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Latest advances in hexapod motion technology for precision alignment and automation in industry and research

By Stefan Vorndran, PI (Physik Instrumente)

Hexapod robotic positioning systems are six-legged parallel-kinematic machines quickly gaining ground in precision alignment, positioning, and automation applications. With advances in control technologies and algorithms, as well as new industrial interfaces such as EtherCat, hexapod performance has taken a significant leap forward, to the extent that electromechanical and piezoelectric hexapods are now viewed as more versatile, efficient, and accurate systems compared to traditional serial linkage or stacked multi-axis positioning systems, including those commonly used on robotic arms.

The latest versions of hexapods are capable of delivering unprecedented ultra-precision resolution to as low as 10 nanometers, and some can provide velocity of several 100s of mm/sec, making them a preferred solution for industrial testing, micro-assembly, microscopy, photonics wafer testing , biotechnology, and even astronomy and automotive applications (Figures 1, 2).

Figure 1: Hexapod six-axis precision motion systems are available with load ratings from 2 kg to 2000 kg and travel ranges from just a few to several hundred millimeters, providing precision in the sub-micrometer realm. Applications include alignment of Silicon Photonics components, semiconductor manufacturing, as well as the positioning of entire body parts in automobile production. [Image: PI]

 

 

 

 

Figure 2: Waterproof Hexapod for astronomy applications. [Image: PI] In the ALMA (Atacama Large Millimeter Array) telescope, up to 64 antennas will be equipped with hexapods to align the pickup perfectly to the large primary reflector. More information is available here. [Image: ALMA]

 

 

 

 

Hexapods
A hexapod, also known as a Stewart platform or Gough/Stewart platform, is a six-legged parallel mechanism structure. In its most common form, it consists of two platforms, one fixed and the other movable, which are connected and supported by six actuator legs (struts) that expand and contract, acting in parallel between them. Coordinated motion of these six struts enables the movable platform, and devices mounted to it, to move in any direction, operating with six degrees of freedom (DOF) relative to the other base platform. With six DOF, the movable platform is capable of moving in the three linear directions of x, y, z (lateral, longitudinal, and vertical) and the three angular directions (pitch, roll, and yaw) singularly or in any combination. Because hexapods have all six degrees of freedom, they can perform manipulations that cannot be done with any other traditional motion system.

In addition to the variable-strut-length hexapod of the Gough/Stewart flavor, other designs with fixed-strut length are also available (Figure 3).

Figure 3: Comparison of variable-strut-length hexapod design to fixed-strut-length design (platform shown in two positions). This fixed-strut-length hexapod uses six linear modules to move the basis of each joint up and down. Other designs that move the joints in one plane are also available. [Image: PI]

 

 

 

 

Most hexapod struts are equipped with cardanic joints. Flexure joints provide the highest precision and are completely free of backlash. However, load capacity and travel range are rather small compared to cardanic or sphere joints. Cardanic joints with Z-offset provide the best load characteristics (Figure 4 -- left). These joints are much stiffer than gimbal-type joints, however they require more sophisticated control algorithms to take into account the complex geometry of each actuator.

Figure 4 (left): Cardanic joints with Z-offset provide the best load characteristics. (Right) Spherical bearings are not recommended due to their lower stiffness and directional dependency. [Image: PI]

 

 

 

 

Hexapods come in many configurations and sizes, capable of handling loads from a few pounds to more than 2 tons (Figure 5). Advanced designs include servo-motor-driven systems for moving large optics or mirrors, piezo-based units for nanometer precision control of processes, and non-magnetic and vacuum-compatible versions. High-load hexapods usually use brushless DC motors and high-precision roller screw drives for high stiffness and self-locking capabilities. For highly dynamic applications such as image stabilization or pattern generation, direct-drive hexapods with voice-coil motors can be used (Figure 6). While most hexapods have a cylindrical shape, square or rectangular versions have also been used.

Figure 5: High-load hexapods. Load capacity to >1 ton with sub-micron resolution are feasible. [Image: PI]

 

 

Figure 6: High-dynamics hexapod with flexure joints and voice-coil direct drives, for dynamics to 200 Hz. A video is here. An application for Google's Pixel 2 cell phone camera development is shown here. [Image: PI]

 

 

 

 

Hexapods have a tremendous potential for streamlining many manufacturing processes by improving accuracy and speed, and reducing set-up and processing time.

