Experimental Physics and Industrial Control System
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Does "cost effective solution" apply to parts only or include labor? Are
you willing to build your own solution, or are you looking for something
that works out of the box? If the former:
I've worked with voltage-based analog temperature sensors (TMP36)
before. In that case, I designed a Raspberry Pi shield board that
included an 8-channel 10-bit ADC addressable via SPI. See
http://www.mackenziegems.com/2013/03/14/chillmon-board/ I imagine you
could do the same thing with resistance-based PT100's if you set up a
voltage divider for each input. The board fabrication and component
price was $22 for 8 channels, although you'd need to add in a Raspberry
Pi, SD card, power supply, etc. For your application, I might consider
dispensing with the Pi altogether, making a shield board for an AVR
microcontroller, and have that communicate the channel data with an IOC
via USB or TTL serial. I'm amazed at how small, inexpensive, and
powerful these AVRs have become, in particular the Teensy:
https://www.pjrc.com/teensy/
Does it have to be PT100? I strongly prefer digital sensors if possible.
I'm currently using DS18B20 OneWire temperature sensors for my brewing
project. They are individually addressable and can be strung together in
a bus. Resolution is 1/16 degree C if you're willing to wait 750 ms for
a conversion; you can configure them to use lower precision which will
reduce the conversion time. I don't know how far these 100 sensors are
spaced out; you may run into bus length limitations with OneWire, but
I've seen documents that talk about improving OneWire performance for
long or noisy buses. I'm using an AVR microcontroller to bit bang the
OneWire bus, and this is queried via USB serial. OneWire has the ability
to detect a broken bus, a non-reporting device, or a transmission error
via a CRC. The AVR can check this and raise an EPICS alarm if the output
becomes invalid. You could also use one microcontroller to run several
independent OneWire buses. This approach is likewise very inexpensive.
Ryan
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ANJ, 15 Jul 2016 |
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