Identifying frame grabber core competencies: do you know where your frame grabber functionality is located?
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 straight-forward. 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.
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 standard 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.)
[FIGURE 1 OMITTED]
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 OMITTED]
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.
Further operations such as image flipping and data formatting are useful if your application requires them and if the host processor can't perform the operation for some reason. For instance, the additional processing time required might preclude running an important real-time analysis routine.
LUTs provide a very fast hardware-based data-formatting method. For example, intensity weighting could be accomplished by using 8-b pixel intensity data to address a LUT. The LUT output values could be designed to shift the whole intensity curve upwards or change its sensitivity to compensate for a known camera characteristic. LUTs typically are specified as 8-8 or 16-16, for example, meaning that an 8-b (16-b) word addresses them, and the output also is 8-b (16-b) data.
The Cognex MVS-8600 Series is a good example of a flexible, two-channel Camera Link product. The MVS-8602 accommodates two independent base-level cameras or one medium-level camera with up to 12-b image data. Onboard 12-b LUTs, pipelined processing, and image buffers support overlapped DMA transfer to the host during simultaneous new-image acquisition.
Although frame grabbers initially may have dealt only with image capture and data transfer, their role today has become much broader. Many times, a frame grabber is considered a single-board solution to machine vision applications. You know that you can't directly connect your camera to the host computer, so if a frame grabber is needed, it had better include all the additional functionality that's required.
This theme was elaborated on by Kirk Petersen, the director of marketing and communications at DALSA Coreco. "Not only do today's frame grabbers incorporate acquisition control features such as trigger inputs and strobe outputs, they also provide I/O control signals and tag incoming images with unique time stamps and can format data from multitap cameras into seamless image data.
"Beyond this, some frame grabbers are taking on pre-processing heavy lifting in an effort to ease the burden on busy host PCs," he continued. "Image correction and processing algorithms such as Bayer inversion filters are being done on the frame grabber rather than by the PC."
What about those really difficult image-processing applications that can overwhelm a PC, even with a frame grabber's help? According to VMETRO, you may want to consider using a large FPGA perhaps working in conjunction with one or more DSPs. The company's PMC-FPGA03 Board is in the PCI mezzanine card (PMC) format defined by IEEE 1386.1.
When interfaced to a PC via a suitable PCI carrier board and used with the CAML-MOD3 Camera Link Adapter Module, the PMC-FPGA03 supports a camera's full 680-MB/s data rate into a large user-programmable Xilinx Virtex II Pro XC2VP50 FPGA supplemented by fast quad data rate (QDR) SRAM and double data rate (DDR) SDRAM. Communication from the carrier to the PC is via a 64-b PCI interface.
In very advanced frame grabbers, performance is limited as much by your imagination as by the hardware. For example, JPEG2000 compression/decompression acceleration, image authentication based on the 256-b secure hash algorithm (SHA-256), and image filtering are a few of the functions that can be performed.
Rather than anticipating that a separate frame grabber will be needed, some camera manufacturers are supporting direct high-speed image transfer to host memory. Similar to the frame grabber changes that occurred when fast PCI-based data transfer became available, gigabit Ethernet (GbE) and Express PCI (PCIe) are challenging traditional frame grabber architectures.
Together with ever-faster microprocessors and DDR memory, these two interfaces make the direct transfer of images from a fast digital camera to host memory very attractive. What about all the other things that a frame grabber adds to an application? If you opt for GbE or PCIe data transfer, all other functionality is split between the PC and camera.
For example, Pleora Technologies has introduced products that support direct GbE connection between a camera and a standard GbE network interface card (NIC) in a PC. The iPORT[TM] PT1000-VB is a small PCB intended to be mounted inside a camera. The board replaces a camera's existing back-end circuitry and is said to provide lower overall cost, longer distance reach, greater networking flexibility, and more scalable PC processing compared to Camera Link solutions. In addition, the board includes triggering and auxiliary I/O capabilities and complies with the automated imaging association (AIA) emerging GbE vision standard.
Clearly, the frame grabber market has changed. You still can address low-cost applications by using analog cameras, and in fact, your choices have become broader. You may decide to use a conventional frame grabber, and through modern ASIC technology, many products offer multiple channel image-handling capability together with synchronization and triggering functions.
Alternatively, cameras are available with IEEE 1394 (FireWire) interfaces that simplify connection to a PC. As National Instruments' Kyle Voosen, product manager for NI Vision, explained, "FireWire cameras provide digital image quality, simple cabling, standard software, and a low cost. They are replacing analog cameras, and today they are supported by a range of machine vision I/O boards that provides frame grabber functionality as well as digital I/O."
One reason that frame grabbers are changing so quickly is the rate of PC change. The PCI bus, for example, has been developed into at least three parallel versions and most recently the serial PCIe bus. Bill Tanner, CEO of Sensoray, discussed some of the features required in the video-surveillance, traffic-monitoring, and medical-imaging markets. He pointed out that changes to the PC's internal bus create a risky environment for new frame grabber designs.
"In an attempt to combat this problem, Sensoray's latest frame grabbers are external modules that use the USB or FireWire bus," he commented. "Large systems of at least 24 channels can be assembled by daisy-chaining multiple frame grabbers. Also, some products achieve MPEG image compression of over 100, facilitating image transmission over low-bandwidth communications lines."
Because of the auxiliary functionality that a frame grabber may include, you can't choose the best one for your application by considering only image-related parameters. You must look more widely at your entire machine vision problem, especially any actuation, triggering, and synchronization aspects, to determine if you even need a frame grabber. Similarly, if a large amount of signal processing is necessary, then that aspect of any proposed solution must be well understood.
Although frame grabbers have been in use for many years, this is anything but a one-size-fits-all product. What a frame grabber comprises depends on the problem it is addressing as well as your point of view. Both software-centric and hardware-centric solutions can address many of the same applications. Make certain that you fully understand all the details of your particular application before deciding which approach is more appropriate.
FOR MORE INFORMATION ON FRAME GRABBERS Active Silicon www.rsleads.com/512ee-178 American ELTEC www.rsleads.com/512ee-179 Cognex www.rsleads.com/512ee-180 DALSA Coreco www.rsleads.com/512ee-181 Data Translation www.rsleads.com/512ee-182 Matrox Imaging www.rsleads.com/512ee-183 MuTech www.rsleads.com/512ee-184 National Instruments www.rsleads.com/512ee-185 Pleora Technologies www.rsleads.com/512ee-186 Sensoray www.rsleads.com/512ee-187 VMETRO www.rsleads.com/512ee-188
by Tom Lecklider, Senior Technical Editor
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|Title Annotation:||FRAME GRABBERS|
|Date:||Dec 1, 2005|
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