Tuesday, September 13, 2011

Trends in machine vision cameras

CameraLink provides the highest robustness and data rates, FireWire has been superseded by GigE, and USB is irrelevant for machine vision. Oh, and by the way, resolutions are going up.

Well that’s what I told an engineer who emailed me a few days ago asking if he should stick with FireWire cameras, but let’s take a moment to dive deeper.

Few in machine vision would argue that CameraLink, for now at least, provides the best overall performance. It can handle the highest data rates, (up to 6Gbps with 2 cables,) works over cable lengths as long as 10 meters, (enough for most industrial applications,) and never misses a frame, but it does have downsides. Chief among these is cost. CameraLink needs a framegrabber, which can easily cost $500 or more. It also needs special or custom cables – yet more cost. And since Power-Over-CameraLink (PoCL) doesn’t seem to have really caught on, perhaps because it’s limited to 4W, figure on running two or three cables out to the camera.

So what about the alternatives? Well let’s start with USB. Every computer has a USB port, so there’s no need for a framegrabber, making it seem attractive. But when you dig down into the details the problems start to emerge. The basic issue is that USB is something of a processor-hog, thanks to its employing a master-slave architecture. There are also bandwidth issues, meaning that a significant proportion of its 480Mbps is not available to transfer image data. For these reasons few manufacturers of industrial cameras have ventured into this standard. (Scientific and astronomical cameras are a different matter of course because they don’t need high data rates.) The maximum cable length of 5m can also be bothersome in many applications.

And then there’s FireWire, IEEE1394, to use its proper name. 1394 cameras, which need no framegrabber and take their power – up to 45W – from the single cable, come in two flavors, a) and b), the difference being speed. Roughly speaking, a) handles 400Mbps while b) hits 800Mbps. That’s more than enough for a 2Mp camera running at 5 fps, so what’s the problem?

Well first, cable length is limited to 4.5m, which can get to be a problem in even the simplest of industrial settings. Second, and I’m speaking from personal experience here, it’s just not totally reliable. I’m not up in all the details of error detection methodologies, but it’s my understanding that 1394 doesn’t have any. That means a frame can be dropped and you’ll never know. Now in many applications that might not be a problem, but if you need to be 100% confident in your system, think carefully before going this way.

That brings us to GigE, strictly speaking, GigE Vision. This uses plain old network cabling, though you should upgrade to the better shielded CAT 6, to transmit up to 1Gbps of image data as far as 100m. You will need to buy a special purpose card for your PC – the regular network card can probably not handle Jumbo Frames and in any case, you need it for internet and intranet access – but these are far less expensive than a CameraLink grabber. And if power cabling is a concern, Power-over-Ethernet (PoE) will deliver up to 13W – enough for most cameras.

Downsides? Well really, just the fact that you’re limited to 1Gbps, which is more than FireWire b) can offer, so it’s enough for run-of-the-mill applications.

As for resolutions going up, well you knew that already, didn’t you?

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