Re: checking weight on a moving belt

In article <1139515661.503254.266100@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
"PD" <TheDraperFamily@xxxxxxxxx> wrote:

Marcel wrote:
Hoping this is the correct group to post this question...

I have an ongoing discussion with a friend in regard to an industrial
check weigh system. it goes like this:

A product is moved at a certain speed on a conveyor belt over a scale
which registers the product's weight.

The system has to be set up with the following main settings:

-minimum weight
-maximum weight
-product length
-speed of the belt

my understanding is, that the speed of the product (read: the speed of
the belt over the scale) effects the registration of it's actual
weight based on the gravitational pull(???)

I'm struggling to understand any *physical* reason why this would be
so.

I would suggest that the scale consists of a single roller mounted in a
bearing assembly connected to a load cell (transducer). If the item
travelling along the conveyor is longer than the distance between
adjacent rollers, only part of its weight will be exerted on the
measuring roller. It's then necessary to integrate the reading over
time to derive the true weight.

In order to get an instantaneous total weight, the scale's pan has to
be at least as long as the item being weighed, so the pan has to be
equipped with its own rollers to keep the item moving. Furthermore,
those rollers have to be powered; you can't keep the item moving by
pushing it with the next item, because that will also affect the total
weight.

And you have to ensure that only one item can be on the pan at one
time, so you don't want the pan any longer than the item, and you have
to time the readings so that you don't read two partial units (one
leaving the pan, and another arriving). This makes the process
complicated, error-prone, and the whole setup would have to be reworked
for each different item type.

It is certainly true that if there is any change in vertical
*direction* of the belt as it passes over the the scale, then there
will be an effect on the weight reading. This is similar to the feeling
you get at the top or bottom of a Ferris wheel, and there the faster
the motion of the Ferris wheel the larger the effect.

But if it's a straight path over the scale, I would think the only
thing that would matter would be the response (settling) time of the
scale. Typically scales are damped oscillators with an exponential
envelope. The exponential envelope has a time constant associated with
it. If you don't wait for several time constants before taking a
reading, you can count on a measurable undershoot of the reading. The
good news there is if you know the time constant and you know the
actual time spent before recording a reading, you can correct it to
find out what the final value would be even without waiting. Is that
what you're looking for?


therefore the system requires to calculate a certain formula including

-the size of the product
-the speed of the belt over the scale
-the actual weight of the product

I am wondering if the size (length) of the product or the speed of the
belt influences the actual weight the scale registers (therefore the
requirement to apply a certain formula to calculate all of the above
factors)

I hope I explained the scenario well enough...

could anybody clarify this for me - your help is very much
appreciated!!!

marcel

If my comments are valid, you need to get enough readings (and PD's
comments re response times hold) to work out a total. I don't see that
the speed or length affect the readings directly, but the combination
of speed, length and response time will determine how many good
readings you can get as the item passes by. The amount of vibration in
the line will also be an issue.

My 2c; hth.
.

Relevant Pages

  • Re: checking weight on a moving belt
    ... A product is moved at a certain speed on a conveyor belt over a scale ... which registers the product's weight. ... scale for less time than the time constant, ...
    (sci.physics)
  • Re: checking weight on a moving belt
    ... A product is moved at a certain speed on a conveyor belt over a scale ... which registers the product's weight. ... good news there is if you know the time constant and you know the ...
    (sci.physics)
  • Re: checking weight on a moving belt
    ... check weigh system. ... A product is moved at a certain speed on a conveyor belt over a scale ... Then the weight would be directly transmitted to the scale and sliding friction is less of a problem. ...
    (sci.physics)
  • checking weight on a moving belt
    ... A product is moved at a certain speed on a conveyor belt over a scale ... which registers the product's weight. ...
    (sci.physics)
  • Re: Has Electric wire gotten smaller for the same guager?
    ... old and weighed it on my postal scale. ... As the insulation has gotten better and more heat resistant has the wire ... tap the scale with the weight on it, to see if the reading changes. ...
    (alt.home.repair)