Benefits of hexapods vs. serial kinematic positioning systems
Parallel-kinematic mechanism (PKM) motion systems have a number of advantages over standard serial kinematic (stacked) positioning systems (Figure 7). It is easy to see most benefits by just looking at the basic designs.

Figure 7: Six-axis positioners. Serial kinematics (stack of six stages, left) compared to parallel kinematics (hexapod, right). A video explaining the difference is here. [Image: PI]

 

 

 

 

Benefits include:
a) Elimination of cumulative error. In serial kinematic positioning systems, wobble and guiding errors in the bearings of each axis accumulate; the bottom stage supports its own moving platform plus all stages above it. Each actuator is assigned to one degree of freedom. Integrated position sensors assigned to each drive measure only the motion caused by that drive and in its direction of motion. All undesired motion (guiding error) in the other five degrees of freedom is not seen and therefore cannot be corrected in the servo-loop, which leads to cumulative error.

b) Automatic compensation for velocity and vector. As different from serial multi-axis positioning, hexapods require that all six struts alter their lengths if a change in only one axis is intended to occur. However, this is completely transparent to the user. The hexapod controller automatically runs the required coordinate transformations directing individual velocity and vector adjustments and transmits new positions to each of the six actuators hundreds of times per second.

c) One-third the parts for lighter weight, lower friction, and higher reliability. Compared with serial multi-axes systems, hexapods have one-third the number of parts, which means lighter weight and lower friction. Friction and torque in serial systems, caused by as many as five moving cables (cable management), limit accuracy and repeatability.

d) Reduced settling times. Because of the low mass of the moving platform on hexapods, positioning operations can be performed with far lower settling times than with conventional, stacked multi-axis systems.

e) High nominal load-to-weight ratio. The high nominal load-to-weight ratio is a very important advantage of the hexapod over stacked systems. The weight of a load in the hexapod platform is approximately equally distributed on the six parallel legs. This means each link carries only one-sixth of the total weight. Additionally, under certain loads the legs on the hexapod act longitudinally, exerting either tension or compression on the struts and reducing axial forces.

f) Large, central aperture. This is critical for optical applications, thru-light applications, or access to the back of a workpiece.

g) User-programmable, freely selectable pivot point (center of rotation). In a traditional positioning system, motion is confined by bearings -- and the center of rotation is related to the fixed radius of each rotation stage and goniometer used. In order to move the center of rotation, mechanical changes have to be made or fixturing needs to be modified. With modern hexapods and controllers, one software command is enough to set the center of rotation to any location inside or outside the hexapod envelope -- critical in many alignment applications.

h) No dragged cables/cable management issues. With traditional multi-axis positioners, as a stage moves, cables need to be dragged along. Friction and torque generated by bending forces contribute to parasitic motions, reducing accuracy and repeatability. Hexapods avoid moving cables altogether.

New features support sub-micron ultra-high precision
The latest high-resolution hexapod systems are being designed with high-efficiency, precision-controlled brushless motors and high-resolution encoders. For nanometer resolution, piezoceramic motors are also available. Different drive principles are being employed, depending on the application.

For example, new hexapods utilizing NEXLINER piezoelectric drives from Physik Instrumente, a leading industry manufacturer of both ultra-precision hexapods and piezoelectric motors, make for a positioning device that is completely non-magnetic. Such a system is critical for medical equipment and device applications that require magnetic non-interference. Other uses include high-energy physics applications (Figure 8).

Figure 8: Non-magnetic, UHV-compatible miniature hexapod with piezo-ceramic-motors. The PiezoWalk motors work reliably in strong magnetic fields. [Image: PI]

 

 

 

 

In addition to classical hexapod structures, parallel-kinematic designs are also beneficial to sub-nanometer precise positioning mechanisms, required for AFM (atomic force microscopy) applications and other novel super-resolution microscopy systems. With the integration of non-contact capacitive sensors, run-out errors on a nanometer and sub-nanometer scale can be determined. The sensors continually measure the actual position against the stationary external reference, and a servo-controller can compensate the error in real time (active trajectory control). Resolution and repeatability can attain 0.1 nanometer in such systems.

