Thanks for the replies.

I checked voltage across S+ and S- with Osciloscope and yes, I could
see the signal. The signal was ranging between 0 to about 5mv (full
scale). The signal was a bit noisy.

I'm planning to amplify it to 0-5V to fit my ADC voltage range. And do
the averaging to eliminate noise in processor. I guess I need a
instrumentation amplifier to achieve this. Any suggestion what
amplifiers that fit to this requirement?

Could anybody direct me to web site links that useful for this
applications?

anyway come into my mind that maybe it's easier to tap the final result
from digital scales's lcd, the lcd is 4 digits 7 segment. Physically
connected to PCB processor by 16 conductors (elastomeric conductor). Is
it feasible to do this? 


Thanks,
Cedi

Cedi wrote:
> I intend to make a weight aqcuistion digital system. For the sensor
> I bought commercial digital scale that have range 0-5kg with 1gr
> resolution. (TANITA KD-403) 
> ...
> And when I measured voltage between S+ and S-, the reading is always
> zero.

Strain gauge signals are very low-level. Your voltmeter may not have
the necessary sensitivity.

This came up recently in a Circuit Cellar design contest, which was
subsequently written up as an article:

   http://www.dtweed.com/circuitcellar/caj00191.htm#3288

The author looked further downstream in the scale's own circuitry
and found a PWM siganl on a marked test point whose duty cycle was
linearly related to the weight on the scale. Perhaps your scale
has something similar.

-- Dave Tweed

Cedi wrote:
> I intend to make a weight aqcuistion digital system. For the sensor
> I bought commercial digital scale that have range 0-5kg with 1gr
> resolution. (TANITA KD-403) 
> ...
> And when I measured voltage between S+ and S-, the reading is always
> zero.

Strain gauge signals are very low-level. Your voltmeter may not have
the necessary sensitivity.

This came up recently in a Circuit Cellar design contest, which was
subsequently written up as an article:

   http://www.dtweed.com/circuitcellar/caj00191.htm#3288

The author looked further downstream in the scale's own circuitry
and found a PWM siganl on a marked test point whose duty cycle was
linearly related to the weight on the scale. Perhaps your scale
has something similar.

-- Dave Tweed

On 23 May 2006 20:41:13 -0700, "Cedi" <cedi@bdg.centrin.net.id> wrote:

> So I did some research to tap signal from the
>sensor. I measure voltage (with ADC) between V+
>and V-, and the reading was toggling, between 0 and about 2.5V (with or
>without load on the gauge).
>I think this is intended for battery saving and for minimizing self
>heating effect on the sensor.

This sounds like a chopper stabilised amplifier. Any DC amplifier will
suffer from drift, but if you can use an AC connected amplifier, this
should not be a problem. 

When driving the sensor bridge by a square wave, the retrieved sensor
output voltage will switch between 0 V and some very low voltage (a
few mV). This voltage swing is amplified by an AC amplifier and a
synchronous detector is used to put the "0 V" setting as the reference
setting and using the other value as the actual measurement value X. 

Any drift in the amplifier will affect both the "0 V" level as well as
the "X" voltage level, but the difference X-0V is proportional to the
value of interest.

Paul
  

"Cedi" <cedi@bdg.centrin.net.id> wrote in message
news:1148442073.407316.213520@j73g2000cwa.googlegroups.com...
> Hi Group,
>
>
> I intend to make a weight aqcuistion digital system. For the sensor I
> bought commercial digital scale that
> have range 0-5kg with 1gr resolution. (TANITA KD-403) After I opened
> the casing, I found that the load sensor
> and the processor connected by 4 wire: V+, V-, S+, and S- (this written
> on the pcb). I guessed that the sensor is strain
> gauge with bridge. So I did some research to tap signal from the
> sensor. I measure voltage (with ADC) between V+
> and V-, and the reading was toggling, between 0 and about 2.5V (with or
> without load on the gauge).
> I think this is intended for battery saving and for minimizing self
> heating effect on the sensor. And when I measured
> voltage between S+ and S-, the reading is always zero. what I don't
> understand is, the value is not changed as I change
> the load on the gauge. The theory I know says that (assume the sensor
> is really strain gauge) that the voltage across s+
> and s- would depend on the load given (because this is the one that
> indicates how strong force given to the sensor).
> Strange thing is the scale was still working perfectly (show a correct
> weight on its own lcd display) when I tap the signals.
>
> Hope someone has deeper knowledge on this. Is there possibilities that
> the sensor is not strain gauge?
> Or there's a tricky modulation using regular strain gauge so the
> voltage across s+ and s- doesn't need
> to depend on the load? Some pulse width modulation or delayed response?
> I'm really blank on this.
>
> Any help would be greatly appreciated
>
>
> thanks,
> Cedi
>

A typical load cell will give an output of 2mV per volt of excitation at
full scale. With 2.5V excitation you can expect about 1mV per kg of load
from your scale - were you looking for such a small signal ?
They may be pulsing the excitaion to save power or as part of an auto
zeroing system.
Try searching Linear Technology's web site for app notes about load cell
signal conditioning and HBM's for specs on load cells.

Michael Kellett

www.mkesc.co.uk

Hi Group,


I intend to make a weight aqcuistion digital system. For the sensor I
bought commercial digital scale that
have range 0-5kg with 1gr resolution. (TANITA KD-403) After I opened
the casing, I found that the load sensor
and the processor connected by 4 wire: V+, V-, S+, and S- (this written
on the pcb). I guessed that the sensor is strain
gauge with bridge. So I did some research to tap signal from the
sensor. I measure voltage (with ADC) between V+
and V-, and the reading was toggling, between 0 and about 2.5V (with or
without load on the gauge).
I think this is intended for battery saving and for minimizing self
heating effect on the sensor. And when I measured
voltage between S+ and S-, the reading is always zero. what I don't
understand is, the value is not changed as I change
the load on the gauge. The theory I know says that (assume the sensor
is really strain gauge) that the voltage across s+
and s- would depend on the load given (because this is the one that
indicates how strong force given to the sensor).
Strange thing is the scale was still working perfectly (show a correct
weight on its own lcd display) when I tap the signals.

Hope someone has deeper knowledge on this. Is there possibilities that
the sensor is not strain gauge?
Or there's a tricky modulation using regular strain gauge so the
voltage across s+ and s- doesn't need
to depend on the load? Some pulse width modulation or delayed response?
I'm really blank on this.

Any help would be greatly appreciated


thanks,
Cedi