7 posts in this topic

Posted (edited)

Hello guys, I have a system of 3 load cells (SP4M-7Kg) attached  to a Micrologix 1500 analog inputs. in the same PLC i have 3 coils for a vibrator, each vibrator is working with signal generators. and the load cells with an external 24vdc Power Supply.

i see my load cells readings fluctuating too much even with the vibrator not working. if it is set lets say 70 Grams it is jumping from 68.4 to 74.8 going up an down never never stabilizes in one number, so i suspect am getting noise somewhere put a 100uF capacitor PLC inputs for the load cells but weight just increases more and more.

How can realize where is the noise comming?

what can i use as a filter?

how can i improve this system? should i changed it to digital? 

 any help welcome! thanks. 

questions?

   

Edited by marckTony

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How long are your cables? Is there any other cables nearby? Are you using shielded twisted pair for these cables? Is the shielding terminated correctly (isolated at the device side and grounded at the PLC side. If there is a junction box, the shield has to be connected through at the junction box but not grounded).

Using a millivolt output over any measurable length is highly susceptible to noise. Take a voltmeter and measure the DC voltage between any 2 arbitrary points and you'll most likely find there is measurable ambient noise in the millivolts. If your cabling is good as per above and you are still have noise issues, you should consider either a digital load cell, or install a transmitter that converts the millivolt signal to a much more reliable 4-20 mA signal as close as possible to the load cell.

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MrAutomation The load cell cables are 4 meters long and the shielding looks correct but the ground side for the vibrators, wasn't  connected to the ground line. try to connect that and see what happens.

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Also, what module are you using to measure the load cells? You must be using a custom module such as this one

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YES sir I am using that module. i connect the ground wires for the vibrators (which wasn't connected ) but no difference at all. i checked all ground cables and shields connected. i will try using 100uF capacitor on excitation lines for load cell and 1000 uF for  power supply outlet 24VDC

i dont know what else can i try? any suggestions?  

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Posted (edited)

http://www.hardysolutions.com/tenants/hardy/documents/HI1769-WS_User_T.pdf

 

Here's the manual for this unit. I've never used the module before, but it has a "Wavesaver" function that filters out noise. What is the current setting (see page  4-11 in the manual)? Try turning it all the way to 0.25 Hz and see if this helps. You will likely see a slower response time in measurement changes.

Edited by MrAutomation

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Hi colleagues

This may sound crazy but don't forget to check for mechanical disturbances like wind, vibration coming from the floor, etc.

Check if the weighing device has any kind of damper.

Check if your load cell is well fitted and doesn't move

To determine if your noise source is electrical, check grounding, check module inputs isolation, check noise coming from your power supply. The last can be checked with a voltage tester in mV AC.

Good luck
 

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