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The ros_control packages are all closed loop controllers of one sort or another, designed for working with encoders and DC motors. If you're motors are driven by open loop stepper drivers without encoders then you'll need a different control system.

The maths behind a two wheel differential drive system is fairly simple. First you calculate the wheel velocities needed to produce the desired linear speed, then calculate the wheel velocities needed to produce the desired angular rate. Adding the two pairs of velocities together will tell you how fast the left and right wheels need to turn in order to produce the desired x linear and z angular velocities from a geometry_msgs/Twist message.

For an open loop stepper driver you'll want to experimentally determine your maximum acceleration possible without losing steps then choose a threshold a certain safety factor beneath this. You'll then want to use the velocities calculated above but with a limiter to make sure that you don't exceed the maximum acceleration of the motors.

However if you don't have encoders there will always be a risk of losing steps if there is more resistance to motion than expected due to slopes or rough surfaces. The usual solution is to significantly over-rate the motors.

Hope this doesn't throw a spanner in the works too much!