The serial arm: one chain, like a human arm
A serial manipulator is built the way you might build a snake out of toy blocks: one rigid link connects to the next through a motorized joint, and that next link connects to the one after it, all the way out to the hand. Because every piece hangs off the one before it, the result is a single open kinematic chain — there is exactly one path from the base to the tip, with no loops. Your own arm is the perfect model: shoulder, then elbow, then wrist, each carrying everything beyond it.
Most joints here are revolute joints (they rotate, like a hinge), though some arms slide along a rail with a prismatic joint. Stacking six of them is what lets the end-effector — the gripper or tool at the tip — reach almost any point and twist to almost any angle inside a big, roughly spherical workspace. That generous reach is the serial arm's signature strength.
The parallel arm: many chains sharing one platform
A parallel manipulator flips the idea around. Instead of one long chain, several short chains all connect a fixed base to a single moving platform at the same time. Picture three or six legs gripping one tabletop from underneath — that closed loop of legs is why it is also called a closed-chain mechanism, the opposite of the serial arm's open chain. The end tool rides on the shared platform, lifted by the whole team of legs at once rather than balanced on the tip of a cantilever.
Two designs you will meet again and again: the delta robot, whose three thin arms dangle a light platform and flick it around at blinding speed, and the Stewart platform (or hexapod), whose six adjustable legs let a heavy top move with all six degrees of freedom — three translations and three rotations. In both, the motors usually sit down on the base, so the moving parts stay light — another reason these machines can accelerate so hard.
Head to head: payload, accuracy, speed, reach
It helps to think of two everyday objects. A serial arm is a fishing rod: long, springy, able to dip the tip anywhere around you — but flex it and the tip drifts. A parallel arm is a camera tripod: short legs braced into a triangle, rock-steady, but the platform on top can only move within a small zone before a leg runs out of travel. The trade-off is the same in both: reach buys you flexibility, bracing buys you stiffness, and you rarely get both at once.
- Reach and workspace: serial wins easily — a big, dexterous, roughly spherical volume. The parallel arm's workspace is smaller and oddly shaped, hemmed in by its legs.
- Stiffness and accuracy: parallel wins. Sharing the load across many braced legs keeps the platform from sagging, so it can hold position to within a fraction of a millimetre.
- Speed and acceleration: parallel usually wins, because its motors stay on the base and the moving platform is featherlight — fast deltas can complete hundreds of pick cycles a minute.
- Payload relative to its own size: parallel tends to carry more, since several legs share the weight; a serial arm's far joints are weakest because they hold up everything beyond them.
Real-world picks: welders versus pick-and-place deltas
Walk a car factory and you see the split clearly. The welding and painting robots are tall six-jointed serial arms, because the job demands reach: they must snake a torch deep inside a car body, twist around panels, and follow a long curved seam. No tripod could fold itself into those tight corners. Reach is everything, and a little flex is tolerable because the weld path is forgiving.
Now stand over a packaging line where chocolates or cookies stream past on a belt. Above it hangs a delta — a parallel arm — snapping pieces off the conveyor and dropping them into trays at a furious, blurred pace. Here the moves are short, flat, and endlessly repeated, so the giant reach of a serial arm is wasted. What matters is raw speed and pin-sharp repeatability, exactly what a stiff, light parallel platform delivers. This is the classic home of pick-and-place work.
So the same family of links and joints, arranged as one open chain or several closed ones, splits the factory floor neatly in two. Ask first what the task needs most — to reach into awkward places, or to slam light parts down fast and exactly — and the geometry chooses itself.