What is the maximum frequency that I can get reliable weight data from a strain gauge load cell?

Question

I need to build a scale that uses a strain gauge load cell such as the one below:

https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/TAL220M4M5Update.pdf

The weight data will need to be stored with timestamp in the memory of the microcontroller. The weight on the scale will be subject to quick changes. For instance, someone might add items to the scale in quick succession, say once per 0.5 second. I wonder if this is too quick of a weight change to be reliably captured by the load cell in that the load cell need some time to "stabilize."

Moreover, as I understand, the ADC will provide an excitation voltage to the load cell and receive an output voltage from the load cell. Does this consume significant energy if done at a high frequency (say, 10 sample per second) over hours? The device is battery powered so power consumption is important.

Thanks

Answer 1

There's nothing in the theory of operation that would limit the bandwidth of a load cell. However, in the situation that you describe, I think your greatest problem will not be electrical, but mechanical. In other words, the scale physically bounces, and that takes a while to settle.

Meanwhile, the load cell faithfully captures what the scale is, in fact, actually doing. A slow display might appear random until it settles, but if you capture it fast enough, you can get a much more accurate sense of what's really going on.

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This is entirely an analog function, with a conversion to digital just tacked on at the end. The excitation is constant, DC, so the power consumption of the load cell itself is also constant, regardless of sample rate.

If you turn off the excitation, then of course you can save power, but it might take some time to stabilize before you can trust the reading. If you spend most of your time waiting to stabilize, then you're better off just leaving it on. How long that time is, probably needs to be determined experimentally, if it's even a problem at all. If the battery lasts "long enough" with it powered on all the time, then you're done.

Answer 2

Almost any load cell will be able to handle a 2 Hz load change rate.
The frequency response of the overall system is liable to be the limiting factor. As a rough rule of thumb you can measure load variations at say 10% of the resonant frequency.
Suggestion which I've not tried :-) - strike the cell sharply and note the resonant response. Staying 10+ times below that should be OK.

Using a cell with an extra temperature compensation strain gauge orthogonal to the strain axis is liable to be very worthwhile.

I bought some low cost domestic 2kg x 2g increment kitchen scales with a temperature compensating gauge included. They are linearly accurate to 1 part in 1000 (N x 2g reads correctly across thewhole range). They are just as good for temperatures across say 10-40 degrees C.
I went back and bought eight :-) (all the remaining stock).

See here for some useful related notes.