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 December 18th, 2017, 01:21 PM #1 Senior Member   Joined: Apr 2014 From: UK Posts: 954 Thanks: 342 This has me beat... I have a function which takes an input between 1 and 6, and divides a variable (let's call it Z) by that number. The result is therefore a number between Z/6 and Z. Z/6 is a result that is 16.6667% of Z, Z is 100% of Z. What I want to do is linearise the control input so that I can specify a 0 to 83.3333% reduction in the result out of the original function. I cannot change the original function, this needs to be a new function which manipulates the control to the original. To clarify, I want an input with range 0-83.33, this must control an input with range 1-6 which gives an output of 100% to 16.667% of a variable, Z. Z is arbitrary. If it's any easier, the control input can be thought of as a 0-5, I simply add 1 along the way. I can't figure out how to generate the control signal from my 0-83.3% input, I've been playing with a table in Excel for ages and can't come up with a solution. Any clues? Last edited by skipjack; December 18th, 2017 at 02:54 PM.
 December 18th, 2017, 03:05 PM #2 Global Moderator   Joined: Dec 2006 Posts: 20,927 Thanks: 2205 Subtract the input value from 1 to give a value from 1/6 to 1, the reciprocal of which is your desired control signal.
 December 18th, 2017, 10:43 PM #3 Senior Member   Joined: Apr 2014 From: UK Posts: 954 Thanks: 342 OK, that works Now to complicate the problem, I'm trying to do it without dividing by variables (vhdl). The original function is a piece of analogue electronics which can't be changed, really, really annoying right now, lol.
 December 19th, 2017, 01:37 AM #4 Global Moderator   Joined: Dec 2006 Posts: 20,927 Thanks: 2205 How accurately do you need to approximate the result?
 December 19th, 2017, 02:49 AM #5 Senior Member   Joined: Apr 2014 From: UK Posts: 954 Thanks: 342 I honestly don't know but 0.1% sounds about right. I am looking into doing a reciprocal in vhdl but it's making me cry.
 December 19th, 2017, 04:31 AM #6 Global Moderator   Joined: Dec 2006 Posts: 20,927 Thanks: 2205 1/(1 - x) = 1 + x² + x³ + . . . could be used, but you might need dozens of terms. I also tried 1.0039 + 0.7843x + 1.9062x^2 + 1.1937x^3 - 0.414x^4 - 1.5278x^5 - 0.8095x^6 + 2.9826x^7 + 11.052x^8, which seems to be within 1%, but not 0.1%. Thanks from weirddave
 December 19th, 2017, 08:23 AM #7 Senior Member   Joined: Apr 2014 From: UK Posts: 954 Thanks: 342 Good grief! That's quite some numbering there. It's clear there's no easy solution, I shall attempt to weigh up the pro's and con's of this method and doing a proper divide, it could go either way! (tho the divide makes me die a little inside). Thanks for the insight

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