ros-ukraine/leobot

RPM calculation from encoder value

Opened this issue · 7 comments

vyosypenko commented

Hi!
Could you please help me with RPM calculation formula?
Currently, I'm able to read the raw value from the encoder. I've found two formulas to the calculation.

  1. https://www.vexforum.com/index.php/16114-calculating-rpms/0
  2. https://electronics.stackexchange.com/questions/375971/how-to-calculate-rpm-given-timer-frequency-and-counter-on-stm32

they both look similar, except divider. Which formula is suitable for us?

Thanks,
Vasyl

AndriyPt commented

Hey @nemo8 :)
As for me the second links looks reasonable. @LyubomyrD please confirm my thinking :)

LyubomyrD commented

Hello all.

I guess that the formula moight be the following

Real_RMP_OF_WHEEL = (EncoderCurr - EncoderLast) / sampling_period / CPR / gear_ratio /encoder_correction_multiplier

*encoder_correction_multiplier depends on the setup of the timer. Commonly for stm32f4xx it is either 2 or 4

vyosypenko commented

@LyubomyrD,
thank you!

encoder_correction_multiplier depends on the setup of the timer in my case = 4.

one motor rotation give me 480 pulses.

what is CPR ?

Could you please take paramenters in formula to bracket? (like (((a+b)/4)/8) ).

LyubomyrD commented

The CPR is counts per revolution - you can find this value in the datasheet of the encoder.

Here is the formula with brackets

Real_RMP_OF_WHEEL = (((((EncoderCurr - EncoderLast) / sampling_period)/ CPR) / gear_ratio) /encoder_correction_multiplier)

  • 👍1
vyosypenko commented

image

Single Output 480 Pulses Per Revolution:
Gear Ratio: 30:1
Each Loop Output Pulses: 16
30 * 16=480

reference: https://www.amazon.com/12V-DC-Geared-Motor-Encoder/dp/B073QXQW5Y

If I understand the timer settings correct, the timer in encoder mode will be incremented by 1 every rotation (when receives 480 pulses from the encoder).

So, I think we can make formula simpler just rid off gear_ratio. (gear_ratio already defined in encoder settings.) Am I correct?

LyubomyrD commented

I guess that You should set the value of the ARR equal to 65535 otherwise the timer might hangup every rotation. Besides, the polarity for the second chanell should be falling, I guess.

vyosypenko commented

Now I can read a value of the timer, but I need to investigate more about this timer in the encoder mode...

http://d1.amobbs.com/bbs_upload782111/files_10/ourdev_265522.pdf (page 18)