Cavendish Experiment

In Fall of 2017, I was commissioned by the Physics Department at New Mexico Tech to create a LabVIEW program that would record data from a Cavendish apparatus for their senior lab.

The Cavendish experiment has a set of ball bearings, balanced at opposite ends of a beam that is suspended by a fishing line.  If the beam is disturbed so that it oscillates, the period of this oscillation can be used to calculate the gravitational constant.

The existing system was a manual pen recorder that moved the pen in response to the voltage generated from some capacitive sensors near the balance.

Calibration of this system is done with a small potentiometer and voltmeter.  Before the beam is disturbed, the potentiometer is adjusted until the voltage is zero.  This could be done in software, but it was decided to leave this hardware in the loop as to not make the system too simple for students.

Data is collected through an Omega OM-USB-1608G.  I had to find drivers for this device that would talk nicely with LabVIEW.

The user is greeted with the front panel shown below.  There will be a graph corresponding to the voltage from the capacitive sensors.  The graph can be manipulated by the tools shown below the graph, including the “Reset Zoom” which will undo all existing zooming.  This program will write the data to a tab-delimited text file as well, for future analysis.

The block diagram of this code is pretty straight forward.  The biggest challenge was dealing with the Omega drivers, but they are in place.  The second biggest challenge was developing the “Reset Zoom” routine.

Before the loop executes, the header for the text file, clears the chart and initializes the Omega device.  Once in the loop, the data is recorded at the specified sampling rate (4 Hz in this example), printed to the chart and the text file.  The routine checks the status of the “Reset Zoom” button.  If it has been pressed, the chart resets.  Once the “STOP” button has been pressed, the loop ends, the file is closed, the Omega device is disconnected, and any errors are handled.

One future work item will be incorporating a laser and some optical sensors.  The laser will be aimed at a tiny mirror on the beam, and then reflected backwards to the optical sensors.  As the beam turns, the reflection will move along the optical sensors, giving a position of the beam location.  The period of the beam’s rotation can be measured this way as well.

Last Update:  2/17/18