Lawrence,

 What about some sort of strain guage setup?

                            Regards,

                              Jim





On Mon, 30 April 2001, Lawrence Lile wrote:

>
> I'm thinking about ways to measure weight, on the cheap.  The idea is to
> have a rack in a cooking oven, with one rail of the rack support supported
> on a spring, to measure the weight of the object being cooked ( never mind
> why we are interested in that... long story)
>
> So the problem of measuring weight comes down to measuring the position of
> this rack, as it rides up or down on it's spring.
>
> Now we can't tilt too much, or we'll dump Mrs. Smith's pie all over the
> inside of the oven.  1/4"[6mm] or 3/8" [9mm] would be about the maximum we
> could tolerate, for maybe a 5 or 6 LB  [2.5 kg] food item.  I'd like to be
> able to measure to 1/4 Lb accuracy
>
> So we are trying to measure how far this oven rack drops after the item is
> placed on it.  Here's what I've thought about so far:
>
> 1.  Actuate a lever, which cranks a small potentiometer.  Probs:  Pots
> travel round in a circle, you might not get more than 45 degrees of rotation
> out of this scheme, limiting your abitlity to discriminate small weights.
> Small pots tend to have a lot of hysteresis, and this would show up as error
> if you are not moving the pot many degrees.  Any slop and tolerance in your
> original mechanism gets multiplied by the lever arm ration (assuming
> mechanical disadvantage) and results in more errors and hysteresis at your
> output.
>
> 2.  Actuate a lever, which cranks a liner pot.  problem: Linear pots are
> kinda expensive.   Cheap ones tend to skip and have dead spots.   (i.e.
> 1,2,3,4,5,0,0,0,0,7,8,9,10 like that)
>
> 3.  Ala' digital micrometers:  print a fine gauge grid of lines on an
> otherwise clear piece of plastic.  Place it between an optical switch
> (led/phototransistor) shielded through a small slit.  Count the number of
> light/dark variations as it moves.  Problems:  Where is zero?  Place your
> item in the oven, then plug it in - how does this sensor know where zero
> was?  Did Rube Goldberg think of this method?
>
> 4.  Actuate a low-pitch screw thread which rotates a potentiometer, say the
> whole thread turns 270 degrees for 3/8" travel, thus driving the pot through
> it's range.    Hmmm.  Gut feeling says this would hang up somehow.   This
> might work OK for longer travel - 5-6 inches.
>
> 5.  Run a springy shorting bar across a series of fine-pitch silver plated
> tracks on a printed circuit board, and measure the position of the shorting
> bar by various methods.  (port input, shorting out a series of resistors,
> keypad-like array, etc. etc. )   So far I can't think of too many problems
> wioth this, except I suspect it might be expensive to implement and the
> tracks might wear out.
>
> Any other ideas?
>
> -- Lawrence Lile
>
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jim@jpes.com

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