Beatrice Parker
University: Massachusetts Institute of Technology
Gradation Date: May 2011
Hometown:
My Project: Contributions to the Electronic Kilogram Experiment: Measuring the acceleration of gravity and configuring a motion control system
The kilogram is the only base unit in the SI (International System/ Metric System) whose definition is based on a physical artifact instead of quantum references and the fundamental properties of nature. The scatter of the primary prototype artifacts is about 50 parts per billion after a century, affecting the uncertainty of other physical constants based on the mass standard. The goal of the Electronic Kilogram Experiment is to improve the measurement of Planck’s constant h (by measuring the ratio of mechanical and electrical power in a watt balance) and provide an alternate definition to the mass standard.
Many measured values and quantum constants go into the mathematics of the watt balance, and the acceleration of gravity still holds a substantial uncertainty. Vertical gravity gradients at a standard location (U.S. Geological Survey marker) on the laboratory floor and 4 m up to the reference mass on the watt balance, as well as a comparison between the two locations, was necessary to “transfer” the absolute value from the standard location to the watt balance. Measurements with a portable relative gravity meter reduced the uncertainty from 12 parts in 109 (2007) to 6 parts in 109. These gravity and uncertainty values have proved to be reproducible within the better uncertainty.
There are two measurement modes needed to obtain the power comparison, one of which measures the force on a kilogram mass. This implies that a mass must be placed on and off the balance. A system of two stepper motors controls the vertical translation of the two separate masses. The second goal of this project was to configure a new motion control system for the two motors and encoder position sensor. New hardware was installed and re-wired, and the LabVIEW control program was updated and debugged for the new system requirements. Finally, the new system was tested, with further adjustments made to increase efficiency and smoothness of operation.
