What happens to the accuracy of a vision system when an unexpected change in line speed occurs?
In a typical vision system a “part presence” sensor detects the leading edge of the part and sends a signal that tells the “image acquisition” sensor to take the picture. A brief time delay exists between when the part presence sensor detects the part and the image acquisition sensor. This time delay depends upon how far apart the two sensors are and how fast the line is moving.
Sensors provide a way for vision systems to take pictures based on the factor of time. An example of why this can cause problems: Line speed suddenly doubles with no adjustments made to the vision system. The time delay between sensors is now different than what was originally programmed into the vision system. This causes the system to see a different part from the one detected by the presence sensor, or see no part at all. In order to maintain inspection quality in this scenario, the physical distance between the sensors would have to double or the inspection time would need to be cut in half. Typically it is more difficult to adjust sensor distance that it is to rewrite time delay code in software.
Using a signal system based on distance, rather than time, is one way to “Do it Better” with a vision system for production lines with frequently changing speeds. An encoder is an electromechanical device that transforms rotational information into a series of pulses to provide output signals based on distance. When an encoder is installed on a conveyor it produces a series of pulses that are proportional to the motion of the belt. When encoder input works in conjunction with the part presence trigger the distance-based image acquisition can compensate for changes in conveyor speed.
The distance based approach allows the vision system to operate independent of line speed and part remains centered in the frame.
For more information on how EPIC can “Do it Better” with your Vision application visit https://epicsysinc.com/