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DIY uber accurate scales

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LAN_403

Conundrum

Sat Jan 22 2011, 10:34AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

heh...
Interesting idea, using this setup.
For those wanting to duplicate this, you can find linear HE sensors in many old video recorders and also 5 1/4" disk drives.
Another source is the small SOIC chips in most standard PC DVD/CD drives as they are often used for sensing motor rotation and position for precise speed feedback.

Interestingly, you could dispense with the magnets entirely and use two opposing magnetic fields from a pair of coils, with an onboard Lipo battery to supply the energy.
this would have the advantage that there would be no degradation in accuracy over time.

-A

Steve Conner

Sat Jan 22 2011, 11:35AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, it's a nice project, but with a FSD of 20g and a resolution of 1g, it's not "uber accurate". It's about a factor of 10 worse than those cheap digital scales used for weighing certain substances, let alone a real precision balance of the kind used in chemistry labs.

There is nothing wrong with the method in itself, I've seen papers about balances made in the 1950s that used it to great effect. Hall effect chips didn't exist then of course, so a light beam was bounced off a mirror mounted on the balance, onto a pair of photocells, creating an error signal that drove an electromagnet.

I think the main source of error in the system would be friction between the weighing platform and the clear plastic tube used to stop it falling sideways.

Killa-X

Sat Jan 22 2011, 02:32PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Never seen the concept before, I do think it's rather interesting, but I don't fully see a point in it, in some ways. Since its electromagnetic it couldnt be used with everything...For example, I have a 1000 gram scale with a resolution of 0.1 grams. I use it for weighing and mixing chemical powders, but some of them are 600mesh, and magnetic..So a scale like so, would be bad for that!

Other than that, I still think its cool that he was able to get a scale made with magnetism. The tube does balance it, so I can see where friction plays a role. But seeing I got my scale for $10 and its been accurate to any mass-block I've put on it, Id have to say its a cool concept, but easier to get a scale

Martin King

Sat Jan 22 2011, 06:52PM

Registered Member #3040 Joined: Tue Jul 27 2010, 03:15PM
Location: South of London. UK
Posts: 237

Killa-X wrote ...

Never seen the concept before, I do think it's rather interesting, but I don't fully see a point in it, in some ways. Since its electromagnetic it couldnt be used with everything

It's a method that's used widely on high accuracy balances (although the hall sensor could be replaced by any number of optical or capacitive etc. sensors) :-

You get away from the problem of weighing magnetic materials by making sure the magnet is far enough away from the measuring pan/sample to not be a problem. A friend was recently involved in designing the electronics for a high sensitivity balance which used this method. I think he ended up using a capacitive displacement sensor. The main difference between the method in the OP and the commercial method is that he is keeping the current constant and measuring the displacement whereas the commercial scales measure the current required to keeps the displacement at zero. I assume there are good reasons for using the later method, possibly down to linearity issues in the displacement sensor being greater than linearity issues in measuring the coil current?

EDIT Thinking about it later, by measuring the displacement you are limited by the mechanical movement of the balance, but with the other method you're only limited by the current the coil can take (and you can supply) thus potentially increasing the useful measurement range.

Martin.

Conundrum

Sun Jan 23 2011, 08:58AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Hall effect sensors didn't exist???!!!! eh?
i thought that they were invented in the late 1920's, perhaps you mean they weren't practical without the low noise on-chip CMOS amplifiers.

Did you know that bismuth filaments made from a length of capillary tube (or normal glass filled with bismuth and pulled into a fine hairthin filament) exhibit magnetoresistance?
Doable at home if you have even simple glass blowing equipment.

-A

testtest

Sun Jan 23 2011, 03:35PM

Registered Member #3271 Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159

Now here is what I call an uber accurate scale....

Fig 10 shows the tidal variations that have to be taken into consideration.....

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