On 2012-03-14, RogerN <regor@midwest.net> wrote:

> One concern I'm having so far is sometimes the weight reading go down with 
> the addition of a little weight, not sure if this is because of a bad load 
> cell or if there is some misreading from the AD7730 chip.  I tried using all 
> SPI modes from the arduino library, I couldn't get mode 0 to work at all, 
> mode 1 works most reliable, mode 2 and 3 both work but the chip locks up 
> after a few readings.

That part is based around a sigma-delta DAC and so has no reason not
to be monotonic.  I'd try substituting a different source that you
know to be monotonic - eg: a pot in series with a big resistor -
and repeating your measurements. bracketing the discontinuity may give
some information as to what the cause is.



-- 
&#9858;&#9859; 100% natural

--- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net ---

I put an AD7730 chip on the breadboard, got it interfaced to the Arduino, 
and am getting numbers from a load cell.  This took some playing with to get 
working almost correctly but I'm getting readings that look good, 5 grams on 
a 30 Kg load cell is producing about 25 counts from the A/D value / 128. 
Should be able to do a little better with actual calibration instead of 
internal reference calibrations.

Next I'll write some calibration and tare functions plus conversions for 
lbs, oz, grams, Kg's, grains, etc.

One concern I'm having so far is sometimes the weight reading go down with 
the addition of a little weight, not sure if this is because of a bad load 
cell or if there is some misreading from the AD7730 chip.  I tried using all 
SPI modes from the arduino library, I couldn't get mode 0 to work at all, 
mode 1 works most reliable, mode 2 and 3 both work but the chip locks up 
after a few readings.

RogerN

On Thu, 08 Mar 2012 09:15:01 +0000, MK <mk@nospam.co.uk> wrote:

>On 08/03/2012 04:40, WoolyBully wrote:
>> On Wed, 07 Mar 2012 20:22:48 -0800, josephkk
>> <joseph_barrett@sbcglobal.net>  wrote:
>>
>>
>>> The signal coming out of a load cell is only a few mV/V full scale.  Keep
>>> the IA.
>>
>>
>>    Millivolts per what?  You fucking dope?
>
>Oh Woolly - how can you be so ignorant and so arrogant at the same time?

It's the only thing Nymbecile excels at!

>
>Strain gauge load cells are almost always specified in mv/V at full 
>scale so Joseph is following the usual practice. (about 2mV/V being 
>typical for a precision load cell)
>
>What it means is that the load cell will give you an output of x mV for 
>each V of excitation at full scale load (where x is the sensitivity in mV/V)
>
>They are specced this way because the output is linearly dependent on 
>the excitation.
>
>Joseph is also correct in suggesting that the OP will find it much 
>easier to get a good result by using a separate amplifier before the 
>ADC. I have never managed to get good results with the all in one 
>devices from AD and others.
>
>Michael Kellett (my real name)
>
>
>

On Wed, 07 Mar 2012 20:40:52 -0800, the renowned WoolyBully
<WoolyBully@arcticicemasses.org> wrote:

>On Wed, 07 Mar 2012 20:22:48 -0800, josephkk
><joseph_barrett@sbcglobal.net> wrote:
>
>
>>The signal coming out of a load cell is only a few mV/V full scale.  Keep
>>the IA.
>
>
>  Millivolts per what?  You fucking dope?

It's a strain gauge, so mV per volt of excitation voltage (delta in
output voltage from no load to full-scale load). A standard way of
specifying sensitivity. 


Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

On Thu, 08 Mar 2012 09:15:01 +0000 MK <mk@nospam.co.uk> wrote in Message
id: <4F587895.5060701@nospam.co.uk>:

>On 08/03/2012 04:40, WoolyBully wrote:
>> On Wed, 07 Mar 2012 20:22:48 -0800, josephkk
>> <joseph_barrett@sbcglobal.net>  wrote:
>>
>>
>>> The signal coming out of a load cell is only a few mV/V full scale.  Keep
>>> the IA.
>>
>>
>>    Millivolts per what?  You fucking dope?
>
>Oh Woolly - how can you be so ignorant and so arrogant at the same time?

Practice makes perfect!

On 08/03/2012 04:40, WoolyBully wrote:
> On Wed, 07 Mar 2012 20:22:48 -0800, josephkk
> <joseph_barrett@sbcglobal.net>  wrote:
>
>
>> The signal coming out of a load cell is only a few mV/V full scale.  Keep
>> the IA.
>
>
>    Millivolts per what?  You fucking dope?

Oh Woolly - how can you be so ignorant and so arrogant at the same time?

Strain gauge load cells are almost always specified in mv/V at full 
scale so Joseph is following the usual practice. (about 2mV/V being 
typical for a precision load cell)

What it means is that the load cell will give you an output of x mV for 
each V of excitation at full scale load (where x is the sensitivity in mV/V)

They are specced this way because the output is linearly dependent on 
the excitation.

