allow gripper collision on the PR-2

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I want to allow the PR-2 gripper to collide with objects on a table. I am designing linear pushing motions. I am using motion_planning_msgs/CollisionOperation message in the MoveArmGoal message, but it does not seem to work.

Here are the message definitions. I use their example of object1 = "gripper" and object2 = COLLISION_SET_ALL. Any suggestion will be very helpful!

# A definition of a collision operation
# E.g. ("gripper",COLLISION_SET_ALL,ENABLE) will enable collisions 
# between the gripper and all objects in the collision space

string object1
string object2
string COLLISION_SET_ALL="all"
string COLLISION_SET_OBJECTS="objects"
string COLLISION_SET_ATTACHED_OBJECTS="attached"

# The penetration distance to which collisions are allowed. This is 0.0 by default.
float64 penetration_distance

# Flag that determines whether collisions will be enabled or disabled for the pair of objects specified above
int32 operation
int32 DISABLE=0
int32 ENABLE=1