Best practices "normalizing" sensor data into a min---max scale.

Projects Programming
1

Hello,

I am harvesting load cell data with a frequency of 80 samples per second.
The raw data comes in a form of long values, starting at 8590300 +/-100 (zero load) and maxing out around 8850000 (with my current spring load configuration).

I am trying to map the data to a joystick axis in Windows and facing the following problems:

  1. My max-min delta is very insignificant compared to the integers I am reading: 250000 values on a scale of 9 million values is less than 3%.
  2. I don't need all 250000 values for my application. 4096 resolution steps (12bit) would be more than enough.

In my previous post there was a great suggestion to chop extra bits off the 24-bit sensor read data.

 dividedValue = Hx711.read() >> 12

This definitely helps with the issue #2, but it doesn't resolve the first problem where my delta will still be 3% of the absolute min-max.

What are the practices used to "narrow down" the absolute minimums and maximums so that my load is more significant on the data scale?

Thanks!

2

constrain to keep any overloads within range, then map to rescale.

int32_t loadCellMin = 8850000;
int32_t loadCellMax = 8590200;

auto loadCellValue = Hx711.read();
loadCellValue = constrain(loadCellValue, loadCellMin, loadCellMax);
int joystickValue = map( loadCellValue, loadCellMin, loadCellMax, 0, 4096);
3

Jiggy-Ninja:
constrain to keep any overloads within range, then map to rescale.

int32_t loadCellMin = 8850000;

int32_t loadCellMax = 8590200;

auto loadCellValue = Hx711.read();
loadCellValue = constrain(loadCellValue, loadCellMin, loadCellMax);
int joystickValue = map( loadCellValue, loadCellMin, loadCellMax, 0, 4096);

Thank you sir!

Looking at the last line of your snippet with the map() function - does that mean that I can remap to my new scale 0-4096 and no bit chopping is required (the line of code I included in OP)?

4

I just tested your suggestion and it worked like a charm. I answered my last question when testing the code. Thanks again!

5

Important note:

if the x is outside the range [a..b]

then map(x, a,b, c,d) will be outside the range [c..d] (linear interpolated)

you could do some print test if there's some change if you not shift bit or not. if all bit are zero, I think you can avoid it. but should always double check...

Regards.

Nitrof

6

nitrof:
Important note:

if the x is outside the range [a..b]

That is why I recommended constraining first.