Poorly-Defined Transition Errors
If one infers the speed v from the a solid object of width W given a time of interruption T, as v = W/T, there will be a small error due to the difference in digital transition time between the rising and falling transitions (td=~250 ns) and a significant error resulting from the difference in the orientation of the shadow (right edge versus left edge) causing a difference in the position at which the digital transition occurs (w ~ 1 mm) relative to the position of the edge of the object. If the speed of timed object through the gates is v, the timing error would be
terror ~ w / v
If a cart is moving at a speed of 1 m/s, then the error would be 1 ms!
For best photogate timing results, only measure the time between the same type of photogate transition (either light-to-dark or dark-to-light) from an interrupter that shadows the same side of the photogate. These criteria require a beam interrupter with at least two bands of known spacing.
We offer the following photogate accessories with equally-spaced interrupters to avoid this problem: