Identifying Frame Grabber Core Competencies

Do you know where your frame grabber functionality is located?

Automated parts inspection is a good example of a machine vision application. In a typical case, the parts may be moving on a conveyor belt and have unknown orientation. The inspection system must capture a picture of each part and determine the presence, absence, or quality of a feature such as laser-etched text. How does such a system work in detail?

Typically, the image-sensing device would be an area scan CMOS or CCD camera. CCD sensors long dominated the camera market, but the CCD process never was a good match to mainstream semiconductor processing. From that viewpoint, CMOS sensors are much preferred, but their performance has been inferior until recently. Today, high-resolution CMOS cameras are available with very good performance.

For this product inspection example, a camera with square pixels arranged in a conventional rectangular sensor area would be appropriate. Cameras with square pixels make computations straightforward. If the product being inspected had been a web of material instead of a solid object, then a linear CCD-based line-scan camera would be a better choice. Pixel resolution across the web can be more than 8,000 points, and the motion of the web provides scanning in the other direction.

Obviously, there are many more camera specifications to consider, lighting can be critical, and there�s a computer to specify as well. But the largest undefined hardware requirement is for the frame grabber, the functional block that synchronizes image acquisition and transfers data from the camera to the computer.

Background
When only analog cameras were available, frame grabbers performed digitization, synchronization, data formatting, local storage, and transfer to the computer. The advent of the peripheral component interconnect (PCI) bus provided high-speed data transfer and reduced or eliminated the need for local storage for some applications. More recently, so-called smart cameras have integrated the frame grabber functions and computing necessary to perform relatively simple tasks.

Nevertheless, frame grabbers are very much alive and well, just different than they used to be. For one thing, higher-end applications generally use digital cameras. Digitization in the frame grabber is no longer required, but these cameras often have multiple taps to allow simultaneous parallel data output from several sections of the CMOS target. Consequently, the overall data rate can be hundreds of megahertz and exceed the capacity of common PCI-32 buses.

The Camera Link standard governs the camera-to-frame grabber interface. Based on National Semiconductor�s channel link technology, the specification provides from four to 12 low-voltage differential-signaling (LVDS) serial data channels, four control signals, one to three clock lines, and a duplex serial link. Base-level cameras use four channels, medium-level eight, and full-level 12, corresponding to one, two, or three Camera Link chips, respectively. Composite data rates as high as 680 MB/s can be handled today, and higher rates are on the drawing board.

One of the major physical problems solved by the Camera Link standard is cabling. Prior to the standard�s introduction, LVDS interfaces were being used for their high speed, but different camera and frame grabber manufacturers adopted a variety of connector types. With Camera Link, there now is a stand-ard cable that simply works.

Nevertheless, many non-Camera Link LVDS systems were built, and frame grabbers are available to cope with them. For example, MuTech�s Series MV2500 products use a MDR 68 Connector and cater to 16 b of LVDS data. In addition, they provide synchronization, event input, strobe output, trigger, and RS-232 I/O functionality. The company also produces the MV2600 Camera Link-compatible series with one Hirose and two MDR26 Connectors (Figure 1).

An interesting perspective on the evolution of frame grabbers was given by Pierantonio Boriero, a product line manager at Matrox Imaging. �Any machine vision system, whether it is a high-performance custom system or a cost-efficient smart camera, requires common components: lighting; lenses; image sensors, capture, processing, and analysis; and communications. Where they differ,� he continued, �is in the level of integration and performance, not functionality.�

On this basis, you can consider frame grabber functionality to include digitization; camera control; image acquisition, storage, time stamping, processing, and analysis; synchronization; data transfer to the host computer; and communications related to performance monitoring. When cost is very important, cheaper analog cameras may be used, so most of these functions reside on the frame grabber board although image storage may be via the PCI bus to part of the computer�s memory.

The relative cost advantages of analog cameras and smart cameras were discussed by Dr. Reinhard Borst, vice president of engineering at ELTEC Elektronik AG. �Digital interfaces and computing power are integrated into smart cameras. These are additional cost factors, and while it�s true that a smart camera may be lower cost than a camera and a frame grabber, this is only the case when considering a single camera.

�If four or eight cameras are attached to one frame grabber, the additional cost of a smart camera solution is very noticeable,� he explained. �In that case, using analog cameras and a frame grabber is simply lower cost.�

In high-end digital systems, the frame grabber may use an onboard look-up table (LUT) to reformat the image data ahead of local storage. If the frame grabber includes a DSP, it could run a Bayer filter algorithm to interpolate colors before transferring the data to the computer. And, the byte order could be swapped to suit Big Endian computers.

Triggering and Control
Especially in applications involving motion, it�s important to trigger image acquisition when the UUT is in the correct position. In the case of products on a conveyor belt, for example, a shaft encoder might trigger the frame grabber shortly before the UUT reached the correct position. Some frame grabbers include a programmable delay function that allows the image to be positioned accurately relative to the encoder output. A camera�s asynchronous reset feature also can be driven by the frame grabber to ensure synchronization.

�Modern frame grabbers now include many of the special features that required extra circuitry on older models,� said Colin Pearce, the managing director of Active Silicon. �A considerable amount of circuit integration is used either in the form of custom silicon or field programmable gate arrays (FPGAs). Thus, such features as triggering and I/O tend to be integrated onboard as standard.

�Data formatting also is an important feature and contributes to optimum system performance. For example,� he continued, �a frame grabber can format the data so the red, green, blue (RGB) data appears in memory in the best order for processing or display.�

A frame grabber that accepts standard analog video can synchronize itself to the video stream. This may be done via an analog phase-locked loop (PLL) or much more quickly via digital synchronization circuitry. Alternatively, some frame grabbers operate in the sync mastering mode in which they provide horizontal and vertical synchronization to the camera.

In this case, the frame grabber is the center of the image acquisition system, accepting triggers from sensors specific to the application, determining how the camera should be operated in relation to those triggers, and providing the required control signals.

The Data Translation Model DT3162 Frame Grabber is a good example of a 10-b resolution analog video frame grabber. Its programmable 40-MHz input timing supports a variety of standard or nonstandard video formats on up to three input channels. Synchronization information is stripped from the video, provided separately by the camera, or supplied by the DT3162. Eight digital I/O lines are available as well as trigger, strobe, external pixel clock, and exposure control signals (Figure 2).

Figure 2. Block Diagram of an Analog Video Frame Grabber
Source: Data Translation

The software you need to program operation comes as part of the Imaging Omni CD that ships with the DT3162. Included are an ActiveX control used to develop your own application software, a device operation verification utility, a device driver, and evaluation copies of DT Vision Foundry and GLOBAL LAB Image/2. The first application provides click-and-drag program development, and the second is a complete image analysis package. Although hardware supplies the processing horsepower, you need comprehensive software to control a frame grabber.

If a frame grabber is required in a system, it�s logical to ask what else it can do. Triggering and synchronization are necessities, but what about providing a number of auxiliary digital I/O lines as in the DT3162 example? Many applications require some type of actuation before or after an image is acquired, so if the frame grabber can fill the need, the cost of a separate I/O board can be saved.

Industrial applications may involve RS-485 differential serial communications or some other RS serial link. Interfaces to these can be provided by the frame grabber. Similarly, some frame grabbers incorporate watchdog timers and inputs for the quadrature signals from a rotary encoder.

Offloading the Host
Capabilities more closely associated with the captured image include cropping and definition of the area of interest. The two functions may provide the same end result, but cropping reduces the amount of video data to be transferred after the entire image has been captured while selecting the area of interest may cause the frame grabber to capture only pixel data from a certain area. Alternatively, some cameras can be programmed to output only pixel data from a selected area.

The differences among these modes of operation may be important depending on the application. If a camera outputs a smaller amount of data per frame, it may be possible to achieve a faster frame rate. If the frame grabber isn�t required to capture complete images, a larger number of smaller image segments can be held in memory. And, transferring cropped images requires less bus bandwidth and host memory.