Laser Inspection of Surfaces - Vibration Management !?
Laser Inspection of Surfaces - Vibration Management !?
(OP)
Hi All,
hopefully someone will have experience of something like this !! - I am using a laser/camera displacement sensor to measure surface defects. The laser is accurate down to .25um so picks up quite a lot of the vibration as it is being moved over the surface (movement is achieved through an x-y table arrangement just like a machine table). Up until now this hasn't been a problem as the defects that I have been scanning have been big enough to be detected over the vibrational 'noise', however I am now trying to improve the resolution of the system. Can anyone advise me what is the best way to filter out the noise in the system WITHOUT buying expensive linear motion slides (outside dept budget I'm afraid). It seems to me that using a hi/low or bandpass filter might work well, but if anyone else has any ideas I'm open to suggestion.
Many thanks in advance,
DC.
hopefully someone will have experience of something like this !! - I am using a laser/camera displacement sensor to measure surface defects. The laser is accurate down to .25um so picks up quite a lot of the vibration as it is being moved over the surface (movement is achieved through an x-y table arrangement just like a machine table). Up until now this hasn't been a problem as the defects that I have been scanning have been big enough to be detected over the vibrational 'noise', however I am now trying to improve the resolution of the system. Can anyone advise me what is the best way to filter out the noise in the system WITHOUT buying expensive linear motion slides (outside dept budget I'm afraid). It seems to me that using a hi/low or bandpass filter might work well, but if anyone else has any ideas I'm open to suggestion.
Many thanks in advance,
DC.





RE: Laser Inspection of Surfaces - Vibration Management !?
Could you do a "dry run" without actually measuring any surface, get the value obtained as an offset and then run the sensor again (measuring) and deduct the offset?
Filtering up to your spec could do the trick but may you be masking one error with the other?
I could be totally wrong...
HTH
Saludos.
a.
RE: Laser Inspection of Surfaces - Vibration Management !?
TTFN
RE: Laser Inspection of Surfaces - Vibration Management !?
thanks for the response. Ok, I will always have some degree of background noise, but at the moment it corresponds to an output below that of the lasers resolution which is fine. I do agree though that a reference surface would help somewhat and it is my intention to calibrate the system using this (ie, by scanning a flat surface with a finish higher than the resolution of the laser) - problem is, it's an open loop system and as a stand alone option its probably not the best idea although I will couple one with the other.
Slowing down the scan speed will more than likely reduce the noise yes, but surely if I operate at a constant scan rate (say using a simple harmonic or lissajous type motion) across the part I'm scanning, then the frequency of vibration will be pretty constant. If THAT was the case I could use a bandpass filter and make sure my sampling rate was lower than the frequency of vibration - which could also be a problem. Rather than slowing down the scan speed, I'd rather make the system more adaptive/intelligent.
RE: Laser Inspection of Surfaces - Vibration Management !?
It may also be possible to put your instrument on an isolation system that has a low enough natural frequency to reject the vibrations coming from the motion platform
TTFN
RE: Laser Inspection of Surfaces - Vibration Management !?
Also you could limit the scan to the middle of the path eliminating the high inertia efects at the ends of the travel.
For what it's worth....
Speedy
RE: Laser Inspection of Surfaces - Vibration Management !?
At the moment I am using vibration damping but it still doesn't absorb all the vibration from the head during scanning, at least not at the resolution that I'm hoping to work from. That is a good point though to try and keep the scanning in the middle of the table to cut out the high intertia. Never heard of a notch filter before though - what exactly is it ??
RE: Laser Inspection of Surfaces - Vibration Management !?
A notch filter is essentially a narror, band-rejection filter, sort of the inverse of a band-pass filter.
TTFN
RE: Laser Inspection of Surfaces - Vibration Management !?
Another thing I noticed on the system is that whoever set it up originally has grounded the digital ground (from the camera) directly to the analog ground. Couldn't this cause problems ?? Shouldn't I have a capacitor between the two ?? If so, how do I calculate what capacitor to use ??
Sorry for all the questions but this thing is a real can of worms.