Linear positioner for fluid mechanics

As an addition to its cutting-edge fluid mechanics research facilities, the Department of Aeronautics at Imperial College London recently commissioned LG Motion to build and install a high-speed belt driven linear slide and motion control system for a new 9m water flume.

The servomotor driven positioner is based on two parallel LR series linear guides from LG Motion's associate MiniTec. The mechanical system is configured as parallel mounted belt-driven and non-driven slides on stainless steel precision shafts, supporting a 600mm x 600mm carriage that transports test models with precisely defined velocity/time waveform profiles at speeds up to three metres per second over an eight metre travel range.

The complete system comprises the LH series linear slides driven by a powerful brushless servomotor with resolver feedback and high precision gearhead, plus a cabinet mounted single axis servo drive and motion controller.

The new 9m water flume has two main modes for fluid mechanics research at the Department of Aeronautics. The first concerns understanding the behaviour of complex structures such as oil platforms in sea state studies by simulating current and tidal conditions to create vortex induced vibrations through turbulence.

In the second mode the water tank is used in similar way to a wind tunnel except that water rather than air is used as the forcing medium. An area being studied in this way is flow-induced behaviour of cylindrical models that may represent surface piercing pipelines or marine based tower structures such as wind turbines, and how different surface features and coatings may influence their prolonged use.

For both of these modes, the positioning system drives the carriage mounted model to follow a precise waveform profile initiated from an analogue demand signal from the flumes’ main controller or, for set-up and other assessment purposes, directly from the motion controller via simple BASIC-like PC programming. The waveform simulates the required modelling conditions whilst a series of high precision sensors collect the relevant structural behaviour information with the results presented through a dedicated data acquisition system.

For a system with such high speed and high acceleration capability, safety was a main concern. The linear slide includes a light curtain operating over the full length of its main working side. Adjustable overtravel limit and datum switches are conveniently mounted on the T-slots of the extruded aluminium profile LH slides. The light guard and limit switches are wired to the cabinet mounted motion controller which includes hardwired and software programmed safety features to ensure safe shutdown under emergency conditions.

Working closely with Dr. Nigel MacCarthy, experimental services and wind tunnel manager at the Department of Aeronautics, LG Motion designed and prepared the assembly in sections at its Basingstoke manufacturing plant for simplified transportation before commissioning the complete system at the College. The modular assembly is based on MiniTec’s Profile System which is widely used at the facility.

"These modular extruded aluminium profiles and machine building components allow us to take a ‘building block’ approach for each project, providing a straightforward and convenient means of adapting and even re-using equipment for the diverse needs of the department,” says MacCarthy.

LH series linear slides are available for customised travel lengths with belt transmissions and various motor technologies available to suit the application. The addition of double-row angular contact bearings on stainless steel precision shafts increases the load carrying and speed/acceleration performance for the 9m water flume.