The simulation of complex devices
is a bit easier, thanks to
analysis software that simultaneously
handles several physical
disciplines. An example of these
new capabilities comes from sensor
developer TransTech Systems
Inc. in Schenectady, N.Y.
To gauge the density of such construction
materials as asphalt and
soil, its devices measure electromagnetic
properties that include
conductivity and permittivity.
The design of these sensors employs
FEA software called Comsol
Multiphysics 3.4 from Comsol
Inc., Burlington, Mass.
“The FEA software lets us
combine a material model, a CAD
model of the sensor, and a Spice
model of the external circuitry,”
says Trans Tech R&D Director
Ron Gamache. “In the past, we’ve
attempted to model sensors as
simple fixed electrical circuits.
But high-frequency sensors are
more complicated than such
models suggest. For example,
some behavior is influenced by
how the device interacts with the
constituents of the material being
measured such as water, stone,
and bitumen. Also, the external
electronics strongly influence the
sensor response. You can’t solve
for the response of many such
sensor/electronics combinations
analytically in closed form, so it’s
necessary to use FEA,” he says.
Gamache once calculated many multidisciplinary sensor
parameters by hand. “I was trying
to solve for just the material
properties independent of contributions
from the sensor and
electronics. But the sensor model
was simplistic and results didn’t
match predictions, so the devices
could not be calibrated in a
straightforward way,” he says.
Now, the procedure is to import a CAD model of a proposed
sensor into the FEA software followed
by a Spice model of the
electronic circuitry. “In Comsol,
the Spice circuit is added as a
set of differential equations and
global variables such as node
voltages and loop currents. The
predicted response closely duplicates
that of the actual sensor,”
he says.
Materials such as soil, which contain many constituents, exhibit
electromagnetic responses that are a function of all the constituents.
This effect, called “dielectric mixing,” prevents the separation of individual
contributions at any single frequency, explains Gamache. But
A phenomenon called the Maxwell-Wagner Effect provides a way to
take them apart. Materials that contain water have a strong permanent
dipole that rotates to line up with any electric field applied to them.
Injecting an electromagnetic signal into the material, starting with a
low-frequency sine wave, makes the pole flip back and forth in time
to the excitation. As the frequency rises, at some point the thermal inertia
and the tendency to randomness of the molecules will overcome
the ability to align to the field. This is called a “relaxation.”
A typical response below the relaxation point shows higher than
expected permittivity. “But when relaxation happens, the permittivity
drops to the value predicted by dielectric-mixing theory,” Gamache says.
“It turns out that permittivity changes in a unique way for each material.
So with the right signature analysis on raw spectral data, it’s possible to
get enough detail to separate the contribution of each constituent.”
Gamache says FEA models are proving quite accurate. “Over time,
I’ve gained enough confidence in the software to reduce or eliminate
time-consuming physical testing,” he says. “I know sensors will perform
as FE models predict.”
Make Contact
Comsol, comsol.com
TransTech Systems Inc., www.transtechsys.com