| ??? 10/31/11 09:00 Read: times |
#184472 - Q/A 2 Responding to: ???'s previous message |
However, a Weigh-Batching system needs much higher resolution than that, possibly around 10,000 counts. To achieve that much of 'displayable' resolution, the internal resolution should atleast be 10x of that, amounting to 100,000 counts or more. This translates to 17-bits, while it is better to have a 18- to 20 bits of resolution. Yes , AD7730 class can provide this. But do not forget - Loadcell's precision is dominant here. My main target was, to design an individual summing amp providing a voltage (0-5 or 0-10V) OR current (0-20mA or 4-20mA) so that i could use it in my project, and if teh results are good, i can replace the 'regular passive' amps on another system at the same plant with these to get better performance So - You are not targeting fast solution for damaged installation. You want to tune-up existing system? If target is to modify Loadcell-Simatic PLC connection You can go this way - with amplifiers, but calculations should be in mind. Your chances to build good amplifier are not high, i think. Buy one from reliable vendor and replace existing old amplifier, check health of loadcells and replace damaged ones. This will allow plant to produce goods (and make money for improvements). Next - think about better performance and improvements: 1)Its a good idea to double controller - i preffer (where applicable) to build and connect new controller in such way , which allows in case of fail to start old controller in few minutes. 2)For me is no doubt , that to build amplifier with better characteristics ,that AD7730class have ,is allmost impossible (for me, again). So if You want to preserve PLC controller and add secondary controller (PC based, GUI), You should build black box with: high class ADC, microcontroller,temperature compensation , D/A - interface to old PLC. Or You can buy weighing indicator with all these installed/ready to use, and concetrate Yourself on system integration. 3)Other fast approach for blackox is to use device like XTR106 (check TI's datasheet). So now if i select the cells, i would atleast consider a safety factor of 1.25 for sizing the load cells. So if : Total desired material weight = ~950 KGs Dead weight of hopper = 250 KGs, then total weight = 1200 KGs Applying safety factor of 1.25 = 1200 * 1.25 = 1500 KGs For 3x load-cells, each load cell capacity = 1500 / 3 = 500 KGs For me good sizing in Your case is 750kg capacity for loadcells, better is 1000kg. I mean safety. You should imagine all possible fails in system behavior - for example fail in silo's door closing - and all overloading scenarios. As i mentioned - C3 class is limited to 3..6 k divisons. On other hand we are building truck scales with 8 loadcells ,C3/ 30t each, passive summing box, AD7730 in front, with 'legal' division 20kg. Problem with comprasion between this truck scale and Your hopper is hidden in loadcell's quolity. So once again - check loadcells type, specs and health. However, if this is not possible, then ofcourse at the end of the day, i will have to make one completely integrated system, and then i can directly use the scheme suggested by Eric / Kai or Micheal to use a 4x channel ADC and connect all load-cells directly to it and do the summing / averaging within uC Code Can You describe procedures for calibration/checking in this case? |
