Thule selects Moog for vibration resistance tests

When a supplier of car rack systems decided to invest in a test facility, it chose Moog FCS to develop a test rig to simulate the conditions its products are likely to face on the road.

Swedish company Thule, took delivery of the test rig in February 2008, a month before it opened its new test centre in Hillerstorp, Sweden.

'We chose Moog FCS because we liked its technical solution,' said Thule's technical group manager Fredrik Larsson.

'They were very easy to work with and their expertise in testing helped us to develop our approach.' The two companies first started to work together in May 2006 to develop a test system for this application.

Thule was looking to carry out accelerated high-frequency vibration testing on its rack systems.

Previously, the company had been outsourcing the work to an external engineering company, but decided to bring the work in house.

Moog FCS supplied a complete test rig including a multi-axis shaker table (MAST) and the software modules to run a wide range of tests.

Software includes the Smartest program for system installation, and Fastest operator software for preparing and running tests to simulate travelling over various types of road surfaces, as well as operating data acquisition and analysis systems.

The Hexapod configuration used by the Moog FCS rig was designed to achieve test acceleration, attain force or displacement inputs, and reproduce data collected on proving grounds.

Rather than using a conventional orthogonal system, which has a table that is moved by actuators mounted on its base and on its sides, the Moog FCS MAST is based on a six-leg Hexapod (also known as a Stewart platform), originally developed for flight simulators.

The Hexapod has two equilateral triangular frames set one above the other, offset at 30deg.

Each apex of the top triangle is connected to the two apexes below it on the lower triangle via Moog actuators.

Moog FCS developed the Hexatest MAST over four years, having completed a benchmark study of all competing designs.

Like traditional systems, it has six degrees of freedom - the table moves in the x, y and z axes, with rotational movements over all three (pitch, roll and yaw).

'We took what the competition could do in terms of acceleration and displacement as a benchmark, and set out to beat their performance specifications,' said Pim van den Dijssel, market manager test systems, Moog FCS.

'The Hexapod is a proven technology but we are the first to use it in this application.

'It's quite complex to control each of the individual degrees of freedom, because even a movement along a single axis involves the use of all six actuators.

'They all have to be controlled exactly in terms of speed and time, and this is especially tricky when the table has to vibrate at high frequency.

'The Moog FCS Hexatest MAST can vibrate at frequencies anywhere from 0.8 to 80Hz.' To enable the MAST to run at very low frequencies, the seismic mass incorporates 'active level control'.

This is because it has a resonant frequency of around 1 to 1.5Hz.

Air springs under the MAST are automatically activated as the mass begins to resonate, compensating for its motion.

The Hexatest MAST can vibrate with amplitude of over 140mm along the x, y and z axes, while maximum rotational displacement is +/-10deg.

It supports loads of up to 450kg and can run for several days without stopping, which is a period equivalent in real-life use to hundreds of thousands of kilometres.

The Hexapod unit is less costly to maintain than alternative designs.

The main reason for this is that, in contrast to orthogonal designs, its six actuators are exactly the same, so the spare part requirements are much less.

Moreover, the geometry of the assembly of actuators offers a convenient working height for the operator to mount and inspect the test specimen when no work platform is required.

'What Thule needs to establish is that the rack or box stays firmly attached to the vehicle and that it stays in one piece,' said van den Dijssel.

'They will be testing the clamping device as well as the rack or box.

'Tests are done with the clamping units attached to a section of the car roof provided by the OEM.

'The new unit will be a showcase item for Thule as well as for Moog FCS.' Moog FCS worked with Thule to time delivery of the system to coincide with Thule building its new test facility.

In the first few months that Thule has been using the Moog FCS MAST it has carried out several real-life tests.

'We are very happy with it,' said Larsson.

'It's compact, clean and user friendly.

'This is particularly important, given the minimal staffing levels at the facility.' Larsson works together with two engineers and two experts in virtual reality test software who develop the CAD models before the company moves to manufacture real prototypes.

'With in-house test capability, we will build up our knowledge base.

'The MAST solution gives us the possibility to develop new tests faster than ever,' added Larsson.

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