These devices are used almost exclusively for measurement of vibratory displacement in those (many) cases where a fixed reference for relative-displacement measurement is not available.
The basic principle of seismic (absolute-) displacement pickups is simply to measure (with any convenient relative-motion transducer) the relative displacement of a mass connected by a soft spring to the vibrating body.
Professor, Dept. of Applied Electronics and Instrumentation Engineering, Heritage Institute of Technology, Kolkata
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B. Sc. (Physics Hons) CU, B. Tech. (Instrumentation Engineering, CU), M. Tech. (Instrumentation and Control Engineering, CU), PhD (Electronics and Instrumentation Engineering, JU)