Abstract
The requirements which a wind tunnel balance must meet are becoming increasingly
stringent. The wind tunnel testing community is calling for balances which; offer higher
resolution, are stiffer, are immune to electro-magnetic interference (EMI) and provide
compensation for thermal effects. It is proposed that in order to meet these requirements,
balance design philosophy needs to be further expanded to include different manufacturing
methods, materials and sensor technology. This study investigates the design and
development of a six component wind tunnel platform balance incorporating Optical fibre
Bragg grating (OFBG) sensors.
Under an applied load, conventional balances measure strain at the surface of a material, by
means of a foil strain gauge. For this reason, sections of material in the balance are purposely
made thin, in order for the strain in that section to be sufficiently high to offer adequate
resolution. This may compromise the stiffness of the balance.
A platform balance is designed which incorporates OFBG sensors using the two-groove
method of strain measurement. The optical fibres are spanned between two probes. One probe
protrudes from the metric end of the balance, the other protrudes from the fixed end. Under
an applied load, the gap between the two protrusions will change, which will induce a strain
in the fibre Bragg grating spanning it. This strain in the fibre Bragg grating will cause the
Bragg wavelength to shift proportionally to the magnitude of the strain in the fibre.
Therefore, the balance is designed around the idea of measuring displacements within the
structure of the balance. This displacement is comparatively larger than the deformation at
the surface of a material. Therefore, strain induced in the fibre spanning the gap would be
larger than the strain at the surface of the material. The balance to which these fibres are
bonded can be made stiffer, while still offering a comparable relative resolution.
The two-groove method uses two fibres to measure one load component. Each fibre is part of
a pair for strain measurement. Each fibre Bragg grating has a different reference Bragg
wavelength. These are spanned across two separate gaps. The balance has been designed such
that, under an applied load, one fibre of a strain measurement pair experiences a tensile strain,
and the other experiences a compressive strain. The final output is determined by calculating
the change in difference between the two fibres’ respective Bragg wavelengths. This method
compensates for unwanted force and thermal interactions...
M.Ing. (Mechanical Engineering Science)