Strain Gauge or ...?

J

Jack B

Guest
I want to measure (indirectly) the force applied to an arm.

Think "bicycle crank arm"... force is applied at one end of the arm (as
with a pedal), and the arm pivots about a perpendicular axis at it's
other end.

Givens: I may not measure the force directly at the end of the arm;
environment is mild temperature-wise (humans can function ok), but lots
of dirt, dust, vibration; sensor must be small and non-intrusive;
measurements should be good, but need not be lab-quality.

Two issues:

- assuming some sort of strain/flex sensor firmly attached to the arm
(e.g. epoxy), what sort of sensor? Thin foil-type sensors seem to need
an "S-"bend to function properly; other commercial ones seem too big.
Would some sort of piezo device work?

- how to transmit the data to an analysis device? RF?

--
Jack
 
On Sat, 29 May 2004 13:41:15 -0500, Jack B
<jackspam@ssppaammbohnhoff.com> wrote:

I want to measure (indirectly) the force applied to an arm.

Think "bicycle crank arm"... force is applied at one end of the arm (as
with a pedal), and the arm pivots about a perpendicular axis at it's
other end.

Givens: I may not measure the force directly at the end of the arm;
environment is mild temperature-wise (humans can function ok), but lots
of dirt, dust, vibration; sensor must be small and non-intrusive;
measurements should be good, but need not be lab-quality.

Two issues:

- assuming some sort of strain/flex sensor firmly attached to the arm
(e.g. epoxy), what sort of sensor? Thin foil-type sensors seem to need
an "S-"bend to function properly; other commercial ones seem too big.
Would some sort of piezo device work?
Click to expand...
---
Make your sensor capacitive, and part of an RF oscillator, and you'll
acquire your data and transmit it in one swell foop!

--
John Fields
 
Jack B <jackspam@ssppaammbohnhoff.com> wrote in news:jackspam-
C26F08.13411429052004@news.mpls.visi.com:

I want to measure (indirectly) the force applied to an arm.

Think "bicycle crank arm"... force is applied at one end of the arm (as
with a pedal), and the arm pivots about a perpendicular axis at it's
other end.

Givens: I may not measure the force directly at the end of the arm;
environment is mild temperature-wise (humans can function ok), but lots
of dirt, dust, vibration; sensor must be small and non-intrusive;
measurements should be good, but need not be lab-quality.

Two issues:

- assuming some sort of strain/flex sensor firmly attached to the arm
(e.g. epoxy), what sort of sensor? Thin foil-type sensors seem to need
an "S-"bend to function properly; other commercial ones seem too big.
Would some sort of piezo device work?
Click to expand...
Assuming you are measuring the stress/strain in the arm assumes that there
will be enough stress (load) to produce measurable strain (bending moment).
If there is enough load to produce a measurable flex, commercial foil-type
strain gauges should work after a careful selection. Gauges should be
configured in a Wheatstone Bridge. Full Bridge configuration is best,
half-Bridge is next best. Gauges are bonded to the substrate by several
different types of adhesives. Epoxy might work, also a cyanoacrylic (super
glue) is used. For more information, check out the Omega Instruments web
site (www.omega.com) or Vishay Micromeasurements
(http://www.vishay.com/company/brands/micromeasurements/). The latter has
a literature series about strain gauges and load cell design.

- how to transmit the data to an analysis device? RF?
Click to expand...
That would probable be the most straightforward method. The problem with
doing it this way is to counterbalance the weight of the data collection
system. But there's not an easy or simple/cheap way to get data from a
rotating assembly (that was not designed for it from the beginning).


--
Sooner dot boomer at gbronline dot com
 
In article <lprhb0ls86h78afo3l8ljiih2uefkvt866@4ax.com>,
John Fields <jfields@austininstruments.com> wrote:

On Sat, 29 May 2004 13:41:15 -0500, Jack B
jackspam@ssppaammbohnhoff.com> wrote:

I want to measure (indirectly) the force applied to an arm.

