PSN-L Email List Message

Subject: Re: Questions about shackleford
From: "Larry Cochrane" lcochrane@..............
Date: Thu, 1 May 2003 17:34:08 -0700


> Hi,
> I am a teacher in a turkish highschool and I try, after having finished
> alread a lehmann, to build a shackleford.
> We follow the article in scientific american and the sugestions made buy
> Larry Cochrane.
> But there are a few things I don't understand:
> - In my opinion, the main purpose of the damping coil is to enlarge the
> period, as the force acted by it is proportional to the elongation and
> not to the velocity. Is this right?

No, the damping coil and feedback loop are used to dampen out the natural
oscillation of the pendulum.

> - What is the purpose of the capacitor just before the damping-coil in
> the original plan and Larrys plan. Does it acts as a low-pass-filter?
> Does it has to be an electrolyt or is keramic possible?

This is a high-pass filter that is used to stabilize the damping of the
pendulum. You may need to change the parts values if the pendulum oscillates.
You can use two back-to-back electrolytic capacitors for this filter.

> - is it right, that the integrator just acts as a low-pass and has no
> influence on the periode of the pendulum?

That's correct, the integrator acts as a low-pass filter and is not in the
feedback loop, so it does not influence the period of the pendulum. What the
integration does is convert acceleration to velocity.

> - In Larry Cochranes Plan is a buffer amp in front of the dampingcoil.
> What are bufferamps for? I can find here only 741 ops. Is it possible,
> to use them as bufferamps?

741 op-amps are not very good. To drive the coil you need an op-amp that can
supply some drive current. I use a NE5534 but other low impedance output
op-amps can be used in this part of the circuit.

Larry Cochrane
Redwood City, PSN


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