PSN-L Email List Message

Subject: Re: Velocity Sensor Designed Specifiically for diff input
From: "Geoffrey" gmvoeth@...........
Date: Fri, 5 Feb 2010 21:02:56 -0700

Hello Chris,

electro mechanical Sensor only
and not the associated electronics.

----- Original Message ----- 
Sent: Wednesday, February 03, 2010 4:02 PM
Subject: Re: Velocity Sensor Designed Specifiically for diff input

> In a message dated 03/02/2010, gmvoeth@........... writes:
> Are you  able to provide us with a mag/coil design with three wires 
> specifically  designed to work
> with a differential amplifier.
> This would mean a center  tapped coil with two wire each one going to + and 
> - and the third  wire
> going to neutral ground ?
> Hi Geoff,
>    Sure, but is there any problem in doing this?
>    The ordinary single channel opamp with 4 resistors  to give a 
> 'differential' opamp circuit normally has a single 'floating' coil eg  LT1007. The 
> feedback maintains the same voltage on both the +ve and -ve inputs,  so the 
> coil drives a resistance of 2 x Rin + Rcoil. With high efficiency  NdFeB 
> magnet blocks, this low impedance can significantly damp the mass and  can even 
> damp it beyond critical. The input impedance relative to analogue  ground is 
> different for the two inputs.
>    If you use a true differential input opamp, like  the INA118, the input 
> impedances to both inputs are very high both relative to  one another and 
> to analogue ground. You need to use two reels of wire to wind  one coil. It 
> is an advantage if you can arrange for the two wires to be twisted  together. 
> Alternatively, you can buy dual core insulated wires. The centre  of the 
> coil is earthed.
>    You can't separate the coil design from the  amplifier circuit design. 
>    I hope that this helps.
>    Regards,
>    Chris

What i mean is a coil like a center tapped inductor


CO part and the IL part have to be identical.
I do not know if you can wind such a thing
all at once or need two identical yet seperate
coil magnet assemblies.

Forget the amplifier part altogether,
use what you want.

This arrangement is the what i believe to be the only
proper one for MY application.

I have never built such a singular thing
and can only guess where to begin.

I do know that inital signal strength and
signal to noise ratio is paramount for
any sensor at all and not just for areas of
our own interest.

Easiest way to get a good signal to noise
ratio seems to be to use a two ton (avd.) mass
instead of a two pound (avd.) one. The highest density
material you can afford.

The electronics is moot since I know what
I want to do there.

Superconducting wire sounds ideal
yet none at room temp as far as i know.

At the moment i am thinking two of your
coil/magnet ideas in additive fashion
connected to the same mass.

Two coils should double the signal
right from the start.
Four would quadruple it.

Ideally you use these things in sets of two
and only one is what I call halfassed.

I am only interested in vertical sensors.

Thanks for your responses.


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