Parameter Identification in a Magnetic Suspension Force Transduction System

In force transduction systems, magnetic suspension can be used to facilitate the measurement of a force acting between two objects in different environments, namely, between the force transducer in one environment and the test object in another environment. In the past, it has been employed in densimetry and equations-ofstate measurements of fluids and gases. An instrument being constructed at the National Institute of Standards and Technology (NIST), referred to as the magnetic suspension mass comparator (MSMC), also employs force transduction between two environments to provide a direct means of vacuum-to-air mass dissemination. Within this work, we discuss the identification, through a combination of finite element analysis and experimental measurements, of the system parameters used for feedback control of the suspension. A Hall effect magnetometer is used to determine the position of the suspended object. Emphasis is placed on experimentally and numerically characterizing the magnetic field and its appropriate gradients to determine the set point of operation. The information presented here is useful for a variety of applications that utilize magnetic suspension for force transduction.