Simple Maths Going Wrong

[aratti]

Hi Darell, could you explain me how you get the average?

Temp = SIN DIR+127 here variable Temp is equal to S since S=SIN DIR+127

Than

S=(S*(AvgCount-1)+Temp)/AvgCount

which is:

S=(S*9+S)/10

So S is still S

I must have lost something.

Al.

[Darrel Taylor]

The results of the SIN and COS functions are a Signed number ranging from -127 to +127.
PBPW can't multiply or divide negative numbers, so they have to be scaled up to be worked with.

Since Temp is a Byte sized variable, adding 127 brings it up to a range of 0 to 254, which can then be averaged easily.

Then after averaging, 127 is subtracted to restore the signed numbers for use with ATN.
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For the averaging ...

S=(S*9+Temp)/10  ; Temp is the new corrected sample

The current average is multiplied by one less than the AvgCount, the new sample is added in for a total of 10 samples. Then it divides by 10 (AvgCount) to get the new average. No WORDs needed.
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[rsocor01]

The new ATN function that Darrel mentions most likely stands for arctangent, which is the inverse tangent. It looks like this new PBP version is going to be loaded with a bunch of new goodies.

Robert

[Archangel]

The new ATN function that Darrel mentions most likely stands for arctangent, which is the inverse tangent. It looks like this new PBP version is going to be loaded with a bunch of new goodies.

Robert

Yeah, I think it's more than fresh paint and new chrome bumpers, that's why I ordered mine yesterday. It's got some new array handling routines in there I am chompin' at the bit to try out.

[Bill Legge]

I think I've solved it for an array of 16 readings? The readings

1. A/D from a potentiometer
2. Range is 0 to 255
3. 0 is North, 64 East, 128 South,192 West

The rules:

1. Always work in pairs of vectors. Using EEPROM to store the vectors:

0 to 15 Stores the vectors. 8 pairs

16 to 23 Stores the next pairing. 4 pairs

24 to 27 Stores the next pairing. 2 pairs

28 to 29 Stores the last pair. 1 pair

30 Stores the result

2. If the difference between the vector pairs if <128 then average the vectors normally
3. If the difference between the vectors >128 then average normally, add 128, check for whole revolutions

I've run a fair number of tests and all seems to be OK?

Regards Bill Legge

X  VAR BYTE                ' First vector
Y  VAR BYTE                ' Second vector
Difference  VAR BYTE     ' Difference between vectors, always +
Average  VAR WORD      ' Average of two vectors
J  VAR BYTE                ' EEPROM memory address

DATA 250,230,240,34,200,10,10,23,23,21,125,0,34,255,123  ' 16 vectors	

Main:
FOR J = 0 TO 28 STEP 2	' Read pairs of vectors
	READ J,  X
	READ J+1,Y
	if Y>X THEN SWAP X,Y	' X is always the larger variable
				
	Difference	= X-Y		
	Average	= (Difference/2) + Y' Half difference and add to smaller vector
		 	
	IF Difference>128 then					
	Average=Average+128	' Reverse direction of vector
	IF Average>255 THEN Average=Average-255	' Whole revolution? Subtract 255 if necessary
	ENDIF
	
	WRITE J/2 + 16,Average		
	NEXT J
	
	LCDOUT $fe,$80,"Average: ",DEC3 Average	
END

[aratti]

Darell, I think that the problem given in post one still remain even with adding 1/10 of the variation (with your smooting system) because the variation are so big (1 to 255) that will making swinging also your reading if not filtered.

As far this new release of PBP 2.6 is concerned can you please clearify this trigonometric manipulation:

Wind = (C-127) ATN (S-127) = cos(a) (no math sign ?) ATN sin(a)

I know that tan(a) = sin(a)/cos(a) so (a) = atn (sin(a)/cos(a))

Is this the way that the new PBP use to extract the angle?

Thank you for your help in understanding.

Al.

[Darrel Taylor]

Hmm, I don't know Bill.
I've had a play with your code, and I'm not sure about it.

It does seem to pick a valid point in-between two samples. But it only averages that point, and the previous point found between two other samples. It's not averaging all the samples, so it's still very jumpy.

Your explanation describes EEPROM locations up to 29, but there are only 15 DATA values, although I don't think that affects the problem.

I've modified the program a bit to use the Average as the X value, then read the samples one at a time, also connected a POT to simulate the wind vane, but I get the same results.

Another hiccup, if the average is currently 10, then it receives a string of 255 samples, it stops at 1, and never crosses 0. And for many other samples, it stops 1 number away from the actual reading. 250 might stop at 249 or 251.

I think finding a point in-between 2 samples is fairly easy. But averaging a large number of samples is something altogether different, and a 2-sample average just doesn't do it.

Aratti,
The ATN function is actually an ATAN2 type since it works for all four quadrants. An arctangent would only work with 1 quadrant.

The format of the statement is ...

Result = X ATN Y

There aren't any other "math signs" involved.
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[Bill Legge]

DT. Thanks for taking time to look into the code.

First, I wrote it specifically to average 16 samples - it won't work with any other number because of the counter:

J/2 + 16

This counter takes the average (or average+128 if the two sample are more that 128 apart - and checks for a 'roll-over' above 255) and stores them:

Samples 0 and 1 average stored in address 16
Samples 2 and 3 average stored in address 17
Samples 4 and 5 average stored in address 18
Samples 6 and 7 average stored in address 19
Samples 8 and 9 average stored in address 20
Samples 10 and 11 average stored in address 21
Samples 12 and 13 average stored in address 22
Samples 14 and 15 average stored in address 23

Then, the same J/2 + 16 deals with the next round of averages:

Averages of 16 and 17 stored in address 24
Averages of 18 and 19 stored in address 25
Averages of 20 and 21 stored in address 26
Avergaes of 22 and 23 stored in address 27

And again

Averages of 24 and 25 stored in address 28
Averages of 26 and 27 stored in address 29

And finally

Averages of 28 and 29 stored in address 30

I guess the same could be done for other numbers of samples but 16 seems a reasonable number for wind direction and the code is short and simple. I have spent a laborious few hours with a pocket calculator checking the results - and so far it is Ok. If you, or anyone else spots an error please let me know.

Regards Bill Legge