Posted 11 Apr 2013 Does anyone know of a simple way to calculate cube root with an old modicon PLC 984 Thanks Share this post Link to post Share on other sites
Posted 12 Apr 2013 Swap it out for a ControlLogix PLC. OK, seriously, A cube root is the same thing X^(1/3). Unfortunately the 984LL exponent function EMTH(34) only support integer exponents. If you are computing the cube root of an integer and the integer has a limited range then use a table. Otherwise read the Wikipeida article on cube root - it has a few methods that you might be able to convert to 984LL, but its not going to be easy. Can you shove it off to your HMI, have the HMI computer X^.3333333 and then read it back? Good luck. Share this post Link to post Share on other sites
Posted 13 Apr 2013 this should be faster than Newton (2-3 iterations should be enough) b=a[ (a^3+R)+R] / [(a^3+R) + a^3] where b = current iteration result (root) a = previous result R= value we need to find cube root of for initial guess you can use a=R/2 or a=R/3 Share this post Link to post Share on other sites