Experimental Physics and
Industrial Control System

Ryan Pierce <[email protected]> · Tue, 12 Jan 2016 10:16:24 -0600

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