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Linear motion - the most basic of all motion

07 November 2016

This article examines a couple of diverse applications involving linear motion products to illustrate how Newton’s first law of motion (objects that do not experience any net force will continue to move in a straight line with a constant velocity until they are subjected to a net force) drives the work of linear motion product manufacturers to increasingly design sophisticated applications.

High speed ‘waveform profile’ linear positioner

LG Motion recently designed and supplied a high speed belt driven linear slide and motion control system for a fluid mechanics facility in the Department of Aeronautics at the Imperial College of London.

The mechanical system is configured with parallel mounted belt-driven and non-driven slides on stainless steel precision shafts, supporting a 600 x 600mm carriage that transports test models with precisely defined velocity/time waveform profiles at speeds up to 3m/s over an 8m travel range.

The complete system consists of linear slides driven by a powerful brushless servomotor with resolver feedback and high precision gear-head, plus a cabinet mounted single axis servo drive and motion controller.

Two key modes

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 a 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 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.

Safety first

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.

6-axis vacuum compatible positioning system

Adjustable over-travel 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.

6-axis vacuum compatible positioning system

A second case study involves a 6-axis vacuum compatible positioning system developed and manufactured for AWE at Aldermaston.

The system is used to determine properties of x-ray diffraction crystals prior to installation into x-ray sensitive diagnostics for fielding on laser plasma experiments.

The PC controlled ‘Two Crystal Characterisation Positioning System’ includes three stepper motor driven rotary tables and a 350mm travel stepper driven linear axis that combine to support optical diffraction mounts and sensors, with two 35mm travel manually adjustable linear stages used to pre-position static optical equipment.

Pre-programmed user interfaces allows operator and supervisor level access to set-up and calibrate the optical system, and to load and run coded pre-defined characterisation routines that run across the PC and the motion controller, producing and storing characterisation data.

The complete system includes a control cabinet, housing the motion controller and micro-stepping motor drives, with all positioning mechanics mounted on an aluminium baseplate.

AWE integrated the mechanics into the vacuum chamber with connections through their vacuum bulkhead to the cabinet via 5m cables. The scope of supply included 10-6 vacuum preparation with special lubrication and through-holes or bleeder cross holes to avoid any possibility of cavities forming under vacuum.

All motorised axes used vacuum compatible stepping motors from motor specialist Empire Magnetics who work closely with LG Motion in hostile environment applications.

The rotary and linear stages were vacuum modified standard tables, part of a wide range of standard linear, rotary and elevation stages that are available with stepper or servo motors to form complete multi-axis positioning solutions.

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