Back-emf to RPM (math problem)


Closed Thread
Results 1 to 5 of 5
  1. #1

    Default Back-emf to RPM (math problem)

    Hello everyone,
    I am developing a speed control for brushless DC motor and I need to convert the Back-EMF voltage into RPM.
    In the application note AN893 (equation A-8) from microchip is the equation, but I can not understand how to solve it with PBP.


    RPM = (1- (ADRESH: ADRESL / 1024)) x RPMmax


    I appreciate if someone can help me.


    Best regards
    Marcos.
    (Excuse me for my bad english)

    Name:  equationA-8.png
Views: 424
Size:  5.4 KB
    Attached Images Attached Images  

  2. #2
    Join Date
    Oct 2005
    Location
    Sweden
    Posts
    3,516


    Did you find this post helpful? Yes | No

    Default Re: Back-emf to RPM (math problem)

    All the formula does is convert the ADC result into a value ranging from 0-1 then multiply that by the maximum RPM. When ADC reads 0 you get 0 RPM when the ADC reads 1023 you get max rpm, whatever that is. The formula is great - on paper....

    There are several ways of doing it with PBP, the best/easiest way depends on
    A) What's the maximum RPM of your motor
    B) What's the ADC Reading (back EMF) at that speed?

    Example:
    If the maximum speed is 5000rpm and the ADC returns 1023 (full scale) at that speed then RPM = ADC_Reading */ 1251 should work. So, what's that */ 1251 I hear you ask.....

    We want to scale the ADC value (1023) to 5000, to do that we need to multiply the ADC value by 4.8876. We can think of the */ operator as multiplying by units of 1/256 and 4.8876 * 256 = 1251.

    /Henrik.

  3. #3
    Join Date
    May 2004
    Location
    NW France
    Posts
    3,611


    Did you find this post helpful? Yes | No

    Default Re: Back-emf to RPM (math problem)

    RPM = ((1024 - ( ADRESH:ADRESL))* RPMmax) /1024

    depending on the processor used, you'll need to use or PBPL(ongs) or DIV32 ...

    Alain
    ************************************************** ***********************
    Why insist on using 32 Bits when you're not even able to deal with the first 8 ones ??? ehhhhhh ...
    ************************************************** ***********************
    IF there is the word "Problem" in your question ...
    certainly the answer is " RTFM " or " RTFDataSheet " !!!
    *****************************************

  4. #4


    Did you find this post helpful? Yes | No

    Default Re: Back-emf to RPM (math problem)

    as 1024 and RPMmax are both constants, do that math at compile time.

    George

  5. #5


    Did you find this post helpful? Yes | No

    Default Re: Back-emf to RPM (math problem)

    Thank you all for responding,
    ********************
    Henrik you are right ... I asked * / 1251?

    George RPM monitoring is continuous, therefore I can not do it at compile time.

    They all seem to be correct but the easiest way for me is the response of Alain.

    If possible I would like Alain explain to me how I get to that equation.

    I'll do some tests

    Excuse me for my bad english
    Best regards

    Marcos.

Similar Threads

  1. Replies: 19
    Last Post: - 27th May 2010, 04:56
  2. Pic Interference - EMF
    By CrazyCooter in forum mel PIC BASIC Pro
    Replies: 1
    Last Post: - 10th August 2009, 14:50
  3. RPM - DIV32 Problem
    By davewanna in forum mel PIC BASIC Pro
    Replies: 10
    Last Post: - 11th April 2008, 05:33
  4. help read Back EMF of DC motor
    By jetpr in forum General
    Replies: 9
    Last Post: - 23rd January 2008, 02:05
  5. Math problem!
    By in forum General
    Replies: 8
    Last Post: - 8th September 2003, 10:32

Members who have read this thread : 2

You do not have permission to view the list of names.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts