plc5 and trig

Thomas · Feb 26, 2007

Hello all,

I am feeling math stupid today and maybe somebody can give me the nudge on this one. I have a function I am trying to implement in a plc5/30 that works fine in a spreadsheet using the ATAN2. The plc only has an ATAN function. This is creating havoc (mostly with my coffee drinking). How can I do the equivelant of the ATAN2 function in a plc5?

I know this should be obvious to me, but I'm not getting it.

With great humility,

Thomas

Steve Bailey · Feb 26, 2007

The difference between ATAN and ATAN2 is that ATAN uses the angle's tangent as its argument while the ATAN2 function uses the x,y coordinates.

From the Excel help: ATAN2(x,y) = ATAN(y/x)

Thomas · Feb 26, 2007

I think what I am getting stuck on is the limits. ATAN works for +/- pi/2 while ATAN2 is good for +/- pi radians....

Steve Bailey · Feb 26, 2007

That's the result of the negative argument. ATAN can't tell whether the negative argument was caused by the x or the y coordinate. You'll have to add ladder logic to keep track. If x is negative, the angle is between 1.5 * PI and 2 * PI. If y is negative, the angle is between PI/2 and PI. If both x and y are negative, the argument is positive and the angle is between PI and 1.5 * PI.

jimbo3123 · Feb 26, 2007

I just realized that this is essentially the same as Steve said above me. (The Wiki article will probably help though).

I would do some compare statements on the x and y arguments, then implement it with the atan instruction and modify the output depending on the original quadrant.

This article may help.
http://en.wikipedia.org/wiki/Atan2

Peter Nachtwey · Feb 26, 2007

Code:

if x = 0 then
  if y = 0 then
	 error;			 // UNDEFINED
  elseif y > 0 then
	 a = PI/2;		 // HANDLE SPECIAL CASE
  else
	 a = -PI/2;		 // HANDLE SPECIAL CASE
  endif
else
  a = atan(y/x);	 // NOW DO ATAN
  if y < 0 then
	 a = a + PI;
  elseif x < 0 then
	 a = a + 2*PI
  endif
endif

Thomas · Feb 26, 2007

Thanks all,

I'll chew on it as I get time today

Peter Nachtwey · Feb 26, 2007

I found an inconsistency with my code. Replace a=-PI/2 with 15*PI
The code I show is supposed to return angles from 0 to < 2*PI

Thomas · Feb 26, 2007

Progress

This is what I have so far.

N7:105 X value
N7:107 Y value
F8:23 quadrant value to be added to the arctan

 

SOR BST LES N7:105 0 MOV 0.0 F8:23 

NXB GRT N7:105 0 MOV -3.141593 F8:23 

NXB EQU N7:105 0 BST GRT N7:107 0 DIV 3.141593 2.0 F8:23 

                 NXB LES N7:107 0 DIV 3.141593 -2.0 F8:23 

                 NXB EQU N7:107 0 MOV 0.0 F8:23 BND BND EOR

plc5 and trig

Thomas

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Thomas

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Thomas

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