Speed as a criterion for camera selection
Looking for a machine vision applications best match camera is based on a bunch of criteria. Imaging speed of the industrial camera is a core criterion. Quite often though, sensor speed and camera speed are mixed up. Fortunately, Burst Mode provides the possibility to operate cameras with low-speed interface with much higher framerate than suggested in the data sheet.
Burst Mode in detail
Different speeds of industrial cameras
Speed of sensor
The speed of the sensor can be found in the sensor manufacturer's datasheet. It describes the maximum speed you can read the data from the sensor with optimum camera electronics.
Internal camera speed
Modern sensors permit reading of image data on parallel highways. The more LVDS or Mipi lanes are attached from the camera electronics to the sensor, the faster the image data can be read into the cameras internal memory. The max of internal camera speed is resulting from the teamwork of sensor speed and speed of writing into the camera's internal memory. After some work on the image in memory the data is put to the external interface.
External camera speed (interface)
The common interfaces of industrial cameras like usb, CameraLink, GigE or CoaXPress differ significantly in terms of speed. When specifying a machine vision camera speed the longterm imaging rate of a camera is measured. Naturally, this measurement is done with the the camera's interface.
Every module after the sensor only can reduce the overall speed of the camera. With an external interface being slower than the sensor, in the long run the maximum speed of the camera will be the interface speed but not the sensor speed. Also, triggering will be covered only with this slower speed.
Burst Mode serves as "Turbo"
The emphasizing lies on the meaning "in the long run". Being built nicely, an industrial camera with enough internal memory can buffer sequences of images into internal memory (with maximum internal speed of the camera). Like this, as long the external interface is slower than the sensor, the camera might be triggered with higher frequency than the camera specs will permit. The sequence of images are buffered and will be delivered delayed through the (slow) exteral interface.
This operating mode is called "Burst Mode".
The size of the usable internal memory and the internal speed of the camera are limiting factors. As soon as internal memory is running out of free space, images are dropped or triggering will be suppressed (depends on implementation) for the time of low memory.
Burst Mode needs no driver support
Obviously, as this is solved by hardware design of the camera itself, Burst Mode is a hardware camera feature. It will work with any GigE Vision software driver.
The user application might take advantage of Burst Mode in terms of application safety, interface decision or camera performance.
- Burst Mode provides the possibility to run the camera with imaging speeds higher than specified by the manufacturer
- Burst Mode enables the user selecting a camera with less performance than the requested imaging speed would require
- As speed of the interface is less important with Burst Mode, applications might benefit from selecting interfaces like GigE with its unmatched flexibility in terms of infrastructure
- Delays in data digest, trouble with network transfer (high externally induced network load, high machine vision processing times) will not affect the timing of the trigger
Burst mode example with SVS-Vistek exo174MGE
Machine vision at a conveyor
As an example, take a conveyor with parts to be inspected running in front of a machine vision camera. At a certain position, the camera should take some images with different colours as fast as possible. After this, the inspected part will be continued. Meanwhile the images can be read and evaluated.
In this example, we're going to use a SVS-Vistek exo174MGE camera. Its Sony IMX174LLJ sensor (2.35MP) delivers about 164 fps (frames per second) in full resolution according to its data sheet. The camera comes with 256MB internal memory and delivers about 52 fps through its GigE interface. Writing into its internal memory is done with about 170MB/s, thus it works with an internal speed of 170MB/2.35MB = 72fps.
The interface speed
The exo174MGE is specified with 52 fps (8 bit mode), this data rate being the maximum load of a GigE network cable. With about 256MB of internal memory it is providing sufficient memory for about 100 frames which can be filled with the camera's internal speed of 72 fps.
Burst Mode as turbo
If the camera is supposed to take an image every 9ms (about 110fps), the maximum length of Burst Mode acquisition time can be calculated as follows (Memory = usable memory in frames, Vreq = requested frame speed in fps, Vextern = interface speed in fps):
t = Memory / (Vreq - Vextern)
In numbers, our example will calculate t = 100/(110-52) ≈ 1.7s as the maximum time our exo174MGE will run in burst mode with full resolution (already delivering images to the host computer application while shooting). With internal framerate of 110fps*1.7s=189 images can be taken like this. Delivery time of the images will be t = 189/52 ≈ 3.6s.
Burst Mode is enabling significantly higher imaging framerates than specified by the camera interface. In machine vision inspection systems the decoupling of testing frequency and imaging frequency is leading towards much more fault tolerant applications. Burst Mode functionality cushions the blow of unforeseen events as parameters like testing frequency or network load might be fluctuating.
Engineers targeting projects with slower infrastructure like GigE, with long data cables or who want to be indepenent from high speed interfaces and specific hardware can take advantage of Burst Mode. Some applications only can be realized using Burst Mode.
Burst Mode is a standard at SVS-Vistek
Building cameras with sufficient memory to support Burst Mode together with the necessary internal architecture might turn out to be more expensive than the cheapest models on the market. The customer's benefit of a more fault tolerant machine vision application soon pays off, though. All SVS-Vistek GigE Vision cameras out of the SVCam line do support Burst Mode.