PSN-L Email List Message

Subject: Re: Shunt damping
From: "Geoffrey" gmvoeth@...........
Date: Mon, 9 Oct 2006 07:36:59 -0700

What is the possibility of using one turn of
a heavy guage wire as a single loop
and sharpening the ends to a point
then sensing the voltages at the points
I would think that changing the cross sectional area
would increase the voltages by concentrating
electrons or if you are faster in your thought
holes sort of like having a resistor in the middle
but no power loss.
This is the way you change impedances in antennas
or so i understand and what is a velocity sensor
but an odd kind of antenna.

----- Original Message ----- 
Sent: Sunday, October 08, 2006 5:13 PM
Subject: Re: Shunt damping 

> In a message dated 2006/10/08, Bobhelenmcclure writes:
>> >   A wire size of #38 or less will allow this number of turns to fit 
>> > comfortably within the gap field cross-section. The coil will have a 
>> resistance low 
>> > enough to permit resistive shunt damping of a pendulum weighing up to a 
>> > kilogram, in my opinion. My sensors have a pendulum mass of about 0.1Kg, 
>> and 
>> > critical damping is achieved at about 30 kOhms. Since the coil resistance 
>> is only 
>> > 340 Ohms, the shunt damping imposes negligible loss on output sensitivity. 
>> > Even a kilogram mass would require only about 10% loss of output using 
>> shunt 
>> > damping.
>>        The damping force required also depends on the set period. What 
>> period are you using?
>>        What effect does this damping current have on the input noise in 
>> practice? Can it be significant?
>> Hi Chris,
>>     I bought cheap low temperature enameled magnet wire from Alltronics. I 
>> have both #38 (3.97 thou OD) and #40 gauge (3.14 thou OD). I never tried to 
>> use the #40 wire, as it is difficult for me to see, let alone handle. I strip 
>> by burning off the enamel. My friend Victor frowns on that, and recommends 
>> fine emery paper.
> Hi Bob,
>    Magnet wire is available from Alltronics in 1/4 lb reels, but it is Kynar 
> insulated
>       If you buy the polyurethane insulated wire like Beldsol, you don't 
> have to strip it. If you put a hot iron and solder on it, the insulation just 
> melts - no problems - but I have only found this wire down to 36 AWG - 5 thou OD.
>       With the Kynar insulated wire, you have to strip it first before you 
> can solder it. I usually use the edge of a wax candle flame (or a match) to 
> first burn off this insulation. (You have to be very careful with a butane 
> lighter not to melt the wire) Then I pull it fairly gently several times through two 
> small pads of the very fine wire wool. This both cleans it effectively and 
> hardens it a bit. 
>       I once only wound two coils with 40,000 turns of 44 AWG copper wire 
> with 1 thou paper interleaving to measure paramagnetic susceptability - it took 
> me two whole days.... 
>>   My horizontal sensors are easily shunt damped. On one of them set to 12 
>> second period, I measured a Q of 1.1 with a shunt resistance (including the 
>> amplifier) of 66 kOhms. My formula for Q is Q = K * R / P. The value for K is 
>> therefore 0.2. The mass of the coil and solder weight is about 100 grams. The 
>> coil resistance is 340 Ohms, the number of turns is 1100, the field strength 
>> is ~0.8T, the field length per turn is 0.1m, and the sensor output is 85 
>> v-s/m. If you know your own sensor's output, pendulum mass, and period, you can 
>> work out your own value for K from the above information, and determine what 
>> shunt damping resistance you would need. However, remember that the coil 
>> resistance, in series with the shunt resistance (in parallel with the amplifier 
>> input resistance) is the damping resistance.
>       What is P please?
>> Volts= B*L*(dx/dt)
>> Force=B*L*I
>> I= Volts/R = B*L*(dx/dt)/R
>> Force= B*L*B*L*(dx/dt)/R
>> Force / (dx/dt)= (B*L)^2 / R 
>>   I have not checked yet to see if the above equation is consistent with my 
>> observed damping versus resistance.
>    In the force equation, isn't the force proportional to the number of 
> turns, whereas the inductance is proportional to the square of the number of turns 
> and depends on the magnetic return path?
>       How does the length of the pendulum, the set period and the mass 
> factor into these equations, please?
>>    Shunt damping makes it easy for me to check my sensors. I measure their 
>> natural period by disconnecting the shunt, and discharging a small capacitor 
>> across the sensor-amplifier terminals. The decay of oscillation amplitude 
>> lets me make sure that the partially undamped oscillation has the expected Q 
>> value, and the time between zero crossings gives me the natural period. I would 
>> abandon the sport of seismometry if I could not control damping this way.
>       I prefer to have my damping and sensors on separate fittings and their 
> setup independant, but I can see the attraction of variable resistive damping 
> if you choose very powerful sensor magnets and a low to moderate seismic 
> mass. 
>       I tend to use rather smaller / thinner magnets for sensing and they do 
> not have a serious diamagnetic repulsion problem, although I have observed 
> this type of effect. 
>       I use wide Cu plate for the damping, so that the arm hits the side 
> stops before the outer edge of the damping plate gets close to the outside edge 
> of a magnet. I try to 'design out' problems when possible.
>       If you are using feedback sensors with electronically extended 
> periods, Cu plate damping is a lot quieter than velocity feedback from a 
> differentiated position signal.
>      Have you any comparisons of the input noise due to shunt damping vs 
> plate damping - or the noise when undamped and when damped? Won't the induced 
> current generate additional noise directly?  
>       Regards,
>       Chris Chapman

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