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APPENDIX B - Maintenance
The HS20 laser head will require very little in terms of routine maintenance, but the work that is
required is primarily optical component cleaning.
There are no user serviceable components inside the HS20 laser head
Two forms of optical maintenance may be required during the lifetime of the HS20 laser head. This
maintenance will either be to remove contamination from the optics or to re-establish optical alignment
between the HS20 laser head and the optical elements which make up the linear measurement kit.
The frequency with which these operations need to be undertaken will be a function of the conditions
within which the system is operated, the beam protection ducting and the mounting schemes used for
the HS20 laser head and linear interferometer optics.
Signal strength problem diagnosis
The HS20 system provides two alarms to the controller, which indicates that the optical arrangement
(HS20, interferometer and retroreflectors) needs attention. These signals are termed beam low and
beam block, asserted at 20% of full signal strength and 10% of full signal strength respectively.
The beam low maintenance alarm has been incorporated into the system to flag that either the optics
need to be cleaned or the optical alignment needs attention, the system will continue to operate even
with beam low asserted with no reduction in positional accuracy.
Beam block is a fatal error which will immediately result in the quadrature feedback signals being
removed from the encoder inputs and all of the lines become tristated. Therefore the machine
controller should shut down so that no machine movement is possible whilst beam block is asserted.
These two alarms may be generated from contamination on the optical surfaces, misalignment of the
optical components or air turbulence. To investigate the source of any alarm the following procedure
should be instigated:
1. In the case where beam low is asserted, ensure that the machine is positioned in the region
where the alarm occurs. In a beam block situation the machine cannot be moved until corrective
action has been completed. However, to ensure that the beam block condition is consistently
maintained, reset the HS20 laser head. If the reset clears the beam block condition then the
original cause of beam block has been removed.
2. Remove beam path protecting ducting to enable access to the laser beam to be obtained
between the HS20 laser head and interferometer and between the interferometer and
measurement arm retroreflector.
3. Observe the two beams entering the lower aperture of the HS20 laser head on a white target.
Identify which of the two beams is produced by the reference retroreflector (attached to the
interferometer) by blocking the beam path between the interferometer and the measurement
retroreflector attached to the moving element of the machine. Observe the following:
i. The relationship between the two return-spots and the relationship between these spots and
the return aperture on the HS20.
Under ideal (near field) conditions these two beams will appear as one spot on the target
i.e. the reference beam and measurement beam will be concentrically positioned. The
further away the measurement arm retroreflector is from the HS20 the larger the return spot
will be at the HS20 return-aperture. In this case the two beams should still be positioned
concentrically with respect to one another and the return aperture in the HS20 laser head
shutter. If both beams and the aperture are not concentric in all cases refer to Section 4
HS20 laser beam alignment.
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