A parallel robot (also called parallel manipulator, or parallel kinematic machine, when it is used as a machine tool) is a closed-loop kinematic chain mechanism of which the end effector is connected to the base by several independent kinematic chains (legs) in parallel. Comparing to a serial robot, each limb of a parallel robot (PR) tends to be simpler and shorter therefore more rigid, and the motors are purposely mounted close to the base in order to reduce the moving mass. Therefore, PRs have the following advantages: high stiffness because the external load is shared by multiple legs, high acceleration and high speed due to low moment of inertia of the moving structure, higher accuracy since the joint manufacturing and assembly errors will be even out rather than accumulated at the end effector, and high force capabilities as the output force is contributed by multiple actuators working in parallel. The disadvantage of a PR lies in its limited workspace volume due to the parallel architecture, and there are often singularities in workspace. Due to these advantages, PRs have received wide applications, e.g., flight simulators, high-precision positioning devices, pick-and-place manipulation, metrology and coordinate machine, reconfigurable tooling, and high-speed machining.
The earliest parallel manipulator is the Gough platform invented by Gough in 1947, which allows the positioning and the orientation of a moving platform so as to test tire wear and tear (Fig. 1). Stewart (1965) designed a parallel manipulator (Fig. 2) for use as a flight simulator in 1965. The research area has not attracted much attention until the 1980s. In the last three decades, many types of PRs have been designed and prototyped, and a few of them have been successfully commercialized.

Based on the Stewart platform structure, the Hexcel Corporation developed the R series robots (Fig. 3a) for biomedical applications and P series robots (Fig. 3b) for industrial applications, such as welding, deburring, and milling. Prof. Clavel patented the groundbreaking Delta robot in 1990 (Clavel and Sogeva 1990). As an extension of Delta robots, the IRB340 FlexPicker was introduced by ABB Flexible Automation in 1999, as shown in Fig. 4a. It is equipped with an integrated vacuum system capable of rapid pick and place of objects. Many applications related to Delta robots in the industry have been developed, and one of them is used as Line-Placer as shown in Fig. 4b. Based on the Delta robot, a number of variants of pick-and-place PRs have been designed and developed. Figure 5 shows two examples: the Quattro robot from Adept Technology (Pierrot et al. 2009) and the Diamond robot from Tianjin University, China (Huang et al. 2003). For high-precision and high stiffness machining operations, Tricept from LOXIN, Sprint Z3 head from ECOSPEED, and Exechon machine from Exechon corporation are some successful examples of PRs on the market today (Fig. 6). For more information, readers are referred to www.parallemic.org.