4-20ma PLC Input Only Reading Lower Half of Range

[standeven]

Hi everyone,

I've attached a 4-20ma level sensor to a PLC input that is configured for current. The PLC reads the range 4-12ma just fine, but anything from 12-20ma is just read as 12ma by the PLC.

I did some measuring with the meter, and the sensor is indeed putting out 4-20ma. I also checked voltage, and the sensor outputs between 1.55 volts (when the sensor is putting out 4ma) and 4.55 volts (when the sensor is putting out 12ma). The voltage remains the same at 4.55 volts from 12-20ma.

Any ideas?
Thanks!

 

[Mickey]

It may help if you tell the forum what PLC and analog module your are using. Also how the module is configured.

Post the PLC's program if you can. Zip it first.

 

[Gomez_]

The Model of the Level Sensor would also be helpful.

 

[standeven]

I'm using an IFM CR0232 (http://www.ifm.com/products/ca/ds/CR0232.htm) with Codesys 2.3, and the sensor is a Turck Inclinometer (http://old.turck.us/illustrations/M1534013 (sheet_1).pdf).

The code is very simple, using an IFM library block:

VAR_INPUT
  xChannelNumber:BYTE;(*Channel number for the X angle*)
END_VAR
VAR
  xInputAnalogBlockExtended:INPUT_ANALOG_E;		(*Input block for extended input, X*)
  xTempWord:WORD;
  xError:BYTE;
END_VAR

xInputAnalogBlockExtended(
		ENABLE:=TRUE,
		MODE:=IN_CURRENT,       (*Global constant defined in library*)
		CHANNEL:=xChannelNumber, (*Integer input*)
		OUT=>xTempWord,
		ERROR=>xError);

 

[Gomez_]

My first thought is its a scaling issue, because the IFM is 0-20ma and the Turk is 4-20ma.

 

[standeven]

I don't think it's a scaling issue as the PLC reads absolute values just fine from 4-12ma.

So when the sensor outputs 4ma, the PLC reads 4ma. Same with 12ma. However it just quits reading anything higher than that, which is also when the voltage stops rising.

 

[pal]

Try using a 1K resistor in series with a 5K variable resistor in place of the sensor. With a 24VDC supply , if you get a reading of approx. 4 - 20 mA from the PLC and analog card over the range of the pot , then the problem is the sensor or the supply feeding voltage to the sensor not able to supply 20 mA .

Paul

 

[danw]

PAL is on the right track.

The Turck inclinometer can only drive 200 ohms of load resistance ( < 0.2K ohms)

It isn't clear where the value of 5,000 ohms comes from for the AI input resistance, but if that's the case, then a 200 ohm resistor in parallel will bring the total load resistance down to less than 200 ohms.

 

[ndzied1]

If you try the resistor test make sure they are 1/2 watt or you might burn them up.

 

[GaryS]

it looks to me like you only have a 5VDc supply to the Turck Inclinometer that would be about right for a 200 ohms load
check the supply voltage it needs to be 10volts to 32 volts

 

[DLMUK]

How have you wired the Turck inclinometer up?

From memory the last time I used one there were two wires, one for an X output and one for a Y output. If you only used the X or the Y wire, when you get to half way through the inclinometers range the output just stops.

I would take your inclinometer out of your circuit and test the output you get the angles you have it at is what you expect.

I also found the turck inclinometer to have a very steep curve in linearity at the extremes of the range, i.e. last 5/10degrees in the + and - direction.

 

[standeven]

I haven't tried the resistor test yet (sourcing parts), but I'll add some more information in the meantime:

• The sensor is getting 24 VDC.
• A meter in series between the sensor and PLC input reads half the range (4-12mA)
• A meter in series between the sensor and ground reads the full range (4-20ma)
• The internal resistance of the PLC input is measured at ~380 ohm
• Both X and Y wires are used from the sensor.
• The problem exists on both X and Y wires, and on three different sensors all using different inputs on the PLC.
• The problem exists on both the standard and extended sides of the PLC.
• A 2-wire 4-20 mA pressure sensor connected to the same PLC input works fine, and the PLC sees the full range.

The fact that the sensor works fine when outputting to a ground is interesting. It makes me think either the PLC has a grounding issue, or the added resistance of 380 ohm is what causes the problem. Other sensors also work fine. It's some combination of the four-wire 4-20mA sensor and the PLC that causes the problem.

 

[RET]

I'm fairly certain that it's the 390 ohm input resistance of your device that's limiting the output current of your sensor. You may be able to use a intermediate signal conditioner to get around that problem.

https://www.phoenixcontact.com/online/portal/us?urile=pxc-oc-itemdetail:pid=2864383#Outputdata

 

[GaryS]

some sensors require the ma load to on the grounded leg of the output
that would explain why you get the full scale when you measure the ground leg
(A meter in series between the sensor and ground reads the full range (4-20ma))
also some sensors don't like having 2 metering resistors on there output they can't supply enough voltage to drive the current though both so you can't stack them and of course you can't parallel then
I believe red lion makes a ma splitter to drive 2 or more meters without overloading the sensor

 

[shooter]

390 ohms is too much, must be below 200 as the max voltage is only 4 volts.
so your max input must be lower as 4 volts
if possible use a voltage input and use only half of scale.