Reading a thermocouple ?

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05 May 2017 06:33 #92683 by calvin.d
calvin.d created the topic: Reading a thermocouple ?
I have a 6i25 connected to a 7i76

I want to read several thermocouples, the goal is to set use them in pid loops in the linuxcnc config

does anyone have a suggestion on what the proper approach to this would be ? I know I am going to need to put an amplifier on the thermocouples, and then I can encode the output from them however I want. I am looking for suggestion on how to go from amp output into the mesa space.

I know I can get rs422 from the 7i76, how hard would it be to read the data over serial that way ?

any suggestions would be great

Calvin
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05 May 2017 09:44 #92684 by tommylight
tommylight replied the topic: Reading a thermocouple ?
7i76 can have 4 inputs set as 8bit analog inputs, so that would be fairly easy to use. Or use Texas Instruments voltage to frequency converters and count them as encoders in 7i76, they are cheap but more work, and for this 7i77 might be more suitable .
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05 May 2017 10:58 #92691 by rotwang
rotwang replied the topic: Reading a thermocouple ?
The down side is that the 7i76 analog inputs are 8-bit, 36.3v full-scale and +-5% accuracy.

@calvin.d -- I don't think you can get the 7i76 RS422 interface to do what you want, but don't let that stop you trying.
Roger
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05 May 2017 18:14 #92713 by PCW
PCW replied the topic: Reading a thermocouple ?
It is possible to build firmware with a Packet-UART connected to the 7I76's RS-422 port
(normally this is used with a smart-serial module)
And I know there is at least preliminary driver support for the HM2 packet-UART
so its not impossible to communicate arbitrary data with a remote uprocessor in real time using this method
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05 May 2017 18:33 #92715 by calvin.d
calvin.d replied the topic: Reading a thermocouple ?
I am fine formatting the output and reading them as encoders, what is the proper mesa board to add if I just wanted 4-8 more encoder channels ?
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05 May 2017 18:51 #92718 by andypugh
andypugh replied the topic: Reading a thermocouple ?
I am guessing that this does not need to be fast?

I have used these with Arduino:
uk.rs-online.com/web/p/instrumentation-amplifiers/7861017/
(SPI interface, K-type conditioning, built-in cold junction)

A HAL module exists to bit-bang the SPI interface:
linuxcnc.org/docs/devel/html/man/man9/max31855.9.html

If you need higher speed then the Mesa BSPI module could be used. But the HAL module above can work on any GPIO including parallel port.
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05 May 2017 19:05 #92719 by PCW
PCW replied the topic: Reading a thermocouple ?
if you just need frequency reading you just need 1 pin per channel (the encoder "A" signal)
so you dont really need 4 or 8 more encoder channels (8 encoder channels is 24 signals)
for example if you have a 6I25, you could have a config that used the second 6I25 connector
as 17 encoders all with just channel A brought out
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05 May 2017 19:09 #92720 by andypugh
andypugh replied the topic: Reading a thermocouple ?

PCW wrote: if you just need frequency reading you just need 1 pin per channel


The MAX31855 component only needs 1 pin per channel too, but also needs 2 on top of that for clock and data.
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11 May 2017 02:14 #93004 by calvin.d
calvin.d replied the topic: Reading a thermocouple ?
ok so one config I was thinking of may require another 7I76 to get another stepper motors for what I am thinking

can I setup the second 7I76 inputs to encoders and just break out the A channel as suggested ?

if that is the case what is a good input scheme for the thermocouple output. I am going to put a micro controller between the thermocouple and the 7I76.

so if I want a 0-600C range for temp
what is a good frequency range to map that too ?

ideally I want to plug this into a PID loop in linuxcnc and control the temp that way

Thanks for all of the suggestions

C
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11 May 2017 12:34 #93018 by PCW
PCW replied the topic: Reading a thermocouple ?
If you just need more step motor outputs a second 7I76 is overkill, you could use a 7I85S and gain
4 additional stepper outputs and 12 inputs

Any frequency from a few KHz to maybe 1 MHz or so should be OK

An all "A" encoder connection would require custom firmware but its a pretty simple change
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