[arnejenssen]

A silly question (from a non-HW guy). Why are digital bathroom scales so coarse? Some have a weight resolution of +/- 500 grams. Would a better microcontroller make a weight faster or more presise? I guess this TI micro controller is overkill for a bathroom scale.

[jmiskovic]

Computing power is crazy cheap and does not help. The perceived slowness is because the taken measurements are averaged over second or so. When you step on the scale your weight shifts around and that affects the measurement by far more than half a kg. Averaging gives you better accuracy and more confidence in the result but it doesn't improve the measurement precision.

The scale precision comes from calibration of the measuring mechanism done on factory line at certain weights. If your specific body weight is far from the weights used in calibration, or too much time passed, then the calibration doesn't help much.

A better microcontroller doesn't help at all. Even a cheapest uC from decades ago is good enough. Better ADC and gauge sensor would help, but even more important is good analog engineering to produce self-correcting circuits with decent noise rejection.

In general this category of products is more for tracking changes in your body weight than getting the precise absolute value. And the body weight changes a lot just from hydration level, so the ±.5kg tolerance is considered good enough.

[myrmidon]

Higher resolution would let the user see that your scale is zeroed badly, that repeatability sucks and that the measured weight is not even constant under a constant load.

Decreasing resolution hides all of those.

[numpad0]

Digital scales generally work by warping metal pieces and forcing a flex film resistor glued on the piece to be stretched along, causing its resistance to change, thereby breaking the balance of a Wheatstone bridge, creating force-proportional current to occur, which is finally read out with an ADC.

I guess the overall finickiness of that can't be instantly improved by a better microcontroller alone? They drift and have linearlity issues and show temperature dependancy and all that.

Precise body weight measurement at bathrooms is also probably not that important, 500 grams is one full bottle of soda/water; body weights can easily change that much within a single day.

I would suspect that the "real" reason is combination of both. 100-500g can be a "good" compromise for cheap bathroom scales.

[Skwid]

As others have said, the crap resolution is to mask the crap repeatability. Better processing can help, but good enough processing fits in the cheapest of microcontrollers. ADC quality is more important, signal conditioning and sensor quality more important still. The biggest factor for accuracy is likely calibration though. A sensor that's 2% accurate with a 2 point calibration can easily become a 0.25% sensor with a 5 point, 3 temperature calibration. But that all takes time, and unless you're running huge batches it adds too much to the bottom line of a low cost product. Source: Designed strain gauge sensors and manufacturing processes for a living

[arnejenssen]

I remember that a friend told that the Nintendo Wii Balance board gave her the weight on gram resolution. Which is 10-100x better than most commercial bathroom scales. But keep in mind that precision is not the same as accuracy.

[zipy124]

Mostly quality of the sensor, processing power is not a constraint. Also bear in mind that high range and high accuracy generally aren't very compatible, so to measure 100kg at a 1% accuracy is accuracy to the kg. to get +/-100g accuracy you need a +/-0.1% accuracy.

This is why highly accurate scales usually have very low max weights.

[yoggodoggo]

It's down to the quality of the sensor, the quality of the ADC translating the signals from the sensor, and the implementation of the software that deals with the ADC translation.

[lukan]

Not an expert here, but I doubt it is related to processing power, but quality of the sensor.

[HackerThemAll]

> this TI micro controller is overkill for a bathroom scale

For majority of use cases nowadays it's much easier to use a programmable chip than invent a complex device using discrete analog electronic parts.