4-20ma signal fluctuating

ccopsey

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Join Date
Mar 2018
Location
Ohio
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5
I have installed some Keyence lasers that provide a 4-20ma signal back to a 6-axis Delta motion controller. The 4-20ma signal is "measuring distance" and isn't real stable. It will bounce around while there isn't a perceived distance change. I have already desensitized the laser as much as possible, but the programmer of the Delta controller says he can not handle any fluctuations in the signal.

Is there some circuitry that I can use to eliminate fluctuations or should the controller be able to utilize code to eliminate the minimal fluctuations?
 
How big are the fluctuations? It is not uncommon to see the D to A counts move around a little in a steady state situation. Is it possible that those small movements in the D to A counts are being magnified by a Scaling function or some other function in the Delta Controller?
 
Depending on your application, you can utilize code but some of the resolution will be lost.
From my experience, AD or DA-conversions is always suffering from fluctuations. Anything from wiring to the surrounding light (when using lasers) to stream or flow fluctuations in liquids (pH measurement).

Just curious, why isn´t an encoder utilized instead?
 
If you don't know what the problem is, you can't find a solution. You first need to diagnose if it is the laser giving the problem, or the 4-20mA transmission, or the Motion controller. Before that you need to know if the diagnostic tools you have can see the problem. You probably have a multimeter, you might have an oscilloscope. Using a multimeter you need to measure the 4-20mA that is in the loop, the obvious way is to set the meter to a mA scale and insert it in to the loop. Can you see the variations? If you can then the multimeter is the tool to use, if you can't then you will have to go to an oscilloscope, or the controller has issues.

Once you have a diagnostic tool you can break the problem down. First remove the loop and controller from the problem and concentrate on the laser. You need to know if it has an active or passive output so you can decide if you need to power the short test loop. The simplest way is to connect a variable resistor across the output, the laser documentation should give you a minimum and maximum loop resistance if it supplies the loop power. If you get current flowing then you know that it is an active output. If you don't then you will need a power supply to power the loop. Can you see the variations in the short loop, if you can then it is a laser problem and you will have to look at the choice of type and make of sensor.

If the variations have gone then you need to look at the 4-20mA loop. The most common problem is a ground loop where different parts of the circuit have different 0V levels, you can solve it by tying 0Vs together or by fitting loop isolators. Another problem is too great or too small loop resistance, check the laser documentation. Next if there are two things trying to power the loop there can be issues, read the documentation for each device in the loop.

If you clear the laser sensor and the loop then you have a club with which to beat the controller suppliers.
 
Depending on how fast you need the response of your actuator to be, you could use the Track (I-PD) instruction on the Delta Motion controller which gives a smoother response when following a reference that can change suddenly at the expense of the overall bandwidth of the system. Alternatively you could use a low pass filter on the input signal with a fairly high time constant.

I imagine the problem you are seeing is the result of the fluctuations in the reference being magnified through to your control output because of the velocity and acceleration feedforwards which are eliminated when you use I-PD control. If you go down the route of using a low pass filter, I would be tempted to turn the acceleration feedforward down to 0 too as it doesn't work well when your reference is a fluctuating signal instead of a smooth mathematically generated trajectory.
 

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