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an external synchronization (EXSYNC) signal to the camera, where one pulse is generated when the
object moves by the size of one object pixel. See 'External Trigger Mode' in the user manual.
Any transport motion that is not correctly reflected in the EXSYNC pulses will cause image
distortion in the scan direction. For standard line scan cameras, this type of image distortion may
not greatly affect edge sharpness and small defect contrast, thereby having minimal impact on
defect detection. However, TDI image quality is more sensitive to object motion synchronization
errors. The following subsections discuss various causes for poor image quality as a result of
EXSYNC not accurately reflecting object motion.
Continuously Smeared, Compressed or Stretched Images
When accurate synchronization is not achieved, the image will appear smeared in the scan
direction. If the EXSYNC pulses are coming too fast, then the image will appear smeared and
stretched in the machine direction. If the pulses are too slow, then the image will appear smeared
and compressed. Check the resolution of the encoder used to generate the EXSYNC pulses along
with the size of the rollers, pulleys, gearing, etc. to ensure that one pulse is generated for one pixel
size of travel of the object.
It is also important that the direction of image travel across the sensor is per the camera's scan
direction set by the user. See 'Scan Direction' in the user manual for more information. If the scan
direction is incorrect, then the image will have a significant smear in the scan direction. Changing
the scan direction to the opposite direction should resolve this problem. Refer to 'Camera
Orientation' in the user manual to determine the correct direction orientation for the camera. Note
that the lens has a reversing effect on motion. That is, if an object passes the lens-outfitted camera
from left to right, the image on the sensor will pass from right to left. The diagrams in the user
manual take the lens effect into account.
Randomly Compressed Images
It is possible that when the scan speed nears the maximum allowed, based on the exposure time
used, the image will be randomly compressed and possibly smeared for small periods in the scan
direction. This is indicative of the inspection systems transport mechanism dynamics causing
momentary over-speed conditions. The camera can tolerate very short durations of over-speed but
if it lasts too long, then the camera can only maintain its maximum line rate and some EXSYNC
pulses will be ignored, resulting in the occasional compressed image.
The over-speeding may be due to inertia and / or backlash in the mechanical drive mechanism
causing variations around the target speed. The greater the speed variation, the lower the target
speed needs to be to avoid over-speed conditions. If the speed variation can be reduced by
eliminating the backlash in the transport mechanism and / or optimizing the motor controller
characteristics, then a higher target speed will be achievable.
Distorted Image when Stopping and Starting
The camera has a stop / start capability unique to TDI imaging and one that customers may not be
familiar with. Further, this feature may expose motion issues in a system that were not visible
previously. The camera is sensitive to any mismatch in object motion with respect to EXSYNC, as
this will cause a smeared, distorted image. When the user is using the camera in a stop / start
condition, great care must be taken to ensure that the EXSYNC pulses accurately reflect the object
motion when stopping and starting occurs and that there is no momentary backwards motion. Any
backlash in the transport mechanism could cause these types of problems due to acceleration
transients when stopping and starting.
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The Piranha XL XDR Camera
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