using move_group.setJointValueTarget() to control gripper

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hello, guys, i'm using moveit to control panda arm and now want to use setJointValueTarget to control gripper open and close, but when i set panda_finger_joint0=0.03 and panda_finger_joint1=0.03 and then execute setJointValueTarget and move_group.execute(myplan), it doesn't work. and in the console it turned out: GripperServer: commanding out of range width! max_width=0 command=0.006， i don't have idea where is the problem, can you give me a help. Thank you in advance!