieve much the same thing.

You've probably already considered this; but I have seen cases of massively overpowered heaters resulting in huge overshoots... esp. during tuning.

Just a thought.

Steve

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Reply to
Steve at fivetrees

Robert Sneddon ha scritto:

here i am, my 2cents

for such a narrow range NTC probes are useful: cheap, accurate and easily sampled by uC onchip ADC, without any expensive precision analog conditioning.

yes you can. looking at wall boiler controller or the like you will find similar product (probes+PID+front_end+actuators) well below 50 (parts cost).

mmmm, +- 0.5C onto K type thermocouples are +- 20uV , 0.2% for RTD. both need precision analog conditioning. regards

Reply to
lowcost

With thermocouples you also have to measure (and assure isothermal conditions under operation conditions) a local temperature within that error budget. So, if you're trying to do 0.5°C you might have to measure the block temperature to within +/-0.2°C and have no more than

500nV/K drift and a few uV Vos to get close to your spec over a moderate Ta range.

RTDs are relatively easy. Best regards, Spehro Pefhany

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"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Spehro Pefhany

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