Precision positioning during assembly and quality assurance

Jon Lawson

Today, microproduction technology makes high-precision, mostly multiaxis positioning systems absolutely necessary for both assembly and quality assurance, and they also have to be as compact as possible to enable them to be properly integrated into the production units. In most cases, only low masses are positioned during this process.

Examples can be found in many sectors: For example, during the production of mobile end devices when certain components in the device must be exactly aligned and held in position for gluing. The same applies for adjusting optical lenses, for example, in objectives, binoculars or even on camera sensor chips such as those used in rear-view cameras in vehicles.

If these workflows are fully or partially automated, the process is dependent on signals from external sensors, cameras or machine vision solutions. Therefore, it should be possible to easily integrate the positioning system into the higher-level automation system.

Precision positioning with up to six degrees of freedom

Experience shows that parallel-kinematic systems are predestined for such cases. Good examples of this are the so-called Hexapods, which are six-axis parallel-kinematic systems capable of exact positioning in the micro- and submicrometre range.

Thanks to their high stiffness, Hexapods have an excellent control response and settling behaviour. They position the loads, which means the components, camera systems or fibres, on six axes in space, three linear and three rotary. Therefore, all drives act on a single moving platform, which results in further advantages compared to serial or stacked systems: Improved path accuracy, higher repeatability and flatness of travel, lower moved mass and therefore higher dynamics, which is the same for all motion axes, no cable dragging and a much more compact design. The pivot point of the Hexapod can also be freely defined.

A typical representative of this class is the H-811 Miniature Hexapod. With travel ranges to 34 mm and 42° on the linear or rotary axes and an actuator resolution of 0.04 μm as well as a load capacity of up to 5 kg, it is suitable for a number of applications in microassembly and quality assurance. The minimum incremental motion is 0.2µm; the repeatability is ±0.1µm. The positioning system is also capable of velocities up to 10 mm/s.

High-performance digital controller communicates with the control system

Due to their parallel-kinematic design, Hexapods require special control. However, the user doesn't need to worry about that because the Hexapods are supplied as a complete solution with a high-performance digital controller.

The user enters the position and drive commands as Cartesian coordinates and the controller performs all calculations necessary for the parallel-kinematic six-axis system by transforming the Cartesian target positions into control of each individual drive. With the exception of the Hexapod axes, the digital controller is also able to control two further axes, for example, linear stages for rough positioning over long travel ranges or a rotation stage for 360° motion.

Easy connection of a higher-level PLC is also possible. The Hexapods can be integrated into virtually any automation system and clock synchronisation with other automation components is also easy to achieve. The control system can then communicate with the Hexapod system, for example, via EtherCat.