Distributed Control System for Crystal Growth
A. E. Kokh, V. A. Vlezko and K. A. Kokh
Institute of Geology and Mineralogy, SB RAS, 3, Koptyuga Ave., 630090, Novosibirsk, Russia
Keywords: Crystal Growth Control System, LBO Crystal, Controller, Load-commutator, Thermal Field Symmetry.
Abstract: Distributed system for control over the crystal growth process is presented. The main advantages of the
system are its low cost, ability to recover after power failure, an application of standard ISaGRAF software
environment and available low-power PC-Controller. One of the option of the system is an ability to control
symmetry and dynamics of the heat filed. This option is the key factor for the progress in growth of
nonlinear optical LBO crystal.
1 INTRODUCTION
Crystal growth systems with various control loops
(rotating, pulling, weighting of the crystal etc.) have
been developed for industry since the middle of last
century. Now there are a lot of commercially
available growth stations with modern control
systems (CS). Our approach in elaboration of CS is
based on ability to nonuniformly heat the
crystallization domain and therefore to apply
nonsymmetric stationary or dynamic heat field. This
provides additional parameters to control over heat-
mass transfer processes which are always have been
considered as a key factor in the growth of high
quality crystal.
The base of our CS is a PC-controller I-7188
and remote input-output modules of I-7000 series
(produced by ICP DAS Company). The use of the
modules of I-7000 series provides quite cost-
effective reliability. They are not unique. A lot of
similar modules are produced by other companies,
for instance by Advantech. Also we have designed
home-made modules with controllers of crystal
rotation and pulling, as well as load-commutator for
control of the heat field.
2 HARDWARE
Here we consider one example of operating CS for
the growth of nonlinear crystal LBO (LiB
3
O
5
). Fig. 1
presents the scheme of the growth station which
consists of three-zone heating furnace, balance
sensor, pulling and rotation drives, contact-meter
and main controller I-7188EG. Temperature control
is realized by three-zone Eurotherm reglator through
the separated RS-485 bus. For that reason additional
serial ports for I-7188EG were added by introducing
mezzanine board X511. A main feature of the CS is
the presence of the load-commutators which may
switch segments of the heating zones through solid
state relays according to the program. The feedback
signal for thermoregulator is provided by four
parallely connected thermocouples placed around
heating zone. This was found to be enough for stable
regulation of temperature in the wide range of
periods of switching (1 sec - 20 min).
3 SOFTWARE
Each growth station with individual IP-address has
its own controller with onboard DOS-compatible
“Mini OS7”. In that way any standard programming
language may be used to realize a project. In our
case we were concentrated on the logic of crystal
growth process, so the I-7188EG controller with
built-in ISaGRAF 3.xx system was used. ISaGRAF
system implements the following functions: data
reading signal preprocessing, realization of control
algorithms, communication between modules and
HOST computer.
A program loaded in the controller is performed
with cycle 0.2 sec. Crystal growth parameters are
slow and have a very wide dynamic range. For
instance, the growth speed may vary from 0.001 to
hundreds of grams/hour.
While a parameter is low-rate changing, the
controller specifies some time interval and calculates
202
E. Kokh A., A. Vlezko V. and A. Kokh K..
Distributed Control System for Crystal Growth.
DOI: 10.5220/0004033902020205
In Proceedings of the 9th International Conference on Informatics in Control, Automation and Robotics (ICINCO-2012), pages 202-205
ISBN: 978-989-8565-21-1
Copyright
c
2012 SCITEPRESS (Science and Technology Publications, Lda.)
Figure 1: LBO growth station with ISaGRAF process controller and SCADA system.
the value. And on the contrary, if parameter changes
very fast, the controller defines how often it will
change the value.
Fig. 2 shows a fragment of FBD program of
process parameters calculation. Output value of
every parameter is the function of crystal length. So
the first part of the algorithm consists in calculation
of length increment as a function of growth speed.
After that other growth parameters are specified as a
function of the increment. In that way limitation of
input/output numbers (not more than 32) for each
block of FBD scheme is avoided.
Crystal growth process may take place up to
several months. During this time there may be some
imperfection in electrical supply while UPS blocks
do not totally protect the system. Or sometimes it is
necessary to reconfigure or even replace some
blocks of the CS. In that view survivability and
reliability of the system is very important. In other
words the controller should contain some algorithms
to restore values of all critical parameters and to
recover the CS after a power failure. It is done with
the current values saved in a nonvolatile RAM while
setpoints and other seldom changing parameters are
saved into EEPROM and are read from there in the
case of system reload with the symptom of
breakdown. The same concept of parameters restore
after breakdown or power off was realized for our
periphery modules like step motor drive MD1-VL
which may be used offline.
A SCADA program InduSoft Web Studio is used
as the program of upper level because of low price
and the presence of Modbus driver for direct
communication between ISaGRAF project and
SCADA. The last reason is also important since
many programs use various OPC-servers which slow
down the system.
A main purpose of SCADA-program is to
interface with operator and visualization of
parameters and trends. The program has 1500
variables which is quite enough for CS of one
growth station. On the other hand, all calculation
function, storage and initialization of variables, and
restore of data after breakdown are implemented in
the main controller.
DistributedControlSystemforCrystalGrowth
203
Figure 2: Block diagram of the piecewise-linear crystal
geometry master.
