| ??? 10/30/11 20:14 Read: times |
#184457 - some Q/A Responding to: ???'s previous message |
My friend was having a lot of variance batch-to-batch - this was one problem how big was variance and what precision was achieved by system? Hopper dead weight = 250 ~ 300 KGs Load-cells on hopper = 3 Load cell specs : Type = Tension S-type Capacity = 500 KG each Output type = 2mV/V Connection = 6-wire (EX+, Sense+, x-, Sense-, Out+, Out-) with sheild. Total measuring range = 1000 ~ 1500 KGs Minimum weight to measure = 0.1KG Accuracy = +/-10% approx that of min. resolution So total max weight is 1800 kg with 3 loadcels x 500 kg each. Loadcells here are undersized, i suppose they are out of health already. 'Legal' trade accuracy with 'standard' C3 loadcells(3x500 kg capacity) is 0.5 kg, in best case - 0.250 kg . This is something what Your friend should be informed about. So recipes/batches should be developed with these limitations in mind - do not lie Yourself and friend that 0.1 kg accuracy is realistic aim. You can achieve such result (0.1kg) but You can't declare it. In the modular concept, theoretically, if a single channel fails, only that particular channel module needs to be replaced. This reduces long-term costs and re-calibration time. If one amp-channel fails, You still will need recalibration with test weights on hopper. Or individual loadcell+amp calibration on test place with test weights (not whole hopper).And here we go to 'digital load cell'. You can buy new set with new digital loadcells and replace old cells. In case of fail You will change D-loadcell. Recalibration will be not needed, but price will be high. Or You can DIY D-loadcells. With 3-4 standard loadcells - passive summing box and AD7730 is enough. There is no need of amp-boards. Main reason for using amp-boards are 2: - noise on cable , between loadcells and PLC - PLC's ADC usually are 'universal' , 12...14 bits. AD7730 (and other in this class) eliminates amp-need. Multi-channel amplifier/multi-amplifier box - is not needed in Your system. Multi- ADC system is good only if You can use: - reliable loadcells with reliable REALISTIC data in passport( Loadcell's passport data / test report by producer)- test report can be used instead test weight. or - test weight to calibrate single loadcell- disconneced from hopper . Think about temperature changes on loadcell's output too. Check datasheets with care. regards |
