[RASMB] Laser delay jump

Tom Laue Tom.Laue at unh.edu
Fri May 8 07:04:14 PDT 2009


Hi-
I still suspect the tach system, but you may be able to get around it. 
The signal from the tach is two pulses, one positive going and one 
negative going. The zero crossing point is considered the reference 
time. In principle, the absolute angle of the zero crossing should be 
speed independent. However, signal filtering leads to a phase shift in 
the peaks, leading to speed dependence above and beyond that from the 
propagation delays. The speed-dependent phase shift is most pronounced 
at speeds below ~3000 rpm, and tends to be minimal at speeds > 5000 rpm. 
Here is what I suggest you try- set the laser delay at 5000 rpm rather 
than 3000 rpm. You should find that the change in delay angle is much 
smaller when you go to 60000 rpm. A simple fix. If this does not fix the 
problem, or if you still have rather large angle adjustments, it would 
suggest a problem in the tach system still exists. The usual culprit is 
marginally weak magnets in the ring. The Beckman field service agent can 
test for this.
Best wishes,
Tom

Zhenli_Luo wrote:
> Dear RASMBers:
> Recently a strange behavior occurred on our XL-I, described as follows:
> Whilom, when using interference system in experiment, to obtain clear fringe
> display, a laser delay was set to 180˚ for cell I at 3000rpm and adjusted to
> about 180.5˚ at maximum speed on a 4-hole rotor, similar for other two cells.
> But now it should be set to 180˚ at 3000rpm and about 177˚ at 60k rpm.
> (The values tend to get smaller). This situation came suddenly since an experiment
> conducted several days ago. A rotor speed at 60k rpm, 20 degree C, both UV-Vis and
> interference detectors were used to collect data at a time interval of 2mins, the
> data collected were alright at the first hundred scans, then very curious data
> appeared, it seemed that there were no signal since then, only noise was contained
> in data detected by both detectors, as shown in attachment. I regreted that I was
> absent at that moment so no more detail can be provided.
> I sought help from Beckman coulter engineer. Possible reasons such as overspeed
> disk nick or tach module (which senses the magnets in the overspeed disk)
> misalignment or mirror #3 shift were suspected, but these suspicion were all
> eliminated by checking the collar offset value after a radial calibration and
> pixels of inner and outer radius of counterbalance.
> Does any AUC user encounter such problem? I beg your suggestion.
> Any word will be greatly thanked.
>
> Best regards.
>
> Zhenli Luo
> 8th, May
>
>
>   
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-- 
Department of Biochemistry and Molecular Biology
University of New Hampshire
Durham, NH 03824-3544
Phone: 603-862-2459
FAX:   603-862-0031
E-mail: Tom.Laue at unh.edu
www.bitc.unh.edu
www.camis.unh.edu




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