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engg.embedded
Joined: 04 Oct 2008
Posts: 9
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 04 Oct 2008 11:47 load cell with 8051 |
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| hi.... im working on a project in which i need to interface sensors with 8051 controllers. i dnt hv prob regardng ADC coz i hv interfaced temp sensor with it and its working fine, bt the prob is with the load cell. I hv a load cell bt i dnt knw hw to gt its output. i hv used LM324 before gvng the input to the ADC bt the output is nt right... i dnt knw whr is the prob.
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laktronics
Joined: 06 Jan 2007
Posts: 898
Helped: 152
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| 05 Oct 2008 6:18 Re: load cell with 8051 |
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Hi,
Normal load cells are available configured in bridge circuit. One pair of wires is provided to apply the excitation voltage and on the other pair you get a small differential voltage proportional to the applied load and excitation voltage. The output voltage is specified as mv/full load/volt of excitation voltage. Typically the load cells provide 20mv for a full load at the rated excitation voltage. You need to amplify this DC signal using a precision differential amplifier to make it up to the level required by ADC.
Regards,
Laktronics
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Zerox100
Joined: 01 Mar 2003
Posts: 320
Helped: 10
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| 05 Oct 2008 6:37 Re: load cell with 8051 |
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[quote="laktronics"]Hi,
Normal load cells are available configured in bridge circuit. One pair of wires is provided to apply the excitation voltage and on the other pair you get a small differential voltage proportional to the applied load and excitation voltage. The output voltage is specified as mv/full load/volt of excitation voltage. Typically the load cells provide 20mv for a full load at the rated excitation voltage. You need to amplify this DC signal using a precision differential amplifier to make it up to the level required by ADC.
Regards,
[url=http://bharathyeju.blogspot.com]Laktronics[/url][/quote]
Do you have a reference design or any example?
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laktronics
Joined: 06 Jan 2007
Posts: 898
Helped: 152
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| 05 Oct 2008 8:15 Re: load cell with 8051 |
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Hi,
You can refer application notes from Linear/National. Here is a reference article and you can get many more on the net:
http://www.edn.com/article/CA6466208.html
For load cell application offset zeroing is not essential since it will always have a manual Zeroing facility, which can be done in the microcontroller.
Regards,
Laktronics
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FvM
Joined: 22 Jan 2008
Posts: 2635
Helped: 431
Location: Bochum, Germany
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| 05 Oct 2008 10:48 Re: load cell with 8051 |
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| For load cell application offset zeroing is not essential since it will always have a manual Zeroing facility, which can be done in the microcontroller. |
Better to say, not required in most applications but essential in some anyway. Consider a load cell mounted at a tank, how do you want to zero it after a power fail?
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laktronics
Joined: 06 Jan 2007
Posts: 898
Helped: 152
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| 05 Oct 2008 11:13 Re: load cell with 8051 |
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Hi,
That's right, thanks FVM, I was biased by common weighbridge applications.
And for more reference circuits visit following pages from Linear :
http://search.linear.com/search?q=bridge+amplifier&entqr=0&filter=p&ud=1&sort=date
Regards,
Laktronics
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arbj
Joined: 05 Oct 2008
Posts: 16
Helped: 2
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| 05 Oct 2008 13:47 Re: load cell with 8051 |
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Consider using CS 5520/CS 5516 both these are good quality load cell amplifiers with SPI interface, the datasheet and application notes are very informative.
The application note also contains circuits that you can use readily.
thanks
aa
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engg.embedded
Joined: 04 Oct 2008
Posts: 9
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| 06 Oct 2008 12:16 Re: load cell with 8051 |
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@ laktronics
thanks...iguess u hv got the type of load cell i have. its sensitivity is 1.9960mv/v and max weight is 10kg.
im using a LM324 ckt before interfacing it with a ADC 0809, bt evn thn thr is sm prob. im uploading the ckt digram. plz go through it, n help me if there is ny harware prob...
thanks...[/img]
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laktronics
Joined: 06 Jan 2007
Posts: 898
Helped: 152
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| 06 Oct 2008 18:50 Re: load cell with 8051 |
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Hi,
First of all you should clear up the problem with your amplifier. LM324 is not good enough opamp for such applications, however, for educational purposes may still be used.
What problem you are facing? In order to test the amplifier, you do not need a load cell, just use a resistive divider with around 10mv differential voltage and connect to the terminals marked OUT+(IN-?) and OUT- (IN+?) points. Check for the output voltage at the point marked ADC.
By the way, I have noticed some noise problem with the amplifier with pins 12,13 and 14. It was attributed to some layout problem and it was solved by using another unused amplifier from the same package.
You should also use a 4.7uF tantulum cap in parallel to a 0.1uF ceramic cap across Power supply pins close to the IC. Small capacitor values of 470pf across feedback resistors also help to avoid unexpected oscillations in the amplifier.
Since you are using on single supply, you have to make sure that all amplifier outputs swing to +ve side only, else you should use dual supplies.
Only after establishing the proper working of the amplifier you should proceed with the remaining circuit.
Regards,
Laktronics
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