Precision Robotics and Automation: Hexapods Advance Production Processes

Summary

Hexapods, six-legged parallel-kinematic machines, are quickly gaining ground in a broad range of industrial automation applications after “learning” how to directly communicate with PLC or CNC controllers via fieldbus interfaces. As far as the semiconductor and electronics industry, automobile industry, and precision assembly are concerned, many production processes have become inconceivable without them. Today, the six-axis positioning systems are available with load capacity from 2kg to 2000kg and travel from 10 to hundreds of millimeters, while maintaining sub-micron precision. Hexapods are used for aligning the smallest optical components in the latest Silicon Photonics production processes, as controlling automated labeling machines, and positioning entire body parts for automotive production. The intrinsic Hexapod features contribute to a wealth of new possibilities in robotics.

Mechanics, Controls, and Software

While the mechanical design is critical for stiffness, accuracy, repeatability, and lifetime, and many details (such as spherical rolling joints) that seem like a good idea at first glance turn out to be a dead end, the flexibility and versatility depends on the controller and software.

For example, unlike with serial kinematics, if you want to move a platform in just one axis, the controller has to change the individual lengths of each strut – some will extend and some will contract. The required coordinate transformations and path calculations need a powerful controller capable of running 100’s of thousands of operations per second, because there is no linear dependence on the current coordinates. The stiffest mechanical designs are based on cardanic joints with an offset between the axes, making the math even more complex.

Generally, there is no analytical solution, and CPU-intensive, iterative algorithms are required to recalculate the complex Hexapod kinematics for each step. All of this is transparent to the user who does not need to worry about implementing the Hexapod kinematics on the PLC.

A dedicated digital Hexapod controller, continuously improved over 3 decades, serves motion with sub-micron precision by tightly controlling each actuator in real time. Linear and rotary motion of the platform is simply commanded in Cartesian coordinates.

All PLC standard language functions are available for controlling the Hexapod systems; the controller communicates with the Hexapod via a standard protocol. Established fieldbus protocols, such as EtherCAT or ProfiNET, are available for this purpose; additional RS232 and TCP/IP interfaces are integrated as well. With the help of these interfaces, it is possible to exchange time-synchronized data, such as target and actual positions or status information, in real time both with the Hexapod system and any other components in the network.

In a typical automation application, the corresponding Cartesian target positions, or trajectories, are generated by a master controller (e.g., a standard software PLC with TwinCAT) and transmitted to the Hexapod system, via the EtherCAT protocol or similar, and in return, the actual positions and status information can be polled. System parameters, such as the center of rotation and the coordinate systems, can also be conveniently configured. The Hexapod system behaves as an intelligent multi-axis drive on the bus.

Examples of Use in Manufacturing and Quality Assurance

The areas of application for the versatile Hexapods are broad-ranging, because there is no difference between precision positioning of semiconductor components or mechatronic components or heavy loads.

For example, automated 6D alignment systems take care of important tasks during testing and manufacturing of accessories for MEMS (Microelectromechanical Systems) camera sensors and photonics components, including the alignment of optical fibers and fiber arrays, the production of optical lenses.