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Stable, predictable and reproducible latency with CoaXPress

Posted by Benny Koene on Thu, Jan 12, 2017

In a lot of machine vision applications it is important to know the exact time at which a camera records its image. For example if a strobe light is used, the camera should record when the strobe is on and not when the strobe is off! This is done via accurate triggering of the camera.

Another situation where latency or accurate triggering is important is if multiple cameras are being used. To be able to relate images with each other and to construct for example 3D images from a set of 2D images, such as with 3D All Optical Inspection (3D-AOI), it is necessary that the images are recorded at exactly the same time. This is especially true if the object of interest is moving along your inspection machine.

With CoaXPress it is possible to trigger the camera over the CoaXPress cable. It is thus not required to connect an additional trigger cable. The use of less cables gives more system design freedom requiring less space, allowing for more mobility of a measurement head, etc.

The trigger over CoaXPress protocol is designed to provide a stable, predictable and reproducible latency. In its definition there are some features which make the moment at which the integration of an image occurs predictable. In this blog we will briefly describe these features.

CoaXPressTriggerSchemeWithScope.pngFigure 1. The Trigger over CoaXPress scheme. An external trigger is fed to the CoaXPress frame grabber. The frame grabber forwards the trigger to the multiple cameras. Alternatively most frame grabbers allow for creating the trigger signal on the card. We connected a scope to the trigger source and to the four camera sensors to demonstrate the stability of the trigger scheme. The measurement result is shown in the next figure.

Trigger over CoaXPress prevents delays caused by galvanic isolated I/O connectors

When triggers are supplied to the camera via galvanic isolated I/O connectors there might be an additional delay caused by the method used to achieve galvanic isolation. For example, if opto-couplers are being used they require a certain response time.  As CoaXPress allows sending a trigger over the CoaXPress interface, these delays in trigger are circumvented.

CoaXPress has a jitter reduction mechanism.

Unfortunately jitter in pulsed systems is very hard to prevent. But it is possible to make its influence minimal. This can be done by actually introducing a latency that is longer compared to the jitter. In this way you know for sure that the system is ready the moment the latency period is over. In most cases a slightly longer, stable, predictable and reproducible waiting time (latency) is much less a problem compared to an unstable, unpredictable and not reproducible jitter.

Trigger signals have a high priority in CoaXPress

In general the timing of when the integration of an image starts is much more important than when an image is exactly read out. Although processing has to occur often live in a system and thus the processing has to be finished before the next image arrives, it is possible to use buffers to take care of time delays. This in contrary to trigger events that are often meant for activities to happen at the same time!

Exactly for this reason, the CoaXPress protocol gives a higher priority to trigger signals. Trigger signals will be identifiable by the CoaXPress protocol such that it can transmit these signals before continuing with, for example the read out of image data.

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Figure 2. (Left) Recorded trigger signal at the sensor controller for four CoaXPress cameras. (Right) Recorded trigger signal at the sensor controller for three CoaXPress cameras and the reference trigger signal (top yellow line).

Topics: Interface Technology

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