Improving accuracy of photogates when used with a sphere

This article provides guidance in setting the photogate height when using objects other than a photogate flag.

For applications where a marble or ball is used with the photogates, the height the photogates are placed above the track is critical.  The timing of the ME-9498A is capable of measuring timed events down to 0.0001 s resolution.  Because the diameter of the ball that the photogate sees changes very quickly with the height that the photogate is set to, it is important for the closest work with the photogate to check the position that the ball causes the photogate to activate and deactivate.  Uncertainty in the detected diameter of the ball will outweigh the uncertainty of the time measurement in experiments using an object with curved edges.

1. Place the photogate at a height at which the photogate receiving ports are at the middle of the ball to be observed when it is on the track.

2. Slowly advance the ball along the track until the light (LED at the top of the photogate turns on.  This shows the position in which the front edge of the object has been detected.  Place a mark (use a post-it note) along the track to note this position.

3. Continue advancing the ball down the track until the light (LED) at the top of the photogate turns off.  This will mark the position where the photogate is no longer seeing the object.  Place a mark (use a post-it note) along to track to note this new position.

4. Measure the distance between these two marked position.  This will be the effective diameter of the ball. 

5. Adjust the photogate head height above the track and repeat steps 1-4 until the distance between the points of the photogate until the two marks on the track most closely match the diameter of the ball. 

6. Once the height of the photogate has been set, use the distance between the on and off positions for determination of the velocity of the ball as this is the effective width of the object within the photogates. 

Our Smart Gates and our Wireless Smart Gates are less susceptible to this issue as these use two beams that are set at the same height.  This avoids the necessity of determining the point where the photogate beam is blocked as the two beams will intercept the object at the same height and both use the leading edge of the object for determining the start and end of the timing event.