Lahr's Magnetic Damping
based on Chapman's Design

 

Naval Jelly  (phosphoric acid) for cleaning the 1/4" steel plates.  Rust-resistant paint.  1/4" nuts & bolts.  1/4" drill bit.  Two of the 1x1/2/1/4" magnets.  Copper plate.  1/4" brass rod and brass nuts.

Damping system in place (end view).

Damping system in place (side view).

Due to the filtering of the AS-1 "black box" it's a bit difficult to determine when the system is properly damped.  The ratio of the actual damping to critical damping, called the "damping ratio," should be between 0.7 and 0.8.  To make adjustments, I switched to a 100x amplifier with no filtering and a DATAQ DI-154RS ADC.  I then was able to measure the amplitude of the first two swings of a lift test.  To find the damping ratio, I entered these amplitudes into the script here:  http://www.jclahr.com/science/psn/damping/ and pressed the Damping Ratio button.  Note that only a small portion of the copper plate needs to be between the magnets to achieve proper damping.

Although Chris Chapman specified 1/16"-thick copper plate (0.0625"), the thickest available at the local hardware store was only 0.025"-inches thick.  I folded this in two for a total thickness of 0.05".  However, given how little of this flag needs to be between the magnets, it seems likely that a thickness of 0.025 would be adequate.

Adjustment of damping magnets.

I spread the magnets farther apart so that the damping vane could be centered within the magnetic field.  I used the DATAQ DI-154RS ADC fed by a 100x amplifier, as above.  The upper figure below show a standard weight-lift calibration when the magnets are too far apart.   With proper damping, the steel plates were 21 mm apart (lower figure).  The second swing of the response is more rounded in this case.

Weight-lift calibration with damping ratio of 0.53.
Weight-lift calibration with damping ratio of 0.75.  The space between the steel plates was 21 mm.  This left a space of 21 mm minus 1/2 inch (8.3 mm) for the copper damping vane.

 

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