| Products: PX500, PX510,
PX610, PX610A
A resettable camera is typically
used in situations where the event that needs to be captured may happen
at times that are not synchronized to the timing of the video stream.
The output from a standard video camera is a continuous sequence of frames
each consisting of two fields separated by a vertical blank. If the event
to be captured occurs midway through a field, there is no way to "reset"
the camera back to the top of the field in order to catch the event. A
resettable camera provides a mechanism to "reset" the normal sequence
of video frames, and more importantly, it also resets the exposure on
the CCD array. All you need is a well lit object and the camera’s
controllable exposure timing to stop motion crisply, no need for annoying
strobe lighting.
A resettable camera typically
has no video output, just sync, until it receives a reset signal. Then
it outputs only one frame or one field depending on its configuration.
The reset signal is usually a TTL signal that is applied to a special
trigger input on the camera. The TTL signal could be generated by a device
like a photo detector, mechanical switch, or other event sensor.
As an example of a problem
whose solution lends itself to the use of a resettable camera, consider
a line of soda bottles traveling on a conveyor belt. As each bottle arrives
at a specific location (e.g. point A), it needs to be illuminated, scanned,
and captured. One possible solution would be to use a standard camera
and a strobe light. However, strobe lights on a manufacturing floor can
be annoying or dangerous. In addition, the bottles will not always appear
in exactly same location in the image. Now the software must find the
bottle in the image in addition to the job it was designed to do.
This problem can be solved
with a resettable camera, a PX, and a device that generates a TTL signal.
Place an electric eye near the conveyor belt just prior to Point A. As
the soda bottles arrive at Point A, it will interrupt the light beam and
generate a TTL signal that will reset the camera, and the PX
- Connect the video
output of the camera to the video input of the PX
- Connect the TTL signal
to both:
- the PX trigger
input, and
- the camera trigger
input
- Put the PX in triggered
grab mode
When the light beam is broken
by a passing soda bottle, a TTL signal is generated which triggers both
the camera and the PX frame grabber. Once the PX is triggered, it will capture
the next frame it receives. After the camera is triggered, it scans the
soda bottle and generates a frame.
Notes and Software
Techniques
The camera shutter speed
may need to be adjusted to compensate for the speed of the conveyor belt.
The PX software library
has a GrabTriggered() function that can be used to control the frame grabber.
The GrabTriggered() function can be used in either immediate or
queued mode.
In the immediate mode,
the function will not return to the calling program until the frame grab
has completed.
GrabTriggered(fgh, buf, 0);
In the queued mode,
the function will return to the calling program immediately and the frame
grab will occur later. The IsFinished() function can be used to determine
when the grab has completed:
hGrab = GrabTriggered(fgh, buf, QUEUED);
/* the grab is completed if the condition returns TRUE */
if(IsFinished(hGrab))
The above techniques can
also be used to grab single fields, depending the capabilities
of the camera.
Single field immediate
grab:
GrabTriggered(fgh, buf, SINGLE_FLD);
Single field queued grab:
hGrab = GrabTriggered(fgh, buf, QUEUED | SINGLE_FLD);
/* the grab is completed if the condition returns TRUE */
if(IsFinished(hGrab))
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