Choosing the Right Interface…
In today’s competent machine vision industry, ranges of interfacing standards are available which have evolved over a period of time. Though added choices are welcome, one must know the advantages and limitations of each interface to select the most appropriate one for your application.
Before one attempts to select an interface, one should know what exactly the requirements of your vision system are. This includes the requirements of the camera, power, speed capabilities and additional inputs like lighting, triggers etc. which are needed for proper image capture during the actual work process. Once this is analyzed, one should be able to make a sound judgment in selecting the interface.
With the broad range of requirements addressed by machine vision systems, there is no single interface that can address them all. Each interface has its own unique strength that makes it more effective and efficient for certain kinds of applications.
Now let us look at the most commonly used interfaces one by one.
Firewire was originally developed as a consumer interface with ease of use in mind. It is also known as IEEE1394 standard and is compatible with almost all operating systems. Some PC’s especially many industrial ones may not come with Firewire ports, however these can be easily installed with Firewire hardware available in the market. IIDC specification provides a global register map for all FireWire cameras to follow, allowing third-party applications to become camera vendor agnostic. System integrators can switch between camera models and camera vendors with minimal impact to their application as long as the cameras support IIDC. FireWire is also highly effective for multi-camera systems. FireWire cameras use a peer-to-peer communication protocol and provide the ability to synchronize trigger broadcast with other cameras on the same bus. Thus for a wide range of applications, Firewire comes through as a viable choice for machine vision systems.
With the wide spread deployment of Gigabit Ethernet on PC and laptops, it has become widely popular among vision engineers. It attempts to standardize camera control and image acquisition so that cameras and software from multiple vendors may inter-operate seamlessly. The most attractive feature of GigE Vision is the support for longer cable lengths. Since each camera can be addressed using an IP address, there’s no limit on how many cameras can operate on the same network. The low cost of Cat 5e and Cat 6 cables makes GigE the most cost effective solution when long cable length is required.
The development of the USB 3.0 interface was originally driven by the consumer need for a PC based interface with higher bandwidths. The USB 3.0 interface came to market in 2009 with support for up to 440 MB/s of bandwidth; 10 times faster than USB 2.0 and 5 times faster than FireWire-B.USB 3.0 supports Direct Memory Access which allows the data to be written directly to memory reducing CPU processing load. Multiple camera systems can be deployed using USB 3.0 hubs. Similarly to GigE Vision, USB3 Vision follows the same robust framework using the GenICam programming interface and XML description files. With this commonality between the two standards, system integrators will be able to easily learn one after working with the other. With its high bandwidth, ease of use, and low cost, USB 3.0 is ideal for wide variety applications within machine vision.
CameraLink is an established interface for industrial imaging applications. It offers the highest data throughput (over 680MBytes/sec) and lowest latency of all interfaces. It offers more deterministic image transfer trough a dedicated frame grabber, which may be expensive.
Camera Link supports Direct Memory Access which reduces the load on the CPU when capturing images. Camera Link also separate control signals with video signals, providing low latency triggering and data delivery. Compared to other interfaces, system costs are higher due to frame grabber and a power supply is needed for every camera in the system. However, having a frame grabber in the system does off-load some of the processing done by the CPU. Frame grabbers can also buffer images, with some high-end frame grabbers even offering some hardware image processing capabilities. Despite low consumer acceptance and higher system cost, Camera Link is widely used for applications requiring real time processing and high bandwidth.
|FireWire -IEEE1394b||Gigabit Ethernet||USB 3.0||Camera Link|
|80 MB/s||125 MB/s||440 MB/s||680 MB/s|
|Difficulty of system Integration||Medium||Low||Low||High|
|4.5 m||100 m||3 m||10 m|
|45 W||15.4 W1||4.5 W||None|
|Theoretical No. of Cameras||63||Unlimited||127||2|
Though these are the machine vision standards which have found wide range acceptability with all camera and other industrial vendors, there are sufficient effort being taken in developing custom made interfaces which provide better suitability to certain applications. Some of them may very well become the next standard interface over a phase of time. CoaXPress is one such new digital interface which promises future with ultra-high bandwidth combined with versatility.
After considering the application requirements, camera performance, data throughput and ease of Integration, one can arrive at a choice for the right interface. In general, the following guidelines can be followed. However, consulting with machine vision experts is advised.
|SELECT||IF requirements are|
|– Wide range of applications-Low Cost|
|GigE||-Low cost-High frame rate-Inter-operability-Long cable lengths|
|USB 3.0||-Ease of Use-Low cost|
|CameraLink||-Highest frame rate.-Real time data transmission/Low latency|