Assigning percentage values to nonlinear analog input

Projects Programming
1

I am attempting to assign a percentage value to a raw analog input that is nonlinear over its range.
But I am thinking there may not be a straightforward solution.

When the sensor is at the bottom end of its scale it reads 190 and at the top end 575
for a total count range of 385.
The sensor detects changes in water level on a 10 trace grid; each grid is 1 1/2" in length. When it barely contacts water at the bottom of the grid it starts reading around 180-190 counts.

Water 1st Detected: 180-190 [0.0"]
1/4 range : 450
1/2 : 515
3/4 : 550
Full range: 575 [1.5"]

I would like to convert the counts to a percentage value to represent how full the container is. But because the output is nonlinear I cannot simply assign X counts = X percent.
I looked at the map() function but I do not think it compensates for nonlinearity.

Otherwise I am left to try and create some sort of table that references a specific percentage (%) value based on the input count value.

ideas?

2

Right. map() is linear.

If you don't need more than 1% resolution a "table" should work. (Or a series of 100 if-statements).

Or you might try approximating the curve with 10 straight-lines and 10 associated map() calculations.

3

I was trying to avoid the if statement route and use as a last resort.
Is there any good links that would cover populating a table and then open the table and find a search result?

4

Try - GitHub - RobTillaart/MultiMap: Arduino library for fast non-linear mapping or interpolation of values

5

You had an excellent link there. And you being the creator
makes it even more appropo.... :+1: :+1: :+1:

6

Is the pot nonlinear, or is the tank nonlinear, or is it a bit of both?

7

A table with interpolation between the points. If you wanted to play with the STL:

#include "Arduino.h"
#include <map>

double interpolate(uint16_t count);

void setup() {
	Serial.begin(115200);
	delay(1000);

	for (uint16_t cnt = 175; cnt <= 600; cnt += 25) {
		Serial.printf("%hu, %lf\n", cnt, interpolate(cnt));
	}
}

void loop() {
}

double interpolate(uint16_t count) {
	static std::map<const uint16_t, const double> sensorMap {
			{ 190, 0.0 },
			{ 450, 0.25 },
			{ 515, 0.5 },
			{ 550, 0.75 },
			{ 575, 1.0 }
	};

	auto lowerItr = sensorMap.lower_bound(count);
	if (lowerItr == sensorMap.end()) {
		return (*(--lowerItr)).second;
	}
	if (lowerItr == sensorMap.begin()) {
		return (*lowerItr).second;
	}
	if ((*lowerItr).first == count) {
		return (*lowerItr).second;
	}

	uint16_t countUpper = (*lowerItr).first;
	double levelUpper = (*lowerItr--).second;
	uint16_t countLower = (*lowerItr).first;
	double levelLower = (*lowerItr--).second;
	double levelRange = levelUpper - levelLower;
	uint16_t countRange = countUpper - countLower;

	return levelLower + (count - countLower) * levelRange / countRange;
}
1 Like
8

its actually a sensor....

DIYables Water Sensor

9

Another interesting approach you have generously provided!
Between you and Rob I have plenty to study and experiment with.
Thank you sir.... :ok_hand:

10

What is required to run the map() function? I figured it was in the standard library
with all the other functions.

C:\Users\Ed\AppData\Local\Temp\.arduinoIDE-unsaved2024431-4120-rhlqsi.fxby\sketch_may31b\sketch_may31b.ino:2:10: fatal error: map: No such file or directory
   // put your setup code here, to run once:
          ^~~~~
compilation terminated.
exit status 1

Compilation error: map: No such file or directory
11

What board are you compiling for?

12

UNO WiFi Rev2

13

Ahhh ... that's the problem. The STL (including Containers like map) is not available on AVR-based boards.

15

Rut row.... that sucks....

16

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