Author Topic: Any load cell experts on the forum?  (Read 4240 times)

Offline awemawson

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Any load cell experts on the forum?
« on: April 23, 2015, 04:53:00 PM »
I'm puzzled  :scratch:

In my chuck gripping force gauge there resides a load cell in a cylindrical format. It's connections are a conventional bridge with two power connections and a pair of differential outputs, so nothing fancy there.

What puzzles me is how you place the necessary strain gauges on something suitably shaped to deform in a controlled manner that it gives readings when gripped in either a three jaw or two jaw chuck, so has either three or two point contact with the outer surface of the cylinder.

The load cell incorporated in the gripping force meter is externally just a plain cylinder giving no clue as to it's internal construction.

Pictures of the device in this thread :

http://madmodder.net/index.php/topic,10608.0.html

Andrew Mawson
East Sussex

Offline sparky961

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Re: Any load cell experts on the forum?
« Reply #1 on: April 23, 2015, 05:30:29 PM »
You can't just use your finger to see how hard it's gripping?  :poke:

(Sorry, couldn't resist)

Offline DMIOM

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Re: Any load cell experts on the forum?
« Reply #2 on: April 23, 2015, 07:06:14 PM »
Andrew - it isn't effectively just acting as a pressure gauge is it?

Dave

Offline awemawson

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Re: Any load cell experts on the forum?
« Reply #3 on: April 24, 2015, 01:56:06 AM »
Dave that's possible I suppose. It would have to be a relatively thin walled tube with a diaphragm on which the strain gauge sits, all filled with a fluid, but there is absolutely no sign of a means of filling.

I did wonder if it is a series of concentric flextures each with a strain gauge and the strain gauges wired in series but no amount of googling for radial load cells comes up with anything similar
Andrew Mawson
East Sussex

Offline DMIOM

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Re: Any load cell experts on the forum?
« Reply #4 on: April 24, 2015, 03:34:54 AM »
The outer cylindrical skin must be cable of accepting/transmitting the deformation, and if there are no specific 'target' zones then, if not pressure based, then I could envisage a couple of other concepts: One method might be to have a length of spring steel sheet, like those used for lead-screw guards, one end anchored to the skin and the other, monitored, end to an arbor - as the cylinder is squeezed the spiral will try to wind up?  Alternatively, I wonder what results you would get if you wound a lap of strain gauge wire on a compressible cylinder, just like a single-layer-winding solenoid; you would then face the challenge of slipping that inside a compressible outer sleeve and filling the annular void with a medium that ensured the pressure was transmitted to the inner cylinder & coil....  Or maybe there's no coil -might it be an air-spaced cylindrical capacitor - one plate = fixed inner arbor, other plate = deformable skin?

However, I've just been back to look at the pictures in the original thread - and there are specific two-jaw and three-jaw adapters with pads for the jaws to bear on. The adapters' purpose could be to avoid marking the head but they look to have sprung fingers with "pips" to engage in specific dimples on the measuring head; hence pressure will always be at the same specific points on the circumference - so maybe a set of six strain gauges and appropriate adapters could accept 2/3/4/6 jaw chucks?

So, maybe it isn't a generic "any point on the circumference" cylindrical strain gauge at all!

Dave

Offline PekkaNF

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Re: Any load cell experts on the forum?
« Reply #5 on: April 24, 2015, 03:51:15 AM »
So it connection behaves like standard four wire and four strain pads in bridge?

But there are those weird spider adapters and pins that go on spesific places as well? Looks like there are small holes to locate the pins and concentrate the force on certain area? Or did I got something wrong?

If it is really universal direction /concentric I would think of a structure that flexes axially when loaded and this axial direction would be then returned to standard one way force measurement.

Pekka

Offline awemawson

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Re: Any load cell experts on the forum?
« Reply #6 on: April 24, 2015, 05:24:45 AM »
The dimples in the load cell are to locate the 2 or 3 jaw adaptors, but the jaw force isn't applied to the pips that go in the dimples, but rather onto the 2 (or 3) pads between them. The destructions refer to the adaptors as 'protectors'

When I measured the arms of the bridge when I was fixing it they read about 75 ohms each iirc so not capacitive.
Andrew Mawson
East Sussex

Offline DMIOM

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Re: Any load cell experts on the forum?
« Reply #7 on: April 24, 2015, 05:56:29 AM »
Sorry if I wasn't too clear, with the "adapters" I wasn't implying pressure on the locating pips, but what I was trying to convey was that, courtesy of those pips/dimples, the jaw-receiving-pads will always fall in a finite number of locations (6?) around the periphery, so if there were discrete sensors those could be their sites...

Dave

Offline PekkaNF

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Re: Any load cell experts on the forum?
« Reply #8 on: April 24, 2015, 07:49:20 AM »
Dave here might be into something.

I'm familiar only commercial four wire / full bridge type (and some inductive ones that strain changes permeability of the iron) but there seems ton be "rosettes" I.E. multiple strain gages on one strip. I'm having difficulties visualising shape of the cavities and beam to measure this.

This type I'm familiar with:
http://www.weighing-systems.com/TechnologyCentre/BalancesZVZ/rekstroo.gif
or
http://www.mensor.pl/indukcyjnosciowa_metoda_pomiaru_masy/new_inductive_method_for_measure_of_mass_clip_image030.jpg

I even know that torque of a shaft is easy to measure using straingages in diagonal...but static force measurement I know allways have to have "spring" stiff in one direction to minimise unwanted signal and flexible enough on measurement direction.

I really can't figure out any other "universal" direction measurement than displacement, pressure sensor could have a straingages and "fluid" could be more like teflon plug or high viscose liquid more like dried out silicon....but then don't know the use of these mysterious "pads" other than protecting the surface of the transducer.

fascinating.

Pekka

Offline awemawson

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Re: Any load cell experts on the forum?
« Reply #9 on: April 24, 2015, 07:58:35 AM »
It's one of those puzzles that realistically doesn't need solving, but to not solve it leaves irritating loose ends  :bang:
Andrew Mawson
East Sussex