Every robot is a chain of rigid links connected by joints. The joint decides what kind of motion is possible, and how many independent ways the arm can move. Here are six of them, in 3D. Orbit any panel, then drive it with the sliders.
Pick up any robot arm and you will find the same two ingredients. Links, the rigid pieces that hold their shape, and joints, the connections that let one link move relative to the next. The links are the bones. The joints are where all the motion lives.
Each joint type allows a specific kind of movement, and a specific number of degrees of freedom, the count of independent ways it can move. One degree of freedom means a single number describes its state. Three means you need three. Drag inside any panel below to orbit the camera, and use the sliders to actuate the joint.
Rotates about a single axis, like a hinge or your own elbow. One angle describes everything it does. It is the workhorse joint of almost every robot arm.
Slides along a straight line instead of rotating. Think of a drawer, a hydraulic ram, or the carriage on a 3D printer. Its one number is a distance, not an angle.
A revolute and a prismatic joint sharing one axis. The collar can spin around the shaft and slide along it at the same time, giving two independent degrees of freedom.
Like a cylindrical joint, but the rotation and the sliding are locked together by the thread. Turn it and it advances, exactly like a bolt. Two visible motions, but only one degree of freedom.
A ball and socket, like your shoulder or hip. The link can point in any direction and spin about its own axis, packing three rotational degrees of freedom into one compact joint.
Chain a shoulder and an elbow, two revolute joints, and the tip can reach anywhere in a working plane. Move each joint and watch the reach. This is the heart of the 2DOF Robotic Arm.
The rest of the family. Six joints cover most of what you will meet, but there are a couple more. A universal joint bends in two directions but cannot spin, giving two degrees of freedom. A planar joint glides across a flat surface and turns, giving three. Every one of them is just a different answer to the same question. Which motions do we allow, and which do we lock.
That single question is the foundation of all of robotics. Get it into your hands on a real arm, and the rest follows.
Two revolute joints, a real servo driven arm, and a guidebook that derives every joint, parameter, and equation from first principles.
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