force sensor...

[Andrew W. Macfadyen]

The S shape is a very good style for a load cell due to the stress concentration factors, in
the UK the supplier RS Components were selling small ones I haven't got the latest catalogue
so can't check if they still do or have expanded the range.

Building load cells isn't too complex  I designed some about 7 years back to measure torque
and axial  force based on a simple X form shape  -- NC machined from solid billet it looked
a bit like  the clasic siceience fiction movie space station in minature, an inner stub
shaft connected on the outer ring by 4 members with strain gauges on the side and top and
bottom surfaces of one of the X members.   We used a standard aluminium alloy for the the
cell and chose  ordinary  wire strain gauges. The tested  accuarcy and repeatability once
calibrated was very very good   although it took a couple of cylces through the full loading
range to "scrag" it in.
The only worry I had was it might suffer fatigue failure from cyclic loading  although we
did allow for this by using generous fillet radi.

Will McGonegal wrote:

> I design and build dynamometers at our emissions lab and we use load cells on most of
> our dynamometers (engine and chassis).  We have been using the 'S' shape style most of
> the time.  These load cells have a strain gage bridge on them to measure the force
> applied through them at the end of a torque arm attached to the pivoting power
> absorption unit (water brake in your case).  These units require a constant excitation
> voltage (typically 10 volts) and the give a signal of about -30 to + 30 mV full scale
> proportional to force.  We had a water brake dynamometer come in a few years ago with a
> spring loaded wire-wound potentiometer (very large spring, dampened in oil) to measure
> the force.  It was quickly replaced with a load cell.  These are simple and easy to use,
> and very repeatable.
>
> We have purchased from a few makers including:
> Interface: 602 948-5555 Super Mini load cells (e.g.: SSM-AJ-1000)
> HBM (Hottinger Baldwin Messtechnik): 508 624-4500
> Omegadyne (http://www.omegadyne.com/) - LC101, LC105
>
> Prices start around $300 US, maybe not so good for the home DIYer.
>
> Will McGonegal
> Project Engineer/Dynamometer Developer
> Emissions Research and Measurement Division
> Environment Canada
>
> > From: "Daniel R. Henriksson" <rotax at ludd.luth.se>
> > Date: Tue, 21 Jul 1998 16:28:12 +0200 (MET DST)
> > Subject: Re: force sensor...
> >
> > I recently acuired a water-brake dynamometer.
> > I'm now looking for some force sensing device,