Speed as a criterion for camera selection
An important criterion when looking for the right industrial camera is not only the resolution of the sensor but also the speed at which it reproduces images. Here, the speed of the sensor and the speed of the camera are often confused. Fortunately, the Burst Mode offers the possibility to run cameras with a slow interface much faster for a short time.
The Burst Mode in detail
Industrial camera speeds
Speed of the sensor
The speed of the camera sensor can be found in the data sheet of the sensor manufacturer. It describes the maximum transfer rate at which the image data can be read from the sensor with an optimal connection to the camera.
Camera internal speed
The more LVDS or Mipi lanes are connected to the sensor by the camera electronics, the faster the reading into the internal memory takes place, since the lanes read the data from the sensor in parallel. The maximum internal camera speed results from the interaction of sensor speed and the transfer rate of the write to the internal memory. The camera then performs operations on the image in the memory and places the data on the interface to the outside.
Camera speed external (interface)
These interfaces of industrial cameras to the outside such as USB, CameraLink , GigE or CoaXPress have significantly different transmission rates. If the speed of a camera is specified for Machine Vision, then it is measured which data rate the camera delivers constantly over a longer period of time. The measurement is necessarily made via the interface of the camera.
Each module downstream of the sensor can only reduce the speed of the camera. If, for example, the interface transmits slower than the sensor, the camera will only deliver the maximum data rate of the interface, but not the maximum data rate of the sensor. This means that in the long term an industrial camera can only be triggered with the maximum data rate of the interface.
Burst Mode as 'Turbo'
The emphasis here is on long-term (in the range of seconds). A camera with enough built-in internal memory can cache images in the internal memory (at the camera's maximum internal speed) if designed appropriately. So the sensor can be triggered with higher frequency than the specified frame rate of the camera allows - as long as the internal speed is higher than the interface speed. The fast following images are stored internally and then delivered (delayed) via the slower interface.
This mode of operation is called 'Burst Mode'.
Here, the size of the available memory and the internal speed of the camera are the limiting elements. As soon as the memory is full, images are discarded (depending on the implementation) or triggering of the sensor is prevented for the time without sufficient memory.
Burst Mode does not require driver support
Burst Mode is a hardware feature of the camera. As such, it is independent of the software driver used and can also be used with third-party GenICam drivers.
Benefit for the user
- Burst mode gives the user the option of selecting a slower camera than the short-term frame rate requires
- In case of change / disturbances in data processing or data transport (high externally induced network load, high processing time of the program for individual images), the behavior during triggering is not affected
- Cameras with slow interfaces such as GigE are often less expensive, more flexible in terms of infrastructure and offer independence from the hardware used than high-speed interfaces.
Practical example with exo174MGE for the use of Burst Mode
Image processing at the conveyor belt
A standard case in industry is, for example, a conveyor belt on which parts are guided past a camera for inspection. If an inspection part comes into the detection range of the camera, several images are to be taken quickly one after the other with changing illumination and then the inspection part is transported further. During this time, data can still be read out.
In this example we use a SVS-Vistek exo174MGE camera. Its Sony IMX174LLJ sensor (2.35MP) delivers about 164 fps (frames per second, 8bit) according to the datasheet. The camera with 256MB internal memory delivers 52 fps over the GigE interface. The internal memory is written with approx. 170MB/s, so the internal speed is 170MB/2.35MB = 72fps.
In continuous operation, the exo174MGE can only deliver 52 fps, as it completely utilizes an Ethernet Gbit line with 52fps and 8bit. However, with 256MB of memory, the camera has enough internal memory for up to approx. 100 frames, which can be filled with the camera's internal speed (72fps).
Burst Mode as Turbo
If the camera should take a picture every 9ms (approx. 110fps), the maximum length of the burst recording time can be calculated roughly like this (Memory=available memory in frames, Vsoll = target speed in fps, Vextern = the speed of the interface in fps):
t = Memory / (Vsoll - Vextern)
In our example, this gives t = 100/(110-52) ≈ 1.7s as the time an exo174MGE can run in Burst Mode at full resolution while delivering images to the application. That's with 110fps * 1.7s = 189 images. A maximum of 100 images can be stored. The delivery will then take t = 189/52 ≈ 3.6s.
The Burst Mode allows users to record at significantly higher frequencies than the interface actually allows. This is particularly useful for users who appreciate the flexibility and hardware independence of the GigE Vision interface with low-cost infrastructure and long cable lengths. In addition, the decoupling of acquisition frequency and inspection frequency (as in the example) ensures significantly more fault-tolerant overall systems, since changing parameters such as inspection frequency or network load are cushioned by the camera's burst mode functionality.
Burst Mode is standard at SVS-Vistek
Building cameras with this architecture and sufficient memory is costly. Unlike many cheaper industrial cameras, the GigE Vision cameras of the SVCam line from SVS-Vistek all have Burst Mode functionality. The user benefits from increased fault tolerance and maximum flexibility in the Machine Vision application.