Already in 1996 the USB interface was developed by Intel and introduced onto the market as USB 1.0. The aim was to harmonise and simplify the different interfaces of the PC for peripheral devices (mouse, keyboard, modem, printer, scanner, ...). Due to the maximum data rate of 12 Mbit/s, the use in machine vision was unthinkable.
The USB 2.0 interface was specified in 2000 and shortly afterwards integrated into mainboard chipsets. For a number of years, several USB 2.0 ports have been a standard in every PC. USB 2.0 corresponds to a data rate of 480 Mbit/s (approx. 40 MB/s net) which is sufficient for many applications. The voltage supply is also integrated in the cable. In case of industrial cameras, too, this interface is well established on the market and predominantly serves for cheap applications.
In 2009 the first chipsets for the next generation of USB were presented to an expert audience. USB 3.0, also called super-speed USB, is supposed to increase the transmission speed to up to 4.8 Gbit/s. A wide market launch is expected for 2010. To which extent USB 3.0 is going to become prevalent for industrial cameras is not yet clear. Due to the high data rate even shorter cable lengths are to be expected in spite of more elaborate cables (one separately shielded pair of conductors each (shielded differential pair, SDP) for both transfer directions as well as a common shielding for the entire cable).
Advantages of USB 2.0 over analogue systems
FireWire and USB 2.0 have become a competition and gain particularly compared to the analogue image interface in recent years.
- The digital image transmission provides a loss-free signal transfer and therefore a better image than analogue image transfer.
- Omission of the expensive frame grabber card. Since 2003, all PCs have provided several integrated USB interfaces.
- Extremely cheap standardised cable and connector technology with integrated voltage supply of the camera. They are significantly less expensive than analogue Hirose cables, but normally not lockable.
Characteristics of USB 2.0
- USB 2.0 provides a bandwidth of 480 Mbit/s, corresponding to an effective image data rate of 40 MB/s. This is sufficient performance in order to operate several cameras with standard resolution or higher on one computer and to inspect several parts per second.
- The technology offers hot plug / hot unplug: devices can be connected and detected during operation.
- Integrated voltage supply (5 VDC) for devices in the 4-pole cable. Devices complying with the USB specification may consume a total of 500 mA from the bus. Devices with a power of up to 2.5 W can therefore be supplied via the bus.
- According to the specification a USB cable must only be 4.5 m long at the maximum. By using high-quality cables, a length of approximately 10 m can be realised as well. If greater distances must be bridged, hubs can be connected in between.
- The thin flexible 4-pole cables have a simple design. In addition to two conductors for the voltage supply, two other conductors (D+ / D-) are required which are twisted. The data transmission is always symmetrical, one conductor transmits the data signal, the other always the inverted signal. The signal receiver evaluates the voltage difference of both signals. The signal level between 1 and 0 is thus twice as high, interferences can widely be eliminated. USB cables are therefore quite interference-free, have a thin cross-section and can easily be laid in an industrial environment.
- Data transmission is possible in both directions: camera control, triggering, parameterisation is equally possible during operation.
- The bus specification involves a central host controller, this master takes over the coordination of the connected peripheral devices called slaves. Up to 127 different devices can theoretically be connected to the USB bus: this is done in the form of a star topology. As always only one USB device can be connected to one USB socket, distributors (hubs) must be used. The network structure can be up to six levels deep. A daisy-chain operation like in case of FireWire is not permitted.
- Practical experience with USB camera systems in industrial facilities, however, reveal that operating several cameras, which simultaneously capture and send images, may lead to interferences and problems with overall cable lengths, etc. A particular bandwidth on the bus can indeed be exclusively assigned and guaranteed to a USB camera. This, however, is no longer available for any other connected device. For real multi-camera systems only one camera is ideally connected to one USB bus. Additional USB plug-in cards are cheap and easy to retrofit.
Disadvantages of USB 2.0
- Due to the master-slave principle of the USB bus, the camera is always a slave device and the host PC is the master device which must control the data traffic on the bus. In case of a triggered operation of the camera, the PC must request the images, which results in performance problems if several cameras with synchronous image acquisition are connected.
- Cables and plugs are unindustrial. Cables suitable for drag chains and robots are hardly available. The achievable cable lengths from 5 to 10 m without repeaters are often not sufficient for larger systems.
- USB as a standard defines the data traffic, plugs, signal level, etc., however, it does not natively support a standardised cross-manufacturer protocol for image transfer and camera control. Proprietary USB camera drivers must always be installed which bind the user to one hardware supplier.
USB 2.0 provides the user with a practical interface which represents the cheapest solution for non-standardised image transfer for simple applications involving not too many cameras. Unfortunately it is not standardised, with the result that all manufacturers require their own USB driver. Ideal for use on laptop, PC and embedded devices (like Raspberry Pie Gen. 2 or Gen.3) less suitable for rough industrial environments or complex applications with many cameras. The cable length is limited.
Modern cameras are increasingly being developed on the basis of the USB3.x interface, which are mostly available as USB3 Vision devices and thus compatible with many cameras from other suppliers.