Since your suspension is rigid mounted I find the use of a strain gauge
interesting. To measure strain you must have "yield". I would guess that
every part of your suspension is extremely strong and not likely to be in a
yield-range small enough, even for a staring gage. You might be hesitant to
"weaken" something just to get a reading.
There are a number of non-contact measurement sensors that work in many
industrial applications. Ultrasonic, MIR (Miro pulse radar), Radar,
etc. are all NON-contact methods. A Goggle search (using the advanced
search feature) on something like "non-contact measurement" or micro pulse
radar will provide plenty of reference material.
I would like to suggest two other approaches. (1) getting a ground distance
measurement or (2) ascertaining important information from your tire.
Number 1 may be straight forward. A sensor (see links below) could be
pointed at the ground to measure vehicle-to-ground distance. Of course,
sensor noise, course deviations, tire growth, would all have to be filtered
and a trend established. Deviations from the tolerance band could
be identified to warn the driver.
Number 2 may be the most accurate approach and fraught with less
variables. There are two predictable characteristics that can be mapped
about your tires. One, is the change in shape due to centrifugal force and
two, is the change in FOOT PRINT due to LOAD. I don't know the weight of
your car and I don't know the N2 pressure you use. However, I imagine there
is a small foot print (even with 75 psi). It may not be discernable by eye,
but it surely would be by a sensor. Obviously, the footprint will become
smaller as you approach aero lift. A measuring laser or some other
non-contact sensor could monitor tire size changes and trigger the
necessary output. (See links)
Measurement for growth, by centrifugal force, can be easily done on a spin
tester and mapped into a look-up table. The foot print dimensional change
can be done statically with a floor jack. Your sensor set-up would be used
in both situations for calibration. I think all the elements are there.
The most difficult part is how you interpret the information and apply it
to your unique vehicle. -Elon
http://www.tokyokeiso.co.jp/english/technology-e/tech_level/l-0001/
They make a non-contact liquid level sensor.
http://www.krohne.com/html/dlc/HB_LEVELRADAR_e_72.pdf
Fundamentals of using radar technology for non-contact measurement.
http://www.automationsensors.com/frames/indexUSsel.html
Ultrasonic sensing solutions for solids, liquids, etc. Various distances
from <1ft to >20ft.
http://www.rosemount.com/products/level/3300tdr.html
Time Domain Reflectometry (TDR). Uses a reflected pulse for measurement.
http://www.andinst.com/dairy-level-products.php
Anderson is a quality company. Read about the different instruments.
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