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Old 06-17-2012, 12:44 AM   #1
kladue
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Default I2C Chips for brewing controls

I have finally found a analog IO chip with I2C interface that will support 0-10V outputs to drive 0-10v input proportional control ball valves. The TI AMC7812 provides 16-12 bit analog inputs, 12 analog outputs, and 6 GPIO output/inputs. With a couple more devices like the LTC 2487 from Linear to do thermocouples, a PCA9535 for digital outputs, and a PCA9685 for PWM control, most of brewing applications could be handled with 4 IC's that would run about $30. This is overkill for 99.9% of the systems out there as brewtroller/BCS is all that is needed, but for the terminally crazy automation folks this would open up all sorts of possibilities. I expect the code and calculations to be beyond the Arduino platform capabilities, but with the ARM9 mini 2440 platforms or Raspberry PI platform there should be enough capability to make this work.

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Old 06-17-2012, 04:29 PM   #2
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Yeah, i2c is easy to use and you can find tons of drivers already written in C for most platforms. I've used it for RTC applications on the PIC platform and it works great.

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Old 06-17-2012, 08:01 PM   #3
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I wonder if anyone else has worked with the TI-AMC7812 before?, most of the rest are already available on individual boards.

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Old 06-17-2012, 09:00 PM   #4
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Hi

Looks like a nice chip. Price is right if you hit the "free sample" button .

My main concern with anything this big is soldering it down. That's not a problem if I'm at work. It's a bit more painful if I'm here at home.

My only other concern would be noise / drift. The data sheet is missing the usual noise histograms, linearity charts, and drift data. Most of the rest of TI's portfolio is full of that sort of information. The noise and drift numbers they have in the data sheet are not bad for a 12 bit part, I just wonder why no data.

This is more what I'm used to seeing from them:

http://www.ti.com/lit/ds/symlink/ads1271.pdf

Could just be the TI / Burr Brown divide showing it's self.

Were you planning to use the 3906 temperature sensor option?

Bob

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Old 06-17-2012, 11:31 PM   #5
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It appears what the are willing to share is in the manual, not bad for the intended use it will see. The 2N3906 sensor is interesting, but I was looking to standardize analog input ranges 0 - 5 V and use precalibrated voltage output temperature sensors. I am debating whether to use onboard 250 ohm shunt resistors for use with 4-20 Ma loop powered devices, or just make that an external feature.
The automated system uses loop powered pressure and flow transmitters, voltage controlled mass flow controllers for gas flow to burners, and voltage input proportional valves for liquid flow control. With these chips I should be able to copy part of the IO functions performed by the Opto22 hardware in a much tighter package.

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Old 06-18-2012, 12:31 PM   #6
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Hi

One issue with current loop is weather you need to galvanic isolate it or not.

Bob

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Old 06-18-2012, 12:52 PM   #7
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As long as the sensors are electrically isolated from the process there should be no problems, outputs are going to be photo isolated. The goal is not to repeat the design problems with direct attachment to the microprocessor like the current controllers, rather use satellite boards where the noise can be dealt with and microprocessor disruptions prevented.

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Old 06-18-2012, 01:23 PM   #8
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Quote:
Originally Posted by kladue View Post
I have finally found a analog IO chip with I2C interface that will support 0-10V outputs to drive 0-10v input proportional control ball valves. The TI AMC7812 provides 16-12 bit analog inputs, 12 analog outputs, and 6 GPIO output/inputs. With a couple more devices like the LTC 2487 from Linear to do thermocouples, a PCA9535 for digital outputs, and a PCA9685 for PWM control, most of brewing applications could be handled with 4 IC's that would run about $30. This is overkill for 99.9% of the systems out there as brewtroller/BCS is all that is needed, but for the terminally crazy automation folks this would open up all sorts of possibilities. I expect the code and calculations to be beyond the Arduino platform capabilities, but with the ARM9 mini 2440 platforms or Raspberry PI platform there should be enough capability to make this work.
I've used the PCA9532 in a bunch of designs. If you want code, just ask. I write in C. I think this part is very similar to what you are considering using.

For thermocouples, by far the easiest way is to use the MAX6675. It's SPI, not I2C. I've used this a couple of times and it is FAR easier than the custom circuits that I've created before - no trim pots, no calibration, no junction errors, etc. Nice (but $$$).

I don't know what the Arduino does, but I can tell you (because I've done it many times) that a simple 8 or 16-bit processor is more than suitable for almost any brewing app. My favorite small processor is the TI MSP430 series, but if you needed to do a lot of signal processing theres a zillion ARM7's out there with internal flash and ram that would get the job jone fine also. ARM9 is serious overkill. I can see the advantage though if you are looking for USB/Ethernet/Wifi and that kind of connectivity.
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Old 06-18-2012, 04:09 PM   #9
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The arm9 is most certainly overkill for just a brewing controller, but meant for an application built for run time configuration and sufficient safety measures built in to prevent mayhem. This is not targeting the Brewtroller/BCS market segment, but for systems that are more complex and need proportional control and analog outputs.
I have the application and code to make this work, just taking time to design and make sure it all works correctly in spite of what a user should not but probably will do to control system.

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Old 06-18-2012, 04:57 PM   #10
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Quote:
Originally Posted by kladue View Post
As long as the sensors are electrically isolated from the process there should be no problems, outputs are going to be photo isolated. The goal is not to repeat the design problems with direct attachment to the microprocessor like the current controllers, rather use satellite boards where the noise can be dealt with and microprocessor disruptions prevented.
Hi

It's the odd pop when a sensor accidentaly goes "un-isolated" that worries me. You can have a lot of voltage on the 250 ohm sense resistor when it does. Consider that people like me get employed to put in sensors (40+ years ago...).

Bob
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