[ros-users] [Discourse.ros.org] [General] Why don't we use ROS?

Chris Albertson ros.discourse at gmail.com
Wed Dec 13 21:43:12 UTC 2017

In the case of a high DOF robot, like say Boston Dynamic's "Atlas" humanoid robot.  One could use Machine kit to synchronize the two to three dozen motors.  What MK is good at is running many axis n synchronization to hit trajectory points in "n-space".   It abstracts the differences between types of motors.

In a typical application of MK user cars about tolerances of 0.0005 inches.  Think about a 5-axis milling machine that is making something like a turbine fan blade.  It is making very smooth compound curves 

What MK adds to a robot is the ability to synchronize moments to the millisecond or better level.   ROS is not good at this at all

Here is an application.   Let's say we want to make amble robot is play "dodge ball" with a group of humans.  The humans have baseballs and through them as fast as they can at the robot.  The robot has to avoid being hit.   ROS would fail at this.   Message passing is simply to slow.  You need a controller that can do sub-millisecond level.

The reason MK can work so fast is it does not use massage tasing.   There is a control loop that runs off an timer.  Lets say it is a 1KHz timing that causes an interrupt.  The output control voltages to every motor in calculated inside that loop.   It's a fast servo controller.   

To interface MK.  ROS would compute a trajectory through space and specify some velocities to be hit at points along that path.  MK would then do the microsecond or millisecond level calculations to force the robot to the specified path and velocity.   

The curent meth is that the servo control loop is closed inside ROS and corrections are send as messages to motor controller.   This is to slow.  Using MK would close the servo loop inside the 1KHz loop.  (or there 10KHz loop if you had a fast enough computer.

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