On 11/30/25 20:58, Leonardo Marsaglia wrote:
I think they may be bending beams and strain gauges.
Judging by the range they claim they can reach I think it's not the case.
Apparently they use the compressed air as a means to sense the position of
the arm. I still think RVDT could be a very good solution. I also thought
about using a rotary capacitor but those are more sensitive to harsh
environments and have less linearity than RVDTs apparently.
Less linearity is a gross understatement of the problem, because the
capacitance is several times more sensitive to the axial placement due
to the square law. where any axial displacement adds capacitance at a
much faster rate on the side getting closer, than is lost by the
increasing spacing on the other side of the plates, so you get the
calculated capacity only if perfectly centered. This isn't correctable
by shaping the plates as eccentric although there have been at least 100
attempts to do that in order to linearize the dial on the common AM
receiver with it's 3/1 frequency range here in the USA.
Using a variable leakage hole also suffers even worse from air louver
sorts of non-linearity because an air louver flows 50% of its air flow
capability with fixed hp fans when only open 5 degrees. I would not
look for a solution involving air leakage, but might consider a piston
and hall effect, which if the resisting spring is calibrated correctly,
could result in a hall effect output that might be 5% accurate. The
spring, properly formed could result in the square law response of the
hall effect device being /mostly/ canceled. So the RVDT solution might
be the best solution out there. Pricy I'd imagine.
El mar, 25 nov 2025 a las 11:11, andy pugh (<[email protected]>) escribió:
On Tue, 11 Nov 2025 at 01:07, andy pugh <[email protected]> wrote:
I think they may be bending beams and strain gauges.
For once the AI on Google is spot on (and better than the web results)
I made a lot of these when working in the field described. (fatigue
testing of CTS specimens).
Despite being extremely simple, they are very accurate and surprisingly
linear.
-----------------begin-AI------------------------------
"Clip gauges" generally refers to clip-on extensometers used in
materials testing, specifically for measuring crack mouth opening
displacement (CMOD) or large strains in specimens. Fabricating one
involves installing strain gauges on a simple flexure device, which
requires specific materials and careful procedures.
Materials Needed for a Basic Clip Gauge
Spring steel strips for the main body of the gauge.
Strain gauges (e.g., EP-series for high elongation measurements).
Terminals for wiring connections.
Connecting block for fixing the strips.
Cyanoacrylate (CN) glue for bonding the strain gauges.
Cleaning agents: sandpaper, cotton, acetone, iso-propyl alcohol (IPA).
Tools: Tweezers, low-tack cellophane tape, light scribing tool or 4H
pencil, soldering iron and wires (for connecting wires).
Calibration tools: A system for applying known displacements and
measuring the output (e.g., a comparator bar).
Step-by-Step Fabrication Process
Prepare the spring steel strips: Clean and abrade the areas where the
strain gauges will be installed using sandpaper. The position should
be as close as possible to the region of maximum bending stress.
Clean thoroughly: Remove all abrading debris using cotton and acetone,
then clean again with IPA. Avoid touching the prepared surface with
your fingers afterward.
Mark alignment lines: Use a light scribing tool or 4H pencil to mark
alignment lines for the gauges.
Position gauges and terminals: Use tweezers to place the strain gauges
and terminals against the alignment lines. Secure their relative
position with low-tack cellophane tape, ensuring the metal foil grid
faces up.
Bond the gauges: Roll one end of the tape back to expose the backing
sheet. Apply a small drop of CN glue to the backing sheet and stick
the tape back in place, applying even pressure.
Wire the gauges: Solder the connecting wires to the terminals. The
gauges are typically wired into a half or full Wheatstone bridge
configuration to maximize the signal.
Mount the gauge "feet": Design the device so that it can be mounted to
the test specimen by bonding or spot welding, depending on the
material.
Calibrate the finished gauge: The clip gauge is a nonlinear device and
requires calibration against known displacements before use. Monitor
the "zero" reading during calibration to check for permanent offsets,
which may indicate localized yielding.
For a detailed guide on the design and calibration, academic resources
like the paper on the "Optimum Design of a Ring-Shaped Clip Gauge"
provide an analytical framework.
---------------------------------------------
https://link.springer.com/article/10.1007/s40799-020-00417-1#:~:text=To%20design%20a%20gauge%20with,available%20in%20commercial%20mathematical%20software
.
This video shows a completed one, but is otherwise not as good as one
might hope.
https://www.youtube.com/watch?v=5ry-7iqQIEw
--
atp
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designed for the especial use of mechanical geniuses, daredevils and
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