Compass deviation alarm

[evoortman]

Hi,

I am working on a project with a compass sensor. The sensor output is 0° to 359°.
I need to activate an alarm if my heading is more than 20° off the compass.
Let's say my desired heading is 50° ± 20° then I can just use the code below.

If heading > desired_heading + 20 Or heading < desired_heading - 20 Then
  alarm = 1
Else
  alarm = 0
EndIf

However, this code will only work if my desired heading is more than 20° from the 0° point on the compass (> = 20° and <= 340°)
But what if my desired heading is 5°?    5 - 20 = -15  which is 345° on the compass.
Even if my desired heading is 350°?  350 + 20 = 370  which is  10° on the compass.

Can anyone give me a push in the right direction how to make the calculation if the desired heading is less than 20° from the 0° point of the compass?

Thanks!

[RGV250]

Hi,
I have not got a lot of time to give a better answer right now but if you test for greater than 360 and if so subtract it. If the value is negative then add 360.

Bob

[trastikata]

Hope this helps.

Code Select Expand

Dim InitialHeading As SWord
Dim MinHeading As SWord
Dim MaxHeading As SWord
Dim CurrentHeading As SWord
Dim alarm As Bit

GoSub Calculate_My_Limits

Main:   
    While 1 = 1
        If CurrentHeading > MaxHeading And CurrentHeading < MinHeading Then
            alarm = 1
        Else
            alarm = 0
        EndIf
       
        'Recalculate limits in case it is needed
        GoSub Calculate_My_Limits   
    Wend

   
   
Calculate_My_Limits:
    MinHeading = InitialHeading - 20
    If MinHeading < 0 Then
        MinHeading = 360 + MinHeading
    EndIf
   
    MaxHeading = InitialHeading + 20
    If MaxHeading > 360 Then
        MaxHeading = MaxHeading - 360
    EndIf
Return   


[evoortman]

Thanks. Sorry for the late replay.
I will give it a try next week.

[evoortman]

Hi trastikata,

It seems your code almost works correctly :-)
The alarm only works correctly if the initialheading is between 340° and 20°. If not then the alarm is never active.

If i change the AND for OR in the comparison in the while loop, then the alarm only works correctly if the initialheading is between 20° and 340°. If not then the alarm is always active.

Do you have any idea why?

Thanks for your help!

[trastikata]

Do you have any idea why?

Yes, because you are working in different quadrants.

The alarm only works correctly if the initialheading is between 340° and 20°. If i change the AND for OR in the comparison in the while loop, then the alarm only works correctly if the initialheading is between 20° and 340°.

You asked for a push in the right direction, I gave you one. Where is the problem finishing the code - look carefully what have I chosen to quote  !

[evoortman]

It was not my intention to criticize your code. I looked at it and really didn't see why. (I still don't exactly actually)
Maybe I assumed too much that I could immediately use the code completely working in my application.

[Pepe]

test it


Dim InitialHeading As Word
Dim MinHeading As SWord
Dim MaxHeading As Word
Dim CurrentHeading As Word
Dim alarm As Bit

Symbol Error = 20

GoSub Calculate_My_Limits

Main:

   While 1 = 1
   
     If CurrentHeading >= Error And CurrentHeading <= 360 - Error Then
                   If CurrentHeading > MaxHeading Or CurrentHeading < MinHeading Then
                                                                                      alarm = 1
                                                                                 Else
                                                                                      alarm = 0
                   EndIf
           Else
                   If CurrentHeading > MaxHeading And CurrentHeading < MinHeading Then
                                                                                       alarm = 1
                                                                                  Else
                                                                                       alarm = 0
                   EndIf
                                   
     End If 
        'Recalculate limits in case it is needed
        GoSub Calculate_My_Limits   
    Wend

   
   
Calculate_My_Limits:

    MinHeading = InitialHeading - Error
    If MinHeading < 0 Then
        MinHeading = 360 + MinHeading
    EndIf
   
    MaxHeading = InitialHeading + Error
    If MaxHeading > 360 Then
        MaxHeading = MaxHeading - 360
    EndIf
Return

[trastikata]

As Pepe said, but he made a typo:

"If CurrentHeading >= Error And CurrentHeading <= 360 - Error Then"

should be

"If InitialHeading >= Error And InitialHeading <= 360 - Error Then"

[Pepe]

Thank for the correction