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Author Topic: Finding a Sensor for Measuring Weight/Mass  (Read 6514 times)
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Hello, oh vast interwebs!
I'm having difficulty finding a new (to me!) type of sensor for my next project.  I am looking for a sensor that can measure small weights.  My range of mass must be as low as 100mg, must be up to at least 2000mg, and optionally up to about 5000mg.  Required precision would be 5mg or better. 
I'm not sure if I'm describing the specs right-- feel free to tell me I described it stupidly, if I did.

The sensor will be used in an automated dispenser as part of a small, non-commercial production line.  Unfortunately, the precision demanded can only be achieved by measuring weight, not volume.  And no, I'm not a drug dealer.   smiley

My questions for all you wonderful makers in the world: what's the name of a sensor that measures weight?  Where can I find one that would be in the micro-weights I'm needing?  And has anyone used this kind of sensor yet with an arduino or raspi?

Thanks!
-A
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My suggestion would be to buy some of the scales which operate in your range
and sell for about $10 on ebay,   and try to hack into their circuit somehow.

Otherwise, it doesn't seem like an easy problem.  There are force sensors that
exist,  I don't know how sensitive they are,  and you would have to rig some
kind of platform on top of them to hold the object being weighed, and then
calibrate it somehow.   Doesn't sound very easy.
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My suggestion would be to buy some of the scales which operate in your range
and sell for about $10 on ebay,   and try to hack into their circuit somehow.

Otherwise, it doesn't seem like an easy problem.  There are force sensors that
exist,  I don't know how sensitive they are,  and you would have to rig some
kind of platform on top of them to hold the object being weighed, and then
calibrate it somehow.   Doesn't sound very easy.

That's great that it sounds like a difficult project!  The really easy ones tend to be less satisfying smiley smiley.  The other components of the project will be a geared-down DC motor to rotate a dispensing tube, and there will be an automatic dump when the correct weight is achieved.

I guess buying & hacking a $10 scale could help me learn how those interface.  However, their precision in that price range is a little lacking.

Does anyone know what the sensor component is called?  And has anyone used one before?
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Google "Load Cell" and then Google "Instrumentation Amplifier" this may help.

wade
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A good quality kitchen scale is in that weight and accuracy range.

Building one yourself is hard, and hacking an existing scale is not always possible.
Some digital weigth scales have a communication port. I don't know how expensive they are.

I have collected a few links for you:

Hacking a scale:
http://www.open-electronics.org/wi-fi-body-scale-with-arduino/
http://www.avdweb.nl/arduino/hardware-interfacing/scale-interface.html
http://sciencestuff.xperiment.mobi/2012/04/19/measuring-weight-via-usb-with-electronic-kitchen-scales-and-the-arduino/
http://hackaday.com/2010/01/31/hacking-a-digital-bathroom-scale/

About weight scales on this forum:
http://arduino.cc/forum/index.php/topic,128031.0.html
http://arduino.cc/forum/index.php/topic,126257.0.html
http://arduino.cc/forum/index.php/topic,80223.0.html

Load sensor:
https://www.sparkfun.com/products/10245

Let us know how you progress. It's an interesting subject.
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http://cerulean.dk/words/?page_id=42

Getting the gain right is a bit tricky, so make sure you have ~100ohm variable resistors on hand, preferable a multiturn type.

Once the inamp amplifies the output of the load cell you still have to convert its analog output to a digital value. If you use the Arduino's ADC then you only get 10 bits of precision (actually 8bits of accuracy). You'll need an external ADC with "more bits" if you need more precision.

If you need a load cell you can snatch one from a small digital scale. Buy these on eBay for less than $10. I can pretty much guarantee that you won't be able to hack these though; all the electronics are on a custom chip and the ADC won't be accessible.
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LOAD CELL!!  That's what I was trying to find!  Thanks, Wade!  Micro load cells are what I need.  The ranges I'm working in are well below that of most scales available-- a few orders of magnitude smaller than kitchen scales.

I think I found my load cell:
http://www.phidgets.com/products.php?category=3&product_id=3132_0

And I found my 24-bit ADC:
ADS1210P
http://www.digikey.com/product-detail/en/ADS1210P/ADS1210P-ND/254605

Chagrin, thanks for the notes from experience!  If I could find an affordable load cell in the 0-10g range, 8bit would be OK, but even so I hadn't thought about that yet. If a 24bit doesn't do it, then I'll move on to another project-- ha. 

Thanks for all the input, folks.  It's time to wait for parts to arrive.  smiley
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However, their precision in that price range is a little lacking.

Not sure why you think that.  Think you can do better ?
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However, their precision in that price range is a little lacking.

Not sure why you think that.  Think you can do better ?

I think that the precision on a $10 scale is a little lacking because I require 0.005 gram precision and the $10 scales I found provide 0.1g or 0.05g precision.  The precision offered is insufficient for my needs.  I also need high repeatability, and the cheap scales either had poor or no specs.  Is the lack of precision due to the structure of the load cell or a limitation of the controlling ICs-- I don't know.  But since I found a $7 load cell as a component with known specs and that meets my requirements, I think that's a better option than going with an unknown salvage.

Do I think I can make a more precise load cell for that price?  Nope.  I don't even care to try.
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Measuring weight isn't that difficult: a strain gauge for example.

Measuring mass is very challenging, electronically.
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But since I found a $7 load cell as a component with known specs and that meets my requirements, I think that's a better option than going with an unknown salvage.

Yeah but that load cell isn't capable of .005g accuracy. It states that its repeatability is rated at .390g max.
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The other components of the project will be a geared-down DC motor to rotate a dispensing tube, and there will be an automatic dump when the correct weight is achieved.

Go to a gun store and buy a powder trickler.
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because I require 0.005 gram precision

yeah, ok, so the load cell you linked to the website for, above,  has a precision and repeatability of 390 milligrams  and you want 5 milligrams precision.

That 390 milligrams just happens to be half of 1 percent of its full-scale range of 780 grams.   Thats about in the ballpark of the precision
you are going to get from most kinds of sensors.   If you are requiring 5 milligram precision from a sensor with a range of 0 to 780000 milligrams,
that's actually quite a big ask.

I'm not sure that you actually need a 780 gram sensor if you only want to weigh objects less than 5 grams.
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Measuring weight isn't that difficult: a strain gauge for example.

Measuring mass is very challenging, electronically.

That's very true,  particularly if you want your device to work on other planets !  Any device which relies on the measuring the deflection of a spring or any other part of the mechanism due to the gravitational attraction of what you are trying to "weigh",  won't work.

The only sort of scales, which will work on another planet,  are the ones where you have to balance the two arms of the scale by comparing the object to be weight,  with one or more known, calibrated, masses.
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and the $10 scales I found provide 0.1g or 0.05g precision.

You didn't look very hard.   Go to eBay and type "digital scales 0.01"  or  "digital scales 0.001"  in the search box.
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