How to dampen movement within a magnetic field?

[frascati]

I'm not sufficiently educated on this stuff to properly describe the question. Sorry.

Consider the following. a primitive gauge is set up in order to observe the differential air flow between two sources. Assuming static pressure linearly corresponds to differences in flow. so balancing air flow is, for sake of argument, sufficient to determine vacuum balance. Just let that be a given for now.

insert a 1 inch by 1/8 inch cylindrical neo magnet into a six inch tube of slightly larger ID just sufficient to let the magnet slide freely.
insert 1/8" x 1/4" neo magnets in either end of the tube in polarity that repels the 1 inch magnet and contains it between them
insert ventilated stops at either end to prevent the smaller magnets from escaping.
attach vacuum/flow tubes to either end.
observe the 1 inch magnet is held in the middle by the other 1/8 x 1/4 magnets but acts as a flow meter from the center mark
so far so good
the meter is wonderfully sensitive, and maddeningly sensititve to pulsation.

Is there any means other than friction dampening the 1 inch magnet to dampen its movement against pulsation?

 

[kuruman]

Is there any means other than friction dampening the 1 inch magnet to dampen its movement against pulsation?

Electromagnetic braking. Watch the video.

 

[frascati]

also I am familiar with damping magnet in tube with eddy current. But in this case the eddy current would be insufficiently strong to dampen pulsation, and the tube must be clear glass or plastic.
another method possible is using a sufficiently large volume canister in line with each input tube that the air volume acts to accumulate pulses and smooth them out.
I'm wondering if there is a way of damping the 1 inch magnet sliding in the tube by influencing it by some other means. something like the eddy current damping, but stronger and directly affecting the 1 inch magnet itself, not the pulses acting against it.

 

[gneill]

If you apply an external magnetic field, will the magnet in the tube not try to twist twist and thus increase friction against the tube walls?

 

[frascati]

any means other than friction dampening

friction damping the magnet itself would only work under ideal conditions. any irregularity or contamination on the tube inner wall would ruin the sensitivity of measurement/movement

 

[gneill]

Is it imperative that the entire tube be transparent? You might try covering only a portion of the tube with copper.

 

[frascati]

nn20,307 / 2,524
Staff: Mentor
"Is it imperative that the entire tube be transparent? You might try covering only a portion of the tube with copper."

but even the weak eddy current of a complete copper tube would be insufficient to damp against pulses. What's needed is something akin to filling the tube with oil... not possible since we're measuring air flow... but instead emulating that amount of damping electrically/magnetically.
It's probably not possible, but I thought it wouldn't hurt to ask.
What is probably going to happen is an accumulator, maybe 500cc of air or so, inline with each connecting tube.
I really didn't want to do this for space considerations.

 

[DaveE]

OK, here's a more complicated way that would probably work:
Instead of measuring the displacement of the center magnet, you could create a controlled magnetic field and a feedback system to always keep the magnet in the central position. Then what you would measure is the force (field strength) required to stabilize the magnet. You can search for "servo-balance", "force-balance" or similar terms to learn more. This will essentially eliminate (or control really) the mechanical oscillations of the magnet by transferring that problem to the electrical domain of the control system.

 

[jrmichler]

On one hand, it sounds like you have a slightly different arrangement of a rotameter. This company has a large selection of rotameters: https://www.omega.com/subsection/rotameters.html. Commercially available rotameters work very well. The one time I designed a rotameter, it did not work well at all. Learning experience number (large): There are details of construction of the tube and the shape of the float that are critical to good operation. If your goal is to measure flow, you would be far better off to use a proven, commercially available, flow measuring device such as a rotameter.

On the other hand, you say:

differential air flow between two sources

A close fitting sliding piston does not measure differential air flow. It can, however, measure differential pressure. Is that what you are trying to do? If so, there are differential pressure gauges. Dwyer Instruments is a good source: https://www.dwyer-inst.com/Product/Pressure/DifferentialPressure/Gages/. Omega also sells differential pressure gauges. If your differential pressure is low enough, a water manometer is self calibrating and dirt cheap - just a piece of plastic tubing with some water in it.

If you are measuring differential pressure, and the pressures are too variable, a simple needle valve restrictor is the cure.

 

[frascati]

A close fitting sliding piston does not measure differential air flow

As described it actually does, and quite nicely, with the condition that it's level. Perhaps not "differential" precisely. As I said, I'm not trained in this vernacular. I'm trying to find the point of balance ONLY. I'm not interested in measuring any difference in that balance. Leveling the gauge is just a matter of placing it such that the middle magnet is centered before any measuring is done. I made one and tested it.

