Machine Vision Frame Grabbers

Frame grabbers are used to deliver images from a machine vision camera's output to the memory of a computer to be further processed and/or displayed. The development of frame grabbers has run on course with the development of computer technology. PCI and AGP ports in modern PC's allow real-time display of images via a frame grabber without the previous need for additional graphics hardware.

There is a huge range of frame grabbers available and can basically be split into three main categories – Standard, Advanced and Intelligent. We will explore each of these categories and where applicable the sub-categories within in a moment. First lets look at the processes which can take place within a frame grabber, a simplified version of which is shown in the block diagram below.

The incoming signal from the vision camera is sampled at an rate specified by a fixed frequency pulse, which can be generated in the frame grabber itself or received from the camera. If the signal is not already digital it passes through an analogue to digital converter, and stored in the buffer until a full image has been converted/received.

Standard Frame Grabbers

We begin exploring the different level of frame grabbers by first looking at Standard Frame Grabbers which are low cost devices with a high enough level of intelligence and software support for inspection applications. They can only be used with standard analogue interlaced video sources, non-standard video sources (i.e., progressive scan, mega pixel, non-standard cameras and digital sources) are not supported.

This type of grabber often does not include memory to buffer images on so the video data is sent to the CPU via the PCI-bus line by line which is processor intensive. Top of the range standard frame grabbers can include some memory and are preferable in applications where each frame must be displayed.

Standard grabbers can be triggered to grab the next image although their response is not instantaneous and there will be a random delay of up to one frame, which remains satisfactory for most applications. They also contain a multiplexer allowing more than one camera to be connected and used in turn.

Advanced Frame Grabbers

Advanced Frame Grabbers are high performance frame grabbers which support non-standard cameras and therefore are dominant in most machine vision applications. Apart form increased accuracy, the distinct feature of an advanced frame grabber which sets it apart from standard level grabbers, is the ability for asynchronous image capture. This is achieved via the use of synchronisation mechanisms between the grabber and camera resulting in instantaneous capture also known as an asynchronous reset operation. This operation interrupts the sampling clock and resets the exposure and readout cycle so that a full image can be generated at any time.

There are four main types of advanced frame grabbers:-

• Single channel monochrome
• Red, Green and Blue (RGB)
• Multi-channel asynchronous monochrome inputs
• Digital input

The first three variants of advanced frame grabber can be considered the same in terms of processing mechanisms as the RGB and multi-channel grabbers have a number of the single channel operations on board. Therefore we will simply look at analogue advanced frame grabbers in general followed by digital advanced frame grabbers.

The analogue advanced frame grabbers utilise very high quality components to acquire highly accurate images from any type of analogue video signal received from standard and non-standard camera formats. They reach very high clock speeds and can contain an abundance of on board memory for buffering DMA (Direct Memory Access) transfers for standard and high pixel count cameras. The software on board these grabbers is comprehensive allowing maximum control flexibility.

Digital advanced frame grabbers have the same high standard digital parts and receive image data via RS422, LVDS, FireWire or Camera Link on 8, 16 0r 32-bit inputs. The facilities for timing control and synchronisation are complex allowing cameras to be controlled via a RS232 link.

Intelligent Frame Grabbers

Intelligent Frame Grabbers effectively contain advanced grabbers but also contain additional on board processing hardware to provide the grabber with a form of intelligence. Escalating the grabber from merely a messenger to a device with processing capabilities built in.

Intelligent frame grabbers can be split into three predominant types:-

• Intelligent capture.
• Pre-processing engines
• Expandable processing engines

Intelligent capture grabbers remove interaction from the host during the acquisition cycle owing to time critical applications. Only the actual data transfer requires the host processor and as such the grabber notifies the host that there is new data residing in the grabber's memory.

Pre-processing grabbers free up even more host processing time by performing some of the functions the host would normally do before the data would be ready to process. These functions include flat field correction, image arithmetic, convolution filters and data reduction.

Expandable processing grabbers can form up to 30 processors into one computing engine increasing the processing power of the host. They excel in applications where the sampling rate is higher than the PCI-bus bandwidth (120MBps) or where the processing can not be accomplished on a single or dual processor host.

The choice of frame grabber type is application driven and can depend on many conditions as detailed below.