Process automation instruments

Questions on other types of hardware and getting it talking to the ARM CPU
mmcginty
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Joined: Wed Dec 05, 2012 11:03 am

Process automation instruments

Postby mmcginty » Thu Jul 18, 2013 4:40 am

There is a fairly large family of automation monitor and control devices that communicate by varying current. The transmitter allows the receiver to draw from 4 - 20 mA at 24V, and the receiver infers the transmitted value from the level of current it's able to draw. Seems to me it'd be difficult to be any more analog than this! :-)

How difficult would it be to make a SuperPro perform either of those roles, as either a simulated transmitter, or a receiver able to translate mA into numbers?

TIA,
MM

basicchip
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Re: Process automation instruments

Postby basicchip » Thu Jul 18, 2013 5:29 pm

We use 4-20 mA sensors all the time.

Basically connect the low side to ground through a 165 ohm 1% resistor and you have converted that to a 0.6 to 3.3 V signal. Yes you are giving up some dynamic range (18%). But for most applications it is fine. Add a small capacitor if you have long cable runs, which we did when we sent pressure sensors down a well as part of a custom design instrument. And it is a good idea to protect the AD with either a diode to the supply or a 5V zener, in case someone hooks up the sensor wrong.

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You can add an op amp circuit to subtract the 0.6V out to get the full range

basicchip
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Joined: Fri Oct 19, 2012 2:39 am
Location: Lake Tahoe, CA
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Re: Process automation instruments

Postby basicchip » Thu Jul 18, 2013 5:32 pm

On the other side, a lot of sensors are just 2 wires, so you have to do all the processing with that 4mA. Maybe it is possible if you run the ARM real slow, but you would also have to somehow calibrate the current used by the ARM and keep it constant.

There are some 3 wire sensors which have a power supply connection and an independent 4-20 mA output. This could be done, and there are a lot of specific parts that convert a voltage into a 4-20mA output.


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