Mechanical , Sensor and Control System Design for an Accelerometer Calibrator with One Part Per Million Accuracy

This thesis presents the design and implementation of a low frequency accelerometer calibrator that will be used to calibrate United States Primary Standard Transducers at the U.S. National Bureau of Standards, Gaithersburg Maryland. Accelerometers convert a rate of change of velocity into a measurable quantity, typically voltage, and must be calibrated to find their sensitivity as the ratio of volts to acceleration. Calibration of a Primary Standard Transducer must be based on fundamental units of length and time. Currently this is accomplished by mounting the test accelerometer on the moving coil of an electrodynamic exciter and driving it with a sinusoidal signal of known frequency. The amplitude of oscillation is measured using laser interferometry techniques. Knowing the frequency and amplitude of oscillation, the acceleration can be derived and subsequently the sensitivity calculated. The uncertainty in the calibration is ±1.0% in the existing system while it is limited to a 1 7/8 inch double amplitude displacement and 2 to 49 Hz bandwidth. This thesis presents four designs as the solution to a new, more accurate calibrator. The worst case design has a predicted 10 parts per million accuracy, while the best design has a predicted 1 part per million accuracy, 10,000 times better than the existing calibrator. The worst case design was the least expensive to implement and therefore a prototype of this design was built using the following components: linear air bearing with 20 inch travel, linear brushless DC motor with continuous force output rating of 18 Ibf and laser interferometer transducer system with 0.1 microinch resolution and maximum allowable slew rate of 70 in/sec. This design has a 8.5 times increase in double amplitude displacement, a predicted 1,000 times increase in accuracy and 1/2 times decrease in bandwidth in comparison to the existing calibrator. The large double amplitude displacement is required to study the bandwidth characteristics of an accelerometer under a constant peak accelerations, this is not possible with existing calibrators. Thesis Supervisor: Professor Alexander H. Slocum Title: George Macomber Assistant Professor of Civil Engineering