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Stefan Kanev
10/31/11 03:00
Read: 551 times

#184472 - Q/A 2
Responding to: Zeeshan Ashraf'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?

List of 58 messages in thread
Load-cell conditioning / summing      Zeeshan Ashraf      10/29/11 10:29      
   what is that?      Erik Malund      10/29/11 15:43      
      RE: #184430 - what is that?      Zeeshan Ashraf      10/29/11 15:45      
         then say what you mean      Erik Malund      10/29/11 15:53      
            your description leaves something unclear      Erik Malund      10/29/11 16:02      
               RE: #184435 - your description leaves something unclear      Zeeshan Ashraf      10/29/11 16:27      
                  where are the loadcells???      Erik Malund      10/30/11 06:08      
                     RE:      Zeeshan Ashraf      10/30/11 09:01      
      RE: #184430 - what is that?      Zeeshan Ashraf      10/29/11 15:56      
         some Q/A      Stefan KAnev      10/30/11 14:14      
            RE      Zeeshan Ashraf      10/30/11 15:27      
               Q/A 2      Stefan KAnev      10/31/11 03:00      
                  RE      Zeeshan Ashraf      10/31/11 04:18      
                     re re      Stefan KAnev      10/31/11 05:00      
                     get rid of that summing amp      Erik Malund      10/31/11 07:09      
               you will be lucky to get 16 bits      Erik Malund      10/31/11 07:04      
                  Precision again...      Kai Klaas      10/31/11 16:58      
                     Comparison      Per Westermark      11/01/11 01:12      
                     RE      Zeeshan Ashraf      11/01/11 02:20      
                        why do you keep babbling about a summing amp      Erik Malund      11/01/11 06:41      
                        Priorities...        Kai Klaas      11/01/11 07:07      
                           RE      Zeeshan Ashraf      11/01/11 07:45      
                              cirrus, ti      Stefan KAnev      11/01/11 08:42      
                                 maxim, linear tech, Analog Devices      Erik Malund      11/01/11 10:09      
                                 RE      Zeeshan Ashraf      11/01/11 17:36      
                              it takes the same time ...      Erik Malund      11/01/11 10:14      
               500 running foot of cable      Stefan KAnev      11/01/11 22:25      
                  RE      Zeeshan Ashraf      11/01/11 22:57      
                     Go for ADC with multiplexed inputs instead of multiple ADC        Per Westermark      11/02/11 07:14      
                        Good Advice      Michael Karas      11/02/11 08:00      
                           It depends...      Kai Klaas      11/02/11 08:48      
                              This is a MODULAR concept !      Zeeshan Ashraf      11/02/11 19:57      
                                 it depends      Stefan KAnev      11/03/11 00:50      
                                 don't be ridiculous      Erik Malund      11/03/11 07:17      
                                    Consider other parameters too.      Zeeshan Ashraf      11/03/11 08:16      
                                       calibration      Stefan KAnev      11/03/11 08:46      
                                       if you do not buy bootleg parts ...      Erik Malund      11/03/11 08:50      
                                          Voltage reference      Per Westermark      11/03/11 09:08      
            Accuracy...      Kai Klaas      10/30/11 16:40      
               RE      Zeeshan Ashraf      10/30/11 16:55      
                  Keep the signal paths as simple as possible.      Michael Karas      10/30/11 20:34      
                  sun umbrella      Stefan KAnev      10/31/11 03:10      
   Separate Gain Paths - Not The Best      Michael Karas      10/29/11 23:33      
      RE: #184439 - Separate Gain Paths - Not The Best      Zeeshan Ashraf      10/30/11 00:11      
         Error Budget Calculations?      Michael Karas      10/30/11 09:50      
            Hysteresis      Kai Klaas      10/30/11 10:14      
         This should alarm you!      Kai Klaas      10/30/11 09:58      
            RE      Zeeshan Ashraf      10/30/11 10:14      
               No contradiction...      Kai Klaas      10/30/11 10:22      
                  RE      Zeeshan Ashraf      10/30/11 10:40      
                     Digital Summing        Michael Karas      10/30/11 12:55      
               with DC exitacion ...      Erik Malund      10/31/11 07:20      
            welcome on board      Erik Malund      10/31/11 07:14      
      RE:      Zeeshan Ashraf      10/30/11 00:16      
   Look at ADuC845      Oliver Sedlacek      10/31/11 01:44      
      ADuC845      Russell Bull      11/16/11 20:54      
         Noise V good      Oliver Sedlacek      11/17/11 04:39      
            Ok, then      Erik Malund      11/17/11 06:26      

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