how to decrease move_arm speed during execution

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If you're using the PickAndPlaceManager's move_arm_to_side function, it tries to use move_arm first, which would use the limits Sachin described. It also, however, currently moves open-loop if move_arm fails, which, if you're concerned about safety, you probably really don't want to do. (The speed in that case is based on the default in the function 'command_joint_trajectory' in controller_manager.py (in pr2_gripper_reactive_approach/src/pr2_gripper_reactive_approach), which sets the trajectory times and velocities based on a max joint velocity of 0.2 m/s.) I would suggest writing your own move_arm_to_side function (if you want everything else in the PickAndPlaceManager, just inherit from it and overload that function) and setting the trajectory filter limits on the param server (look at http://www.ros.org/wiki/roslaunch/Tutorials/Roslaunch%20tips%20for%20larger%20projects).

Depending on how you are telling the arm to move, you could increase the duration of the movement. The JointTrajectoryPoint message has options for specifying the desired velocities, as well as the time_from_start parameter to control how long the path motion will take.