redundancy resolution
Redundancy resolution is how a robot chooses among the many ways it could do the same task when it has more joints than the task strictly needs. Your own arm is the perfect example: keep your hand pinned flat on a table and you can still raise and lower your elbow, swinging it through the air while the hand stays put. The hand's job is fully specified, yet the arm still has a leftover freedom to play with. A robot with that same extra freedom faces a choice every instant — and redundancy resolution is the rule it uses to make that choice sensibly.
The leftover freedom is a gift, not a nuisance, because it lets the robot pursue a second goal for free while still nailing the main task. With its hand glued to the right spot, the arm can quietly reshape its elbow to dodge an obstacle, stay clear of a kinematic singularity (a stuck pose), keep its joints away from their limits, or simply move in the way that costs the least effort. Redundancy resolution is the recipe for picking, out of infinitely many valid arm shapes, the one that also serves whatever secondary aim you care about most.
Mechanically, these extra motions live in what is called the null space — the set of joint movements that shuffle the arm around without moving the hand at all. Redundancy resolution works by letting the main task fully determine the hand's motion, then using whatever null-space freedom remains to satisfy the secondary goals. That is how a graceful, human-like robot arm reaches into a cluttered shelf: hand locked on target, elbow weaving politely around everything in the way.
A seven-joint arm must hold a full coffee cup level while carrying it across a crowded counter. Holding the cup upright and steady pins down most of the arm, but one spare freedom remains. Redundancy resolution spends that freedom on swinging the elbow aside so it never knocks the sugar jar — all while the cup stays perfectly level.
Extra joints let the arm chase a second goal — here, dodging the sugar jar — without disturbing the main task.
An arm is called redundant for a task when it has more joints than the task needs — for example a seven-joint arm doing a job that only needs the six directions of hand motion fully specified.