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