Hi all,
I searched a lot to find a user manual or datasheet for Rosbot 2R motors (Xinhe Motor XH-25D), but I couldn’t find one. I appreciate it if you share it with us.
Best,
Ramin
Hi @RafalGorecki ,
Yes, I’m looking for a relationship between motor torque and angular velocity. I want to convert torque to rpm.
Best,
Ramin
Hi @Ramin_Esmzad,
Unfortunately, there is no direct way to convert the values of torque and rotational speed of the motor. However, it can be done by linking them through an additional parameter, which is voltage or current.
Referring to the specifications provided in the previous message:
Please let me know if you find this information helpful.
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Hi @RafalGorecki ,
Thank you for your answer.
There is a torque-speed curve in DC motors datasheets. That curve has a linear representation as
T = T_s - Ts/wn*w
where T is the motor torque, T_s is the stall torque, wn is the no-load speed, and w is the motor angular velocity.
Can we use this equation to convert torque to speed?
Best,
Ramin
The torque-speed curve can be defined, approximately, with the following equation (using your symbols):
w(T) = w_0 - (w_0/T_s)*T
where:
T is the variable for motor torque,
T_s is a stall torque,
w_0 in the no-load speed,
w is the variable for actual speed,
w_0/T_s is a speed/torque gradient.
It can be transformed to:
T(w) = T_s - (T_s*/w_0)*w
where:
T_s/w_0 is a torque/speed gradient.
In our case (ROSbot 2R), you do not know the actual torque, unless you have a way to measure or estimate it - for example by measuring the actual friction and/or measuring the slope to drive on.
You can read the current speed from the encoders and estimate the torque using this equation.
Two more things:
a) The nominal values (2.9kg*cm, 165rpm) are defined for 6V motor voltage. In our robot, this voltage can vary from 0V up to ~12V (with the full battery) because of the variable PWM signal for driving motors. The motor voltage will change both stall torque and no-load speed, rather proportionally.
b) The no-load speed is given with the gearbox. Without the gearbox internal friction, the theoretical no-load speed is ~180 rpm (~6100 rpm on the motor).
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Hi @radeknh ,
Thanks for the useful information.
In my case, I’m currently designing a controller for Robot 2R using its dynamic model to compensate for various dynamic effects. The output of my controller is torque. So, I want to change that torque to speed and feed it to the Rosbot.
As you stated, there is a problem with this approach as the motor voltage can change over time and as a result, the proportion of w_0/T_s will change. Is there any way to find out this proportion w_0/T_s online approximately?