Planar Type High-Speed Positioning Platform

4 planar type three degrees-of-freedom redundantly actuated parallel mechanism positioning platform is developed. It is originally developed to be used as a main platform of a new PCB depaneling system which consists of a stationary separating unit using punching system and three degrees-of-freedom positioning system. Besides the application for the PCB depaneling system, it can be applied as a main positioning platform for surface mounting machines, semiconductor industry, flat panel industry and MEMS field. The positioning platform has three degrees of freedom, of which translational axes of are in x- and y-axis with its stroke of 420 mm each. The maximum velocity is 150 m/min with its acceleration of 2g. The velocity of 150 m/min is one of the highest one achieved among the positioning platform with the stroke around 500 mm. Another unique feature of the platform is that it has one rotational axis along z-axis with its capability from -90 to +90 degrees. The main specifications of the positioning platform are shown in Table 1.

Fig.1 A photograph of the positioning platform

Fig.2 Rotational capability of the positioning platform (0°, -90°, +90°)

Table 1.Specifications of the positioning platform
Overall size	1,900 (L) × 1,220 (W) × 600 (H) mm
Moving platform size	230 (L) × 230 (W) × 15 (H) mm
Dexterous workspace	190 × 190 mm
Maximum velocityt	150 m/min/span>
Maximum acceleration/span>	2g (19,600 mm/sec2)
Number of axes	4 (One actuation redundancy)
Stroke of each axis	420 mm (-210 to 210 mm)
Link length	285 mm
Actuators	Linear servo motors 1.5 kW (Mirae, Korea)

The positioning platform is based on a new symmetric 2-PPR parallel mechanism, which has four actuators. Since it has three degrees of freedom and four actuators, it has basically one actuation redundancy that is essential to achieve high-degree dexterous motions. It is crucial to eliminate the actuator singularity in the workspace from the results of kinematic analyses. The dynamics analyses for the redundantly actuated parallel mechanism that include the inverse dynamics, vibration and resultant force analyses have been performed. They helped to develop a fastest positioning platform. As a result, the positioning platform satisfies target velocity and acceleration. In addition, the mutual fighting problem of the redundantly actuated parallel mechanism is solved by a new calibration method. After calibration, the positioning accuracy is under ±0.005 mm. The detailed results are shown in Table 2.

Fig.3 A schematic of the parallel mechanism and CAD model of the positioning platform

Table 2. Positioning accuracy and the repeatability
Index	X-axis	Y-axis
Repeatability (Forward) (4σ)	3.495 micro meter	4.388 micro meter
Repeatability (Backward) (4σ)	4.027 micro meter	4.996 micro meter
Positioning accuracy	4.261 micro meter	7.698 micro meter