pre-grasp and approach
Pre-grasp and approach is the careful run-up a robot's hand makes just before it actually touches an object — the part of the motion where it lines up its fingers, opens them to the right width, and glides in along a chosen path. Watch yourself pick up a coffee mug: you do not slap your hand straight at it. You first spread your fingers, swing your hand around to come at the handle from the open side, and slow down as you near it. A robot does the same in two named pieces. The pre-grasp is a waypoint pose parked a short distance in front of the object, with the gripper already oriented and pre-opened. The approach is the final, usually straight-line, glide from that waypoint in to contact.
Splitting the motion this way solves a real problem: the spot where the robot wants its fingers to end up is often crowded with the very object it is reaching for, plus the table, the bin wall, or neighbouring items. By first flying to a clear pre-grasp pose and only then approaching along a clean, collision-free line (often straight down a gripper's closing axis, or in along a suction cup's normal), the robot avoids swatting the target away, clipping a wall, or arriving at a good final pose but through an impossible path. The approach direction also sets which surfaces the fingers will land on, so a thoughtful pre-grasp is half of getting a stable grip.
In practice a grasp plan is not just a single target pose but a short recipe — pre-grasp pose, approach vector and distance, then close — and a motion planner is asked to reach the pre-grasp first and the approach second. If the straight-in approach is blocked or the pre-grasp is unreachable, the system rejects that grasp and tries another candidate, which is why these two steps are baked into almost every pick-and-place and bin-picking pipeline.
A bin-picking robot spots a bottle lying against the bin wall. It computes a pre-grasp pose hovering ten centimetres above the bottle's middle with the gripper aligned to its length, swings the arm there freely, then approaches straight down until the fingers straddle the bottle — never touching the wall — and closes.
A pre-grasp waypoint plus a straight downward approach lets the robot reach a tucked-away object without hitting the bin.
Pose here means a position plus an orientation together — where the hand is and which way it is turned. The short backing-off move after the grip closes, used to lift the object clear, is the mirror image of the approach and is often called the retreat or lift-off.