Internet PLC Forum

General => Technical support => Topic started by: Santiago Garza on November 25, 2005, 10:47:03 AM

Title: Square root
Post by: Santiago Garza on November 25, 2005, 10:47:03 AM

Hi. I am trying to do a program where I need to use the circle equation.
I couldnt find the square root function and how to write a square number either.

Can you help me?

thank you

Title: Re:Square root
Post by: support on November 27, 2005, 01:16:43 PM

The PLC does not support floating point so Square root function are not supported. For limited range of value you could pre-store the data in DM and use the index to find the corresponding value. This is called a look-up table.

Title: Re:Square root
Post by: Bruceh on November 27, 2005, 03:16:55 PM

I came across the same problem when having to calculate real time RMS currents and got a reasonable solution by using a technique called "newton's iteration" - here is a link that describes the technique - http://www.jimloy.com/arith/sqrt.htm

For my purposes it was not too computationally intensive and I got a pretty good estimate in 5 to 7 iterations.

Bruce

Title: Re:Square root
Post by: evanh on December 14, 2005, 07:27:39 PM

I haven't tried to compare but the method I've used in the past uses a multiply instead of divide per iteration.  It's a simple binary search - If you want a 16 bit result from a 32 bit source then it always takes 15 iterations and is not an estimate so it's as accurate as 16 bits can be.  Combined with the multiply, which is often a lot faster than a divide, then the binary search method might be a better solution.


The search loop goes something like:

square=12345678 ' The number we want to find the square-root of.
sqroot=0 ' The resultant square-root.
index = &8000 ' The bit mask indexing.

sqrt_loop:
   sqroot = sqroot + index
   if sqroot * sqroot > square then sqroot = sqroot - index
   index = index >> 1
   if index<>0 then sqrt_loop



Evan

Title: Re:Square root
Post by: evanh on December 16, 2005, 03:29:52 AM

And if you want rounding to nearest, instead of rounding down, it changes in detail but not really all that much:

square=12345678 ' The number we want to find the square-root of.
sqroot=0 ' The resultant square-root.
index = &8000 ' The bit mask indexing.
check = 0 ' The lookahead sqroot - Added for faster execution.

sqrt_loop:
  check = sqroot + index
  index = index >> 1
  if (check * check - check) < square then sqroot = check
  if index<>0 then sqrt_loop


Evan