National Instruments ISC-1762, Руководство пользователя

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5-4
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Chapter 5 Image Acquisition
The incoming trigger is synchronized to the line rate of the smart camera. This adds an additional
delay that can vary on a frame by frame basis. The maximum variability is shown in Table 5-1.
The amount of time required from the assertion of a trigger to the start of the light strobe and
image exposure varies by application. For example, if a sensor that detects the presence of a part
is positioned before the smart camera on a conveyor belt, a trigger delay will be necessary so
that the smart camera waits to expose the image until the part to be inspected passes in front of
the smart camera. In this case, specifying the trigger delay in terms of quadrature encoder counts
allows the smart camera to expose the image when the part is positioned in front of the smart
camera regardless of changes in speed of the conveyor belt. For other applications, a delay
specified in milliseconds is sufficient.
If you are strobing a light, there is a short delay while the lighting controller turns on the light.
This delay is represented by the lighting turn-on time in Figure 5-1. Table 5-2 lists the lighting
turn-on times.
After the lighting turn-on time, the exposure begins. The width of the exposure pulse determines
how long the sensor is exposed. The exposure time can be adjusted by setting the
Exposure
Time control in Vision Builder AI, setting the Exposure Time property in LabVIEW, or by
setting the Exposure Time control in MAX. The lighting strobe deasserts at the end of the
exposure pulse. The end of an exposure starts the image readout from the sensor.
The maximum trigger rate is determined by the maximum frame rate for your configuration.
Refer to the
Maximum Frame Rate
section for information about the factors that affect the
maximum frame rate.
Table 5-1.
Trigger Synchronization Variability
Smart Camera Model Trigger Synchronization Variability
NI 1722
NI 1742
NI 1762 31.2
μ
s
NI 1744
NI 1764 71.6
μ
s
Table 5-2.
Lighting Turn-On Time
Smart Camera Model Lighting Turn-On Time
NI 1722
NI 1742
NI 1762 156
μ
s
NI 1744
NI 1764 143.2
μ
s