Scale Magnometer to Servo

Question

I'm trying to determine what is the best way to scale a magnometer to a servo, I am using the BNO055 from Bosch I get a heading say 50 degrees, and I want to be able to input 45 degrees and have the servo motor move accordingly. I am using a 180 degree motor. I would like to be able to input 45 degrees, and then the motor if 50 degrees was 95, would decrease in value step by step until the magnometer read 45; I'm having difficulty creating the logic to do this.

Bosch output is between 0 and 360; so I could end up with servo 0 is 120; and servo 180 is 300. Meaning the values are 300 - 360, 0 - 120 making the full 180, so I am not sure how I could use something like map in arduino to solve this issue.

Answer 1

Okay, lets say the output of your BNO055 is called x and the value for your servo is y. We need to translate x to y and you are correct to use the map() function. But what you are forgetting is that you do not need to map all the values of x to all the values of y. You can split it up.

In your example the values of the BNO055 relate to the servo as follows:

x   | y   
0   | 0
120 | 0
300 | 180
360 | 180

The servo only rotates from 0 degrees to 180 degrees between the BNO055 sensor values of 120 and 300. So you only need to map those values, while all values below should result in 0 and all values above should result in 180 degrees for the servo.

This translates into the following code:

//x is the output of BNO055 sensor
if(x < 120){
  servo.write(0);
}
if(x > 300){
  servo.write(180);
}
if(x > 120 && x < 300){
  y = map(x, 120, 300, 0, 180);
  servo.write(y);
}

Another way of coding this is:

if(x < 120){
  servo.write(0);
}
else if(x > 300){
  servo.write(180);
}
else{
  y = map(x, 120, 300, 0, 180);
  servo.write(y);
}

Answer 2

You need to remember that you are working with a circle. You have degrees 0 to 360, but only if counting clockwise. If you work anticlockwise you also have degrees 0 to -360.

So your angles of interest are, clockwise from 0, 0-120, and anti-clockwise from 0, 0 to -60

To make things easier on you you need to create a range that is wrapped around so that anything in the "left" half of the circle is counted as anticlockwise from 0 and anything in the "right" half of the circle is counted clockwise from 0.

So values 0-180 are 0-180, but values of 180 to 360 are -128 to 0. Then your entire range spreads from -180 to +180 instead of 0-360 (if that makes sense).

It's simple enough:

if (angle > 180) angle = 0 - (360 - angle);

So if angle is 300 you get:

0 - (360 - 300) = 0 - 60 = -60

If angle is 330 you get:

0 - (360 - 330) = 0 - 30 = -30

Now you can map the new values using the map function @Len has detailed above:

servo = map(angle, -60, 120, 0, 180);

It is also possible to combine it all into one line should you wish, using the C "inline if" format:

servo = map((angle > 180 ? (0 - (360 - angle)) : angle), -60, 120, 0, 180);

But that is kind of cryptic :)