Low cost weight sensor ?

Yes, there are piezoelectric load cells. They may be difficult to use because they require very high impedance analog circuitry to read the output. ("Charge Amplifiers") See:

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May be... How stable will that be?

Reply to
Roberto Waltman
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Anton, I just read again your original post: "I am trying to provide a indication of the amount of paper available. The empty container is approx. 1kg, and approx. 10kg full of paper."

(Stepping out from the box and trying to think.) You choose weight as the parameter to be measured. What other options are there? It would be very easy to measure the height of the stack of paper. (Measure the angle of a hinged rod lying on top of the stack, ultrasound sensors, etc.) Light transmitted through the stack will probably not work with 10kg of paper. Anybody has any suggestion other than weight?

Reply to
Roberto Waltman

Anton,

As Peter points out, a piezo sensor is a "change" sensor: it produces an output when its state changes. If you want to use this aproach to measure something static (e.g. paper stack size) you have to find or create a changing characteristic (e.g. movement in response to vibration).

If this were a mechanically-created stack one could use a simple photointerruptor to count the sheets coming in and those going out. You'd still want some way to "zero out" the up/down counter for those occasions when the count stopped reflecting reality, though.

If you're _not_ going to count sheets, that leaves thickness and weight (and three other characteristics I haven't thought of or forgot ). The first two vary across types of paper (20lb?

24lb? cheap copier I-don-t-know-how-heavy?), which makes an exact measurement tricky.

Thickness: Someone previously mentioned capacitance varying according to the stack height. Several photocell approaches (or even a simple switch) can detect "paper present" vs. "no paper present".

If you can handle the mechanics (and the situation allows), you could add a "paddle" that lays down on top of the stack and use that to mechanically translate height to light intensity, resistance, or encoder ticks. (Paddle needs to be out of the way when adding more paper, of course.)

Thickness also translates to light intensity, but only up to a limit, and it'll vary _strongly_ with paper color. I'd expect resistance and moisture content to vary even more widely.

Weight: Not sure I can add much here to previous postings. Convert it to air or fluid pressure and measure that? (but remember that waterbedds leak from time to time. )

Again, good luck.

Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)

-- "Very few things happen at the right time, and the rest do not happen at all. The conscientious historian will correct these defects." -- Herodotus

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Reply to
Frnak McKenney

Get a normal mechanical weighing machine, with the big dial, and pointer and put a 10K put on the shaft of the needle. ADC, Done!

martin

Reply to
martin griffith

We have used a switch that indicates when the paper is less than a certain amount. Both mechanical and optical. The system is a ticket dispensing machine - typically used in parking lots. The people employed to operate it, is minimum wage employees (Which is about US$0.70 per hour in South Africa). The systems are not well maintained, the environment is extremely dirty and dusty, and the enclosures in which the machines and paper are housed are never cleaned. I am trying to think of a way to make a cheap robust sensor which would work reliably for the 10 year life expected of the equipment. Being able to measure more than just a specific quantity of paper left would be a bonus. Counting as suggested by someone else would not work since the paper is periodically added, without any way to know exactely how much had been added. The paper is also fairly low quality that generates lots of paper dust, that loves to cling to things like photo diodes and transistors.

Keep the ideas coming. Even if I cannot use it, I am sure someone someday would get something they could use out of the archives.

Regards Anton Erasmus

Reply to
Anton Erasmus

--snip--

Anton,

Thanks for the detail; it helps a lot. A couple of questions more:

1) Are these tickets something whose construction/printing are under your control, and if so, to what extent?

Barcodes, at least the traditional type, wouldn't help give you quantity, but there may be some options based on the tickets themselves.

2) How precise does the measurement have to be, and what will be done with the information? That is, if all you care about is being able to turn on a LED indicating that it's time to refill the dispenser, what is it that makes you want more detailed information on the remaining ticket count? 3) How is the level monitored now? (This overlaps a bit with (2)). 4) The tickets _are_ stacked, and not dispensed from a reel or spool, yes? (Sorry, but I had to ask ).

If, for example, the attendant periodically walks around the lot and looks at each dispenser, you could use a simple mechanical "flag" attached to a lever spring-pressed against the top of the stack to give a "gauge" of how full the dispenser was. The flag would bevisible through a transparent window and would show "50% full", "25% full", "near empty", and "empty" or even more detail if desired.

Any better?

Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)

-- "We don't just borrow words; on occasion, English has pursued other languages down alleyways to beat them unconscious and riffle their pockets for new vocabulary." -- James D. Nicoll

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Reply to
Frnak McKenney

maybe you can find idea in this application note:

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r.

Reply to
raivo leini

Hello Anton,

Looks like you may need a few pressure sensors. Example:

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

--snip--snip--

(Hmph. Following up on my own post. I guess it's okay, as long as I don't start answering... )

Anton,

It's amazing what just a few hours' sleep can do for one's thinking. Scratch all that "sophisticated" stuff.

Why not just put a transparent (plastic, glass) window in the side of the ticket dispenser so you can _see_ if any tickets are left? No added mechanics, no electronics, and, best of all, it would make my high school math teacher proud :

"Answer by inspection!"

Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)

-- Before drawing conclusions from _any_ poll, it it absolutely vital to read the questions with greast care -- and that goes double for summaries released to the press, and triple for headlines based on the handouts. This is especially true when the poll was commis- sioned by a campaigning organization and even more so when the results were not what the sponsor hoped for, -- Pete Shanks / Human Genetic Engineering

Reply to
Frnak McKenney

"Anton Erasmus" schreef in bericht news: snipped-for-privacy@4ax.com...

Ultrasonic distance sensor?

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Thanks, Frank.
(remove 'q' and '.invalid' when replying by email)
Reply to
Frank Bemelman

There are optical distance measurement components that you can mount above the stack of paper to see how thick it is. (Not anything fancy like LIDAR, just an IR beam coming in at an angle and a position sensitive detector to look at the parallax where the beam hits.)

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David M. Palmer  dmpalmer@email.com (formerly @clark.net, @ematic.com)
Reply to
David M. Palmer

We have the tickets pre-printed on one side, adding a black mark for detection, and typical legal mumbo jumbo wanted by the customer. The size, etc of the tickets are fixed, since it depends on factors such as cost, typical size wanted, printing mechanisms used, acceptor used etc. The amount of paper in the stack should be measurable within the current paper definition.

We print a dynamically generated barcode on each ticket. The dispenser does not have a barcode reader.

If I can have 5 or 10 different levels it would be ideal. In smaller sites an attendant walks around and fills up as required. On the bigger sites, the units are networked, and there is a central office, where one can supply information. The idea is to be able to indicate when units have used enough paper to be able to take a typical extra stack as normally supplied, and when the paper situation is becoming critical. Some gates are used more than others, so a "critical" paper situation for one can mean only 5000 tickets left, while for another "critical" is only 100 tickets left.

No monitoring is currently done. We have done some prototypes with field trials in the past using optical sensors, but they proved to be unreliable in the long run . On the smaller sites having an attend check it periodically is good enough. On the larger sites running out of paper when it is busy is unacceptable, and because of paying minimum wages a "supervisor" needs to know when it is running out to send someone to add paper. Systems that worked

100% in the past when they did no pay minimum wages, are now unreliable and the landlords are prepared to pay for a technological solution, in stead of paying more and getting better employees.

Yes it is a fan-fold stack which feeds from the top. One adds paper to the bottom.

As described earlier, this sort of solution worked fine in the past. Now a "supervisor" must play policemen to make sure things are done. The "supervisor" does not want to walk around the lot, hence wants an indicator in his office. This situation has arisen since the parking lot operator companies became public companies. Short term profit is now the only thing that counts.

The weight sensing will work. Getting something cheap reliable which is good enough is going to a bit more complicated than I thought it would be.

Regards Anton Erasmus

Reply to
Anton Erasmus

I was thinking of using an electronic version of this. Using a piezo sheet or other sensing material in stead of the transparent window. As long as one can get some sort of "position" indication when the paper is fed, one should be able to sense how much paper is available. The type of material used for robotic hands should work. I am not sure what sort of price one would expect to pay for these sorts of materials.

Regards Anton Erasmus

Reply to
Anton Erasmus

This might work if I can get a module at low enough cost. The one your URL points to, is too expensive. I think there is a chance of getting quite a sophisticated ultra-sonic or light distance measuring modules at low cost.

Regards Anton Erasmus

Reply to
Anton Erasmus

"Anton Erasmus" schreef in bericht news: snipped-for-privacy@4ax.com...

"We have used this sort of setup, but for the current product I would want something in the US$20 range."

They offer a 5-pack for $100.

Since you mentioned the dust problems, anything using light seems out of the question. Depending on the mechanics, you could also use a lever and hall sensor to get your analog value of the bin content.

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Thanks, Frank.
(remove 'q' and '.invalid' when replying by email)
Reply to
Frank Bemelman

How about the low-cost MEMS "Air pressure" sensors, attached to a small plastic bag with a plastic hose. A rectangular plastic bag the size of the tickets would only need to have about 1/4 inch of air in it and would only deflect a small amount as the load varied from 1 to 10 Kg. Cheap parts... Probably linear within 5 percent.

Regards, Terry King ...On The Mediterranean in Carthage (Back ...In The Woods In Vermont for the Summer)

Reply to
terry

How about these:

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Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)
Reply to
Stef

All sorts of clever ideas... but I'm still curious about one of the ideas the original poster mentioned: A hinged rod resting on top of the stack (presumably attached to a pot or some discrete switches or something like that). What didn't work with that setup?

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Charles Allen
Reply to
Charles Allen

It wasn't me that mentioned the hinged rod. In my case this would not work since although I am using a stack of paper, it is a fan-fold stack. The stack is placed below the dispenser, and the paper feeds into the bottom. The slope of the paper varies approximately

+- 40 degrees from the vertical.

Regards Anton Erasmus

Reply to
Anton Erasmus

This is a followup to a later posting (also by Anton), the original for which seems to have been lost by news.east.earthlink.net during its recent multi-week spate of news server hiccups. Sorry about the incorrect threading...

Thought I'd give this problem one last shot. Based on the above, here's what I envision:

Dispenser ===================+ -----------------\ | Ticket +======= \ ========+ being | \ @ | dispensed | \ | | \ @ | | \ | | / @ |

Reply to
Frnak McKenney

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