Pre-grasp Pose

An intermediate pose where the gripper pauses near an object, aligned with the grasp direction, just before the actual grasp.

Arm grasping usually happens in stages: move to the pre-grasp pose, approach the grasp pose in a straight line along the approach direction, close the gripper, then lift and retreat. The pre-grasp pose is typically obtained by backing off the grasp pose a short distance along the approach direction (usually the direction the gripper faces), with the gripper pre-opened to a suitable width. This separates ‘large-scale travel’ from ‘fine motion near the object’: the first leg can be handled quickly by motion planning, while the second is short and straight, less likely to bump the object or its surroundings. ROS's MoveIt describes this in its Grasp message with pre_grasp_approach (direction and distance) and pre_grasp_posture (hand shape before grasping). In dexterous-hand research, ‘pre-grasp’ also often refers to the finger shape formed just before the hand closes.

ExampleIn the MoveIt grasping tutorial, a Panda arm first stops about 11.5 cm from the grasp pose (desired_distance of 0.115 m) with the gripper open, approaches and grasps along the x-axis, then retreats about 25 cm upward along z.

Also called

Approach Pose, Pregrasp Pose

Related

Grasp Planning · Grasp Pose Detection · MoveJ / MoveL · MoveIt Motion Planning Framework · End-Effector Pose · Grasping

Sources

MoveIt Tutorials: Pick and Place
moveit_msgs/Grasp.msg