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Automated Precision Inc.

CMSC

The Somewhat Different Robot

Highly accurate hexapods are gaining popularity outside their specialized applications

Published: Tuesday, May 20, 2014 - 16:16

Hexapods: The word sounds alien, but we see them every day. A hexapod (“hexa” is the Greek word for six; “pod” is Greek for foot) is simply a spatial motion machine with six driving elements. They can be seen at fairgrounds, as the motor for carnival rides, or on flight simulators for pilot training. In both cases, the cabin is mounted on six stilts whose length can be changed and rotated against each other, allowing the platform to move in all six degrees of freedom.

Essentially, hexapods are robots. They are used in many manufacturing sectors, including automotive. Hexapods have a parallel kinematic structure. Machines with a different kind of structure—known as serial kinematics—are far more popular in industry. Serial kinematics, particularly those that make up an articulated arm, have become well established in industrial production.

Hexapods, meanwhile, are used only in special cases, but their accuracy in these areas is unmatched. Their parallel kinematics offer higher positioning accuracy because the position errors of the axes are not cumulative, as in serial kinematics, but are only proportional to the overall movement.

This increased accuracy allows hexapods to handle particularly demanding applications that involve dynamics and low moving mass with the same high payload. These include studies into the wave movements of water, which are needed in shipbuilding research, or in the positioning of satellite antennas in anechoic chambers. In all cases, a hexapod is used to determine the exact positioning of structures and components in space. The largest hexapods reach an average height of about 3 m (9 ft).

A hexapod is used to study the influence of waves on a ship’s hull in the wave pool at IFREMER in Brest, France.

Measurement and compensation of the axis

At Symétrie, manufacturer of high-precision positioning and motion hexapods headquartered in Nimes, France, one of the leading laser trackers is used in production and assembly to ensure the hexapods’ precision of movement through every angle and position.

The AQUILON Hexapod can accelerate a payload of 6 tons with 1 g. As was found at Symétrie, laser trackers are the ideal measurement systems for measuring the positions of the robot or its payload in space. While setting up the systems, and during their installation and calibration at customers’ sites, portable measuring systems play a crucial role.

In its element with special applications

The larger hexapods are traditionally used for movement simulation (these are known as motion hexapods). For example, submarine crews train for emergencies while the simulator, which is based on a robot with parallel kinematics, simulates the movements of the boat. The French are experienced with simulating the motion of waves swelling, while other customers use hexapods to prepare satellite antennas that are installed on ships and must be stabilized via positioning systems. In another application, loading cranes are tested for simulations at sea.

The realistic simulation of the waves by a robot is also important for companies that manufacture insulation for tankers that transport liquefied natural gas (LNG). In liquid form, cooled to –162 degrees Celsius, natural gas takes up only 1/600th of its original volume, which makes the transportation of large quantities of this fuel economical; more than 25 percent of the natural gas transported worldwide is liquefied. But binding international standards on the construction and equipment of liquefied gas tankers requires extensive—and expensive—testing. Thanks to their ability to withstand certain types of damage from collisions or strandings, these tankers are standardized worldwide. Symétrie’s motion hexapod makes it possible to investigate the influence of surge or sloshing effects of the liquid load on a transport ship.

The influence of waves on a ship’s hull is also studied in the wave pool at IFREMER in Brest, France. The Deep Wave Basin is Europe’s deepest wave pool for hydrodynamic investigations, with a depth of depth of 20 m (66 ft). These and other underwater acoustical measurements are carried out here, with the hexapod robot installed upside down for the applications.

Symétrie is also responsible for another customized solution with hexapods. One that can be operated under vacuum-like conditions has been constructed for the Maritime Research Institute Netherlands (MARIN). In a 240 meter-long basin, tests are carried out on the underwater stability of ships’ hulls, as well as on the influence of cavities and other fissures caused by steam bubbles on propellers. The tests are performed in the basin on models that are built at a smaller scale, but the results correspond with the actions of full-size ships. These measurements are taken in almost vacuum-like conditions, with pressure as low as 2,500 Pascal (the average air pressure of the atmosphere is 100,000 Pascal).

Testing the underwater stability of ships’ hulls at the Maritime Research Institute Netherlands (MARIN)

Measuring with a laser tracker

A laser tracker is the ideal measuring system for measuring robot positions in space. Its unlimited portability allows it to be set up in the immediate vicinity of the object to be measured, without the latter having to be transported. This represents an unbeatable advantage over stationary measuring machines. Its compact design enables it to be transported in a car, so that measurements can be carried out even at a customer’s premises. A measurement target, known as a spherically mounted retroreflector (SMR) is the only equipment that is needed to measure structures or geometries. Laser trackers from API can dynamically track targets, taking continuous fluid measurements as the SMR moves across a surface—hence the name “trackers.”

Another tracking method is to fix the SMRs position, using a magnet, to an object that can then move freely while being tracked by the laser beam. A tripod is usually needed for the setup, although laser trackers can be mounted directly on, or in, the immediate vicinity of the object to be measured (for example, using a magnetic base). Trackers can even be mounted at a right angle or upside down from the ceiling. Standard software packages from established manufacturers are used. Customers particularly appreciate the high accuracy of laser-based measurement systems, which can’t be matched even approximately by any other mobile measurement method.

Move. Test. Simulate.

Hexapods are also in demand for testing satellite antennas. The antennas and receiving devices are installed together in anechoic chambers for electromagnetic waves. The position of the antenna or the receiver can be changed by the hexapod robot in the open space, and the “far field” of the antenna can be calculated from the measured “near field” part using mathematical models. Similar tasks are performed by a parallel kinematic robot for controlling telescopes; other applications can also be found in nuclear and synchrotron research, as well as in defense activities.

In summary, hexapods represent an ideal solution for 6D motion simulations, thanks to their high dynamics and a simple statics. Moreover, through the parallel arrangement of their drives, they have a better ratio of payload to weight than serial robots. It can therefore be seen that hexapods are almost exclusively used in demanding test environments that require a movement simulation of this kind.

Special solutions

Symétrie offers custom engineering solutions for position and attitude measurements in many industrial sectors. Geometry measurements are carried out by the company on behalf of customers, using laser trackers, laser interferometers, and specialized software solutions. The company is ISO 9001-certified, and is authorized to perform calibrations in accordance with international standards. For many unique applications, the company has its own software development department. Here, developers write programs for motion simulation and hexapod control that are not available on the market.

These innovations allow Symétrie to introduce hexapods to a growing number of industries and applications. Ultimately, hexapods will be even more visible—even to people who are still unfamiliar with the word.

Article by Anne Duget. First published on API’s Learning Center.

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About The Author

Automated Precision Inc.’s picture

Automated Precision Inc.

Founded in 1987 by Kam Lau, Automated Precision Inc. (API) is the inventor and original patent holder on the first laser tracker systems. API develops and manufactures laser trackers; portable coordinate measuring machines (CMMs); laser tracker accessories, which include probes, sensors, targets, cameras, SMRs, and scale bars; and machine tool calibration products. API Technical Services include contract measurement equipment rental, onsite laser tracker calibrations, machine tool error mapping, machine tool alignment, and training for 3-D measurement systems and analysis software. With its world headquarters in Rockville, Maryland, API has offices in China, Germany, and India.