You could use the average of all 4, but this assumes that the equipment is
level and the weight is evenly distro accross the platform, thus you will
have to do sme very strainge calculations to get the adverage wieght
accoring to a few nasty little vecotors and assumptions. Unless the
equipment is large, there is no reason why one load cell could not be used
(See more latter).

Putting all the cells in paralell is not a good idea as each will have its
own inductove pickup and the noise would be inserperable from the real signal.

This is not a simple task, think very carfully about what you are measuring
and the actual results that you ontain. As you only are looking for a
differnece then you could get away with one load cell. However the 4 cells
ensure that the device being measured moves by the same amount on all 4
mounting points when the load is changed etc (That should give you a hint
in what your looking for i.e. You are looking for the difference between
the static weight of any one (Or all) of the 4 load cells and then the
weight difference when the load is introduced etc.)


Have fun....


Dennis





At 12:14 16/08/99 +1200, you wrote:
>Hi,
>
>I'm working on a new project that will be using 4 load cells to
>measure the weight of a piece of machinery during operation.
>
>I've seen weighing platforms that use a load cell on each mounting
>foot - presumably they just average the four readings to get the
>total weight eg. (A+B+C+D)/4 ?
>
>Can I minimise the electronics by putting the outputs of the four
>strain gauge bridges in parallel (maybe with some series resistors)
>and then just use one bridge amplifier and one ADC input on my
>micro? (Rather than 4 x bridge amps, 4 x ADC's)
>
>Would this affect linearity much?  My dynamic range of weight is
>very small, I'm looking to measure 0-20kg change (product) on top
>of a constant 500kg or so (the machinery itself), and 0.1 or 0.2kg
>resolution would be fine enough.
>
>Also, what are your favourite bridge amplifier's for a job like this?
>
>Hope these are simple questions to answer for people with
>experience here, and interesting for the rest of us!
>
>Thanks in advance, Brent.
>
>Brent Brown
>Electronic Design Solutions
>16 English Street
>Hamilton, New Zealand
>Ph/fax: +64 7 849 0069
>Mobile: 025 334 069
>eMail:  brent.brown@clear.net.nz
>
>