Shackleford-Gundersen Seismometer

 Construction Notes

The Pendulum:

The length of the pendulum should be 10 to 12 inches (~25 to 30 cm) long. I used a piece of 1/4 inch brass tube with a lead sinker soldered to the bottom end.

The following components are mounted on the pendulum:

Magnet: The magnet is used for damping the pendulum, and should be mounted somewhere between 1/2 and 2/3 of the way down the pendulum. Its location is not that critical.

Oscillator and Transmitting Antenna: I used a standard 4Mhz crystal oscillator,[circuit] with the components soldered to a piece of ‘Vero board’, however I have done a [pcb layout], [with overlay].The circuit and pcb can be seen here. The SG filter board, has output terminals to power the oscillator. +12 volts DC output, and ground.

The power to the oscillator needs to go through the hinge that holds the pendulum. You can use some very small wires to pass the power to the pendulum holding the oscillator, or, use the power pass through hinge (see the pictures that I used)

The oscillator should be mounted near the top of the pendulum transmitting antenna. Soldering the tab of the 2SC1096 to the antenna holds the satisfactorily and couples the Rf to the antenna. The transmitting antenna is mounted at the end of the pendulum. If you are using double sided PC board material for your transmitter antenna, you must solder a wire to join both sides of the board.

Pendulum Weight: The lead weight should be around 95 grams.

Damping Coil:

Coil: Use a solenoid coil salvaged from a washing machine 240V water valve.

Magnet: A number of small Neodymium magnets (3mm) stuck to a small steel bolt mounted through the pendulum shaft, seems to work fine.

Location: The damping coil should be mounted on the frame and located so that the magnet, mounted on the pendulum, is sticking in about 1/2 the length of the coil. You need to make sure that the magnet is not touching the coil as you swing the pendulum back and forth a little.

Wiring and Polarity: A 2 pin connector located on the SG filter board supplies the signal to the damping coil. If the pendulum, oscillates when power is applied then you will need to reverse the wiring to the coil.

You may also find that the Damping Adj trim pot can not reduce the signal enough for proper dampening, or, may not be able to supply enough signal. The amount of signal need for proper damping depends on the weight of the pendulum, the strength of the magnet, and the number of turns of wire in the coil. If the Damping Adj pot can not supply enough signal then you should reduce the weight of the pendulum. If the trim pot can not be turned down enough, then you should add a resistor in series (not across the coil) with one the of the wires (it doesn't matter which one) feeding the coil. A 4.7k 1/4 watt 5% resistor is a good starting point. The resistor should be located near the coil.

 For more information on damping please see below

Receiving Antenna Plates:

Spacing: The receiver plates should be spaced about 3/16 of an inch (~5 mm) on each side of the transmitting plate. After hanging the pendulum you should level out the base plate and then secure the antennas to the base plate so that they are the proper distance from each side of the transmitter plate.

Receiver Plate Tuning: Mount a transmitting plate the same size as the receiving plate on a fixture. Then connect the transmitting plate to the oscillator. Locate the receiving antenna plate about 3/16 on an inch (~5 mm) away from the transmitting plate. Using a DVM (Digital Volt Meter) measure the output voltage and the capacitor for maximum output. Each antenna should produce over 9 volts if you use an RF probe, and produce about the same voltage, but one should produce a positive voltage and the other a negative voltage. Note: This procedure should be done with the receiving antennas disconnected from the SG filter board.

SG Sensor Damping Adjustment

This describes the method, by Larry Cochrane, http://www.seismicnet.com to adjust the damping on the SG sensor.

You will need the following:
Some sort of coil. I use the coil from a relay, or you can wind one. It should have 1000 turns or more of any small wire size. The coil must not have any magnetic material, just the bobbin and the wire. Attach enough wire to the coil, speaker wire will work fine, so you can do the damping testing way from the sensor. You will also need a DC power supply or battery, and some way of viewing the data. You can use your SDR or EMON system, or, you can also use a storage scope if you have access to one.

Setup:

After setting up the sensor, mount the coil behind, or on the end of, the normal damping coil used by the sensor. You can use some tape or other sticky substance (I use something called "Stik Tak" available a hardware store) to hold the coil on the end of the sensors damping coil.

The best thing to do is run your data logging system in another room so you will not disturb the sensor. Run the wire from the damping adjustment coil into the other room and connect up one end to the power supply. You will be temporarily connecting the other wire to the power supply or battery to excite the pendulum.

Damping Adjustment:

After covering the sensor and letting it settles down, your ready too go. Start with the Damping Adjustment trim pot set to minimum. Minimum is when you turn the pot counter clockwise until you can hear a click.

Set your data logging software to an X scale of 1 so that you can see what happens when you excite the pendulum.

Now quickly touch the second wire to the power supply or battery. You will need to experiment with the voltage of the power supply or battery and how long you touch the wire to it. One thing you can do is charge up a electrolytic capacitor and discharge it through the coil. A 10uf / 25V cap should be a good starting value. It is important not to saturate the sensor by exciting the pendulum too much. You can control the amount of movement by varying the voltage to the excitation coil or the amount of time you connect the coil to the power supply or battery.

Using the two WinQuake screen shots below as a reference, adjust the Damping Adjustment trim pot so that it matches the second screen shot.

Under Damp System

Note that it takes several cycles for the signal to die down.

Properly Damped System

Note that there is no, or very little, undershoot in the negative direction.

Problems:

If you have too much damping with the trim pot turned to minimum, you will need to add a resistor in series with the coil. With the trim pot all the way to minimum add enough resistance so that the system becomes under damped. Then use the trim pot to adjust the damping to the proper point.

If you can't get enough damping with the trim pot all the way to maximum then you will need to do the one, or a combination of, the following:

Reduce the weight of the pendulum.

Add more turns of wire to the damping coil.

Get a stronger damping magnet.