Magnetic Damping to Minimise LF Vibration

[dhwilliams]

I am currently prototyping a new device which is a software controlled electromechanical unit - the central part of which is a counterbalanced radial arm which rotates about a central shaft alternately clockwise and anti-clockwise through 180 degrees taking about 2.5 seconds to move from 0 to 180. I have found that this arm tends to vibrate at a low frequency (possibly 5-10Hz). This looks like a resonant-frequency effect. As luck would have it, there is also a disc of Aluminium (Aluminum?)about 180mm diameter, 3mm thick attached to the shaft. Would it be possible to place 2 powerful magnets (NdFeB?) diametrically opposed such that the disc travels through the gap to dampen this vibration and, if I'm on the right track, what order of magnet size and gap would be a good starting point for testing this? BTW I'm a Programmer and Computer Support Manager so keep it simple!
Thanks,
Dave

 

[drdick]

Eddy current damping will slow the pendulum, but should help smooth things out. Calculations are not precise. Suggest you pick up a bag of magnets from a hardware store and do some empirical work. A pair of .5" disk magnets about .25" thick placed so they attract throught the aluminum disk would be a place to start. Ceramic magnets should work, but Neodymium will too. Try various spacing from the disk. A steel "yoke" (magnet mount) that connects the more distant poles will make the system more efficient. If two magnets are not enough, add more in parallel. If the magnets are to strong, try placing them so they oppose magnetically; this will put the magnetic field in the plane of the disk. Electromagnets would work too, and allow for adjustment and frequency related modulation.

 

[dhwilliams]

Thanks for the suggestions, drdick. At the moment I'm having greater problems with the engineering side of this. I hand-built a prototype from aluminium bar and angle-iron which works better than the "properly designed" and "precision-engineered" unit I have now. The bearings are bumpy (I'm sure there's a more appropriate technical term!) and, as for my drive gearing, the wormwheel isn't sitting straight on its shaft and the shaft with the worm on it slides backwards and forwards through its 2 bearings. When I have these problems sorted I might have time to look again at magnetic damping.
Thanks again,
Dave

 

[hacksaw]

don't believe magnets will help at such low velocities (frequencies).

you need to look at the resonant frequencies of your rotating arm (as a beam). you may need to stiffen it.

the bearing problems can be sorted out, but you may need to control the rate of acceleration of your motor to avoid exciting the criticals of your structure.

 

[dhwilliams]

Thanks Hacksaw, we are actually having more of these built by a different engineering company. They should be able to fit the bearings correctly and line up the worm and wheel. When they are ready I will be able to check them out to see if any of the problems have gone away! If the stiffness of the arm is a problem we could consider a different material, aluminium might be the wrong one to use!

The motor speed is controlled by the software which starts the motor at about half-power then increases to three-quarters then full power (and the inverse for stopping). If necessary I can put in more steps to minimise the excitation but at the moment it looks fairly smooth.