Date: Wed, 9 Jun 2004 22:12:58 EDT
Subject: My second horizontal sensor

  I wanted to send you a photo of my new horizontal sensor, but don't think I did because you were away. It is a close copy of my old original design, with neater execution. The pivots and jeweled bearings were salvaged from an old microammeter.

  It is being operated as a N-S sensor, and is performing well at a period of 11 seconds, which I extend to 24 seconds or more by digital filtering. It is very difficult to adjust for longer natural period, because the diamagnetism of the coil causes a decentering force on the pendulum. At longer periods, once the pendulum drifts very far off center, it just keeps going.

  Not shown in the photo are pieces of iron, 2 inches by 1 inch by 1/2 inch, that I added to the sides of the magnet assembly. The 1/4 inch top and bottom plates of the magnet were saturating, and I needed additional flux paths to avoid that.

  The base plate was cut from a 14" by 16" by 3/4 inch surplus aluminum plate I found on eBay for $35 plus $16 shipping. It cost another $20 to have the plate sawed in halves to yield two 8" by 14" pieces. I purchased the 6/32 12" threaded brass rods from, the 2" by 1" by 1/4" magnets from "ninteach" for $24.50, the #38 copper wire from, and plastic sheet from the local glazier for a cover and the coil assembly. The aluminum angle stock and the aluminum bar stock were items I salvaged from a drum recorder that I had built in my youth. One might find such stock at Home Depot, if not, you can pay a lot and get it from

  I wound the 1100-turn coil dry in a demountable form, and then let polyurethane varnish diluted with turpenture soak into it and dried in an electric fry pan at 150F (used an oven thermometer to find the right setting). I was able to extract the finished coil from the form with only one turn not firmly bound to the coil.

  If you want to buy an amplifier for this sensor from Larry Cochrane, you must substitute a 100K resistor for the 10K resistor normally used across the input stage. Also, you should add a 1.0 microfarad shunt capacitor as well to prevent parasitic oscillations in the input stage. You must also specify a voltage gain of no more than 1000 when using the 12-bit DI-154, as the sensor has an output of 0.86 v-s/cm. (You will probably have to use more gain for the 10-bit DI-194.). Damping of the sensor is accomplished by adding more shunt resistance across the coil. I use about 94K, not including the 100K amplifier impedance.

  Suitable magnets are not always easy to find on eBay. I think the best size is 50mm by 18mm by 6mm, which sells as blk18. That is what I used on the E-W sensor. That size seems to produce less diamagnetic decentering effects.


Bob McClure