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Extracting digital data from a scale

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#1 brian w

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Posted 02 October 2014 - 10:12 AM

Here's my dream:


Three Sartorius digital scales (which have a RS232 interface) supplying data at around a reading per second and having those data plotted as a function of time along with a couple other analog signals.


The issue I'm having is that the data from the Sartorius scales is intended to be sent to a printer. It's a full duplex RS232 full serial interface with a range of transmission rates and adjustable parity.  The character format is 1 start bit, 7 bit ASCII, and 1 or 2 stop bits. The output per communication is 16 characters. The communications can be set to occur at a user defined interval within the balance. 


It looks like all the pieces are there to get this to work but this is well out of my wheel house of competence. Any tips on where to direct my incompetence?


I'd like to think I won't be doomed to countless hours of manually logging scale measurements.


Thanks in advance for any help.



#2 LabJack Support

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Posted 02 October 2014 - 03:59 PM

Before you go down the path of talking to the scales through the T7, have you considered using a normal USB=>RS232 dongles, or even Ethernet to RS232 devices?



#3 brian w

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Posted 02 October 2014 - 04:32 PM

That would make things less complicated.


I also may just pick up some load cells and get an analog output from those so I can keep all the data streams synchronized.



#4 brian w

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Posted 22 October 2014 - 03:39 PM

Instead of fussing with the problem I started here with I decided to pick up some S-type load cells to use with a stated +/- 0.02% full scale combined error. Although they are at least an order of magnitude less precise than the lab balances I started with, they should fit the bill.  The 3 cells I have range in response from 1.96 mV/V to 0.75 mV/V (yuk). The worst cell with 10V excitation should give me a 7.5mV +/-0.0015mV when fully loaded (5000g). That "bad" cell is right about at the acceptable limit for what I want to do.


Experimenting with the load cells, I used the VS from the T7 to excite the load cells. It's pretty sloppy as was noted in other threads I've seen on the topic, so I ordered a 10V reference source which hasn't shown up yet. Hopefully that will clean up the signal some but I'd like to make sure I'm doing everything reasonable to get a good signal out of these things.


Any tips on how to get the best reading I can from the load cells with a T7?



#5 LabJack Support

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Posted 22 October 2014 - 03:57 PM

Set DAC0 to perhaps 4.0V and use that as excitation.  Connect DAC0/GND to +/-excitation and connect +/-signal to AIN0/AIN1.


Or when you get your 10V excitation source, connect 10V/Ground to +/-excitation, connect power supply Ground to T7 GND, and connect +/-signal to AIN0/AIN1.


Set "AIN0_NEGATIVE_CH"=1, and then read "AIN0" which will give a differential reading of AIN0-AIN1.


Connect +excitation to AIN2.  Measure "AIN2" at the same time as "AIN0" and use this real-time reading of excitation voltage in your equations.


Set "AIN0_RANGE" as low as possible.  0.1 or 0.01.
Set "AIN0_RESOLUTION_INDEX" as high as possible.  If time does not matter just set it to 12 which will use the max value for a T7 or T7-Pro.  If you have a normal T7, the default of 0 already equates to the the max value of 8.

#6 brian w

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Posted 27 October 2014 - 11:36 AM

So simple.


That really cleaned up the signal a bunch.


Thanks for your help!

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