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This article provides an example of printed circuit board detection using one-way optical fibers and amplifiers.
Some of the circuit boards are less than one millimeter thick, and there is hardly any space for a sensor in the machine itself. Conventional through beam systems require mounting space above and below the object, but this space is not always available.
One approach is to use through beam optical fibers in a lateral configuration. In this setup, the transmitter and receiver are not positioned above and below the PCB path, but rather to the left and right of the path along which the PCB travels. The compact fiber heads take up very little space, and the actual amplifier - e.g., a WLL80 or a GLL70 - is located outside in the control cabinet.
The receiver is intentionally not mounted at exactly the same height as the transmitter, but with an offset. As a result, the light beam crosses the plane of the circuit board at an angle rather than parallel to the surface.
What’s the benefit? An ultra-thin circuit board, which would barely interrupt the light beam with a straight beam arrangement, has a significantly larger cross-section in the beam path due to the angled alignment.
Another practical feature: The distance between the two fiber heads can be adjusted. If the PCB format changes or a different board size is used, there’s no need to rebuild the entire setup - you simply adjust the distance.
Teach in and device configuration
Response time The response time should be set to a level where the received light level is below the saturation (display dig. shows 9999). It should be fast enough for the process and slow enough to have a well recognizeable difference between PCB presence and no PCB. For example fictional: 2000µs response time -> 5500 dig without PCB and 230 dig. with PCB.
One point teach-in During this process, the sensor records a reference value and uses it to calculate the switching point.
Make sure there is no PCB in the beam path - the light beam should travel unobstructed from the transmitter to the receiver. Then press and hold the TEACH button on the amplifier for more than one second. The sensor saves the current light reception value as the teach point and uses it to calculate the switching point.
What is the teach-in offset?
The switching point is not set equal to the teach point.
Instead, a percentage offset is applied, known as the teach-in offset. SP = TP + X%, where TP is the measured teach point and X is the set offset.
In through-beam mode, the received light value is high when the beam path is clear. When a circuit board interrupts the beam, this value decreases. The switching point must therefore be below the clear-beam value with a negative offset.
In practice, this configuration is primarily used in situations where space is limited and thin objects need to be detected. These are typically assembly lines where, before components are placed, the system checks whether a circuit board is present at all. Or feeding and transport systems on which circuit boards are moved in magazines or on conveyor belts. This setup can also be used in quality control - for example, to verify that a circuit board is correctly seated in a workpiece carrier.
The combination of side-view detection, minimal space requirements, and reliable detection of even the thinnest boards makes this setup a straightforward and robust solution.
