Techniques for extreme attitude suspension of a wind tunnel model in a magnetic suspension and balance system
Although small scale magnetic suspension and balance systems (MSBSs) for wind tunnel use have been in existence for many years, they have not found general application in the production testing of flight vehicles. One reason for this is thought to lie in the relatively limited range of attitudes over which a wind tunnel model may be suspended. This text reports on the modifications made to an existing small MSBS at Southampton University to permit the suspension and control of axisymmetric models over an angle of attack range from less than zero degrees to over ninety degrees. Previous work had shown that the existing arrangement of ten electromagnets was unable to generate one of the force components necessary for control at the extreme attitudes. Examination of possible solutions has resulted in a simple alteration which rectifies this deficiency. To generate the feedback signals essential to control the magnetically suspended model, an optical position sensing system using collimated beams of laser light illuminating photodiode arrays has been installed and tested. An analytical basis has been developed for distributing the demands for force and moment needed for model stabilisation amongst the electromagnets and over the full attitude range. This has been implemented by an MSBS control program able to continually adjust the distribution for the instantaneous incidence in accordance with pre-scheduled data. Results presented demonstrate rotations of models from nought to ninety degrees at rates of change up to ninety degrees per second, with pitching rates rising to several hundred degrees per second in response to step-change demands. A study of a design for a large MSBS suggests that such a system could be given the capability to control a model in six degrees of freedom over an unlimited angle of attack range.