Turn off robot servos from launch file

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No, there isn't a specific ROS command for this. However, you could make use of the on_shutdown feature of the ROS api (if using Python, see here) to call whatever the 'motor shutdown' sequence is when your motor controller node shuts down. However, I would point out that most positional servomotors tend to stay locked at a position irrespective of what code you have running: when powered on, they're attempting to achieve their goal location/velocity.