Robot manipulator design, part 1: fundamentals

A robot manipulator is a robot arm: a chain of links and joints that moves a tool to pick, place, weld, paint or assemble. You find them in car factories, operating theatres, warehouses and research labs. This first part covers the fundamentals you need before drawing a single part: degrees of freedom, joint types and the main arm layouts.

Four disciplines in one arm

Building an arm means getting four things to work together:

Area	Covers
Mechanical design	Links, joints, bearings, materials: strong but light
Electronics	Motors, drivers, sensors, wiring, power
Control	Making each joint reach its target precisely and safely
Programming	Kinematics, motion planning and the task logic

Degrees of freedom

The degrees of freedom (DOF) of an arm are the number of independent joint motions. To place a tool at any position and any orientation in 3D space, an arm needs 6 DOF: three to position the wrist, three to orient the tool.

Fewer is often enough. A pick-and-place arm that always grips from above needs only 4 (x, y, z and a rotation about the vertical axis). More than 6 makes an arm redundant: it can reach the same pose in many ways and use the extra freedom to avoid obstacles, like a human arm with 7.

Joint types

Joint	Symbol	Motion	Example
Revolute	R	Rotation about an axis	Shoulder and elbow of most arms
Prismatic	P	Sliding along an axis	Vertical axis of a SCARA, gantry axes
Spherical	S	Rotation about three axes (ball joint)	Wrists, passive joints in parallel robots

An arm is often named by its first three joints, from the base outwards: an RRP arm has two revolute joints followed by a prismatic one.

The common arm layouts

Layout	Joints	Workspace	Strengths	Typical use
Cartesian (gantry)	PPP	Box	Simple control, very accurate, scales to large sizes	3D printers, CNC, palletising
Cylindrical	RPP	Cylinder	Simple, good vertical reach	Machine loading
Spherical (polar)	RRP	Part of a sphere	Long reach	Early industrial robots
SCARA	RRP (+ wrist R)	Flat cylinder	Very fast in a plane, stiff vertically	Assembly, pick and place
Articulated	RRR + 3R wrist	Nearly a sphere	Flexible, reaches around objects	Welding, painting, general use
Parallel (delta)	Several chains to one platform	Small dome	Extremely fast and light, stiff	Food packing, sorting

Picking a configuration

Start from the task, not from the robot:

$$ \text{Task} \;\rightarrow\; \text{Required DOF} \;\rightarrow\; \text{Workspace shape} \;\rightarrow\; \text{Layout} \;\rightarrow\; \text{Link lengths and motors} $$

Moving parts between points on a table, quickly? → SCARA or delta.
Very large area, high accuracy, simple control? → Cartesian.
Reaching around obstacles or holding a tool at any angle? → articulated, 6 DOF.
Heavy loads at low speed? → short links, large gear reduction, possibly Cartesian.

A useful rule: use the fewest joints that do the job. Every extra joint adds weight, cost, error and control effort.

How much can the arm lift?

Every joint has to carry everything beyond it, so the shoulder works hardest. A first estimate of the holding torque at a joint, with the arm stretched out horizontally, is

$$ \tau \approx g \left( m_\text{load}\, d_\text{load} + \sum_i m_i\, d_i \right) $$

where each mass $m_i$ (links, motors further out) acts at horizontal distance $d_i$ from the joint. This is why designers put motors close to the base and make the outer links as light as possible.

Our experience

When we built our own manipulator, the biggest lesson was how closely the four areas depend on each other: a heavier motor changes the link design, which changes the torque, which changes the motor again. Working through mechanical design, electronics, control and programming together, rather than one after the other, is what finally gave us an arm that performed its tasks reliably.

Coming in part 2

Part 2 looks at workspace analysis, singularities, redundancy, how to measure an arm's performance and how to control it.

Watch the video

For the full explanation, see Robot manipulator design on YouTube.