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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)
Attachment 7987
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.
RPM = ((1024 - ( ADRESH:ADRESL))* RPMmax) /1024
depending on the processor used, you'll need to use or PBPL(ongs) or DIV32 ...
Alain
as 1024 and RPMmax are both constants, do that math at compile time.
George
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.