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'Weight sensors'
2002\12\22@141202 by Augusto Yipmantin - OA4CVT

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Hi all,

Just again with little question,

Need to measure the weight of a perssonel transport bus, in that way may any
one can tell me where to find information about sensors to be attached to an
A/D pin of the PIC16F877 ?

Best Regards, and my best wishes to all in this holidays.

Augusto, OA4CVT

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2002\12\22@160924 by Spehro Pefhany

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At 02:11 PM 12/22/02 -0500, you wrote:
>Hi all,
>
>Just again with little question,
>
>Need to measure the weight of a perssonel transport bus, in that way may any
>one can tell me where to find information about sensors to be attached to an
>A/D pin of the PIC16F877 ?

In general you'd be looking at a load cell and some signal conditioning
to do this sort of thing. It's unlikely you'll find a sensor that can be
connected directly to the ADC. Load cells are mechanical arrangements to
spread the load out and strain gauges, which are resistor networks that
shift slightly as the metal under them stretches (the strain) under stress.
They are very low level (thermocouple level) and often people want to
read them to very high resolution and accuracy.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2002\12\22@171043 by Rich

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There are several manufacturers of load cells.  Here is one
http://www.entran.com/ltoc.htm
But there are others.  The real trick in application of load cells is the
error budget.  To minimize drift, you need a highly stable excitation
voltage.  In high precision you should apply a pulse and capture with a duty
cycle such that the error introduced by self heating is eliminated.
Rich
{Original Message removed}

2002\12\22@174440 by Spehro Pefhany

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At 05:15 PM 12/22/02 -0500, you wrote:
>There are several manufacturers of load cells.  Here is one
>http://www.entran.com/ltoc.htm
>But there are others.  The real trick in application of load cells is the
>error budget.  To minimize drift, you need a highly stable excitation
>voltage.

"Stable" is about right, it does not have to be very accurate- you can get
16 or 18 bit accuracy with a 5% accurate excitation voltage.

>  In high precision you should apply a pulse and capture with a duty
>cycle such that the error introduced by self heating is eliminated.

And consider low-frequency swapping of polarity to remove unwanted
thermoelectric potentials.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam.....interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2002\12\23@125920 by o-8859-1?Q?Tony_K=FCbek?=

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Hi,
Augusto wrote:
>Need to measure the weight of a perssonel transport bus, in that way
may any
>one can tell me where to find information about sensors to be attached
to an
>A/D pin of the PIC16F877 ?

Well as other have noted this is normally done with loadcells. There
are many manufacturers of these, an seach with google should turn up
some.

Anyway to make an 'scale' is not an easy task, the output from the
loacells
is normally ratiometric and labelled as mV/V. One of the most common
outputs
is 2mV/V, i.e. with an excitation of 10V one gets 20mV output signal at
full
rated load. As you see this output is quite 'small' and normally
requires
to be amplified by an PGA. Then there is issues with excitation
stability,
drift, temperature, and thermoelectric effects. I would not do this with the pic A/D as the resolution is normally
considered
to small. Instead I would suggest using an A/D with more precsition,
there
even are some integrated 'bridge trancducer ADC' on the market, search
Crystal or Analog Devices. These have the PGA and A/D integrated and
will
help you avoid some design issues.

Then you also have the issue with the scale platform itself
(mechanical),
weghing trucks normally requires some heavy duty construction. As there
are both performance and safety considerations.


/Tony

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