# Parallel Mechanism Classification

From the perspective of sports form, parallel mechanisms can be divided into:

• Plane mechanism
• Space agency

Further subdivision can be divided into:

• Plane moving mechanism
• Plane moving rotating mechanism
• Space pure moving mechanism
• Space pure rotating mechanism
• Space mixing motion mechanism

## Classified by the freedom degree of the parallel mechanism

(1) 2 degrees of freedom parallel mechanism.

A 2 degrees of freedom parallel mechanism, such as the 5-R or 3-R-2-P (where R refers to a rotating pair and P refers to a moving pair), typically has two degrees of freedom.

The most common example of a 2-DOF parallel mechanism is the planar 5-bar mechanism.

This type of mechanism generally has two moving parts.

(2) 3 degrees of freedom parallel mechanism.

There are various types of 3 degrees of freedom parallel mechanisms, which are more complex in form. Generally, they can be classified into the following categories:

Planar 3-DOF parallel mechanisms, such as the 3-RRR mechanism and 3-RPR mechanism. These mechanisms have two movements and one rotation.

Spherical 3-DOF parallel mechanisms, such as the 3-RRR spherical mechanism and 3-UPS-1-S spherical mechanism. The axes of all the motion pairs of the 3-RRR spherical mechanism meet at a single point, which is referred to as the center of the mechanism. The 3-UPS-1-S spherical mechanism uses the center point of S as the center of the mechanism, and the movement of all points on the mechanism is the rotational motion around that point.

3D pure moving mechanisms, such as the Star-Like parallel mechanism, Tsai parallel mechanism, and DELTA mechanism. The kinematics of this type of mechanism are very simple and positive, and it is a widely used 3D mobile space organization.

Space 3 degrees of freedom parallel mechanisms, such as the typical 3-RPS mechanism. These mechanisms are under-actuated mechanisms, and their most prominent feature is their different forms of movement at different points in the workspace. Due to their special kinematics, their wide application in practice has been hindered.

There are also space mechanisms that add auxiliary members and motion pairs, such as the 3-UPS-1-PU spherical coordinate 3-DOF parallel mechanism used in parallel machine tools developed by the University of Hannover, Germany. Due to the constraints of the auxiliary lever and the motion pair, the motion platform of the mechanism has one movement and two rotational movements (also referred to as three movements).

(3) 4 degree of freedom parallel mechanism.

Most 4-DOF parallel mechanisms are not fully parallel, including the 2-UPS-1-RRRR mechanism.

In this mechanism, the motion platform is connected to the fixed platform through three branches, and the two motion chains are identical, each comprising one Hooke hinge U and one moving pair P. P and 1 R are the drive pairs, indicating that this mechanism is not fully parallel.

(4) 5 degrees of freedom parallel mechanism.

The existing 5-DOF parallel mechanism has a complicated structure, such as Lee’s 5-DOF parallel mechanism with a two-layer structure (a combination of two parallel mechanisms).

(5) 6 degree of freedom parallel mechanism.

The 6 degrees of freedom parallel mechanism is a widely used class of parallel robots, which has been extensively researched by both domestic and foreign scholars. It finds applications in various fields, such as flight simulators, 6-dimensional force and torque sensors, and parallel machine tools.

Despite its popularity, there are still several key technological challenges that need to be addressed, such as finding positive solutions for its kinematics, establishing dynamic models, and accurately calibrating parallel machine tools.

From a fully parallel perspective, this mechanism should have six kinematic chains. However, in the existing parallel mechanisms, there are 6 degrees of freedom parallel mechanisms with only three kinematic chains, such as 3-PRPS and 3-URS. These mechanisms have a 5-bar mechanism attached to each branch of the three branches, which provides the necessary drive for a 6 degrees of freedom parallel mechanism.