“Stethoscopes” for nonintrusive monitoring

This paper provides a survey of example sensors that can be implemented with nonintrusive electromagnetic measurements. Stray electric and magnetic fields exist around many important components in commercial and industrial processes. For example, power cables operate surrounded by magnetic and electric fields. Flow meters operate with cyclically-varying magnetic fields. And stray electromagnetic fields can serve as an energy source for powering sensors wirelessly. These stray fields provide remarkable opportunities for nonintrusive sensing of industrial processes. Sensed information can be used to establish monitoring in new or retrofit systems, or can be used as a backup or redundant source to verify the operation of an installed sensor network. Three different example sensors are presented in this paper for power monitoring, fluid flow tracking, and electromechanical vibration monitoring. All three sensors make use of a common set of circuits for electric and magnetic field sensing. They illustrate approaches that could be applied for many other sensing applications.