Digital Inspection: A Camera Perspective
With all of the options and flexibility available, digital inspection makes sense.
Analog cameras dominated the early years of machine vision systems, offering adequate performance, a simple interface, and a moderate price. Today, digital cameras are the norm for most new and even many legacy applications—including quality inspection applications. Lower prices, standardized interfaces, and opportunities for customized preprocessing have made digital imaging less painful and more profitable. The digital nature of these cameras has also created an opportunity to incorporate functions beyond image capture, increasing system performance.
There are several triggering events that can justify making the investment in a new vision system.
One is a desire to increase the throughput of an inspection system, which is set by the camera frame rate.
Another benefit of digital inspection is the ability gained to augment camera operation with customized preprocessing.
Early digital camera designs used proprietary, high-speed interfaces with low-voltage differential signaling (LVDS). This required large, bulky, and expensive cables that could only run for a limited distance before connecting to the frame grabber or processor. Further, because the camera interfaces were proprietary, and the connectors and pin-outs were non-standard, system developers needed to create a custom cable for each camera/frame grabber combination. This added time, cost, and made the system less reliable (whenever there was a problem—check the cable first!).
Today’s digital cameras offer improved, standard interfaces to simplify system assembly. The rise of standard interfaces has freed developers to mix and match components from different vendors to meet their system requirements. As one of the first standards, CameraLink continues to play a role in vision inspection systems where large amounts of data must be transferred quickly and reliably to a frame grabber for processing.
In recent years, interfaces like FireWire, Gigabit Ethernet (GigE) and USB3.0 have brought several advantages to digital inspection systems, offering system designers a ubiquitous interface choice and, in the case of GigE, extending cable lengths for more remote inspections. A further advantage is the reduction in cable complexity and cost.
Of these interfaces, GigE has proven very advantageous, and the industry has seen a proliferation of GigE Vision systems as a result. The interface also made it possible for industries beyond machine vision—including Intelligent Transportation Systems (ITS) and cinematography—to capitalize on these advances.
The development of standardized hardware interfaces has recently led to standardization in the software and control areas too. Progress continues with standards like GenICam™, which offers a common set of command options for digital cameras so that application programs can also remain independent of camera choice. Applications simply make standard calls to drivers that handle any data format or other hardware-specific differences.
Speed and In-Camera Preprocessing
In addition to improving system interfaces, digital cameras can now easily achieve hundreds of frames per second acquisition, and resolutions beyond 12 Megapixels. The combination of speed and resolution is an enabler for inspection systems, providing both clarity and throughput, allowing manufacturers to maintain a high level of product quality and more efficient delivery with fewer field failures. Print, textile, food and parcel sorting, as examples, all benefit from the integration of more advanced imaging components.
A recent innovation in digital cameras for vision inspection is the availability of image preprocessing in the camera. A pre-processed video signal may still have the data structure of an image, but with changes to the data content. For instance, possibilities include edge enhancement, perspective distortion correction, and threshold filters, as well as time stamp, exposure duration and other information about the image embedded in each image frame.
A side benefit of in-camera preprocessing is that it does not affect the system interfaces or hardware design. The camera can achieve its preprocessing by routing the sensor data through an FPGA for manipulation before passing it on to the rest of the system. From a system design standpoint, all that changes is the interpretation of the data coming in. In many cases, preprocessing can eliminate tasks that would otherwise be handled in the host computer, reducing processing demands and increasing system performance.
As a result of these innovations and the inherently digital nature of the image data within the camera, digital cameras provide vastly greater design flexibility and simpler system design than analog cameras. By ensuring that pixels reliably represent the same point on the image every frame, digital cameras eliminate the calibration that many analog cameras require for their digitizers. Similarly, digital cameras can provide white balance calibration in a single location (the camera) while analog systems may require calibration in both the camera and frame grabber.
System Design and Set-up
The use of digital cameras also simplifies system design by supporting the easy implementation of configuration options. Changing the image resolution of an analog camera within a machine vision system, for instance, forces timing changes in the frame grabber and alters the digitizer clock speed. Plus, depending on the limitations of the sensor in the analog camera, simply changing digitization frequencies to produce more pixels may not provide any additional information. Changing digital system resolution, on the other hand, involves only changing a software-controlled feature of the camera with notification of the host computer to handle the new data structure. The system can readily be designed to adjust to a new camera resolution.
Adopting Digital Reduces Costs
There are several triggering events that can justify making the investment in a new vision system. One is a desire to increase the throughput of an inspection system, which is set by the camera frame rate. A new requirement for color vision can also prompt movement to a new camera system. The need to replace failing components or ones that have become obsolete can be another triggering event. Virtually all of the new development in vision is based on digital camera systems, so a replacement for an obsolete analog camera that offers equivalent or better performance may be difficult to find. A digital camera quickly becomes the obvious choice.
Another benefit of digital inspection is the ability gained to augment camera operation with customized preprocessing. This preprocessing can represent unique value added elements to the vision system’s capabilities, or free the host PC from some image tasks to manage additional work. Digital cameras afford developers an opportunity to either lower the requirements for (and the cost of) the host PC’s performance or increase system capabilities without requiring a new PC.
With all of the options and flexibility available, digital inspection makes sense. With the right camera, machine vision systems can increase productivity, simplify maintenance, and lower their total cost of ownership.