Tapping signal from digital scale's weight sensor

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

Reply to
Cedi
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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

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Reply to
MK

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

Reply to
Paul Keinanen

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:

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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

Reply to
David Tweed

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:

formatting link

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

Reply to
David Tweed

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

Reply to
Cedi

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