Joseph is also correct in suggesting that the OP will find it much 
easier to get a good result by using a separate amplifier before the 
ADC. I have never managed to get good results with the all in one 
devices from AD and others.

Michael Kellett (my real name)

On Wed, 07 Mar 2012 20:22:48 -0800, josephkk
<joseph_barrett@sbcglobal.net> wrote:


>The signal coming out of a load cell is only a few mV/V full scale.  Keep
>the IA.


  Millivolts per what?  You fucking dope?

On Sun, 4 Mar 2012 00:44:28 -0600, "RogerN" <regor@midwest.net> wrote:

>"Nico Coesel"  wrote in message news:4f52b148.3510317656@news.kpn.nl...
>
><snip>
>>>Sure you can.  Averaging will give more bits than there is hardware =
for.=20
>>>I
>>>played that game with Tektronix digitizers 35 years ago, to pull =
signals=20
>>>out
>>>of the digitization noise.  The number of bits gained (assuming=20
>>>uncorrelated
>>>noise) is proportional to the log of the number of samples, so it gets=
=20
>>>boring
>>>fast, though.  ;-)
>>
>>That only works if there is enough noise to flip at least the LSB bit.
>>If the signal sits somewhere between 2 codes you can oversample all
>>you want but you'll never get any extra bits of resolution.
>>
>>--=20
>>Failure does not prove something is impossible, failure simply
>>indicates you are not using the right tools...
>>nico@nctdevpuntnl (punt=3D.)
>>--------------------------------------------------------------
>
>Here's an idea I was curious about to increase A/D resolution.
>
>Lets say for example the LSB represented 1 millivolt.  Could I use a=20
>resistor network (or DAC and voltage divider) to drop 1/4mV, 1/2mV, and=20
>3/4mV.  The idea would be to add these voltages to the load cell signal,=
=20
>read with 0mV offset, next read with 1/4mV offset, then 1/2mV offset... =
 It=20
>would seem that averaging with offset voltages less than the LSB could =
get a=20
>higher resolution and hopefully with linear accuracy.  Just wondering if=
=20
>that would work?  The A/D I intend on using will probably be 24 bit so =
I'm=20
>not going to higher than that, but sure would be nice for the lower=20
>resolution load cells sometimes.  I thought about using a DAC with a =
voltage=20
>divider to make a programmable offset voltage to divide the LSB.  Offset=
 1/2=20
>LSB, compare, offset 1/4 LSB, compare... till I find the fraction of the=
 LSB=20
>where noise toggles it.
>
>So  if I were to do this with the 10 bit Arduino ADC, offsetting 1/64LSB=
 at=20
>a time over 64 readings, would the sum of the readings be (almost) as =
good=20
>as a 16 bit ADC?  Don't know if it would be practical but sounds =
interesting=20
>and educational :-)
>
>RogerN
>
A thought to keep track of: the load cell has only about 0.01% accuracy /
linearity / resolution in the first place.  No matter what you do with =
the
signal afterwards that is all you can get from them.

?-)

On Tue, 6 Mar 2012 11:49:44 -0500, "Oppie" <Oppie@127.0.0.1> wrote:

>If you can, put the ADC in the scale so you don't have to deal with =
noise=20
>pickup in the cabling. Much better to run SPI signals in a cable (with=20
>suitable drivers/receivers and signaling rates) than it is load cell=20
>signals.
>
>If you must have the load cell remote to the ADC, use twisted pairs in a=
=20
>shield. Excitation as one pair and bridge output as the other pair.
>
>Signal is only as significant as your reference is stable. ADC reference=
=20
>should also be the excitation signal.
>
>Depending on your resolution needs, some folks forgo the instrumentation=
 amp=20
>and go directly into a differential input channel of the ADC. I =
generally=20
>prefer to use an IA for the high CMRR but with every stage of =
amplification=20
>you also get noise.
>
>There are a lot of apnotes out there under "strain gauge application"=20
>(strain gauge being the basic sensing element in the load cell)=20

The signal coming out of a load cell is only a few mV/V full scale.  Keep
the IA.

?-)

If you can, put the ADC in the scale so you don't have to deal with noise 
pickup in the cabling. Much better to run SPI signals in a cable (with 
suitable drivers/receivers and signaling rates) than it is load cell 
signals.

If you must have the load cell remote to the ADC, use twisted pairs in a 
shield. Excitation as one pair and bridge output as the other pair.

Signal is only as significant as your reference is stable. ADC reference 
should also be the excitation signal.

Depending on your resolution needs, some folks forgo the instrumentation amp 
and go directly into a differential input channel of the ADC. I generally 
prefer to use an IA for the high CMRR but with every stage of amplification 
you also get noise.

There are a lot of apnotes out there under "strain gauge application" 
(strain gauge being the basic sensing element in the load cell)