What is a Hexapod?
A Hexapod, or Stewart Platform, is a parallel kinematic structure composed of a mobile platform linked to a fixed platform with 6 actuators. This design allows you to move an object placed on the mobile platform with 6 DOF (Degrees Of Freedom). In other words, the hexapod can move an object along the 3 translations (Tx, Ty, Tz) and the 3 rotations (Rx, Ry, Rz); any combination is possible.
A workspace defines all reachable positions of the mobile platform for specified degrees of freedom. An infinity of workspaces exists depending on which DOF are set to be swept and which DOF are set to be constant among Tx, Ty, Tz, Rx, Ry, Rz.
Example of two workspaces: – In yellow, the workspace [Tx=swept, Tz=swept, Ty=0, Rx=0, Ry=0, Rz=0]. – In orange, the workspace [Tx=swept, Tz=swept, Ty=0, Rx=0, Ry=0, Rz=20°]. The orange workspace is smaller than the yellow workspace because the Rz rotation requires extra actuators’ length.
Types of Hexapods
Symetrie, one of the world’s leading providers of hexapod solutions for positioning and motion applications, develops two lines of hexapods:
Positioning hexapods are used to precisely align optical components, samples on beamlines, or mirrors on satellites or telescopes. Positioning hexapods generally have submicrometer resolution but their speed is mostly limited to several mm/s.
Motion hexapods can simulate the motion of a boat, truck, tank, aircraft, etc. in order to test an instrument that will be on board later: sensors, electro-optics systems, antennas, inertial measurement units, gravimeters, etc. These dynamic motion hexapods can go to a speed of 1 to 2 m/s and they can reach 1g acceleration.
Ground-based telescopes are becoming more and more powerful in order to help astronomers to see further and more accurately. As a consequence, telescope manufacturers are looking for improved mirror positioning performance. Hexapods for astronomy are mostly used to realign the secondary mirror relatively to the primary mirror to compensate for the mechanical deformations of the telescope structure due to temperature and gravity changes during the night.
Hexapods can also be used to position telescope instruments or to calibrate mirrors or other optical components during their manufacturing phases.
A hexapod is positioning the secondary mirror of the Indian ARIES DOT telescope with an accuracy of 0.5 µm (pictured above). The hexapod is located inside the black box on top of the telescope manufactured by AMOS in Belgium.
The ESO Extremely Large Telescope (ELT, pictured below) will have a primary mirror of 39 m, made of 798 segments. The optics specialist Safran REOSC is polishing and testing all the M1 segments. In order to calibrate them, the company uses 4 hexapods in order to position the segments and the calibration tools with high stability: 0.1 µrad over 1 hour.
Research & Engineering Applications
Thanks to the flexibility of the six degrees of freedom of the hexapods, these parallel kinematics systems can be used in various fields of research including chemistry, physics, industrial equipment.
A non magnetic hexapod is positioning a vacuum chamber in a 2 Tesla magnetic field for quantum research at the University of Sydney.
Hexapods can be used by space companies and research institutes to accurately test and position satellite antennas, to align and assemble components together, to adjust and calibrate optical benches or to verify the quality of the optical instruments and their subassemblies.
The telecommunication satellite manufacturers test the antennas’ performance inside anechoic chambers (above), in order to reproduce communications between the geostationary orbit and Earth stations.
During the life of the satellite, antennas can be reconfigured and reoriented to communicate with different stations. As the space mechanisms used to reorient the antennas cannot function at Earth gravity, they are replaced by hexapods during the RF tests.
A satellite that is sent to space cannot be repaired easily if needed after the launch. This is why it is crucial that each product has zero defects. Testing devices are fundamental for the success of the missions. Hexapods are very practical to calibrate space optical instruments thanks to their high resolution and their stability over time.
Precise positioning hexapods are particularly suitable for the specific and demanding needs of synchrotrons. They make it possible to align various components such as samples, mirrors, vacuum chambers, etc. with high resolution, great stability over time and high stiffness.
This compact BORA hexapod (above) is positioning a sample with 0.1 µm resolution at ESRF, the European Synchrotron. It is installed on a goniometer and can work in any orientation.
This BREVA hexapod (above) has been customized to have a larger angular travel range and to integrate an extra rotation under its mobile platform. It is installed at APS synchrotron on beamline 12-ID-D.
For optics, the multiple degrees of freedom and the precision of the hexapods make it possible to align components during assembly or test phases: alignment of lenses before bonding, calibration of mirrors and optical surfaces.
Two hexapods are integrated on the optical test bench of MIRIM, the Mid Infra Red IMager of the James Webb space telescope (above). A manual hexapod is positioning the cryostat containing MIRIM, while an electromechanical hexapod is positioning the light source with an accuracy of 10 µm. These tests have been done by the Astrophysics Department of CEA.
Choosing the Right Hexapod
Thanks to HexaSym, Symetrie’s free simulation software, it is very easy for you to check the possible travels and load capacities of each hexapod in their range. HexaSym will allow you to select the most suitable product for your application and to test a variety of possible configurations with it.
With HexaSym simulator, you can test cumulative travel ranges on several axes simultaneously, including the “worst cases” of your applications. It is also possible to change the center of rotation, to define different reference frames, to vary the orientation of the hexapod (vertical, horizontal, other) and the payload.
HexaSym also includes a 3D visualization that will enable you to see how the hexapod moves according to the commanded positions, as well as sliders allowing you to control movements more immediately.
It is also possible to use HexaSym for customized hexapods, for which you will receive a compatible configuration file.
Some specific terms linked to metrology are often used to characterize hexapods. Here are some definitions that can help you understand the specifications of a hexapod.
Resolution – Resolution is seen as minimum incremental motion (MIM). It is the smallest motion increment that the system is able to achieve in a consistent and detectable manner.
Repeatability – Deviation from the average of actual positions when the system is commanded several times to go to the desired position. Repeatability is given as unidirectional repeatability in any point of the axis with ± 1 standard deviation (parameter R+ following ISO 230-2 standard).
Accuracy – Difference between the actual position and the desired position. Defined by ISO 230-2 standard. Accuracy depends on the commanded motion and on the location of the pivot point. This is why it is not easy to give one global accuracy value for one hexapod. It is better to specify it for a certain motion associated with a certain pivot point.
Stability – Defines how much the hexapod deviates from its position over time without any new command.
Stiffness – Defines how much the hexapod deforms when subject to an applied force. Stiffness is determinant to increase the natural frequency.