Has anyone been successful with reading and sending 4-20ma?
I would like to build a needle valve driven by a stepper motor. I will feed its position back to the stamp with a potentiometer driven by the valve stem. I would like to convert·the·4-20ma signal into some form of data the stamp can read and move the valve to some scaled value from the potentiometer. How can I read and scale·a 4-20? And while were at it how can I send a 4-20? Thanks again.
Comments
Typically you read current with a shunt and an ADC with that small of current you will also need an op amp to boost the signal. Depending on the resolution you need the ADC has many choices.
If you are using a stepper then you will know the position of the valve from the number of steps you turn the stepper, also a stepper with a digital position feedback encoder would allow you to verify the position.
Op amps are also used to control current and digital pots can be used to adjust the current output.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
Post Edited (metron9) : 2/10/2009 9:14:56 AM GMT
http://www.emesys.com/OWL2face.htm#4-20 ma
Regards
Chet
This PDF is interesting
http://www.datel.com/data/meters/dms-an20.pdf
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
On the processor side, you need an analog to digital converter that will give sufficient resolution, 12 or 16 bits say, in combination with a shunt.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com
You connect the +24Vdc to the Sensor+. The Sensor- will go to the resistor. The other side of the resistor to gnd/0Vcd. Such sensors work usually with less than 9Vdc. The resistor is R=V/I so even 10V/20mA, 500ohm will give you a reading range of 6V (from 4 to 10) and leaving enough voltage (14V) to the sensor for powering it.
R= 250ohm will give you a range from 1V(4mA) to 5V(20mA).
Take the resistor 1% tollerance
The reason for using a current loop over a voltage loop, is that the current in a series loop is always the same, anywhere·(and everywhere)·in the loop at any point in time, regardless of how many items are placed in the loop. (assuming a sufficient supply voltage, nominally 24 /48 vdc)
In other words, if you put a ma meter in the loop out in the field,·and another one in the control room thousands of feet away, they would both read exactly the same thing. It's the way around voltage drop over long cable runs.
The standard for votage driven equipment was 0.25v to 1.25v· ....· Usually this was control room equipment, but could be field mounted equipment like a valve positioner as well.
All you need to do is put a precision 62.5 ohm resistor anywhere you need··in the line to convert current to voltage·and then measure the voltage differentially across it.
Note that·the actual·loop voltage varies·as the·data transmitting device·maintains the proper current level to match the variable measured. (data from the field to the controller(· (data in loop)
Or the controller maintains the current value to match the output level to position the control element. (control data out loop)
Just use·a 62.5 ohm resister and an ADC in differential input mode.
Darlene
As I re-read your post, seems there's a gap in your information.
I would like to build a needle valve driven by a stepper motor. I will feed its position back to the stamp with a potentiometer driven by the valve stem. I would like to convert·the·4-20ma signal into some form of data the stamp can read and move the valve to some scaled value from the potentiometer. How can I read and scale·a 4-20? And while were at it how can I send a 4-20? Thanks again.
How are you getting this 4 to 20ma signal to come from the valve with a potentiometer, more over, with a stepper to control the stem position, you already know (by the number of steps in either direction), where the stem is in it's travel and at most, would benefit from an encoder in the event that the motor was stalled against the load and pulses didn't create physical rotation.
If you were just using a gearmotor to move the stem, then yes, you'd need positional feedback, but I'm still lost as to where the 4 to 20 thing comes from. Is it a seperate position indication transmitter?
Post Edited (Geekgirl) : 2/11/2009 10:55:50 PM GMT