Mind your I's and O's

Vendors have simplified camera- and software-configuration tasks so most test engineers can easily set up inspection routines. But when it comes time to use a vision system's I/O ports, engineers often wonder, "How do we use those signals to do useful things?"

They may think they can connect solenoids, motors, and sensors on a production line directly to a vision system's I/O connectors. But vision systems don't operate autonomously within a manufacturing plant. Instead, they serve as part of a complete production line, overseen by one or more programmable logic controllers (PLCs).

The design of a real vision system requires a thorough understanding of the types and uses of various I/O signals.

The I/O ports on vision systems operate with 24-V levels and 5-V TTL signals and may provide uncommitted NPN and PNP transistor outputs. You can use a variety of signal types to interface with external devices, but you'll most likely use a PLC, even if you need to turn on simple pass/fail lamps. Inputs also originate from a PLC, which can tell a vision system when a part reaches a set position, what types of inspections to run, and so on.

I/O operations get complicated when a camera operates on a production line some distance from sensors and actuators. The vision system may "see" a defective product, but because the product must travel farther down the line to reach the reject-bin actuator, the vision system cannot command a PLC to immediately push the part into the bin. Instead, the vision system or the PLC tracks the part and signals the actuator to push it in the bin at the proper time.

Tracking may require that system designers use sensors to detect the movement of parts and establish time delays to synchronize operations. After all, several good parts may have to pass the reject-bin actuator before a failed part arrives.

This example involves combinations of devices that connect to the camera and to the PLC and that must operate in sync with each other. Even if engineers master the use of the individual I/O signals, they still must get all the I/O devices to work together.

In cases when a vision system must produce and sense "local" signals, engineers must understand how to control I/O lines that don't involve a PLC as well as controlling those that do. Local signals, for example, can control light sources and sense the arrival of a part for inspection. It makes no sense to run these signals through a PLC, which would skew their timing. Although I/O signals seem easy to understand in theory, their application can get quite complicated.