Precision Positioning During Assembly and Quality Assurance

Hexapods in Microproduction

What do optical components and glass fibers in photonics, mobile devices, and high-quality wristwatches all have in common? More than you would think at first sight: In the end, the objective is precision positioning of the individual components, parts or workpieces during assembly and in most cases, on several axes. But, it’s not only necessary to work with the highest accuracy in the most confined space during this assembly process. It is also important for the measuring probes, optics or camera systems to be exactly positioned for the accompanying or final quality assurance after assembly. The range extends from “manual manipulation” under the microscope up to completely automated, camera-based solutions. This makes it essential to find the right positioning systems.

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 (Image 1). In most cases, only low masses are positioned during this process.

Image 1. Today, microproduction technology demands high-precision, mostly multiaxis positioning systems for both assembly and quality assurance. Thanks to its high stiffness, the Hexapod can be mounted in any direction. (Image: PI)

Image 1. Today, microproduction technology demands high-precision, mostly multiaxis positioning systems for both assembly and quality assurance. Thanks to its high stiffness, the Hexapod can be mounted in any direction. (Image: PI)

Small Components, Very Little Space

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. Photonics also makes the same demands when fibers have to be positioned exactly to achieve the so-called “first light” (Image 2).

Image 2. Photonics also makes high demands when fibers have to be positioned exactly to achieve the so-called "first light". (Image: PI)

Image 2. Photonics also makes high demands when fibers have to be positioned exactly to achieve the so-called “first light”. (Image: PI)

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; a requirement that applies both to microassembly and quality assurance.

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 submicrometer range. Thanks to their high stiffness, Hexapods have an excellent control response and settling behavior. They position the loads, which means the components, camera systems or fibers, 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 (Image 3): 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.

Image 3. In contrast to serial kinematics, all actuators act directly on the same platform in parallel-kinematic systems, it is therefore not possible for guiding errors to accumulate as would be the case with "stacked" systems and this increases the accuracy considerably. (Image: PI)

Image 3. In contrast to serial kinematics, all actuators act directly on the same platform in parallel-kinematic systems, it is therefore not possible for guiding errors to accumulate as would be the case with “stacked” systems and this increases the accuracy considerably. (Image: PI)

A typical representative of this class is the H-811 Miniature Hexapod (Image 4), which is part of the extensive range from the Karlsruhe-based company, PI (Physik Instrumente). 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 0.2 µm; the repeatability is ±0.1 µm. The positioning system is also capable of velocities up to 10 mm/s. Users from the field of photonics will be particularly pleased to learn that the corresponding scanning algorithms for fiber optic adjustment are already integrated. Furthermore, the versatile Hexapod is also available in vacuum-compatible versions.

Image 4. With travel ranges to 34 mm and 42° on the linear or rotary axes and 0.04 μm actuator resolution, this miniature Hexapod, which can be loaded with up to 5 kg, is suitable for a large number of applications in microassembly and quality assurance. (Image: PI)

Image 4. With travel ranges to 34 mm and 42° on the linear or rotary axes and 0.04 μm actuator resolution, this miniature Hexapod, which can be loaded with up to 5 kg, is suitable for a large number of applications in microassembly and quality assurance. (Image: PI)

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 (Image 5). 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.

Image 5. The digital controller performs all calculations necessary for converting the Cartesian coordinates entered by the user, into motion commands for the parallel-kinematic six-axis system. (Image: PI)

Image 5. The digital controller performs all calculations necessary for converting the Cartesian coordinates entered by the user, into motion commands for the parallel-kinematic six-axis system. (Image: PI)

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 synchronization with other automation components is also easy to achieve. The control system can then communicate with the Hexapod system, for example, via EtherCat (Image 6). As master, it specifies the target positions or trajectories as Cartesian coordinates in space and also reports the actual positions back to the fieldbus interface. The Hexapod controller takes care of all other calculations and acts as an intelligent drive with respect to the PLC.

Image 6. Standardized fieldbus interfaces simplify integration. (Image: PI)

Image 6. Standardized fieldbus interfaces simplify integration. (Image: PI)

SpaceFABs – Six-Axis Positioning Systems as Small as the Palm of Your Hand

The principle of parallel kinematics used in Hexapods can also be applied elsewhere: The SpaceFAB principle is based on a combination of serial and parallel kinematics. It is based on three XY stages that position a platform using three struts with a constant length and a suitable joint configuration. This makes it possible to realize fast and high-precision six-axis travel ranges. Products from the SpaceFAB series are so small that they can be easily placed on the palm of the hand (Image 7). They also offer travel ranges of up to 13 mm × 13 mm × 10 mm and angular motion on the rotational axes of more than 10 degrees. The position resolution is 2 nm.

Image 7. Palm-sized, parallel-kinematic SpaceFABs have six degrees of freedom and are suitable for a large number of applications in microproduction and quality assurance. (Image: PI)

Image 7. Palm-sized, parallel-kinematic SpaceFABs have six degrees of freedom and are suitable for a large number of applications in microproduction and quality assurance. (Image: PI)

The design is based on combined linear positioning stages and can be quickly and easily adapted to application requirements, for example, even for use in a high or an ultrahigh vacuum. Piezo-based inertia drives from the Q-Motion range are the driving force. They stand for high resolution in the nanometer range with theoretically unlimited travel ranges, compact design and attractive price. However, piezoelectric inertia drives are definitely not as sluggish as their name might suggest. Depending on the version, they are operated at a frequency of 20 kHz, are silent, and achieve velocities of up to 10 mm/s. The six-axis parallel-kinematic positioning systems therefore cover a wide spectrum of applications in microproduction, for assembly tasks and quality assurance.