4 TO 20 mA Sensors - how handle negative??

How does the 4 to 20 mA output of a sensor respond when the measured parameter goes negative.
Example
Hall effect used for current monitor on a battery. When charging let us call that a positive current flow and positive 4 to 20 proportional to 0 to XX amps battery current.
Discharging current is call it negative and 4 to 20 mA would have to be negative.

How would sensor and the "monitor" ie PLC or digital instrument
etc handle the change (transition?) between
+4 and - 4 mA
when current changes from neg to positive?

I would want a "polarized" signal ie positive when current is positive and neg when current is neg.

I do not recall mention of this question
Is 4 to 20 mA a poor choice for this application?
Am I better off with voltage type signal ie mV or 0 to 10??

This is for electric vehicle and I would want to record battery discharge and charge current. Would like 4 to 20 for best EMI "immunity" and to avoid the small negative values seen on voltage signals when measured parameter is at zero.

Another interesting thought
if I monitor shaft RPM to get velocity and distance what happens when I back up?? In PLC of course I could check for a negative value and throw it out or change to a positive but what about a simple digital readout without this capability?

Uhh is this a realistic concern / question or am I doing hand wringing and Chicken Little here?

Dan Bentler

Make 4ma the maximum discharging current, and 20ma the maximum charging current, with zero current somewhere in the middle. For example, if the max charge/discharge values are the same, then 12ma would be your 'zero'.

For shaft speed, a quadrature encoder will give you speed AND direction.

http://en.wikipedia.org/wiki/Rotary_encoder#Incremental_rotary_encoder



-Eric

Hi Dan

My thumb of rule 4-20 mA usually better then 0-10V.It is not work allays .Immunity you Say.
Regarding the negative value ,it is matter of rang like temp sensor -32:+100 degree.
4mA=(-32) 20mA=100 (C/F) degree.
You can use -10V:+10V to measurer the polarity, depend on your sensor.
To change measurer direction .You can increment up for foreword or decrement for backward.(you know the shaft rotation).

Hope I helped with something

Check out the 'example' in this PDF...

http://www.nktechnologies.com/pdfs/dt_series.pdf



-Eric

Eric - that current sensor is what I have in mind.

Arik and Eric -- Thank you for telling me what I should have remembered and thought of
ie make 4 the lowest ie most negative and 20 the highest and most positive value.

More than likely will use encoder for shaft RPM so maybe should not have used that as an example. On other hand will more than likely want to set a max accelearation value and RPM setting in VFD when in reverse (These settings would be for reverse only)

Conversion of 56 pickup may be started and done (drivable ??) this summer.
Dan Bentler

I have used 4 to 20 mA rang analog temperature sensors. We calibrated the sensor by sticking the sensing probe in two constant temperatures to get a range. For the calibration of the 4 mA, we would use a cup of ice water for a constant of 32 deg F. For the 20 mA, we had a small electric calibration furnace that we could set to stay at a constant temperature. The furnace had small holes in the front of it for various size wands that are hooked to temperature probes. After we set the 4-20 mA range, we would go between the ice water and furnace to make sure the sensor was reading accurately. The temperature probes are connected to an Allen Bradley slc 500 used to control a steam pasteurization system. With that all said, the bottom line is that 4- 20 mA is just a range in which it has to be set to recognize a set point of 4 mA, for the low side and 20 mA for the high side , which we are able to see any temperature between the two. There are sensors that come from companies that are already preset for certain analog function ranges such as temperature, level, pressure, etc. I am sure if you check around, you will be able to find a sensor that meets your needs. Hope this helps some.
Sincerely:
Maintenance Man.