| 摘要 |
<p>The appts. uses a series of markers, near the ends of the bridge, disposed in a horizontal line and separated by decreasing intervals towards the ends of the bridge. A fixed detector senses the passage of the markers to provide an instantaneous indication of the speed of the gate during the end of its opening or closing cycle. The detector commands the braking of the bridge, by inverting the polarity of the supply to the linear motor, as a function of the measured speed. This permits the time taken for the bridge to travel the distance separating two consecutive markers to remain essentially constant. The detector is such that it commands the braking when the time taken between two consecutive markers is below a threshold, but ceases to command the braking when an upper threshold is reached.</p> |
