Nano BLE 33 IMU tutorial code

Hardware Nano Family Nano 33 BLE
1

This code in the Nano 33 BLE IMU tutorial says to use map() to convert accelerations to degrees:

/*
  Arduino LSM9DS1 - Accelerometer Application

  This example reads the acceleration values as relative direction and degrees,
  from the LSM9DS1 sensor and prints them to the Serial Monitor or Serial Plotter.

  The circuit:
  - Arduino Nano 33 BLE

  Created by Riccardo Rizzo

  Modified by Jose García
  27 Nov 2020

  This example code is in the public domain.
*/

#include <Arduino_LSM9DS1.h>

float x, y, z;
int degreesX = 0;
int degreesY = 0;

void setup() {
  Serial.begin(9600);
  while (!Serial);
  Serial.println("Started");

  if (!IMU.begin()) {
    Serial.println("Failed to initialize IMU!");
    while (1);
  }

  Serial.print("Accelerometer sample rate = ");
  Serial.print(IMU.accelerationSampleRate());
  Serial.println("Hz");
}

void loop() {

  if (IMU.accelerationAvailable()) {
    IMU.readAcceleration(x, y, z);

  }

  if (x > 0.1) {
    x = 100 * x;
    degreesX = map(x, 0, 97, 0, 90);
    Serial.print("Tilting up ");
    Serial.print(degreesX);
    Serial.println("  degrees");
  }
  if (x < -0.1) {
    x = 100 * x;
    degreesX = map(x, 0, -100, 0, 90);
    Serial.print("Tilting down ");
    Serial.print(degreesX);
    Serial.println("  degrees");
  }
  if (y > 0.1) {
    y = 100 * y;
    degreesY = map(y, 0, 97, 0, 90);
    Serial.print("Tilting left ");
    Serial.print(degreesY);
    Serial.println("  degrees");
  }
  if (y < -0.1) {
    y = 100 * y;
    degreesY = map(y, 0, -100, 0, 90);
    Serial.print("Tilting right ");
    Serial.print(degreesY);
    Serial.println("  degrees");
  }
  delay(1000);
}

Does this mean that the Nano 33 BLE's accelerometer and the Arduino Library (at github) returns accelerations as degrees around the axes? Or is is map(x*100, 0, 97, 0, 90) just a sloppy estimate of atan2(x,z)*180/pi?

How is x_acc=0.97 supposed to mean "Tilting up 90 degrees"?

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2

It is complete nonsense. Unfortunately, that particular bit of nonsense is ubiquitous on the web, and the tutorial writers clearly don't know any better.

1 Like
3

I could see a fast, cheap approximation being worthwhile in some situations, but in any place you'd might use such an approximation, using the acceleration (times a constant?) would work just as well.

The 97 confused me, and I couldn't think of any reason why that scheme was in any way more useful than:

approxDeg = x * 90;
4

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