A Balancing Act: Newsletter Challenge (04/05/11)

Probably by making the ratio of the horizontal balance beam arm length and the length of the vertical swing arm that holds the pan such that their oscillations will tend to cancel each other.

The balance arm will oscillate up and down. The pan will swing side to side toward the balance arm's fulcrum. It should be possible to get the two so that their oscillations tend to cancel each other out if their respective lengths are right.

My understanding is modern balances use magnetic fields inside a conductor to induce eddy currents as a form of a magnetic dashpot.

I dunno what a single pan ballance looks like.
With the old fashioned chemical balance the large diameter pans offers a lot of aerodynamic resistance to movement and thus acts as a dash pots.
There was also a knob in the front which allowed you to raise the balance slowly which avoided the problem in the first place.

Del

For the single pan balances I used back (way back) in high school and college the damping mechanism was my right thumb. It worked by lightly brushing the pointer at the end of the balance arm with the fleshy tip of my thumb until the pointer came to rest. If the pointer wasn't centered on "0" (zero) I'd adjust the sliding weights until it was. I sometimes had to repeat this laborious procedure twice -- maybe even three times! -- to make the measurement.

Fair enough. I'll get the ball rolling on this one...GA

In order to stop the oscillations, we need to dissipate the kinetic energy of the system. Since we have mechanical pieces in contact with each other, there will be some amount of friction that will dissipate the energy over time. The friction has to be small or the accuracy of the balance would be poor as the system would "hang up" at some off center position if the friction is very large. Therefore, the balance needs to use some type of viscous damping which requires a fluid. The most practical fluid to use is air because a damper can be created that does not have any parts contacting each other. The damping will be greater the larger the magnitude of the oscillations. Another fluid could be used but either it would be something to spill/evaporate or would need seals that would add friction. Once the oscillations become small, the friction in the system will bring the balance to a final stop.

Perhaps a digital mechanism (ie a finger)?

Balances should be read in motion anyhow. Count the divisions of each swing and average them to determine how far from center the indication is.

Back in the old days this was how we were taught to do it. Unless you had a more expensive model that had the apparatus poster #12 talks about.

Since the final balanced position of the horizontal arm is the same, i.e. horizontal, all you need is two limiting bars or stoppers, One up and one down with the horizontal arm resting finally between them.

Then, there will be a piece that swings to jam the arm against one of the stops, during weight additions. Releasing the Jam, will allow the arm to move within the stoppers gap. The arm will jam against the top or the bottom until the weight is balanced when the Horizontal arm will move slowly and does not touch the stoppers.

This is a simple mecanism.

That's how the old-fashioned doctors' scaled worked when I was a child....I think we had smaller units at my junior school (7-11 yo)

Number 12 (with the exception of the magnet not being a horseshoe but rather sliced circle magnet) has described the damper installed on my balances. The energy of the swing is absorbed by the resistance heating of the induced currents in the vane. As a young student before I had a magnetic damper, I would blow on the pan, timing the puff of air to counteract the swing. After a few months of practice, one "blow" would suffice (though not good for weighing powders).

I think the answer probably depends on what sort of single pan balance we're talking about:-

http://www.google.co.uk/images?hl=en&source=hp&biw=1283&bih=783&q=%22single-pan+balance%22&btnG=Search+Images&gbv=2&aq=f&aqi=&aql=f&oq=

Worked for company that did scale work. There are different devices that would be used to control oscillations of the beam. Most prominent one was a lever that pinned the beam in the up position while loading. A slow release would control the oscillations. Others included the dash pot which where hydraulic or magnetic. Which are generally not used as any device that restricts beam motion can reduce accuracy. So they would be used on very low end scales where accuracy was not an issue or on very high end scales where cost was not an issue (design of the dash pot so it would not interfer with beam accuracy) Even saw couple that used the pan as the dash pot. At a balance the pan set just above circular depression in the base. The tight tolerance between the pan and the perimeter of the depression would slow oscillations as the air was used as the fluid.

But all that's been solved it's called a load cell. Maybe the next challenge could present some thing new. Not something that was resolved 50 years ago!

Magnetic dampening. A plate of aluminum which is conductive but non-magnetic is positioned in parallel with the movement of the arm and two magnets are held close to it so that it must move between them as the arm moves. As the plate moves through the magnetic field between the magnets, electric eddy currents are caused to move in the aluminum plate. These fields produce their own magnetic fields that are opposed to the static field of the magnets and resist the motion, causing the oscillations of the arm to slow rapidly.

Two views of our twin pan balance, shows the aluminum plate suspended between the split-ring magnet. Also has a lever arm with felt pads to allow the pans to be dropped slowly. Catenary chain and vernier scale allow measurement to milligrams. Once used to measure tiny tufts of cotten used as a filter, around .010±.002 gram range, I believe.

On most, but not all of these scales, there is a metal plate that passes through a magnetic field between a couple of magnets. This causes an induced electric current in the plate causing a resisting magnetic field which damps the motion of the balance arm. Older models used a plate hanging in an oil filled reservoir to damp the motion, but to maintain the accuracy of the balance the oil level had to be monitored closely due to the buoyancy of the plate and rod that supported it.

Rubber bumper(s) in the right place to limit the swing.Could be retractable. Simple and more stable than a thumb (at least more stable than my thumb!)

So as to not use yesterday's methods, I thought:

Feed the location of the pan to an onboard computer, using one or more sensors; have the computer move the scale so that the lift point is directly over the pan. With the right response times, this could work almost instantaneously and without affecting accuracy.

Hello,

There is a metal extension on the end of the arm that passes between magnets which impede the oscillations.

Thanks,

Fred

Two classic mechanisms for damping an oscillatory motion:

1 a vane, Paddle fixed concentrically to the arm to be damped. The vane rotates in a dash pot enclosure, with as little clearance as possible. The oscillatory rotation of the paddle or vane tries to compress the air in front of the leading edge, but air leaks around the gap an resists the motion of the paddle.

2 Similar idea but the vane is replaced by a electrically conductive disc, usually copper or aluminium, strong permanent magnets are arranged above and below the plane of rotation of the disc, (as close as possible) so that they do not touch the disc but their magnetic field passes through the disc. Oscillatory motion of the scale arm is transferred to the disc and eddy currents are induced in it by the magnetic field that generate an opposing magnetic field slowing the motion of the disc -and thus damp the motion of the scale arm. Voila!

PS. In small instruments the disc is often replaced by a copper cup, with a coaxial fixed magnet - same principle.

Should we build one (or more than one)?

Appears as though a simple shock absorber below the pan should do the trick.

simplest and maybe the cheapest is a rubber ball under the pan

How about the three suspension chains for the pan being unequal in length?

I may have missed someone saying this but if the arm is very flexible surely it will dissipate any kinetic energy in internal friction very quickly

The most simplest form of damping for a beam balance type scale is to not attach the pan support arms, hoop (or even on old gold prospectors scales the chains) directly to the beam. There should be very short arms attached at either end of the beam hanging down and the long pan support arms or chains are hung from these. This method of attachment counteracts and dampens the slight swing of the arms and helps steady the beam to a halt. To explain this in reverse a little further, if a child's swing is attached at the top at single pivot points using chain or rope. the child can swing themselves up quite easily. In some children's playgrounds you may see that some genius? engineers have used a greased trunnion bearing arm attachment for the chain at the top this counteracts the swing much to the child's disappointment.