4 USE OF THE CONTROL
SYSTEM. EXAMPLE OF LBO
CRYSTALS GROWTH BY
KYROPOULOS METHOD
Kiropoulos method consists in the growth on seed
crystal slightly dipped in the melt (or the melt-
solution). The growth proceeds towards the melt by
a smooth decrease in temperature at small
temperature gradients, which makes it possible to
grow high quality crystals. This method is widely
used for production of large sapphire crystals of any
crystallographic orientation, with an extremely low
dislocation density of less than 1000 cm
-2
.
LBO single crystal is a well-known material used
in nonlinear optics (NLO) for the last 20 years. Due
to good operating performance and relatively high
nonlinear coefficients LBO crystals are widely used
for frequency conversion in the visible and near UV
regions. These crystals are of special interest now
since they may be used in the laser systems of
extremal intensity (http://www.extreme-light-
infrastructure.eu/). Growth from molybdenum oxide
fluxes has brought a considerable progress in LBO
growth technology but the crystals with large high
quality parts were still unavailable until recent time.
LBO growth furnace has 3 heating zones. The
bottom and middle zones are composed of 8 heating
elements. Their connection is realized through two
separate load commutators governed by the control
system. Each one allows to simultaneously switch
on any of the heating elements in any sequence and
for any time period (Kokh et al., 2009).
First advantage of such a system is the possibility
of intensive but noncontact mixing. It is very
important since homogeneity of badly-miscible LBO
and the flux seems to be a key factor to large size
and high-quality crystal growth. The melt
homogenization procedure is realized by prolonged
period between switchings (more than 15 minutes).
In this case, extremely high radial temperature
gradient produces intensified thermogravitational
convection. A convective pattern at the surface of
the melt (Fig. 3) clearly indicates a direction of the
prevalent flow which will be turned relative to the
crucible after next switching of the heaters.
Figure 3: Convective flows on the top of the melt-solution
during homogenization procedure.
In order to grow larger crystals the direct scale
up of the setup faces some serious problems. For
example, for large crucibles (>100 mm in diameter)
it is difficult to maintain axissymmetric heating and
hence to fix a coldest point on the melt surface at the
geometric center of the crucible. Otherwise distorted
heat field can lead to highly asymmetric crystal
growth which results in the formation of defects.
The CS allows to set the corrections to switching
time for any group of the heaters. Seeding process is
carried out in quazi – stable heat field when period
of switching is ~3 sec. In that case no temperature
oscillations are observed in the melt. Usually our
furnaces need delay corrections less than 0.5 sec for
some heaters to adjust surface temperature
distribution to axissymmetric one. This process is
controlled by observing the movement of small
pieces of LBO (~ 1-2 mm) thrown onto the melt
surface at a temperature of 10-15 degrees higher
than crystallization temperature (Kokh et al., 2010).
Step-by-step adjusting of time for each pair of
heaters is done until a crystal piece stops to move
and dissolves right in the crucible center.
ICINCO2012-9thInternationalConferenceonInformaticsinControl,AutomationandRobotics
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Large period of switching may result in the
considerable temperature oscillations (~10˚C) in
crystallization region. Until recently such conditions
were considered as unfavorable for crystallization
process. However our results have proved this
opinion to be incorrect (Kokh et al., 2012). The
progress of LBO growth under nonstable
temperature regime has resulted in the crystals (Fig.
4) suitable for fabrication of world largest 65 mm in
diameter optical elements for high energy laser
applications (Mennerat et al., 2011).
Figure 4: LBO crystal suitable for production of Ø65mm
nonlinear optical element; weight: 1290 g, dimensions:
149x131x83 mm.
5 CONCLUSIONS
Elaborated distributed control system provides
reliable growth process. One of the options of the
system is an ability to control symmetry and
dynamics of the heat field. By the example of LBO
crystals, this option was shown to contribute in the
progress of growth technology.
ACKNOWLEDGEMENTS
Financial support form grants RFBR 11-02-12156-
ofi_m and 11-02-12164-ofi_m is highly
acknowledged.
REFERENCES
Kokh A. E., Vlezko V. A., Kokh K. A. Control over the
symmetry of the heat field in the station for growing
LBO crystals by the Kyropoulos method. Instr. Exp.
Techn, 2009, v.52, 747-751
Kokh A., Kononova N., Mennerat G., Villeval Ph., Durst
S., Lupinski D., Vlezko V., Kokh K. Growth of high
quality large size LBO crystals for high energy second
harmonic generation // J. Crystal Growth. 2010,
Vol.312, N10, p. 1774-1778.
Kokh A., Vlezko V., Kokh K., Kononova N., Villeval Ph.,
Lupinski D. Dynamic control over the heat field
during LBO crystal growth by high temperature
solution method. J Crystal Growth. 2012,
10.1016/j.jcrysgro.2011.11.050
Mennerat G., Bonville O., Le Garrec B., Villeval Ph.,
Durst S., Lupinski D., Kokh A., Kononova N., Vlezko
V., Kokh K. Frequency doubling and tripling for
future fusion drivers. Abstracts of the OSA meeting of
50-year jubilee of Nonlinear Optics, Hawaii, July
2011.
http://www.extreme-light-infrastructure.eu/
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