Serial Mechanism

A mechanism whose links connect end-to-end through joints from base to tip, with no closed loop.

A serial mechanism, also called an open-chain mechanism, is a string of rigid links connected end-to-end by joints, running from a fixed base all the way to the end effector, with no closed loop anywhere along the way. Each joint is usually driven by its own motor, and the end-effector pose is simply the product of each joint's transform chained together, so forward kinematics (joint angles to end-effector pose) has a unique solution, while inverse kinematics can have multiple. Six-axis industrial arms, 7-DoF collaborative arms, SCARA robots, and a single humanoid arm are all serial structures. The advantages are a workspace that's large relative to the mechanism's footprint and an intuitive structure; the disadvantages are that error and deflection accumulate link by link along the chain, and each proximal joint has to carry the weight of every motor farther out, giving lower stiffness and payload capacity than a parallel mechanism (where several chains connect to the end effector at once, forming closed loops, as in a Delta robot or a Stewart platform).

ExampleA UR5e's six joints — base, shoulder, elbow, and three wrist joints — connect one after another in sequence, a textbook serial mechanism. A person standing with both feet on the ground, by contrast, forms a closed loop from the ground, up one leg, through the hip, down the other leg, and back to the ground — a closed-chain mechanism.

Also called

Serial Manipulator, Open-Chain Mechanism

Related

Parallel Mechanism · Kinematic Chain · Link · Forward Kinematics (FK) · Robotic Arm · 6-Axis Robot Arm

Sources

Wikipedia: Serial manipulator
Modern Robotics (Lynch & Park), Ch.2 Configuration Space