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Zeeshan Ashraf
10/29/11 10:29
Read: 2057 times

#184428 - Load-cell conditioning / summing

I happened to read about load cell conditioning & problems some one was facing and other people (experienced, like Eric, Kai..) putting in their valuable feedback.

My query is related, but different. I am designing a 2-part system. The birds-eye-view is that there are 4x tanks / silos which will be feeding a weighing hopper. The system will cycle through all the silos / tanks till all the material is in the hopper, and then empty the hopper on a conveyor. The 2 parts system is basically 1) Summing / averaging amplifier AND 2) 8055 board with ADC to digitize the data and convert into real-world (KG / Lb) units.

The hopper is suspended on 3x Tension load cells (S-type). Hence the amplifier. The 8055 control system will be getting the summed / averaged load signal from this amplifier (0-5V or 0-10V). The running cable length between the uC Board and amplifier can be upto 50 feet, or max 100 feet.

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

Thats the back ground and specs. Now i am actually stuck in designing the summing amp at the moment. Most commercial designs of load-cell summing amps that i have seen, including one on my friend's plant, have some low-noise / low-offset opamp as summing amp. The output of load-cells directly terminate on the resistors of the summing opamp. Moreover, the excitation is almost always fixed 10.0VDC with a 10- or 20-turn trimpot for excitation adjustment.

The circuit that i am (trying) to make, i am following a slightly different concept. Here are the specs i am planning to build :

Excitation : 5.0V
Excitation type = DC Excitation with sense lead compensation (Closed loop opamp + Vref based excitation),

I want to place an instrumentation amp on each load-cell input. Once all load-cells's outputs have been brought into single-ended (ground referenced) domain by the IA (and also amplified by a gain of 100), then these will be summed with a opamp in summing-amplifier configuration.

Q1) I need to know if i am on the right track. I know this approach, an IA for each load-cell, is costly upfront, but still.

Q2) I have researched a couple of IAs from Texas Instruments, Analog Devices and Linear, and i have made an excel sheet for comparison of the total error of these amps (ppm of full-scale on standard and elevated temperatures). Somehow i am seeing AD621 standing out in majority of the areas. I just need to know if that is really what i should use or any suggestion. I can send the Excel sheet to interested people.

Q3) Is the Fixed DC excitation OK, or should i consider AC excitation ?

Any help in this regard will be most appreciated.

Warm regards,

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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