4.2 Calibrating the Position Transducer

1. Connect the power supply to the terminals at the bottom of the torsional oscillator apparatus.  You are going to rotate the disc by applying current to an electromagnet which interacts with a permanent magnet on the rotor.
2. Before turning on the power supply, make sure the voltage is set to maximum and the current is set to zero.
3. Turn on the LabView data acquisition program.  Start the program using the arrow in the upper left.  Press the ‘Plot’ button near the bottom right, the press ‘record’.  The plot should fill with voltages that indicate the angular position of the disc.  ecord the voltage from the position transducer as well as the angle of the disk from the ruler on its circumference.
4. Apply approximately 2 A – or some value that will rotate the disc to an easily-read angle.  Record the voltage from the position transducer. Turn the current back to zero.
5. Swap the leads from the power supply to rotate the disk the opposite way.  Apply approximately 2 A again; the disc should rotate in the opposite direction.  Record the angle and the voltage from the position transducer.
6. Repeat the steps above a few times.  We use this range to avoid any hysteresis in the torsion fiber that may arise from only using it in one direction repeatedly.  Use the average of your measurements as a calibration factor for the position transducer voltage.

4.2 Magnetic Damping

1. Make sure the magnetic dampers are all the way disengaged and make sure there is no weight hanging on the rotating disc.  We need to take data with as little friction force acting on the disc as possible.
2. Use your hand to turn the disc slightly (half a radian or less should do it).  Let the disc oscillate a few times before recording data.  Don’t worry, it will oscillate for a long time.  You’ll want about 20-30 seconds worth of data.
3. Turn the knobs until the magnetic dampers are just about to overlap the disc.  Repeat your measurement.  The effect of the damping should be noticeable.
4. Each turn of the knob corresponds 1.27 mm of translation.  Give each knob two turns and repeat the data collection.  Repeat these steps until you have about 7 data sets or the disc stops rotating before it can move through about 5 oscillations.
5. You can use the above data to determine the Q value of the oscillator as a function of the radial overlap of the damper.

4.3 Damping Due to Fluid Friction

1. Completely disengage the magnetic dampers.  Retake the data for friction-free oscillation.  Hopefully your Q value hasn’t changed.
2. Ask the TA or technician to insert the paddles into the apparatus.
3. Make sure the paddles are turned such that the face is vertical.  You want as little fluid friction as possible here. Start recording the position data.
4. GENTLY rotate the disc.  The arms will stop you from rotating beyond VALUE. Release the disc.  You will need about 20-30 seconds of data.
5. Rotate the paddles such that the faces are horizontal.  You want as much fluid friction as possible here.  Start recording the position data.
6. Again, rotate the disc and release.  Make sure you have 20-30 seconds of data.