Researchers sing praises of parallel kinematic machines

  • 04-Nov-2009 03:49 EST

The five-axis horizontal Tau PKM has a 1.46-square-meter planar working envelope, comparing favorably with the 1.0-square-meter footprint of the machine.

Parallel kinematic machines (PKMs) are not widely used in the aerospace industry, but researchers from Boeing, ABB Robotics, and other partners in an EU-funded project called SMErobot have shown through assessments that there is a place for them.

Boeing has successfully engineered the use of commercial-grade light robotics and automation in the manufacturing process of high-volume component parts for, among other implementations throughout the company, the 787 Moving Trailing Edge produced at Boeing Aerostructures Australia in Melbourne. Boeing Research & Technology is now developing light robotics through new and emerging robotic platforms to complement traditional gantry systems and standard industrial robotics.

PKMs have been developed in innumerable formats and have resulted in machines with excellent accuracy, high speed and acceleration characteristics, and high stiffness. However, generally they have suffered from small working envelopes compared to the size of the machine.

ABB has patented a format of PKM, the Tau, which has the desired characteristics of a PKM with a working envelope equivalent to an industrial robot or gantry of the same machine size. With this innovation, it could be seen as the breakthrough characteristic for PKMs in the aerospace industry, where industrial robotics have generally required enhancement in accuracy and deflection compensation to enable application.

The SMErobot project involved construction of two Tau machines in gantry format—one in a horizontal and the other in a vertical configuration. It was found in a series of tests run over a ninth-month period that PKMs generally, and Tau PKMs in particular, delivered significant advantages in performance in the application tested: the fettling of castings. When compared to industrial robotics, the advantages include speed, repeatability/accuracy, acceleration, and stiffness. Compared to gantry robotics, advantages include speed, acceleration, inertia, mounting, and possibly repeatability/accuracy. When compared to contemporary PKMs, advantages in accessible work envelope is a major advance.

The researchers note that the gantry Tau is modular with a nonredundant mechanical mounting of a lightweight arm system, making it very easy to assemble, disassemble, and move the robot when compared to a serial gantry manipulator.

PKMs do have limitations, however, including work envelope. The links structure makes it difficult to find configurations which can reach over objects in the work envelope. For example, items of large single or double convex curvature may collide with the links. However, the design of the end-effector configuration and the placement of the linear axes may aid in this regard. Simulation of the application can be used to debug.

The researchers say the Tau prototypes can be used as the basis for refinement of performance targeting applications other than that studied in the SMErobot project. For the development of a robot targeting machining, for example, joints, links, and framework with higher stiffness need to be used, and linear encoders need to be introduced. Also, the calibration software developed by the University of Lund for the SMErobot testing should be refined, and the same calibration equipment as used for CNC machines today must be adopted.

Generally, the Tau PKM will be suitable for applications that are relatively planar or require little reach-over capability. Alternatively, the workpiece can be actuated, as for bodies of rotation or in some high-stiffness CNC machines. The bulk of aerospace detail product or subassembly can be considered to lie in this domain. The drill and trim of components would possibly be the first application of interest and would exploit the accuracy/repeatability and stiffness attributes of the machine. In addition, the accelerations possible with the machine will lead to cycle time improvements when traversing between positions. This attribute, and machine stiffness, may be exploited further in the drilling of assemblies. However, in assembly drilling, the large frame structures of the Tau PKM may restrict access or visibility of operators to the assembly.

This article is based on SAE technical paper 2009-01-3222 by P.J. Crothers and P.L. Freeman of Boeing; I. Dressler, K. Nilsson, and A. Robertsson of the University of Lund; W. Zulauf and B. Felber of Güdel; R. Loser and K. Siercks of Hexagon Metrology; and T. Brogard of ABB Robotics.


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