Erik,

Even a 0.1 deg shift of temp is not inconsiderable. The periodic nature of
your problem makes it seem likely that the heater/controller has a response
time of about 1/(this period).  This is reasonable as there are always lags
in thermal controllers and you have to make the frequency response very
slow to stop them from oscillating even worse.  The best idea would be to
measure the average power to your heater, disconnect the temperature sensor
and run the system "open loop" after setting the heater current (perhaps
with an external supply?) to the average value that you measured.  This way
the heater isn't turning off and on and, although you may end up at a
temperature not quite what you wanted, it will be much more stable.

Or it could be the temp control on the room air-conditioning. (which can
oscillate in the same way)

Jim Pawley


>"Daniel C. Focht" <[log in to unmask]> wrote:
>> What is the frequency & amplitude of the z axis displacement you are
>> encountering?
>
>Tom Donnelly <[log in to unmask]> wrote:
>> I am not totally clear on the problem.
>> Does the focus drift out or just change from x to x +/- 0.5um?
>
>Thanks for your replies so far.
>I'll make THE PROBLEM more clear now.
>
>The frequency of the z axis displacement is between 2 and 6 minutes,
>depending on the insulation of the objective (better insulation,
>longer period). During such period the focus position first changes slowly
>down (to the objective) and than quickly changes to the first position.
>The amplitude of the shift is between .2 and .5 um. When I swich off
>either heater-controller (objective-heater or warm-table)  the focus
>shifts dramatically towards the objective.
>
>Again, I do not believe that the heater-controllers are not OK. They
>have an accuracy of less than 0.1 C. It must be something else.
>
>This week I'll try the other suggestions. You will hear from me.
>
>Erik Manders
>CRC Nuclear Structure and Function Group
>Sir William Dunn School of Pathology
>Oxford University
>South Parks Road, Oxford OX1 3RE, UK
>Tel: (+44/0) 1865 275529 (/275500)
>Fax: (+44/0) 1865 275501
>E-Mail: [log in to unmask]

                   ****************************************
Prof. James B. Pawley,                                       Ph.  608-263-3147
Room 1235, Engineering Research Building,                    FAX  608-265-5315
1500 Engineering Dr., Madison, WI, 53706
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