Abstract
M.Eng. (Mechanical Engineering Science)
Wind tunnel balance requirements are becoming increasingly stringent. There is a need for wind tunnel balances to offer higher resolution, improved stiffness, are immune to electro-magnetic interference (EMI), provide thermal compensation as well as reducing the cost and lead times associated with the development of a wind tunnel balance. To meet these needs, it was proposed that balance design philosophy should expand to include different materials, sensor technology, measurement philosophy and different manufacturing techniques. Therefore, this thesis investigates the design and development of a hybrid six-component balance that incorporates strain gauge and optical fibre sensors. Conventional strain gauge balances are designed and constructed to measure the local deformation (strain) of a material by utilising foil strain gauge sensors. It is required that sections of material in the balance be relatively thin to ensure that the strain is sufficiently high enough to offer acceptable resolution. This may have a negative impact on the overall stiffness of the balance. Furthermore, foil strain gauges are susceptible to EMI and thermal effects. A hybrid platform balance was designed which incorporated both strain gauge and optical fibre sensors. The strain gauge sensors were configured in a Wheatstone bridge, which are typically used in conventional strain gauge balances. This provides temperature compensation. The Optical Fibre Bragg Grating sensors were implemented using the two-groove method of strain measurement. The optical fibres spanned a gap in the optical fibre transducers. The optical fibre transducers were designed using the strain amplification design philosophy. Under an applied load, the optical fibre transducers bend, therefore, causing the gap to either open or close. This induces a strain in the fibre Bragg grating spanning it. The strain in the fibre causes a Bragg wavelength shift proportional to the magnitude of the strain. The strain induced in the fibre is significantly larger than the strain experienced on the surface of the material. This allows the balance to be made stiffer than a conventional, full strain gauge balance while offering comparable relative resolution. The two-groove method uses a pair of fibres to measure a single load component. Each fibre Bragg grating has a different reference wavelength. As the balance is loaded, one optical fibre transducer bends outwards, causing the optical fibre to experience a tensile strain, while the second transducer bends inwards, causing the optical fibre to experience a compressive strain. The final sensor output is defined by calculating the difference between the outputs of each of iv the fibres Bragg wavelengths. Typically, this method compensated for both unwanted forces and thermal interaction. However, due to the size of the balance, the two-groove method was not used to provide temperature compensation since the space between the transducers was sufficient that each transducer could experience different temperatures. However, it provided interaction compensation. A finite element method (FEM) study was conducted to analyse the performance of the hybrid balance before manufacturing. The FEM study of the design showed that the balance had high sensitivity and stiffness. Furthermore, the FEM study showed that the balance exhibited low interactions for each load component. The performance shown from the FEM study led to the decision to manufacture the hybrid balance. The balance was manufactured, assembled, gauged and tested. The balance was loaded to 60% of the maximum design load to avoid damaging the thin components as well as to avoid damaging the optical fibre and strain gauge transducers. The output of the optical fibre transducers was linear with regards to the wavelength shift in response to an applied load, however, the strain response output of the strain gauge transducers was found to be cubic with wavelength shift. The repeatability of the balance was found to be within 0,619%. However, this exceeded the recommended value of 0,1% set by the wind tunnel testing community. There were errors that resulted in uncertainties in the measurements. These errors were attributed to misalignments in the bonding of the strain gauges to the strain gauge transducers, design flaws with regards to the optical fibre transducers, loading and manufacturing errors. Fibre creep was another factor that contributed to the measurement uncertainty. Furthermore, the optical fibre sensors tended to drift in a time-dependent logarithmic trend. The drift was apparent when a load was applied or removed. This added further uncertainty. The interactions of the balance were larger that the FEM predicted interactions. This was mainly attributed to manufacturing errors and misalignment issues. The balance is a proof of concept that a balance can utilise two different sensors. The balance performance has shown that the balance is not ready to be utilised in a wind tunnel balance as it does not fully meet the criteria set by the wind tunnel testing community. However, the balance is cost effective with short production lead times. Before the balance can be implemented in a wind tunnel, fibre creep must be attended to and optical fibre installation methods must be revised.