Think "bicycle crank arm"... force is applied at one end of the arm (as
with a pedal), and the arm pivots about a perpendicular axis at it's
other end.

Givens: I may not measure the force directly at the end of the arm;
environment is mild temperature-wise (humans can function ok), but lots
of dirt, dust, vibration; sensor must be small and non-intrusive;
measurements should be good, but need not be lab-quality.

Two issues:

- assuming some sort of strain/flex sensor firmly attached to the arm
(e.g. epoxy), what sort of sensor? Thin foil-type sensors seem to need
an "S-"bend to function properly; other commercial ones seem too big.
Would some sort of piezo device work?

---
Make your sensor capacitive, and part of an RF oscillator, and you'll
acquire your data and transmit it in one swell foop!
Click to expand...
Mmm. If I choose piezo, then it could be a/the C element, eh?... as in:

http://www.amp.com/products/technology/articles/interface.stm

....seems good: low cost, low part-count (?)

Else... as Dan Major has noted, foil strain gauges are usually put into
one leg of a Wheatstone bridge... and would require some additional RF
modulation-thing to transmit strain level.

And... additional client requirements:

there could be many of these units in close proximity; cross-talk is
forbidden; units will come and go in no predictable pattern.

--
Jack
 
On Sat, 29 May 2004 19:49:00 -0500, Jack B wrote:

In article <lprhb0ls86h78afo3l8ljiih2uefkvt866@4ax.com>,
John Fields <jfields@austininstruments.com> wrote:

On Sat, 29 May 2004 13:41:15 -0500, Jack B
jackspam@ssppaammbohnhoff.com> wrote:

I want to measure (indirectly) the force applied to an arm.

Think "bicycle crank arm"... force is applied at one end of the arm (as
with a pedal), and the arm pivots about a perpendicular axis at it's
other end.

Givens: I may not measure the force directly at the end of the arm;
environment is mild temperature-wise (humans can function ok), but lots
of dirt, dust, vibration; sensor must be small and non-intrusive;
measurements should be good, but need not be lab-quality.

Two issues:

- assuming some sort of strain/flex sensor firmly attached to the arm
(e.g. epoxy), what sort of sensor? Thin foil-type sensors seem to need
an "S-"bend to function properly; other commercial ones seem too big.
Would some sort of piezo device work?

---
Make your sensor capacitive, and part of an RF oscillator, and you'll
acquire your data and transmit it in one swell foop!

Mmm. If I choose piezo, then it could be a/the C element, eh?... as in:

http://www.amp.com/products/technology/articles/interface.stm

...seems good: low cost, low part-count (?)

Else... as Dan Major has noted, foil strain gauges are usually put into
one leg of a Wheatstone bridge... and would require some additional RF
modulation-thing to transmit strain level.

And... additional client requirements:

there could be many of these units in close proximity; cross-talk is
forbidden; units will come and go in no predictable pattern.
Click to expand...
Couple places make preapproved RF modules. FHSS - so more than one
remote can talk to the master.

www.aerocomm.com is one. I can't remember or find my notes on the
other, so take some key words from AeroComm site and FHSS or
frequency hopping (or hopped) and google it. The one I can't
remember sells the modules for around $90 ea in 100 qtys, IIRC.

Don't assume the data rates quoted are all yours - part of it is
overhead. Lots, actually.

Now these buggers might be a bit large. John's got a good idea, but
don't let that frequency sweep outside the allowed channel bandwidth
of whatever band it is that you're not licensed for, etc., blah,
blah, range, interference, band, ISM...

What I think would be best is one of those RFID systems like those
used for tire pressure monitoring. Use that to get the data off the
rotating machinery and somewhere where you have more room for
circuitry.

