Finding Coordinates of Partial Arc Radius Without Sine and Cosine Functions

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Discussion Overview

The discussion revolves around finding the x and y coordinates of a partial arc radius given an angle, specifically in the context of programming for a CNC lathe that lacks sine and cosine functionality. Participants explore alternative methods to calculate these coordinates without directly using trigonometric functions.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Exploratory

Main Points Raised

  • Mike expresses a need for assistance in calculating coordinates for a partial arc radius without sine and cosine functions.
  • One participant suggests using the equation x² + y² = r² as a starting point.
  • Mike indicates that the Pythagorean theorem provides one equation but leaves two unknowns, necessitating another equation or method to generate sine ratios.
  • Another participant proposes approximations for sine, suggesting that for angles less than 30 degrees, sin(x) can be approximated as x (in radians), and provides a more accurate approximation for larger angles.
  • Further suggestions include approximating cosine using a similar method, with additional terms for increased accuracy.
  • Mike later mentions discovering Taylor series and CORDIC algorithms as methods for estimating sine and cosine values, indicating a realization that the problem is more complex than initially thought.

Areas of Agreement / Disagreement

Participants present various methods and approximations for calculating sine and cosine without directly using the functions. There is no consensus on a single approach, as multiple techniques are discussed and explored.

Contextual Notes

Participants discuss approximations and methods that depend on the range of angles being considered, highlighting limitations in accuracy based on those ranges.

Fergus
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So I think this falls under the stuff I've forgotten file. We're writting a program for the CNC lathe and I need to find x and y coordinates of a partial arc radius given the angle. This trick is the controller doesn't have sine and cosine funcitonality. I'm sure this is a basic definition thing I learned in trig in High School, unfotunately the 15 years since then have killed that memory. Help?

Thanks,
Mike
 
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You probably just need x^2 + y^2 = r^2
 
I looked at that but I'm going to need more. Basically we want the user to input the angle. The Pythagorean theorum gets me one equation and two unknowns, I need to come up with another equation or be able to use something to generate the sine ratio without using the sine funciton.
 
Sine is pretty easy to approximate.

If the angle is less that 30 degrees then the approximation,

sin(x) = x, with x in radians ( equiv to sin(x) = x*pi/180 with x in degrees),

will get you less than 5% error.

If you want better use sin(x) = x - x^3 / 6 (equiv to six(x) = x*pi/180 - (x*pi/180)^3 / 6 with x in degrees) will get you approx 1% max error if x is less than 60 degrees and better than 0.1% max error if x is less than 30 degrees.
 
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Yep wat uart said was 100% correct, and for the cosine function you may want to use cos(x) = \pi - (x*\pi/180)^{2}/2 with similar accuracy. for more accuracy just add ...+ (x*\pi/180)^{4}/24
 
When evaluating the polar form of complex numbers I hate dealing with angles in the 2nd, 3rd and 4th quadrants.
 
Thanks for the help! I actually just stumbled on a site showing how to use a Tayor series to estimate sine and cosine ([w__.homeschoolmath.net/teaching/sine_calculator.php) and how calculators etc. use a CORDIC algorithm to caclutate the value. Guess it wasn't actually a simple thing I forgot. Of course then I went back to one of my old math books and there it was. Thanks again!

Mike
 

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