The revolute joint rotates in one plane only. It has a single degree of freedom (1DOF). The knee and elbow joints on the Xemo robot are revoluate joints. Also known as rotational joints.
The spherical joint is also known as a ball joint. It has three degrees of freedom (3DOF). Two of the DOF help the joint rotate anywhere inside a hemi-sphere shell. The third DOF allows the joint to twist - rotating around its long axis. The shoulder and hip joints on the Xemo robot are spherical joints. The ankle is a modified spherical joint, with only 2DOF. The twist DOF is locked on Xemo's ankle to provide better stability.
The prismatic joint is a linear actuator. The motion is in a line rather than rotating in a circle. Hydraulic pistons seen in heavy equipment are linear actuators.
Force, Rotation, Angular Velocity
The joint rotation in Xemo is controlled in two ways
The engineer specifies a joint position, and the robot moves the joint to that position. Parameters for spring/damping are set to control the force that the joint uses to move to its target. The spring pulls the joint to its desired angle. The further the joint is from its target angle, the more spring force is applied. The damper slows down the joint from reaching its target angle. The higher the damping, the slower the change - it opposed the spring force.
Damping is used to prevent overshoot. Without it, the joint would acclerate to its target position and over shoot, then move backward, perhaps overshooting again. This overshoot back and forth is known as an oscillation. The goal in control is to minimize the oscillation, which will depend on the amount of spring force, the amount of damping, the mass of the limb the joint is moving, and the force of gravity.
Joints in Xemo are also controlled using angular velocity. The engineer sets the desired velocity, and then slows down the joint when it reaches its target position. The robot control system uses a proportional differential (PD) controller for this purpose. We'll talk about PD controllers more in the control section.