Recent hexapod designs provide extremely high stiffness and rigidity of their components and all moving parts, such as its bearings, joints, and drive screws. This results in high natural frequencies (500 Hz at a 22 lb load) that prevent bending in the six struts or in the movable platform, making these new hexapods capable of extreme accuracy and an ideal tool for precision machining, metrology, microscopy, and medical applications.

Silicon Photonics: Silicon Photonics is the fabrication of planar optical devices and circuits, often alongside microelectronics, using materials and processes familiar from integrated circuit microlithography. Testing individual chips on a wafer requires multi-axis positioning systems with extremely high accuracy and controllers with advanced alignment algorithms. One such system is the new FMPA nano alignment system (Figure 9), again from Physik Instrumente, based on two miniature 6-axis hexapods and XYZ piezo flexure nanopositioning scanners for fast fine alignment and tracking. The hexapods employed can deliver more than 10 lb of force and motion in all six degrees of freedom. Encoder resolution is 10 nanometers, and travel ranges cover 40 mm linear and a 60-degree rotation. Peak velocity is 20 mm/sec. These miniature hexapods can also be used for micro assembly and microscopy/neuroscience applications.

Figure 9: Double hexapod for silicon photonics wafer probing/alignment applications, based on the H-811 axis mechanics. Additional piezoelectric scanners provide sub-nanometer resolution and high scanning speed for alignment and device characterization. More information is available in this article. [Image: PI]

 

 

 

 

The system's high level of accuracy is a combination of extremely precise parts, precision assembly and testing, and sophisticated algorithms built into its advanced motion controller that take into account the exact tolerances of each strut and joint, and provide precise coordinates to each of the six actuators.

Hexapod motion is not confined by mechanical bearings, and as such there is no fixed pivot point (center of rotation) as with traditional rotation stages or goniometers. Instead, advanced hexapod designs make use of a virtual, programmable pivot point for rotational alignment tasks, allowing motion around any point, not unlike the human hand. Programming of target positions in six-space is simple. All positions are specified in Cartesian coordinates, and the controller transforms them into the required motion vectors for the individual actuator drives. Any position and any orientation can be entered directly, and the specified target will be reached by a smooth vector motion.

Because of the complex motion a hexapod can perform (Figure 10), it is desirable to have simulation tools to explore the possible motion range of the payload mounted to the hexapod. Figure 11 shows a simulation tool with automatic collision detection/avoidance. The shape and size of the payload can be added by the user inside or outside the hexapod envelope, and the software calculates the distance to any obstacle in its way.

Figure 10: Work space of a hexapod with a cylindrical payload. [Image: PI]

 

 

 

 

Figure 11: Hexapod working-space simulation tool with automatic collision detection/avoidance. [Image: PI]

 

 

 

 

The latest hexapod motion controllers facilitate programming by open-interface architecture providing a variety of high-level commands. EtherCat interfaces allow easy integration into industrial automation environments, with additional motion axes and I/O functionality.

Simulation tools can also verify workspace requirements and loads (Figure 12). Such software provides full functionality for creation, modeling, simulation, rendering, and playback of hexapod configurations to predict and avoid interference with possible obstacles in the workspace.

Figure 12: Hexapod strut force calculation tool. [Image: PI]

 

 

Hexapod-like structures:
In addition to true parallel kinematic positioners, hexapod-like structures have recently emerged. An example is shown in Figure 13. This palm-sized 6-axis nanopositioning is based on three XY linear stages, with integrated linear encoders, driven by piezo linear motors. The three stage pairs actuate a tripod, in a way to provide highly precise motion in all 6 degrees of freedom. Applications of these miniature positioners are in optics and miniature camera manufacturing and testing.

Figure 13: A miniature 6-axis system with piezo motors is based on a combination of parallel and serial kinematics (Model Q-821). [Image: PI]

 

 

 

 

Summary
With the new design developments that hexapod systems are experiencing, design engineers and researchers that have a need for motion with extreme flexibility and accuracy in multiple degrees of freedom can now capitalize on them for improvements within their workplace.

Hexapod technology has advanced considerably in a few short years, so now it is up to industry to embrace these new developments and put them to work to reduce set-up and processing time, overall production cycle times, and ultimately reduced cost of operation.

Published January 2019

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