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Below is an example of the precautions necessary to achieve a smooth start to imaging from a
stopped condition using a non-ideal transport mechanism.
The test bench transport mechanism discussed here has an encoder which is an integrated part of
the servo drive motor. There is a gear reduction and screw mechanism that attaches the motor /
encoder to a slide that supports the object to be imaged. This setup includes some backlash
between the slide and motor / encoder which creates an object motion to EXSYNC mismatch at the
start and stop of travel. The whole test setup also vibrates for some time after it stops due to the
acceleration transient. To achieve the best possible start up image quality with this non-ideal
transport mechanism, the following precautions had to be taken with the transport control:
1. Before imaging was triggered, the transport was moved forward a few millimetres at a very slow speed
and stopped to take up the slack.
2. The transport was then stopped for a sufficient time for the vibrations to die out.
3. Image capture could then be enabled and the transport ramped up to the target speed
4. S-Curve acceleration profiles were used to minimize rapid velocity changes.
Even with these precautions, some image distortion could still be seen. It is assumed that the user
will have much better transport system than this demonstration test bench. However, users may
still experience similar issues, especially when they working with very high object resolutions.
Distorted Image when Changing Direction
The camera achieves its high responsivity by accumulating 4, 8 or 12 rows within in the sensor in a
fashion that accurately follows the object motion. When the scan direction changes the multiple line
TDI accumulation process must reverse to match the reversed image motion across the sensor.
Only when all rows being accumulated have received the same image will the output be correct.
Prior to this some lines have been exposed to one direction and other lines exposed to the opposite
direction in the accumulated output. The camera will output a number of invalid rows immediately
after a direction change as follows:
That is, when changing direction the first 2x #Rows - 1 should be ignored.
Notes:
1. The camera has a single line gap between each light sensitive row, thus the 2x #Rows - 1 invalid rows.
2. The camera does not drop invalid rows as some applications have issues when the number of EXSYNC
pulses sent to the camera does not match the number of lines output from the camera.
Optical Misalignment Issues
As with all TDI cameras, image quality is sensitive to how accurate the image movement is aligned
to the pixels as it tracks over each row of the sensor. The greater the number of rows, the more
sensitive image quality will be to misalignment. The camera's maximum number of selectable rows
is 12, which is far less than some CCD TDI cameras. However, maintaining accurate alignment
should still be a priority.
The Piranha XL XDR Camera
Number of Rows Selected
4
8
12
Number of Invalid Rows
After Direction Change
7
15
23
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