Measure Small Weight?

Anybody aware of some very inexpensive sensors or other devices that can be used to measure weights of about 8 ounces to a 0.01 ounce accuracy? I need the gizmo to be robust, reliable, linear and monotonic.

I have looked at piezoelectric sensors but the ones from Honeywell and SensorOne are way too pricey. The one's that aren't really expensive seem to lack the accuracy or repeatability I want. Any idea on the cost of the devices from Cooper Instruments? Are there other manufacturers (Yahoo isn't much help here...)

I have considered magnetics, beam balances and so on but haven't come up with a strategy that I am sure will work. Any ideas?

Brad

Reply to
Brad
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One hundredth of an ounce in 8 ounces is one part in 800 or 0.125% - say ten bit accuracy.

This sort of stuff is never particularly cheap.

Farnell lists a 3kgm (6.6 pound) load cell for 194.94 euro (about $260) which is probably accurate enough to do your job.

What you probably want is an open-pan laboratory scale with a 0 to

250gram range

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but they cost even more, and are more accurate than you need.

To get the precision you want, you'd probably need to balance the weight of the object by the repulsion between two current-carrying coils, where the moving coil is tightly contrained to move only up and down in relation to the fixed coil so that it neither tilts nor slides sideways - I'd think in terms of berylium copper leaf springs, slightly offset, so that vertical movement would be accomated by a minimal rotation of the moving coil.

If you can keep the area free of ferromagnetic materials (their permeability is temperature dependent) and monitor the current through the coils (and not the voltage drop across the coils, because the resistance of the coils is also temperature dependent) you could get your accuracy easily enough. I'd use a capacitance sensor to monitor the height of the moving coil - traditionally one used to use an optical lever, but capacitance is much nicer.

------------- Bill Sloman, Nijmegen

Reply to
bill.sloman

No, It is not for that type of activity. Why would you make such an assumption about my motives? There are easier ways to obtain a scale to measure that sort of stuff, I'm sure, than making a post to a news group. Thank you very much!

I am interested in BUILDING a scale, not buying it. I have a perfectly legitimate use in mind, but my primary interest, at this point, is purely pedantic. I'm fascinated with technology and how things like this are done.

The best idea I have come up with so far uses an optical sensor or two on an automated beam balance. I can easily achieve the accuracy I am looking for, but an automated beam balance is so pedestrian......

I am looking into the magnetic approaches, but as an earlier responder pointed out, alignment is critical and slop is unacceptable.

I am looking for a linear approach (or nearly linear) to minimize the number of calibration points required. But, the earlier poster is correct, if I use enough calibration points and I can assume piecewise linearity is sufficient, then its not a rigid requirement.

Regards,

Reply to
Brad

Mmmm... Interested in getting into the drug dealing business I see. Drug dealers need scales that measure to around 0.01 "oh-zee" accuracy with a maximum weight of around 8 oh-zees. Strangely there do seem to be legal stores that cater to the "needs" of this part of society.

Reply to
Fritz Schlunder

Take apart a hard drive and use the head positioning servo.

--

    Boris Mohar
Reply to
Boris Mohar

ISTR moving coil loudspeakers, with an opto feedback system seemed like a good idea(at the time).

martin

Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.

Reply to
martin griffith

Another vote for a capacity based system - use the capacitance to vary the frequency of an oscillator, don't try to measure it directly. If you use a differential design of capacitor, then some of the errors cancel out to first order. Getting the accuracy you want will not be easy - prepare for a significant learning curve !

The other approach that might work is a servo'd torsion balance.

Dave

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Reply to
Dave Garnett

Go to a GunShop, and look at the Dillon D-Terminator digital scale. It is cheaper than you could buy the parts for to build it. However you could then dismantle it and see what is used. Some of your 'requirements', are unnecessary. For instance, it is very easy to linearise a non-linear sensor, in the sampling processor, provided the reading is repeatable, and the ADC has more resulution than the final 'requirement'. A simple pair of strain gauges, in the right configuration, can meet your requirements, provided you also sample temperature, and correct the readings. In this case, 'robustness', will be down to providing end stops to prevent the beam being overloaded, and protecting the sensors/electronics.

Best Wishes

Reply to
Roger Hamlett

I read in sci.electronics.design that Brad wrote (in ) about 'Measure Small Weight?', on Thu, 10 Feb 2005:

Look up 'joke' in the dictionary.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
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Reply to
John Woodgate

kG? That puts you quite high in the supply chain...

d

Pearce Consulting

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Reply to
Don Pearce

Ah - we're obviously not talking about the white stuff, then.

d

Pearce Consulting

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Reply to
Don Pearce

Just because he is measuring in ounces, Does Not mean he is a drugs dealer (around here they measure in KG)

martin

Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.

Reply to
martin griffith

Kgs are for the bottom feeders here. You should see when a boat gets lost, loads of little RIB's scouring the sea for contraband. One kitesurfer found some dope floating in the sea, but it was too heavy to carry to shore.

martin

Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.

Reply to
martin griffith

snip

Nope, I'm no longer in the TV biz, so no more temptations. But going back to the OP, I tried to find the SciAm/amateur scientist article about a DIY microbalance(it used a mA meter, an excellent article)

All deleted AFAICS, bastards

martin

Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.

Reply to
martin griffith

that

and

expensive

come

op-amp

8

to me?

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The Scientific American article was a follow up to an earlier S.A. piece. Both versions used gutted galvonometers, and the second was much easier to use. That article is what got me thinking about trying to weigh larger masses. The S.A. devices were microgram scales and were incredible in terms of their capability. You could wet a piece of thread, then watch the weight change as the thread dried out. Very nice! They didn't explain how it was so incredibly accurate, but I think they got a little help from the fact that that the drive circuit kept the galvo constantly vibrating and the vibration could overcome the friction in the bearings.

S.A. has removed all external links to this article. It is available on the S.A. web site along with a hand full of other articles from the Amateur Scientist. I don't have the link, but read the article a week or so ago.

None-the-less, I'm looking for grams and a converted galvo is not robust enough. I'm considering making a similar device using needle bearings/jewel bearings with a more powerful coil. I need to lift about 8 ounces maximum. If the original device can measure a few micrograms I should be able to measure a few hundred grams....

Regards,

Reply to
Brad

Looks like they have removed it. If you have it could you send it to me?

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

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Reply to
Boris Mohar

see:

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martin

NOT EVEN YOUR RINGTONES ARE SAFE.

Reply to
martin griffith

On 10 Feb 2005 03:33:01 -0800, Brad wrote in Msg.

I'd go for the voice coil (loudspeaker) servo feedback approach. Guaranteed to be linear, and cheap. I recall having seen it in Elektor magazine several years ago.

--Daniel

Reply to
Daniel Haude

There was one in the Amateur Scientist columm in Scientific American several years back. It used a gutted moving coil meter and a an op-amp to servo the pointer back to center. Can't recall the resolution and accuracy, but I think it did much better than .01 oz, the top end of 8 oz may be a problem though, and I'm not sure about the "robust" bit.

I could find the article if you are interested.

Barry Lennox

Reply to
Barry Lennox

responder

Elektor

I'm leaning towards this approach. I looked at the Elektor site for any construction article that seemed related and couln't find any. They have them all online from 1998 and later. When was the article? Any idea on the details?

Thanks for the suggestions everyone!

Regards, Brad

Reply to
Brad

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