Digital Camera Technology for Today’s Industrial Imaging Applications

Tuesday, May 01 2007

Parallel signal standards next moved to LVDS (Low-Voltage Differential Signaling), which has a lower signal voltage than RS-422. LVDS also accommodates higher transmission frequencies (>40 MHz) and consumes less power than TTL or RS-422. However, the same issues with cable costs and pin counts remain.

With the availability of serial digital standards like Camera Link, many of the problems inherent in parallel signaling were alleviated. Because it transmits data serially, the number of required signals per tap is greatly reduced. And, since Camera Link is a standard, cables are interchangeable between cameras and frame grabbers. Camera Link allows up to four independent control signals for events such as reset and integration, as well as integrated serial communication for camera configuration and status, so that Camera Link cameras incorporate many of the control features required for machine vision applications. The ability to dynamically reconfigure and query the status of the camera under software control, while sometimes available, is rare with analog cameras.

What to Consider When Selecting a Digital Camera

The initial step in any imaging application is determining what to acquire and what features within that image should be highlighted. As an example, let’s consider an application in electronics manufacturing that defines an object to be acquired, such as a populated circuit board. The overall image will contain many features of interest for inspection. The electronic component features — such as size, identity, quantity, location, and orientation — are all critical for the correct and complete assembly of a circuit board. Other features may include markings on the board and characteristics of the bare board itself, such as material density and routing/continuity of the circuit board traces. The features of interest within the object’s image become the inspection points, and the characteristics of those features are the inspection criteria.

The resolution of the image required for inspection is determined by two factors: the field of view required and the minimal dimension that must be resolved by the imaging system. Using a basic example, if a beverage packaging system must verify that a case is full prior to sealing, it is necessary for the camera to image the contents from above and verify that 24 bottle caps are present. It is understood that since the bottles and caps fit within the case, the caps are then the smallest feature within the scene that must be resolved.

Once the parameters and smallest features have been determined, the required camera resolution can be roughly defined. It is anticipated that, when the case is imaged, the bottle caps will stand out as light objects within a dark background. With the bottle caps being round, the image will appear as circles bounded by two edges with a span between the edges. The edges are defined as points where the image makes a transition from dark to light or light to dark. The span is the diametrical distance between the edges.