Theoretical and Experimental Analysis of Eddy Current Contactless Speed Sensors for Linear Motor Elevators

Linear motor elevators are the strongest candidates for the next generation of elevators. They require a new method for sensitively detecting overspeed, while dispensing with the requirement of ropes that span the building that are used in conventional sensors. In this paper, we report on the theoretical and experimental analysis of a new contactless and ropeless safety device actuator system that uses electromagnetic inductive force to measure the speed of movement with good sensitivity. The sensor is mechanically actuated by eddy current forces and its operation does not depend on electrical power supplies or electronic components, making it suitable as a safety device. We describe two methods for making the speed response nonlinear, in order to increase sensitivity to overspeed. In the first, the eddy current forces are generated through a variable overlap of a magnet over a reaction plate, thus the speed-force relationship is nonlinear. In the second, the eddy current forces are modulated by the speed to obtain mechanical resonance effects. We have also developed a semi-analytic calculation method for the detailed analysis of the electromagnetic field distribution and eddy current forces in the proposed overspeed detector system, and verified it by experiments. Computed and measured values show good agreement, supporting the validity of the analysis.