Measure Small Weight?

[Brad]

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

 

[martin griffith]

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

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.

 

[[email protected]]

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

http://www.balances.com/ohaus/ohaus_navigator.html

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.

 

[Dave Garnett]

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

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

 

[Roger Hamlett]

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

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

 

[Fritz Schlunder]

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.

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.

 

[Brad]

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,

 

[John Woodgate]

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!

Look up 'joke' in the dictionary.

 

[Boris Mohar]

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

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

 

[martin griffith]

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.

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.

 

[Don Pearce]

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

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

d

Pearce Consulting
http://www.pearce.uk.com

 

[martin griffith]

On Thu, 10 Feb 2005 14:53:23 GMT, in sci.electronics.design

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

d

Pearce Consulting
http://www.pearce.uk.com

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.

 

[Don Pearce]

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.

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

d

Pearce Consulting
http://www.pearce.uk.com

 

[martin griffith]

snip

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

d

Pearce Consulting
http://www.pearce.uk.com

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.

 

[Rich Grise]

Look up 'joke' in the dictionary.

With or without the quotes? ;-)

Cheers!
Rich

 

[Barry Lennox]

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

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

 

[martin griffith]

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

see:

http://www.amasci.com/amateur/sciam1.html




martin

NOT EVEN YOUR
RINGTONES ARE SAFE.

 

[Boris Mohar]

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

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) http://www.viatrack.ca

 

[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)

http://www.viatrack.ca

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,

 

[Daniel Haude]

On 10 Feb 2005 03:33:01 -0800,

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

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