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'[PIC] and [OT] Measuring weight using TSL214 with'
2000\06\06@063735 by Rob Bakker

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Hi all
   I am using a PIC to measure the weight of a small animal feeder.
Currently the feeder is hanging from a spring and I'm trying to measure the
change in height as the feeder empties.  Feeder weighs no more than a couple
of kilograms and the amount of feed delivered is in the order of 100 grams -
so I need to detect approx 0-100 gram change.  the feeder rises approx 1 cm
from full to empty...

I am using a linear pixel array (TSL214,  64 photocells in a line ) mounted
next to the feeder and using that to determine the feeders position.  The
problem is, I'm not sure that the PIC is reading the whole array,  it seems
to be just reading the first pixel 64 times!  Has anybody used the TSL214
(linear pixel array) from TI?

Does anybody have any other ideas?  I'm also thinking along the lines of
making a load cell using foil strain gauges...  but the engineering required
may be too expensive/complex   ( I need to make about 20 of these! )


TIA

Rob Bakker

2000\06\06@205331 by Gennette, Bruce

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What's wrong with connecting a wire to the bottom spring mount point,
passing it up through the centre of the spring to operate a normally open,
spring loaded switch via a short rubber band?

No feed on platform - main spring pulls platform up and spring loaded switch
goes open.
Feed on platform (or feeder) - main spring stretches and switch is pulled
closed.  Rubber band in switch closer handles over extensions.

Simple.  Cheap.  Adaptable to multiple switches, rotary pots or gray code
encoders for half full, near empty, etc.

Put the top end of the switching wire and the switches in a box and the
whole lot is inaccessible (except for the small hole in the bottom of the
box where the switching wire enters).

Bye.

       {Original Message removed}

2000\06\06@213357 by Brent Brown

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Rob,

Haven't been watching this thread, but one way of measuring 0-
10mm distance you might consider is IR reflected intensity.

One LED, one photodide, a few discretes and maybe an OpAmp.
Pulse the LED on, AC couple and highpass the photodiode output
so as to reject 50/60Hz and sunlight, measure the intensity of the
received pulse with A/D input, turn the LED off. Intensity is
inversely proportional to the square of the distance. Use a lookup
table to work out the distance. Fairly simple, quick and straight
forward, low parts count, low average power consumption, no
moving parts, make the micro do the hard work.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile: 025 334 069
eMail:  spam_OUTbrent.brownTakeThisOuTspamclear.net.nz

2000\06\08@093346 by paulb

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Rob Bakker wrote:

> I am using a PIC to measure the weight of a small animal feeder.
> Currently the feeder is hanging from a spring and I'm trying to
> measure the change in height as the feeder empties.  Feeder weighs no
> more than a couple of kilograms and the amount of feed delivered is in
> the order of 100 grams - so I need to detect approx 0-100 gram change.
> The feeder rises approx 1 cm from full to empty...

 It might just be interesting to consider the extremely elegant, being
both mechanically simple and robust, mechanism used as I understand in
most precision balances.  One advantage is that unlike the mechanism you
suggest, there is extremely little movement of the load.

 You use a single optical chopper.  The load moves this, but you
constrain its range with stops to perhaps a millimetre or two.  A
solenoid pulls against the load (or may indeed push).  The solenoid is
powered by a linear driver with current feedback.

 In general, the solenoid is feedback controlled to bring the chopper
exactly to its midpoint (ergo, it must have at least two threshold
levels).  At that equilibrium, the solenoid current is directly
proportional to the load.  The solenoid curent can either be measured by
an ADC, or simply determined by a DAC or PWM (In fact, if you use only
digital feedback, the latter is necessary to perform the former anyway).

 The minimum system is a sensor and a solenoid driven using PWM
straight from a digital driver.  You will need a PID algorithm to do
this, but the mechanical aspects certainly should be simple and robust.

 No, I haven't done it.
--
 Cheers,
       Paul B.

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