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Machine Vision Cameras

Overview -

Of all the components in the Machine Vision Arena, cameras have advanced the most in the past few years. There has been a consistent drive to reduce the cost in machine Vision systems. This has gone in a number of directions as machine vision is quite complex are there's never a single solution. It's more than just the camera pricing.

But the bottom line is that people wanted to get the cost of machine Vision down. This meant a number of things.

1 Get rid of the framegrabber

2 Move certain processing functions onto the camera

As a result the camera market today differs dramatically from that of ten years ago.

The pre-eminent camera interfaces are now Firewire for Lo End and Gig E for Hi End .

Low cost smart cameras are now available with the functionality of a very expensive Hi End system of ten years ago.

It's difficult to categorise cameras there are so many formats and categories: Monochrome, colour, Analog, digital, firewire, GiG E, Hi Speed, CCD, CMOS, Area Scan, Line Scan, 3D, Smart cameras.............

For the novice it can be quite daunting. Hopefully the next few pages will give you some guidance and help you choose a camera that's appropriate for your application.

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From Analog to Digital

The majority of machine vision systems sold a few years ago contained analog cameras connected to a PC via a card in the PC called a framegrabber. These are still quite popular in older systems but are being designed out all the time.

They have been overtaken by Digital Firewire cameras for the most part where a cheap firewire card can be used as a framegrabber, thereby reducing the cost of machine vision dramatically. These cameras have other advantages which allow certain parameters like shutter speed, brightness level etc to be adjusted in software, thereby eliminating the need to access the camera to make subtle adjustments.

At the Hi End where large data transfers are necessary, the Camera link interface bacame the norm but this is now being replaced by the Gig Ethernet interface, again, to elimitate the need for specialist expensive hardware in the PC.

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Entry Level Digital - Firewire Cameras

Firewire is now the pre-eminent interface in Machine Vision. No framegrabber is required and cameras can be daisy chained together reducing the need for multiple firewire ports on the PC.

There are two types of camera. Firewire A and Firewire B. The disadvantages of Firewire A are the short cable lengths and the low bandwidth. You are restricted to 10m cable lengths and transfer rates of 400MB/s which took some getting used to if you come from an analog world..

Firewire B supports cable lengths of 100m and transfer rates of up to 800MB/s.

The cameras are ubiquitous, and relatively cheap. And the interface is pretty standard. It faces some competition from USB which is even lower cost but the difficulty here is that the firewire transfer protocols are more widely accepted than the USB for image transfer. So for some of the machine vision SDKs available from large imaging companies, there is no interface written for USB.

And remember the USB protocol was written for printers, scanners and just about every peripheral that hangs off a PC. Whereas the Firewire protocol is designed for transferring image data.

So for critical applications where you don't want to drop frames or write your own interface , firewire is typically better as most of the work is already done for you.

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Hi End Digital - Camera Link

For larger amounts of data transfer where more bandwidth is required, with line scan cameras, for example, it is necessary to move up to the camera link interface. The cameras on the right have a rectangular camera link port. Otherwise it's impossible to tell them apart. Full camera link configuration can carry up to 680MB/s of video data over a 64 bit data bus.

Camera Link has been the standard for Hi Speed data transfer in machine Vision systems for the pas few years. There are a few disadvantages though..

1 Camera Link needs a framegrabber in the PC......No way around it and this adds to the cost.

2 There's a limitation of 10m in cable lengths between the framegrabber and the camera. And with higher camera clock frequencies you can drop frames. It's possible to extend the cable lengths by using repeaters but it starts to get complicated here, and beyond the scope of this site.

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GigE cameras

Cameras with Gigabit Ethernet (GigE) interfaces are cropping up in machine vision applications everywhere, and with good reason. GigE delivers full-duplex connectivity at one gigabit per second (Gb/s) over standard PC infrastructure. It has long-distance reach, supports almost any network configuration that , and allows heavy-duty processing tasks to be handled by scaleable stacks of economical PCs.

They are touted as the answer to the machine vision users dream. The cameras don't need a feamegrabber, the cable length restrictions are not there, they use ubiquitous PC Network Interface card etc. However beware it may be worth investing in a dedicated NIC manufactured by one of the Imaging hardware OEMs, as the standard cards cannot be relied upon to handle high-bandwidth image data in real time.

They all look the same with their RJ-45 jack at the back and they all have to perform the same basic functions:

acquire imaging data from the camera head;

convert the data to IP packets;

queue the IP data for GigE transfer;

transfer the data to the IP link;

deliver control signals from the GigE link to the camera head; and

handle network functions such as boot-up and packet resend

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