PSN-L Email List Message

Subject: Re: (No subject)
From: "rem11560@............
Date: Thu, 1 Jul 2004 01:40:59 GMT

Hi Jason,

  I have some suggestions. Others please feel free to disagree.

1. Greatly reduce the mass of the pendulum. A mass of 5 pounds is probably too much for your superstructure. As the pendulum moves, it exerts a transverse moment on the support structure and baseplate, making it hard to achieve a stable long period. High mass also makes shunt resistance damping practically impossible. 

2. Stiffen the superstructure laterally with cross bracing. 

3. Having a magnet on the pendulum makes the sensor responsive to stray magnetic fields. Put the coil on the pendulum and the magnet on the base. While you are at it, place a second magnet on the opposite side of the coil, with poles reversed. You will get twice as much sensitivity, much better linearity of response, and half as much shunt conductance required for a given damping level. With the coil mounted on the pendulum, you might not need any extra mass, and that would make shunt damping easier as well.

4. Be sure to use a cover which shields the sensor from ambient temperature change and drafts. You may need a heater inside at the top to prevent convection of air upward from the baseplate.

5. Put a one microfarad capacitor across the input terminals of your amplifier. I have amplifiers of differing designs, and they all oscillate internally if there is no shunt capacitance across the input terminals when the source is inductive (i.e., a sensor coil). You cannot see this oscillation at the output terminals; it manifests itself as excess output noise, and in the case of my DC amplifiers, a large and fluctuating bias. 

  If you want to see my sensors, visit John Lahr's web site at the pages he kindly provides for my amateur efforts: 

Bob McClure 
Locust Valley, NY 

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