what is a typical level of noise on a 4 - 20 mA signal?

[egret2008]

I have a "time of flight" laser distance measurement device connected to a NI9203 (AI +/- 20mA) module.

The sensor is configured to output 4 mA at 200 mm and 20 mA at 1800 mm (i.e.  0.01 mA/mm).

The application requires a resolution of 1 mm but I'm currently getting 2 - 4 mm of noise.

The noise is not introduced during the measurement phase because the data obtained over the serial port is good (see attached image comparing serial and analog data).

The noise has an amplitude of 0.02 to 0.04 mA (0.1 - 0.2 % of full scale).  Is this a reasonable level of noise or can I expect better results?

I'm currently testing in an office where I would expect low levels of electrical interference.  I'm using shielded cable and short cable runs.  The final application will be in a noiser environment with much longer cable runs.

To eliminate the sensor as the source of noise I replaced it with a 9 V battery in series with a 820 ohm resister to generate 10 mA signal.  Using the same cable and DAQ channel I get a similar level of noise.

The sensor has a 16 bit DA converter and the DAQ card has a 16 bit AD converter which should be more than adequate for this application (1600mm / 2^16 = 0.02 mm resolution).  My understanding is that current loops exhibit a high level of noise immunity. Is it possible that there is a problem with my DAQ card causing this noise?

[aeastet]

What is the accuracy and resolution of the laser from the manufacturer. You will need to look at all of those specifications to know if you can expect better. If you have a 1800mm laser measurement system 2-4mm is only .002% error Max. That is very good for an analog device. If you can go slower you could introduce filtering and averaging.

[JÞB · Accepted solution]

the Spec states a range accuracy of 0.049mA .  That's the best you can hope for with this device and isn't too shabby! 

What do you mean by "Noise?"

Quantitization uncertainty can be reduced by averaging.

EMI/RFI can be filtered post acquisition

Good connections and cabling (you seem to be aware of) can reduce interferance.

[egret2008]

The sensor I'm using is a Micro Epsilon optoNCDT ILR 1181-30(01)

Specifications are:

linearity: +/- 2 mm

resolution: 0.1 mm

repeatability: =/< 0.5 mm

The serial data (RS232) I have acquired from the unit performs as per these specifications.  But the analog signal (4 - 20 mA) gets lost in all the noise.

I'm finding it hard to understand why the laser sensor is fitted with a 16 bit DA converter if a significant proportion of those bits is "wasted" on noise.  The sensor has a resolution of 0.1 mm and the AO has a resolution of 0.02 mm when scaled over the range it has been configured for (1600 mm).  Therefore, in theory, I should be able to detect changes as small as 0.1 mm. It appears that these small changes are present in the analog signal but they are superimposed with 2 - 4 mm of noise.

This level of noise may be typical of all 4 - 20 mA loops, in which case I will investigate other options such as software filtering or serial comms.  But my preference is to remedy the problem at its source (if there is one).  

Using serial comms is significantly more expensive.  So I need to be sure that the 4 - 20 mA signal is not up to the task.

[JÞB]

To clarify, the Range accuracy spec for the NI device is 0.049mA

[aeastet]

What kind of surface are you measuring on? I found with certain Micro Epsilon Lasers that the do not like black surfaces or materials. I saw noise in my system until I put masking tape on the surface of the system to be measured.

[egret2008]

Hi Jeff, thank you for your reply.

Can you please clarify what the meaning of "range accuracy" is.  If I understand correctly any change in the signal of less than 0.049 mA can be disregarded.  Does this make the effective resolution of the module (20-4)/0.049 = 327 divisions?  This is significantly less than the resolution of the modules ADC (2^16 = 65,536 divisions). If so, this gives me a practical resolution of 1600/327 = 4.9 mm.

Where did you get the figure for "range accuracy" from? I can't find it anywhere in the module specifications.

I'm experiencing two types of "noise" on my signal. 

• The one I'm most concerned about is a band of high frequency oscillations about 2 mm (0.02 mA) wide superimposed on the "clean" signal.  The difference between clean and noisy signals can be seen in the picture attached to my original post.  The analog (noisy) signal is represented by a wide band of data while the serial (clean) data is represented by a fine line.  I would like to acquire a signal similar to the serial data using the AO signal.  Is this possible?  Note there is an offset between the two which I will need to correct for.  
• The other source of noise is a series of sharp spikes present at random intervals.  These appear to be coming from the laser sensor as they are not present when "distance tracking" is turned off (AO retains its last know value without spikes).

 I don't think this a quantization problem because I do not see any discrete steps in the data.

[egret2008]

Hi aeastet,

Thanks for your assistance.

I don't think this is a problem with the surface type or color because the serial data (RS232) out of the laser is good. It has good resolution and no noise.  See the picture attached to my original post.

The noise I'm concerned about is being introduced somewhere after the laser has determined the measured distance.  Either in the laser's DAC (for the 4 - 20 mA output), the cable or the NI module.

[JÞB]

Knowledge base

article discussing accuracy

[aeastet]

Just because you are getting a clean signal from the serial port does not mean that the noise is not there. There could be filtering, It could be sending less samples, there could be a lot of things going on here.

As I suggested earlier you can also use a filter on this data if you measurement system will allow. You could also do sample averaging to help with the noise.

Look for ground loops, make sure that the shield on your cable is grounded and only connected at one end. There are a lot of things that could cause noise. Try using a different surface to see if things get better. Make sure the the lens on the laser is clean and not scratched.

Just some thoughts.