Simple Maths Going Wrong

[Bill Legge]

First of all - a bit of relief that my difficulty is not in the 'daft' category.

1. Thanks for pointing me to the earlier discussion. I see M's solution and agree that it works fine with two readings - but I can't see an easy way of doing it with an array; say 16 readings? And wind direction needs a good bit of averaging to get a good result.

2. Effect of wind speed. I agree that wind direction is meaningless unless the wind is blowing. I have a threshold that discards direction reading unless there is a reasonable wind speed.

3. Trignometric solution. thanks, a neat idea but how to get the an angle once all the SIN and COS have been summed/differenced? I don't think inverse trig functions are available?

Overall, the task seems so visually simple that I still feel that some addition/subtraction of readings will do the trick. Perhaps it's as simple as:

1. Use, say, the first vector as a reference (zero)
2. Always measuring clockwise, add the angle to the next vector
3. divide by the number of vectors
4. Add this to the first vector
5. Subtract 256 if necssary (i.e one revolution)

The trick is to always measure angles in the same direction, say clockwise?

Regards Bill Legge

[rsocor01]

Bill,

Yes, you might be in the right track by measuring angles in the same direction. This might work for two angles, but if you need more than two readings it can get quite complex. Still you need to work out the algorithm and tested with different values.

I know that in order to get the angle you need inverse trigonometric functions, inverse tangent to be specific which is not available in PBP (maybe in future versions). But, I didn't want to throw that in here just in case you were not familiar with these functions. However if you know the signs of the X and Y resultant components that will tell you in which quadrant the angle is and that is a big help.

Note that the method of summing two readings and dividing by two (averaging) works most of the times. The only times that it doesn't work is when one of the readings value range is (0 < reading1 < 64) and the other value range is (192 < reading2 <= 255). When this happens you get bad results. Then, you might need to add or substract 128. Based on this info you should be able to write your algorithm.


Robert

[Darrel Taylor]

Here's one that works real well.
But it requires PBP 2.60 for the ATN function.

The measured wind direction should be in the variable DIR.
Then GOSUB Avg_Wind

It works by averaging the SIN and COS separately for each sample.
Then converts those averages back to an angle with ATN.
Angles range from 0-255, representing 0-359 degrees.

CLEAR

DIR   VAR BYTE   ; Current sample of wind direction (0-255)
S     VAR BYTE   ; averaged SIN
C     VAR BYTE   ; averaged COS
Wind  VAR BYTE   ; final averaged wind direction
Temp  VAR BYTE   ; temporary variable

AvgCount  CON 10 ; amount of averaging to apply

;---------------------------------------------------------------
Avg_Wind:    ; average in one DIR sample
  Temp = SIN DIR+127
  S=(S*(AvgCount-1)+Temp)/AvgCount
  Temp = COS DIR+127
  C=(C*(AvgCount-1)+Temp)/AvgCount
  Wind = (C-127) ATN (S-127)
RETURN

;---------------------------------------------------------------
Set_Wind:    ; Sets average wind direction without averaging
  S = SIN DIR + 127
  C = COS DIR + 127
GOTO Avg_Wind

If you don't have 2.60, the same thing can probably be done with scalerobotics cordic trig routines.

HTH,