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August 13th, 2017, 12:20 AM  #1 
Newbie Joined: Aug 2017 From: India Posts: 15 Thanks: 0  Percentages problem
Very simple problem but very poor in mathematics. This is the problem statement i received. My input initial value is 500 and the final value is 1500 and the output should decrease at the rate of 10% for every 100. The formula i derived FinalOutput = FinalOutput  0.1 * (PresentInputValue  500) works fine if my Finaloutput starts from 100 and ends at 0. But there are cases where my Finaloutput should start at 20 but should end at 0. In that case what equation should I use. Very confused. 
August 13th, 2017, 03:48 AM  #2 
Global Moderator Joined: Dec 2006 Posts: 18,965 Thanks: 1606 
Neither the problem statement nor your formula makes sense. Firstly, "the output" is undefined. Secondly, 10% of what? Thirdly, for every 100 of what? Fourthly, how many times is "the output" decreased? The problem with your formula is that PresentInputValue is undefined. Also, the formula changes the value of FinalOutput, but the problem description doesn't mention "Final Output", only "Final Value", and doesn't mention changing that. You give "the problem statement" as though it's all you are given, yet you later state that "there are cases where . . .", which implies you are given additional information. When you've dealt with those issues, there may be other things that need clarification. 
August 13th, 2017, 04:04 AM  #3 
Newbie Joined: Aug 2017 From: India Posts: 15 Thanks: 0 
Sorry let me explain clearly. The input is my machine rpm which can vary from 0 to 1500 which is my feedback and the output is the power or duty cycle which can be from 100% to 0% or sometimes a variable which can be based on machine some 40% or 20% which i call it full duty to 0%. How to control the duty is when machine starts that is 0 rpm need to apply the full duty as the speed increases i need to decrease the duty. Up to 500 rpm it is full duty and after 500 and below 1500 the duty should decrease by 10% for every 100 rpm increase. After 1500 it should be 0. My full duty is variable. The program executes every one second. I want an equation for the above problem. I also want to develop an equation where 500 and 1500 can also be variables and i want an equation. Please help me how to get the equation.

August 13th, 2017, 01:20 PM  #4 
Global Moderator Joined: Dec 2006 Posts: 18,965 Thanks: 1606 
If the duty starts to decrease at m rpm and reaches zero at n rpm, and if the "maximum" duty percentage is p% and the current rpm (which I assume is always available) is c rpm, calculate r = p/(n  m) (which is 0.1 for your original figures). Now use the formula duty = r(n  c) (subject to c being at least m and at most n). Obviously, this formula reduces to the formula you posted when the values you posted originally are used.


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