Damping oscillations in models

Hi All,

I’ve a strange (possible slightly sad!) application. Imagine, if you can, I’m a guy who likes making big model tanks. The tank turret has a Action Man sized figure poking out of the turret (top half from the waist up). When the tank moves it bounces (slightly) up and down but rocks back and forth and side to side as it drives over rough ground ( + pitching and yawing). I can imagine fixing my figure to a gimbal at its waist with a mass (a few 10s of grams?) slung underneath to maintain stability when the tank moves around. To more accurately simulate the action of a real human tank commander in a real tank (with stomach muscles maintaining a vertical stance and not being thrown around like a rag doll (or Action Man figure!)) I need to critically (or perhaps over-)damp (to human timescalesscale!!) the oscillations arising in the figure from the tank’s movement. This is a low mass (10’s of grams), low frequency (e.g. 0.5 – 5sec) random 3D movements over low angles (normally <10-20 degrees from horizontal).

Can anyone think how I might critically (or even over-)damp (three dimensionally) the upper half of a gimbal mounted (even rubber band mounted) relatively low mass system without resorting to viscosity damped air/liquid piston or electrically actuated drives to achieve this.

As you will have realised, I'm no engineer, but having worked with you guys before, I'm constantly surprised about the degree of imagination you can apply to such simple problems.

Any thoughts would be very gratefully received given there are a frightening number of us sad characters who might be interested in such an innovation.

Thanks for your help

Bob

If you stick your body out of the turret of a full size tank while it's travelling over rough ground, you get thrown about like a rag doll. Try a small counterweight below the figure, mounted on a (more or less centrally) pivoted vertical support (a stiff wire) figure on one end, counterweight on the other. The counterweight may be contained in a liquid filled balloon (tied to the wire) which rests inside a small pot. OK, it's a viscous fluid but no precision engineering required! If the relative weights are similar, inertia will do most of the work, the pot just provides damping.

Ahhh, I fancied a mad invention today.

.

Edited By Nick_G on 10/10/2016 22:02:36

Oh O.K.then, Nick ...

At the scale under discussion, my first thought would be Sorbothane: this has the same general characteristics as the viscous damper mentioned by daveb, with the added advantage that it's a solid.

http://www.sorbothane.com/engineering-design-guide.aspx

MichaelG.

What aboutmaking your counterbalance weight in the form of a simple electric motor with a heavy flywheel which will act as a gyroscope and be self damping.

Edited By JasonB on 11/10/2016 07:23:53

I'm not sure if I understand JasonB's idea. A gyroscope needs to whizz round at umpty-several rpm. And damping is avoided in such designs to avoid driving losses. At the same time a 'simple electric motor' is not self damping but requires a supply of current. Perhaps he was thinking of a dynamo, but a dynamo that swings rather than rotates, and in 3d rather than back and forth...

Can you help with a sketch, please?

Cheers, Tim

Maybe support it on some shape of silicone rtv ? it could turn out to be a simple length and diameter relationship and would be easy to change with this.

The Sorbothane egg video reminded me of some of the can't break an egg video's on ebay.

John

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Tim to keep the figure upright on whatever gimble is used a counter balance weight is needed at the bottom so if this were say a common RC motor like a 540 that should have enough weight to balance half an action man.

If a hefty flywheel were fitted to this motor and spun round at a good lick when it it spinning the flywheel will not want to alter its plane of rotation quickly as it will act like a gyroscope and this is what will damp the free movement of the gimble./action man

Just try holding anything with a rotating mass and then try moving it quickly - there will be some resistance. Move it slowly and it will go where you want it to.

Hi Bob, How about four springs mounted between two discs, one fixed to the turret and the other to the figure.

You would have to work out sizes and strength of springs. The springs would work against each other to control wobble

I think. (springs mounted across diameter of discs) . Best of luck Maurice.

The pendulum in a Wixey gauge is damped by magnet and copper disc. This could be adapted to damping your action man. You'd have to rig it up to work in 2 planes, one magnet fastened to gimbal ring. Works by inducing eddy currents into the copper. I'm sure if you google for long enough you'll find a picture of a Wixey with the back taken off to show the pendulum arrangement.

Hi Bob Carr

I wonder if the right steady cam mount could be found?
You might even be able to mount a camera at the same time!

I found a few examples here:

**LINK**

Regards
John

You only have three variables:

The period of the pendulum (how fast he can swing)

The amount of damping.

The non-linearity of damping (i.e. it gets stiffer until you reach a point where it won't swing any more)

One way of damping might be to let a peg on the end of the pendulum swing between the bristles of a hairbrush! This will rapidly slow it down without limiting its motion, and require a minimum movement before motion happens.

Obviously some of you have never stood in the turret opening of a tank! On flat ground it is quite stable and on inclines etc one braces oneself by holding the turret ring. I would fix the dummy in place and let it follow the tank motion.

One interesting fact i noticed at speed on a flat concrete road was that the vibration set up a dust ball in the center of the crew space. If i had a bag I could have collected it, as the speed dropped so did the dust ball.

Clive