MOTION CONTROL FEATURE
As a powerful and agile motion control solution, hexapods are growing in popularity, as Edward Williams, senior applications engineer at Physik Instrumente (PI), explains
AN UNCOMPLICATED SOLUTION FOR COMPLEX MOTION
F
or many robotics applications, the ability to move in six degrees of freedom is crucial, and this has been achieved using serial set-ups
consisting of stacked stages or multiple links connected by motor- actuated joints. But although these solutions do allow movement in several degrees of freedom, there are numerous drawbacks including the lack of precision and flexibility critical for the manufacturing of complex components or the effective operation of calibration and inspection systems. In addition, coordinating the movement of several connected stages to perform a movement can be both difficult to control and prone to errors, as individual inaccuracies in each stage will propagate and multiply throughout the system. This is where the Physik Instrumente (PI) hexapods come in to save the day.
HEXAPODS EXPLAINED PI hexapods have a single moving platform controlled by six independent, actuator-controlled struts, eliminating the accumulation of guiding errors found in stacked systems and enabling rapid and precise movement in all six degrees of freedom. We already know that the hexapods can perform complicated movements, but what needs to be stressed is how easy they are to coordinate – the fact that the hexapods can be combined with absolute measuring sensors, motion controllers and advanced software makes creating complicated motion profiles a piece of cake. The hexapods can be effortlessly integrated into automated systems, communicating directly with PLCs or CNCs via standard protocols (RS232 and TCP/IP) or fieldbus protocols (EtherCAT). And although most hexapods are designed for use in ambient environments, there are also models that can operate at high vacuum (10-6 hPa) and be used in an outdoor setting.
CONTROL AND SIMULATION TOOLS The success of hexapods is due to the availability of powerful software tools that can be used to both control the system and run simulations. Programming the motion of the hexapod is very straightforward, and is done in cartesian coordinates, even offering users the ability to define their own pivot points, with the necessary coordinate transformations handled automatically by the software. PI’s simulation tool even makes it possible to scan the experimental
set-up for potential collisions that might have been overlooked: all the points on a trajectory that have been flagged for a possible collision are checked before the move command is executed. This tool also checks whether the hexapod will be able to bear the load, which depends not only on the weight of the load, but also on the position of its centre of mass
and which movements are performed, which both affect the torque the hexapod will experience. This makes it possible to identify any mechanical problems with the set-up in advance and, since PI’s simulation tool is free to download, users can check how the hexapod will perform the task they have in mind before purchasing the hexapod.
SCALABILITY WITHOUT COMPROMISE PI hexapods come in a variety of different sizes. The smallest one has a 50mm base, while the largest has a 1m motion platform, and can support a load up to a ton. Crucially, even the largest hexapods still exhibit high accuracy, and can position the loads with sub-micrometre precision. This exceptional precision allows hexapods to be used in a variety of different applications – from industrial precision assembly and metrology to high resolution dynamic positioning for photonics alignment. Hexapods are now used in a variety of different fields, both for
production and testing. It is easy to imagine how a hexapod can be used for production, but they are increasingly being used in development activities, such as the development of camera software for mobile phones. Hexapods can be used to shake cameras – simulating the unsteady hand of a user – in order to assess image stabilisation algorithms. Hexapods are ideal for this type of the testing, since they can easily create defined, replicable movements or vibrations to enable comparative testing. Another interesting testing application is silicon photonics where the quality of the photonic device is checked through wafer probing. This process requires contact-free coupling of optical fibres with nanometre precision, and hexapods can be used to provide accurate, controlled alignment, without the risk of the probe coming into contact with the wafer.
AN AGILE MOTION CONTROL SOLUTION Hexapods represent a powerful and agile motion control solution that is easy to implement across a variety of different applications. As software control of these innovative tools has developed, they have grown in popularity across numerous sectors. However, there are probably a lot of people out there that have still not discovered that a hexapod is exactly what they need!
PI (Physik Instrumente)
www.physikinstrumente.co.uk
/ DESIGNSOLUTIONS DESIGN SOLUTIONS | MAY 2021 9
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