Why does bathroom scales use half bridge load cell rather than 4 full bridge loadcell in parallel? Except for the price, is there any other reasons?

Question

I have been wondering why does bathroom scale uses 4 half bridge loadcell connected like this figure bellow rather than using 4 full bridge loadcell connected in parallel? Except for cheaper manufacturing price or half bridge loadcell is there any other reasons? For example, is it easier to calibrate for each loadcell corner?

Half bridge bathroom scale connection:

From what I know, using 4 full bridge loadcell in parallel will average the sensed voltage from each load cell so we need to calibrate each corner using potentiometer to make sure they sense the same voltage at same load. Is it possible that this half bridge connection get rid of the necessity of calibrating each corner?

Answer 1

If you are using a single load cell, then full bridge is useful, so both sides track.

However if you are using four nominally identical cells, then you already have tracking, and half bridge does everything you need.

To the best of my knowledge, consumer grade scales do not calibrate each corner individually. They rely on the load cells having the same gain, which they nominally have being the same type, and simply add the four outputs together. If you trace the circuit, you will probably find that 'adding' is done by driving both inputs to the differential ADC from different cells, and inverting half of them.

While lack of individual calibration makes the scales less than perfect, they are adequate for consumer use, especially as with bathroom scales you tend to stand somewhere in the middle. Scientific or commercial scales would be likely to be individually calibrated.

If you are concerned about the accuracy, get a heavy weight and move the load to mostly one corner at a time. Take care that there is still some load on the other corners, as you do not want to get into non-linearity if the foot is close to lifting off the floor.

One part per thousand sounds very optimistic for 'bathroom' scales, whether for hysteresis, long term zero drift, or any other notion of accuracy.