Strain gauges: Sensym, if they're still around, makes them. IIRC,
some of their pressure sensors had built in bringe circuitry so it
stands to reason that some of their strain gauges do also.
--
Best Regards,
Mike
 
On Sat, 29 May 2004 19:49:00 -0500, Jack B
<jackspam@ssppaammbohnhoff.com> wrote:

In article <lprhb0ls86h78afo3l8ljiih2uefkvt866@4ax.com>,
John Fields <jfields@austininstruments.com> wrote:


---
Make your sensor capacitive, and part of an RF oscillator, and you'll
acquire your data and transmit it in one swell foop!

Mmm. If I choose piezo, then it could be a/the C element, eh?... as in:

http://www.amp.com/products/technology/articles/interface.stm

...seems good: low cost, low part-count (?)

Else... as Dan Major has noted, foil strain gauges are usually put into
one leg of a Wheatstone bridge... and would require some additional RF
modulation-thing to transmit strain level.

And... additional client requirements:

there could be many of these units in close proximity; cross-talk is
forbidden; units will come and go in no predictable pattern.
Click to expand...
---
Piezo is _not_ a good idea, since you'll only get an output when
pressure changes. That is, for a steady-state change in pressure,
after an initial charge transfer, the output will drop to zero.

For a piezo film transducer, what you'd need to do would be to
mechanically deform the PVDF dielectric to get the capacitance change,
but when you did that you'd get the accompanying electrical output
from the transducer screwing things up until it settled down and all
you'd be seeing was the remaining capacitance.

What I had in mind was more like a temperature compensated capacitor
with an air dielectric separating two parallel plates which got closer
to/farther away from each other as pressure was applied to the
outboard end of the arm.

That way, if you had a fixed and a movable plate you could have a
simple screw adjustment which would move the fixed plate to adjust the
distance between them (and the frequency out of the oscillator) and as
the movable plate moved (with pressure), the frequency of the
oscillator would change. Having the screw adjustment would allow you
to have all the sensors operating in their own bands and thereby get
rid of the crosstalk problem.

But...

For something other than a one-off, the (radiating) oscillator scheme
starts to look less and less attractive as the number of sensors
increases, if only for the multiplicity of receivers or the spectrum
analyzer needed to capture the output frequency of all the
oscillators. Plus, just add all the in-situ calibrations needed for
the sensors, and it all starts to quickly turn into a can of worms.
So, for your situation, I'd think seriously about the strain gage
route, but it's hard to say without more data.

For instance:

Are you allowed to multiplex the data from the sensors?
How good does the data have to be?
How do you want to drive the sensors? (AC, DC, Mains, Batteries?)
Do you have to do sensor driving, data acquisition, signal processing
_and_ data transmission all at each of the sensors, or can you break
it up into separate physically disparate tasks?


--
John Fields
 
Jack B wrote:
I want to measure (indirectly) the force applied to an arm.

Think "bicycle crank arm"... force is applied at one end of the arm (as
with a pedal), and the arm pivots about a perpendicular axis at it's
other end.
Click to expand...

http://www.srm.de/

Of course, SRM has been doing this for over 10 years. Post in
rec.bicycles.racing if you get a working model cheaper than SRM -- SRM
is very expensive.

The PowerTap hub is less expensive, but still pricey:

http://www.cycle-ops.com/index.html
Go to products|powerTap

also
http://www.analyticcycling.com/CycleOps/PowerTapFeatures.html
http://www.yellowjersey.org/COPWRTAP.JPG


Givens: I may not measure the force directly at the end of the arm;
environment is mild temperature-wise (humans can function ok), but lots
of dirt, dust, vibration; sensor must be small and non-intrusive;
measurements should be good, but need not be lab-quality.

Two issues:

- assuming some sort of strain/flex sensor firmly attached to the arm
(e.g. epoxy), what sort of sensor? Thin foil-type sensors seem to need
an "S-"bend to function properly; other commercial ones seem too big.
Would some sort of piezo device work?

- how to transmit the data to an analysis device? RF?

--
Jack
Click to expand...
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top