Board in/out sensors on reflow machines, do they work well?

> They work great as long as your conveyor speed > has been calibrated and checked on some type of > PM schedule. On some oven types these sensors can > warn you to possible problems with the conveyor > speed. Say the oven is calculating that the board > is moving at 28 inches per minute and for some > reason a bearing or pulley is binding on one end > and slowing the conveyor speed down to 26 inches > per minute, your alarm will go off. I would never > ignore an alarm that the machine is giving you. > Most of the time the machine has a just cause for > annoying everyone. The problem may need to be > looked into at a different level.

I often get alarms because the sensor miss counted inputs due to surface reflection vairation; the in sensor will count two boards and the out will count only one. Sometimes a hole in the board will cause a board to be counted as two. Or a board will be pulled back after triggering the input.

Different machines allow different methods to filter out false triggers due to baord texture (reflectivity). But they all allow the user to clear the system which indicates that it is recognized that it is not a perfect system.

What filter method to prevent false triggering of the sensors do you feel works best? Some may be:

1) Entering Board Length

2) Allowing the sensor to be positioned across the width of the oven opening.

3) Global ignor of input triggers within a time period.

4) Light break vs. light reflective sensor type.

5) Proximity sensor vs. light

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