cmd_vel linear and angular relationship

Question

Hi All,

I'm using rqt robot steering to output cmd_vel message which I subscribe to and they control the power of the motors.

Linear is fine and +/-100% is controlling the motors fed and reverse okay. How should I combine that with the rotational input such that the effect is to increase or decrease the rate of turn? Just something simple is fine, this is a 2 wheeled robot.

The examples I keep finding on the internet seem to be simple key based designs. i.e. F=Fwd/L=Left etc...

Many Thanks

Mark

---

Originally posted by MarkyMark2012 on ROS Answers with karma: 1834 on 2014-11-17

Post score: 1

Answer 1

I think the following is what you're looking for. I'm not familiar with rqt's robot steering, so I'm assuming you're using geometry_msgs/Twist messages. If so, and assuming a differential drive robot, the usual method requires knowledge of the wheel separation (i.e., axle length) and wheel diameters.

void twistCb(geometry_msgs::TwistConstPtr &msg) {
  transVelocity = msg->linear.x;
  rotVelocity = msg->angular.z;
  double velDiff = (wheelSep * rotVelocity) / 2.0;
  double leftPower = (transVelocity + velDiff) / wheelRadius;
  double rightPower = (transVelocity - velDiff) / wheelRadius;
}

---

Originally posted by kramer with karma: 1470 on 2014-11-17

This answer was ACCEPTED on the original site

Post score: 4

---

Original comments

Comment by MarkyMark2012 on 2014-11-18:
Thanks Kramer - steering passes through rad/s so I can convert that m/s (v = ωr)