From the perspective of motion, parallel robots can be divided into planar mechanisms and spatial mechanisms; further subdivided into planar translational mechanisms, planar translational-rotational mechanisms, purely spatial translational mechanisms, purely spatial rotational mechanisms, and spatial hybrid motion mechanisms. They can also be classified according to the number of degrees of freedom of the parallel mechanism:
(1) 2-degree-of-freedom parallel mechanism.
Two-degree-of-freedom parallel mechanisms, such as 5-R and 3-R-2-P (R represents a revolute joint and P represents a prismatic joint), are the most typical two-degree-of-freedom parallel mechanisms. These mechanisms generally have two translational motions.
(2) 3-DOF parallel mechanism.
There are many types of 3-DOF parallel mechanisms, and their forms are quite complex. They generally include the following forms: planar 3-DOF parallel mechanisms, such as the 3-RRR mechanism and the 3-RPR mechanism, which have two translational and one rotational kinematic pairs; and spherical 3-DOF parallel mechanisms, such as the 3-RRR spherical mechanism and the 3-UPS-1-S spherical mechanism. In the 3-RRR spherical mechanism, the axes of all kinematic pairs intersect at a single point in space, which is called the center of the mechanism. In the 3-UPS-1-S spherical mechanism, the center point S is the center of the mechanism, and the motion of all points on the mechanism is rotational motion about that point.
3D pure kinetic mechanisms, such as Star Like parallel mechanisms, Tsai parallel mechanisms, and DELTA mechanisms, have simple forward and inverse kinematic solutions and are widely used 3D kinetic spatial mechanisms. Spatial 3-DOF parallel mechanisms, such as the typical 3-RPS mechanism, are underranked mechanisms. Their most prominent feature is that their motion patterns differ at different points in the workspace. Due to this special motion characteristic, their widespread application in practice is hindered. Another type is spatial mechanisms with added auxiliary links and kinematic pairs, such as the 3-UPS-1-PU spherical coordinate 3-DOF parallel mechanism used in the parallel machine tool developed by the University of Hanover in Germany. Due to the constraints of auxiliary links and kinematic pairs, the motion platform of this mechanism has one translational motion and two rotational motions (or three translational motions).
(3) 4-DOF parallel mechanism.
Most 4-DOF parallel mechanisms are not fully parallel mechanisms. For example, in the 2-UPS-1-RRRR mechanism, the motion platform is connected to the fixed platform through 3 branches. Two of the kinematic chains are the same, each with a Hooke hinge U and a prismatic joint P. Among them, P and one R are driving joints. Therefore, this kind of mechanism is not a fully parallel mechanism.
(4) 5-DOF parallel mechanism.
Existing 5-DOF parallel mechanisms are complex in structure. For example, Lee's 5-DOF parallel mechanism in South Korea has a double-layer structure (a combination of two parallel mechanisms).
(5) 6-DOF parallel mechanism.
Six-DOF parallel mechanisms are a major category of parallel robot mechanisms and are the most studied parallel mechanisms by scholars both domestically and internationally. They are widely used in flight simulators, six-dimensional force and torque sensors, and parallel machine tools. However, many key technologies for these mechanisms have not been fully resolved, such as their forward kinematics, the establishment of dynamic models, and the accuracy calibration of parallel machine tools. From a fully parallel perspective, these mechanisms must have six kinematic chains. However, some existing parallel mechanisms have three kinematic chains, such as 3-PRPS and 3-URS, and others have a five-bar linkage added to each of the three branches as the driving mechanism.
