Laser and one-dimensional PDA on the elevator overspeed measuring

This paper provide a new method of elevator overspeed measurement. Helium-Neon laser is mounted vertically on the surface of the elevator car. One-dimensional photodiode array(PDA) is installed at the top of the shaft. The laser beam passes through the beam shaping lens for good homogeneity and proper divergence angle. According to the total height of shaft and the divergence angle θ=0.4mrad, the spot radii in different heights of elevator car are measured. On the basis of small divergence angle a model proposed that the beam radii change linearly with the heights of elevator car. The interval in adjacent detection units is determined in this model. The laser spot is spreading from center to edge when the elevator car is moving down. The time interval of signals in adjacent detection units are measured by timing circuit. When the elevator is running in maximal speed, the time interval of signals is defined as time threshold. Time intervals of signals are compared with time threshold. Time intervals of signals are greater than time threshold that elevator running normally, time intervals of signals are less than t0 that elevator overspeed.