How Robot Arms Work: Joints, Kinematics and Grippers in Plain Words
What a robot arm is made of, what degrees of freedom mean, how forward and inverse kinematics work, and why grasping is hard.
A robot arm is a chain of rigid links connected by joints, with a tool at the end. Your own arm is a good model: shoulder, elbow, wrist, hand.
The parts
- Links: the rigid sections.
- Joints: where links turn or slide. Each joint is driven by a motor, often with gears. See motors for robots.
- End effector: the tool at the tip, usually a gripper but sometimes a welder, a paint gun or a suction cup.
- Sensors: encoders at each joint tell the controller its angle.
Degrees of freedom
A degree of freedom (DOF) is one independent way the arm can move. To place a tool anywhere in space and point it in any direction you need six: three for position and three for orientation. That is why many industrial arms have six joints. Simple hobby arms have three or four and are more limited.
Forward kinematics
If you know every joint angle, you can calculate where the tip is. This is forward kinematics, and it is basic geometry (sines and cosines of the angles).
Inverse kinematics
The harder, more useful question: I want the tip at this point, so what angles should each joint take? That is inverse kinematics. There may be several answers, or none if the point is out of reach. Software solves this many times per second.
Why grasping is hard
Picking up a known box in a fixed position is easy. Picking up different objects from a messy bin is hard, because the robot must see the object, decide how to hold it, and judge force. Vision and force sensing help. This is why many warehouses still use humans for picking.
Types of arms
- Articulated arms: rotating joints like a human arm; the most common industrial type.
- SCARA arms: fast, flat-moving arms used for assembly.
- Cartesian (gantry) robots: move along straight axes like a 3D printer.
- Parallel robots: several arms linked to one platform, very fast for light parts.
See industrial robot and robot arm.
What goes wrong
- If the arm is asked to reach beyond its limits, the maths has no solution and it stops or jams.
- Gears can have backlash, small loose play that makes precise positioning harder.
- Without enough torque, a joint sags under load.
Build one
A three-joint hobby arm with servos is a great project once you have finished a wheeled robot. Start with our first robot tutorial.
Frequently asked questions
How many joints should a beginner arm have? Three or four servos is enough to learn the idea.
Is inverse kinematics hard? For simple arms it is a small amount of trigonometry. Libraries handle harder cases.
What is a cobot arm? An arm built to work safely next to people. See collaborative robot safety.
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