Aerospace and Electronic Systems Magazine October 2017 - 16

Measurement of Wing Deflections in Flexible Aircraft
transmit/receive antenna, and render the sensor electromagnetically active. The sensor's response comprises of a fundamental
frequency and its associated harmonic frequencies, and these frequencies are related to the sensor's geometry. The sensor-antenna
coupling occurs through the magnetic near-field, and the current
induced in the sensor reaches its maximum magnitude near the
sensor's resonant frequency.
The sensor's response is measured by the network analyzer as a
return loss scattering S-parameter, which depends on the energy reflected from the antenna. S11 is the reflection coefficient displayed
on the network analyzer, and is shown as a distinct amplitude resonance signature, which is a function of frequency [17].
Figure 3 shows a sample S11 plot. This plot shows the unique
signature of the resonant fundamental frequency and its harmonics, as exhibited by a 100 mm square 156 turn copper SansEC sensor adhered to the surface of a dielectric material. When the material is bent by 8 degrees, a frequency shift is produced as shown

in the figure. It is this frequency shift that is used to quantify wing
deflection, as is illustrated in subsequent sections.

BENDING-MODE STRAIN SENSING WITH SANSECFLOATING ELECTRODE SYSTEM
The novelty of the SansEC technology requires the functional
dependencies between the sensor parameters and material's performance to be characterized and calibrated by experimental and
computational methods. This work presents experimental investigation of the SansECs strain-sensing capabilities by utilizing a
"floating electrode" approach. It has been observed that such a
design approach enhances the sensor's responsiveness to deformation of its substrate material, in terms of increased measurement
sensitivity and resolution. The floating electrodes imply strip(s) of
conductive material placed in the proximity of the sensor but not
electrically connected to it (see Figure 4). This material attenuates
the electric and magnetic fields produced by the sensor. The degree of this attenuation as well as the sensor's response frequency
depend on the separation distance between the tip of the electrode
and the substrate.
The following formalism establishes the dependence of frequency shift on the displacement Δy. Based on empirical evidence,
a linear relationship exists between the S11 frequency response and
the bending displacement as follows:
f − f0
Δf
= θ
Δθ θ − 0°

Figure 3.

S11 reflection coefficient measurement showing fundamental and harmonic frequencies of a SansEC sensor placed on a dielectric substrate.
Plots are shown for the unbent substrate (FS-0) and with an 8 deg bending of the substrate.

(1)

where f0 and fθ represent the original (undeformed) and bent-state
frequencies, respectively, and θ is the angle by which the substrate
is bent. For the current application, it is reasonable to assume small
bending angles of the order of 10-12 degrees. The substrate curvature can then be approximated as a tangential straight line, and
therefore, θ = θy, (shown in Figure 4b) and it follows that θ = sin-1
((2Δy)/l) where, l is the length of the floating electrode. Thus, the

Figure 4.

Floating electrode approach for SansEC sensor strain sensing.

IEEE A&E SYSTEMS MAGAZINE

OCTOBER 2017

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine October 2017