I wanted to see if I could diy a really simple manometer for balancing the cylinders of a twin cylinder carbureted motorcycle. Mine is a 650 opposed twin. The firing "order" here actually comes close to naturally damping pulses on the gauge. But they're always sufficiently off to cause trouble reading a jiggling magnet.
The best diy example of a twin cylinder manometer is a vertical loop of aquarium tubing, filled halfway with light viscosity oil, and damped with a needle valve at the bottom of the loop. Just for the fun of it I wanted to see if magnets could do the job.

In a six inch long clear plastic tube the magnets at either end repel and keep the middle magnet precisely centered. The end magnets are fixed in place. The gap around all three magnets allows sufficient airflow to pass the end magnets and move the middle magnet from left to right. Its a huge bonus that the sensitivity is progressive since the middle magnet is most sensitive in the small range to either side of center (where the influence of the end magnets is weakest) that is most useful for tuning. The vac/flow has to be extremely imbalanced in order to draw the center magnet nearer the ends. Without this progressive quality the gauge would be useless until the carburetors were nearly perfectly tuned.

This gauge is intended only, exclusively, for establishing flow/pressure/vac between two sources. So any accuracy of scale is unimportant. No scale other than a center point is needed.

I hooked it up to the carburetors ports and it works brilliantly. Problem is that the center magnet wants to dance half a centimeter or so in either direction while its doing its job. If I could damp it without ruining its averaged sensitivity I'd feel like the experiment was a success.

 

[jrmichler]

AHA! What you have is a very lightly damped spring mass system. I expect that the magnet would bounce less at a different RPM. Rev it a little and see what happens. Try a partial copper sleeve and see what happens. In a lightly damped system, a small amount of additional damping goes a long way.

You are measuring differences in intake manifold vacuum. That's differential pressure, and it is related to the air flow through each carburetor.

 

[frascati]

I expect that the magnet would bounce less at a different RPM. Rev it a little and see what happens.

Yes it does. Calms it right down. The pulse frequency eventually can't overcome the mass of the middle magnet and things smooth right out. It helps to make adjustments along a range of rpms however so damping would be very nice. I will try a copper sleeve. I could sleeve the whole plastic tube with copper tubing leaving only a viewing slot. Would be cool if it worked. A heavier mass (bigger middle magnet) would work, to a point, unless its size negates the whole point of simplicity and compactness.

This is mostly an exercise in curiosity. But it would be pretty nice to have something nearly as accurate as a gravity fluid manometer but without messy fluid and compact enough to tuck into a toolkit on the road. My experimental version of it is no larger than a bic pen with a few feet of micro tubing (smallest weed whacker gasline)

It was actually a little thrilling to have it hooked up and watch the center magnet respond to my tiny little adjustments of fuel, air, and mixture at one or the other carburetor.

 

[gneill]

friction damping the magnet itself would only work under ideal conditions. any irregularity or contamination on the tube inner wall would ruin the sensitivity of measurement/movement

The magnet is already sliding back and forth on the tube surface (thanks to gravity). You may only need a really small "twist" to give it just a hint of resistance.

If you go the accumulator route, consider that you might make the accumulators part of the packaging that also stores the lengths of tubing for connecting to your carburetor. The box would have two chambers with your tube bridging the chambers. Make the tight-fitting lid of Plexiglas so that you can see the tube in operation when the lid is sealed shut.

 

[gneill]

Here's an idea that just came to me.

You might try using wire coils around the ends of the tube (leaving the central area free), wired so that they generate fields opposing the motion of the middle magnet. When the magnet moves towards or away from one of the coils a current will be produced that in turn generates a magnetic field that opposes the motion. The faster the motion, the greater the opposition.

You'd have to experiment with the number of turns to get the optimum. Probably want to use some pretty high gauge wire and lots of turns. If the damping appears too great, try placing a resistance in the current path.

 

[Tom.G]

Having run into the same problem on my 650 V-twin, I used two vacuum gauges attached to the carb ports with about 3 feet of rubber tubing each. Then I put a C-clamp on each tube as near to the carb ports as they would fit. Gradually tIghten the C_clamps, pinching the tubing almost closed, until the gauges stop oscillating. I also made a hanger to hold the two gauges next to each other by bending a couple loops in a wire clothes hanger and ran the tubes and gauges thru the loops.

The clamps and tubing end up being a low-pass filter with the constricted clamped section being the resistance and the air volume in the longer part of the tube being the compliance.

Cheers,
Tom