Selection of the right interface
This chapter exclusively explains interfaces for the transmission of images of an industrial camera to computer systems which then evaluate these data.
Mainly in the early days, analogue data interfaces were used for the image transmission of CCIR or PAL signals, etc., which then had to be digitalised by means of a frame grabber card on the PC. This method is still used in many fields of application, as the available technology is proven, robust and sophisticated.
New camera developments of the last years, however, reveal a trend towards digital interfaces, with the image already being digitalised in the camera and then sent to the computer as digital data without quality losses. Many of the interfaces used, such as USB 2.0, FireWire or Gigabit Ethernet, are already integrated in state-of-the-art mainboards today so that an additional plug-in card in the PC can be omitted.
Comparison of different interface types
Today the user can choose from a large number of interfaces which are capable to meet various requirements. In addition to the actual task of transporting data, the user should also pay attention to secondary features. Ruggedness, data security, long-time availability and compatibility are important criteria for industrial applications!
- The data rate which can be transported by means of these interfaces is sufficient for many applications: Applications with one or several cameras with several images per seconds can be realised using more or less any interface. If, however, particularly time-critical applications, systems with high-resolution cameras, many images per seconds or image capture by many cameras at the same time are to be realised, the required bandwidth and the transmission concept should absolutely be studied. The net bandwidth can indeed vary when one or several cameras are operated on the bus. Clarify such details if the bandwidth could be short.
- The maximum number of connectable cameras is often a theoretical value and only specifies how many devices can be controlled directly. In case of multiple camera systems pay attention to how many cameras can send images at the same time without image losses.
- An important aspect for the choice of an interface is also the extent to which the interface is standardised for the purpose of image data transmission and camera control. If this is not the case, the camera hardware and the software behind it remains a proprietary special solution and as a user you bind yourself to a particular hardware and software manufacturer. The question of driver support and maintenance for different operating system versions should also not be missed!
- The use of additional interface cards has advantages and disadvantages. On-board interfaces save money and allow for compact solutions. Additional plug-in cards for FireWire / Gigabit Ethernet often provide high-quality chip sets and possible even a higher bandwidth. Real frame grabber-based systems also have advantages: they are not any interfaces which are partly controlled by the operating system, but real-time capable point-to-point connections with highest data security.
- Particularly in case of industrial applications, cable lengths are an important subject. Compared to the analogue image transmission, digital interfaces are strongly limited in general, as the signal is not only somewhat noisier, but when the length is exceeded, it is simply cut off. Using special cables or by sending images at reduced bus speed, longer distances beyond the specification can also be realised. If this is not sufficient, the path can be extended by means of extenders or optical fibre data transmission. Clarify your requirements specification for cable and plug technology. Cables for drag chains or suitable for robots or screwable cables are not available for all camera and interface combinations!
The forecast is bright to extremely sunny. The development of state-of-the-art interfaces advances continuously and is also adapted to image processing purposes with some delay. USB 3.0, IEEE 1394-2008 (S 3200), 10GigE and new inventions like CoaXPress are already at the ready today. The development of chip sets, cables and plugs is driven by the computer systems mass market and thus becomes cheap for all users.
Interfaces with sufficient "power" are rarely a problem today. Select a universal solution which can be used for as many applications as possible. The individual manufacturers partially only provide certain interfaces so that different SDKs must repeatedly be used to connect the hardware. Therefore focus on suitable software solutions which support a wide range of hardware and interfaces. Then the interface discussion is mostly irrelevant!