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rkukl

Multiple Load Cells

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I am having difficulty reasoning through a project so hoping for suggestions. Platform is Control-Logix I have 4 load-cells that are weighing a large vessel. 1) To calibrate? Empty vessel =0 -- add 400 pounds to vessel and lets assuming load-cells are reading equally distributed weight. Should calibrated weight be 100# per cell then a simple sum or, 400# per cell / 4? 2) Now I want to compare each load cell to see if they are all within tolerance of each other so how to best compare 1-2-3 to 4, 2-3-4 to 1, 3-4-1 to 2, 1-2-4 to 3? I am not looking for anyone to write my code just some ideas. Seem to be hitting a wall here.

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Average the four load cells. The compare to that average. You will have to determine what deviation is acceptable. This will be determined by the normal distribution of material. If you don't have any idea then fill, visually verify that physical distribution of product is within acceptable limits ( an off center in-feed may fill preferentially to one side). Look at the load cell returns. Do they reflect the distribution? That should help you determine an acceptable deviation from the average.

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Load_Cell_Emu_2.ACD here is where i am at thus far. still struggling with the tolerance aspect. hope i attached file correctly??? Thoughts Anyone? Hopper LoadCells.pdf Edited by rkukl

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The limit tests near the end and your naming of the output are confusing. Isn't the output of a LIM true when the test value is between the limits? Why is the output bit labeled 'Out Of Tolerance'?

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You are correct I could have done a better job on descriptions and logical arrangement. I have attached V3 but still struggling how to get tolerance working as i see it in my mind. One of my problems is I’m unable to verbalize it to myself so if I can't explain to myself what i want then how can I get what I want??? Load_Cell_Emu_3.ACD

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I didn't analyze the logic, but as for your instruction comments, I would simply change the description of the LIM instruction output bits to "within tolerance" so that you are describing the condition when the bit is true. As for using comparisons between load cells, you generally want to ensure that all four corners have very close to the same dead load applied to them (usually done with mechanical shims or adjusting screws), and that they all respond with a similar range. If you want a purcha$ed solution that does all the hard stuff for you but gives you plenty of features and diagnostics, then I can recommend the Hardy Instruments HI 1769-WS and their load cell summing board with IT capability. I used it to retrofit the controls on an in motion case checkweigher with excellent results that make it a much more capable machine that the MT controls it replaced. I used the IT summing board with existing non-IT (mettler toledo) load cells. I found the IT (integrated technician) features are great for set up even without C2 (smart) load cells, since you can read voltage levels of each cell (on the HMI in my case) independently and perform your cornering adjustments without having to attach test instruments or even open the j-box. They provide generic code examples and also a module profile you can add-on as well. EDIT: I see now you're using controllogix so look at the HI card for 1756 chassis here. Edited by OkiePC

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Here is what I finally came up with. My Local vendor PAC specialist headed me in the right direction (thanks Shawn). sum 4 loadcells , divide each loadcell input / sum to calculate % variation, limit test % variation to determine good or bad. I will tweak a bit to convert to full % values and make a bit easier to use in my View project. Rich. Hopper LoadCells_6.pdf Load_Cell_Emu_6.ACD

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