GigE Vision Merits
- Supports common camera control interface GenICam. European Machine Vision Association (EMVA) has developed GenIcam.
- It has plug & play, high data transfer rate and low cost cabling. All the above helps system integrators a lot.
- It has wide range of camera for various applications
- cable length supported is around 100m. This feat is not possible by other standards like Firewire, USB3, camera link and coaXpress.
Camera capture system
Real time applications do not necessarily need ultra fast acquisition. But images should be acquired and processed within the stipulated time. Reliability of a real time system depends upon the parameters like jitter and latency. Latency is normally understood as time delay. Here it means the time taken to complete a task from start to finish. Jitter gives the time variation when the same task is repeated multiple times.
Camera capture system consists of a PC with Network Interface Card (NIC), camera and Ethernet link to connect the PC and camera. Hardware or software trigger can initiate camera to capture image. As expected, hardware trigger has lesser latency. Camera-head process the trigger and start the sensor to accumulate incoming light and convert into electrical charges. These accumulated charges are converted into digital and to be placed in the camera buffer memory. This process is called 'readout'. Entire buffer content is transferred to the PC by breaking them into small chunks and adding Ethernet header for each chunk. NIC receives the packet and raise an interrupt to CPU. If CPU is not busy then it will process the packet and put the chunk into the computer memory. The time taken from start of trigger to reception of last packet of the image is included to calculate the latency.
GigE Standard
Single GigE camera connected to PC via direct Ethernet link or multiple GigE camera can be connected to PC through an Ethernet switch. Avoid using hub to multiple cameras
A dedicated wire or electronic signal which is directly connected to input pin of the camera can act as a hardware trigger. To avoid false start, trigger debouncing method is incorporated. The price we pay for safety is one microsecond latency. An application software can send a trigger via camera configuration channel and it has lesser responsiveness than camera pin. If a software trigger comes from an application that runs on a non real-time operating systems (ex. Microsoft Windows) then jitter may vary from few hundredth of microseconds to few milliseconds. So it better to avoid software trigger mode. There are three types of exposures viz. free running mode, horizontal synchronous mode, reset mode and jitter varies from one frame to one pixel depending upon the type of exposure. Latency of camera is depends on exposure time and sensor readout time. Biggest contributor of latency will be readout time. A 60 frame per second camera takes 16ms to do readout.
The normal size of Ethernet frame (A packet in physical layer is called frame) will be 1500 bytes. Jumbo packets with a size of 9000 to 16000 bytes are available. A chunk inside a frame will be called as payload. Then GVSP (GigE Vision Stream Protocol) header, UDP (User Datagram Protocol) header, IP header and at last Ethernet header are added to payload. Appended four byte Cyclic Redundancy Code (CRC) will help to detect any errors that creped while the packet was in transit. 8000 byte sized packet will take 16.3 microsecond to get transferred over network.
Without the involvement of CPU, transfer of data from NIC to memory can be accomplished using Frame Grabber. It contains powerful Direct Memory Access (DMA) engine that helps to reduce latency and jitter to a minimum. Fortunately or unfortunately GigE standard do not have frame grabber. GigE software driver takes care of the role of frame grabber. So choice of GigE software driver plays a vital role in the performance.
Performance Improvement Tips
- Few network adapter allow 'interrupt moderation'. This instead of raising an interrupt for every packet arrival, it waits for certain number packets to arrive then it raises an interrupt. This helps to reduce CPU overload.
- 9000 byte sized jumbo packets are best even though networks may support 16000 byte size jumbo packets. The reason is CRC calculation above 9000 bytes is very cumbersome.
- Increase the receiver buffer size as much as possible. This in turn will reduce CPU usage.
A typical GigE camera will have physical dimension of 5cm x 3cm x 7cm , with 1400 x 1024 image resolution capable taking 75 frames per second (fps). Image exposure duration will be 100 microseconds. The data can be transported over 100m using CAT-5e or CAT-6 cables. It will have a mass of around 120 grams. Monochrome, colour and high speed cameras are available.
Source:
- Automated Imaging Association http://www.visiononline.org
- Allied Vision Technologies, http://www.alliedvisiontec.com
- Whtie paper on "GigE Vision overview" (GigE_Vision_for_Real-Time_0.pdf) This pdf is available with the key word in Google search "Whitepaper GigE".
- White paper on GigE http://www.alliedvisiontec.com/fileadmin/content/PDF/Support/Application_Notes/ApplNote_GigE_Vision_Setting_Standards.pdf
