## motor ratio to rpm

Discussions and questions about the Crazyflie Nano Quadcopter
CrazyGuy
Beginner
Posts: 27
Joined: Fri May 02, 2014 6:03 pm

### motor ratio to rpm

Hi, does anyone know an explicit formula how i can determin the rounds per minute (or second) of a rotor after i've set the ratio of the corresponding motor to a specific value?

Thanks,
CrazyGuy
arnaud
Bitcraze
Posts: 2431
Joined: Tue Feb 06, 2007 12:36 pm

### Re: motor ratio to rpm

Hi,

I am afraid there is not practical way to find exactly such formula. Crazyflie is using brushed DC motors and in theory (from what I remember from school) the revolution speed would be directly proportional to the voltage applied to the motor (w = k.V)

However the factor k will be different for every motors and will have to be measured for each motors, but you might miss internal resistance and other things that will affect rotation speed. Also there is no measurement of the voltage at the motor in the Crazyflie, it can be approximated by the battery voltage multiplied by the motor's current PWM ratio (which is different from the ratio sent by radio, it is corrected by the stabilisation control loop).

So if you want an exact formula, it is hard. If you just need roughly the law between PWM ratio and motor rotation speed, it is mostly linear.

/Arnaud
capriele
Beginner
Posts: 20
Joined: Wed May 21, 2014 8:16 pm

### Re: motor ratio to rpm

Hi,
can someone help me in finding the relation between PWM ratio and motor rotation speed
Great thanks.