I. Characteristics of Parallel Robots
1. No cumulative error, high accuracy;
2. The drive unit can be placed on or near a fixed platform, which makes the moving parts lightweight, high-speed, and with good dynamic response;
3. Compact structure, high rigidity, and large load-bearing capacity;
4. Completely symmetrical parallel mechanisms have good isotropy;
5. Small workspace;
Based on these characteristics, parallel robots have been widely used in fields that require high rigidity, high precision, or large loads but do not require a large workspace.
II. Classification of Parallel Robots
From the perspective of motion, parallel mechanisms 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.
Alternatively, they can be classified according to the number of degrees of freedom of parallel mechanisms:
1. Two-degree-of-freedom parallel mechanisms: Two-degree-of-freedom parallel mechanisms, such as 5-R and 3-R-2-PR, represent revolute joints and P represents prismatic joints. Planar 5-bar linkages are the most typical two-degree-of-freedom parallel mechanisms. These mechanisms generally have two translational motions.
2. Three-degree-of-freedom parallel mechanism.
Three-degree-of-freedom parallel mechanisms come in many varieties and are quite complex in form. They generally fall into the following categories: planar three-degree-of-freedom parallel mechanisms, such as the 3-RRR and 3-RPR mechanisms, which have two translational and one rotational motion; spherical three-degree-of-freedom parallel mechanisms, such as the 3-RRR and 3-UPS-1-S spherical mechanisms. In the 3-RRR mechanism, the axes of all kinematic pairs intersect at a single point in space, called the center of the mechanism. In the 3-UPS-1-S mechanism, the center of the mechanism is the center of S, and the motion of all points on the mechanism is rotational motion around that point; three-dimensional pure translational mechanisms, such as Star-Like, Tsai, and DELTA mechanisms. These mechanisms have simple forward and inverse kinematic solutions and are widely used three-dimensional translational spatial mechanisms; and spatial three-degree-of-freedom parallel mechanisms, such as the typical 3-RPS. One type of mechanism is the underranked mechanism, whose most prominent feature is that its motion varies at different points in the workspace. Due to this special motion characteristic, it hinders the widespread application of this type of mechanism in practice. Another type is the spatial mechanism with the addition of 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 it can be said to have three translational motions.
3. Four-degree-of-freedom parallel mechanism.
Most four-degree-of-freedom 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 three branches. Two of the kinematic chains are the same, each with one Hooke hinge U and one prismatic joint P. Among them, P and one R are driving joints. Therefore, this kind of mechanism is not a fully parallel mechanism.
4. Five-degree-of-freedom parallel mechanism: Existing five-degree-of-freedom parallel mechanisms have complex structures. For example, Lee's five-degree-of-freedom parallel mechanism in South Korea has a double-layer structure that combines two parallel mechanisms.
5. Six-degree-of-freedom 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.
