# MAGNETIC SCALES

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## Introduction: MAGNETIC SCALES

This is just an idea I had a while back. Instead of using springs for a set of scales I thought permanent magnets might do the trick. Unfortunately I didnt think about the properties of magnets, the magnetic field is not uniform and therefore the scale won't go up in even intervals, instead it seems to be an exponential or inverse relationship. But i though I would still publish it since it is somewhat interesting and later on in this instructable I'll tell you how you could get around the exponential problem.

## Step 1: Materials

You need:
1) Meccano pieces
2) two permanent magnets preferably with holes (easy to attach to meccano frame)
3)graph paper
4)thumbtacks
5)A set of coins (find out how much they weigh)
6) plastic lid
7) thin wire and string
8) a sheet of cardboard or plastic

## Step 2: Put It Together

Put it together so it looks similar to mine in the pics, of course it it can be completely different. Play around with it so you get the largest angle possible from your magnets.

## Step 3: Write Measurements Onto Graph Paper

to make the scale on your graph paper it is handy to know that the average thumbtack weighs 0.25 grams and find out how much your countries coins weigh. Luckily here in New Zealand the new set of coins have just come out, and they are of a handy size. the 10cent weighs 3.3g, the 20cent weighs 4g and the 50cent weighs5g. Using these known weights draw up your scale by putting the known weights into the tray on your scale, mark the interval on your graph paper and keep doing this with more weights. I used coins aswell as thumbtacks to check wether they weigh the same amount and to make sure that the scale will be correct. Once you have marked all the intervals on the graph paper your set of scales is finished. mine was very sensitive up to one gram and then the interval rapidly decreased until the magnets touched, just after 10g.

## Step 4: Improvements

I talked earlier about improving the set of scales so that the scale isnt of the exponential type. I had the idea that by using a solenoid a uniform magnetic field can be established. The repulsion force will be the same throughout the middle of the solenoid except for the ends. Another benefit of using a solenoid is that a variable power supply can be used to change the magnetic field strength of the solenoid and therefore the scales could be made more or less sensitive. The bad quality picture may help to explain it

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## Questions

As the solenoid creates a constant field within itself, it should exert a constant force on the permanent magnet... so I don't think your idea would work as planned. Now electronically controlling the power through the solenoid is an interesting concept... I'll try it when I get my ardy.

Nice try, but I applied for a U.S. patent on this concept years ago as a kitchen scale. You don't need a solenoid, if you use disc shaped ceramic magnets in a  clear, plastic holding tube with a short-stroke piston connected to a measuring plate.  Basically, it's a modified damper system. You can then calibrate the scale with a standard weight set by etching the side of the holding tube with weight measurements as the piston is depressed in the tube with one magnet at the base of the piston magnetically levitating in same polarity with an opposing magnet mounted in the base of the tube. As the weight of the food depresses the damper you read the weight marks on the side of the clear plastic tube as the base of the damper descends in teh tube. Simple, yet elegant and cheap.

Rick Dickson

Inventor

Kirkland, Washington

This is a force-feedback mechanism using magnets for the force. The magnetic force will be in exponential relationship to the proximity of the magnets. The scale can be linearised by imposing a new set of lines over the original set originating on the right side of the scale lines and proceeding to the left side of the paper. I'm an engineer too and found your scale very interesting, but it has no commercial application as the magnets are not able to maintain their magnetism so the scale would need to be recalibrated often. Using electromagnets would be much better and scale calibration would remain the same over long periods. Keep experimenting with it, you've come up with something very simple and very good. Some things to look for is accuracy, repeatability and long term stability.

Rare earth magnets would lose less than one percent of their strength in ten years. Would that be good enough for this application? Also, supposing you wanted to use only permanent magnets- could you obtain a more linear relationship between force and distance by using magnets shaped in a curve with a thick end and a thin end?

Magnetic field strength vary by distance--and according to the surrounding magnetic fields and/or materials in the vicinity. Thus, building it open frame steel was probably not such a great idea. The field strength varies inversely as the square of the distance. But the easy way to calibrate it is to do just that, use weights and chart the movement. It will not have equal divisions between equal weights. But it will be accurate for those tics.

What a great idea! I like the idea of using an electromagnet, too. I'll have to build one. +5/5

Hey, this is only sort of relevant: where do you get those "Mecano"/"Erector" pieces (sets, whatever)...can I just go out to my local hobby store and grab some, or did you just have some lying around?

7 replies

I'd suggest using Lego, actually. You don't want the frame to interfere with the magnetic fields, so a plastic frame would probably be better.

If you're looking for magnet sources or other fun projects, check out Dan's Data: http://www.dansdata.com/magnets.htm

mine were aluminium and they aren't magnetic

First of all, cool concept! Kiki makes a good point. As your load lever arm rotates counter-clockwise down, the moment that is produced from the weight of the load grows (not just because of the extra weight), up until the maximum point where the arm is horizontal (because the weight force is perpendicular to the moment arm). You could do the math and possibly make the non-linear moment gain from the angular position of the load arm cancel out the non linear effect of the magnet. This will be tough, though, because the force of the magnet is dependant on the position of the arm, which is dependant on the load. If I get bored tomorrow I'll draw you a free body diagram! :)

oh gee sounds complicated, what if you make the arm curved like on my last page then the moment remains the same at any position. right?. maybe I should give this another go sometime so that it is more accurate

Oops, look like I replied to the wrong post! I'll draw you out what I mean tomorrow morning/afternoon (I dont know why I'm still awake, its 4:30 AM!). I'm just refering to the force from weight, which always pushes straight down. When the lever is horizontal, weight produces a much more significant torque than on a lever that is nearly in the vertical position, for example. This means that as the whole scale rotates from the horizontal position, any given weight will produce a non linear torque. I think the curvature you are refering to is from the magnet and inductor, and it is true that with any given magnetic force it will produce a torque that is irregardless of angular position. This is because the force from the solenoid follows the magnet attached to the lever (purely due to the shape you drew), whereas the weight of an object will always point down. I have another question too. Is the variable voltage solenoid just to change the scale output? Meaning, do you fix the voltage before weighing the object? Or are you trying to vary the voltage as the scale rotates? Just another thought before I go... When a fixed magnet moves through an inductor, it induces a current in the inductor, which resists the motion of the magnet. I'm wondering if this will affect anything... I dont think it will, because this is just a transient effect that will cease when the motion stops. I think it will act like a damper... Gotta go, the suns coming up!

Legos are too weak. (trust me, I have a 2x3 bin full of them, as does my sister)

Try second hand, ebay, trademe or some garage sale

BEAUTIFUL! I wonder if feedback can be used to hold it in balance with more current during weighing, and if a digital (amps) meter reading the current could give a linear weight reading.

2 replies

As prodmod said " how much current it took to accomplish that. And that current is then multiplied by a factor to give you weight in say grams, for example.". So all that you have to do is take a reading off an ammeter and multiply that by a constant (that can be worked out by weighing something and dividing its weight by the ammeters measurement)

Thanks for letting me know the feedback system works. So then you wouldn't need math for reading the weight on an ammeter, just adjust it's sensitivity with a reostat until it reads out the correct weight, unless your solenoid already holds 1 gram with 1 milliamp and 10g with 10ma. Again, it's great you made it work without all that.

It is very interesting that you came up with the solenoid concept on your own (see the last page). I am an engineer working for a digital scale company. And one of the technologies that has been around a while uses a solenoid to resist the weight of the sample. A light sensor provides feedback to ensure the balance lever is brought back to "home" and reports how much current it took to accomplish that. And that current is then multiplied by a factor to give you weight in say grams, for example. Good work.