An electronic controller for a reciprocating rotary crystallizer is described. The heart of this system is the electronic timer circuit. A schematic along with a detailed description of its operation is given

Hester, Howard B.

Kroes, Roger L.

Reiss, Donald A.

English

NASA Technical Reports Server

Electronic Controller for Reciprocating Rotary Crystallizer
CD
Roger L. Kroes, Donald A. Reiss and Howard B.Hester -J
Space Science Laboratory, Marshall Space Flight Cente^
Huntsville, Alabama 35812 3. " ^
i
An e 1 ectron ic . contro 1 ler for a reciprocating rotary (ysystaFil <fcer
is described. The heart of this system is the e 1 ectrMPnic^iimer
circuit. A schematic along with a detailed descripti-cm of its
operation is given.
One important and widely used technique for growing large
single crystals of high quality from solutions involves the use
of the reciprocating rotary crystallizer (RRC). This technique
was pioneered in the 1940's at Bell Labs by A.N.Holden and
others [1,2]. With it they were able for the first time to
produce large quantities of high quality single crystals of
ammonium dihydrogen phosphate (ADP) and ethylene diamine tartrate
(EDT). The former was used for sonar detection during the war
while the latter was used in telecommunications [3,4].
Figure i shows the major features of a reciprocating rotary
crystallizer. Crystal growth takes place on seed crystals
attached to rotary arms which are immersed in a supersaturated
solution of the growth material. As the crystals grow they
reduce the concentration of the solution, so the temperature is
slowly decreased to maintain a relatively constant degree of
supersaturation. The quality of the crystals is largely
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determined by the high stability of temperature control and the
regular reciprocating rotary motion of the seed crystals through
the solution which provides them with an optimum flow of solution
over their surface. This maintains the relatively constant
growth rate necessary for quality growth.
The temperature control is usually maintained to an accuracy
of +0.1 degree C or better and the rotation rate is on the order
of 5-10 rpm. The direction of rotation is reversed with a period
of about one to five minutes. These are nominal values. Actual
values can vary greatly depending on the material being
crystallized and the circumstances of growth. The reason for the
reciprocating action of the rotor is to reduce any leading edge-
trailing edge effects on the growing crystals.
In the past the control of the reversing procedure has
generally been by mechanical means. Usually a cam or a screw
which triggers a micro switch was used. While such techniques
are useful, the equipment often lackes flexibility or is
difficult to fabricate. To overcome such problems we have
designed an electronic reversing system. A block diagram of the
system is shown in figure 2. It consists of a d.c. stirrer
motor, a solid state speed controller with a manual reversing
switch, and the electronic timer controller. The electronic
timer controller consists of a d.c. power supply, the timer
circuit, and the reversing relays. The motor with its speed
controller can be purchased from any scientific equipment
supplier and the components used to construct the electronic
timer controller are also readily available.
The heart of the control system is the timing circuit, the
schematic of which is shown in figure 3. It is based on a NE556
dual timer configured for delayed pulse generation[5]. The two
555 timers on this chip are made to trigger each other through a
O.Oimf ceramic capacitor. When the output goes low, the
capacitor discharges, producing the narrow low-going pulse
required for triggering. The timing capacitors are 220mf
electrolytics. Electrolytic capacitors are readily available at
low cost, and large capacitance values are available in
relatively small packages. They tend to have loose tolerances
and high leakages which affect the accuracy of the timing
circuit, but in this application high accuracy is not required.
A number of different timing resistors connected to a
multiposition switch provides flexibility in setting the interval
between reversals of rotation direction. The duration of each
interval is determined by T(sec.) = I.IR^C^. The maximum
practical value of R-^ is about 13megohms, giving a maximum
duration of about 52 minutes. In our circuit the range is i
minute to about 10 minutes. Shorter times require a smaller
capacitance. A 74121 monostable multivibrator is used to provide
a pulse at the end of each interval during which the power to the
motor is interrupted. This prevents an abrupt change of
direction which can put a great deal of stress on the motor and
the crystal mounting hardware, especially when the crystals have
grown large. Triggering is provided by the low-going outputs of
the 556A and 556B through .OlmF capacitors. The output pulse
width of the 74121 is determined by the timing capacitor and
resistor and is approximately equal to 0.7C-^R^. A pulse of .5 to
i sec. is sufficient.
The timing circuit is used to drive relays which have been
wired into the stirring motor controller in place of the manual
reversing switch. Reversals are accomplished by a latching
relay. Power is applied alternately to the two coils through the
2N2905/2N910 pairs which are driven by the 556 outputs through
the 7406 hex inverter. The ground pin of the relay is connected
to ground through a pair of 2N2222's which is turned on during
the timing period of the 74121. This limits the time that
current flows through the coil, preventing overheating. The non-
latching relay interrupts the power to the motor at the same
time, causing the motor to stop between reversals. This relay is
controlled by a pair of 2N2222's driven by the 74121 through the
7406.
When the controller was first used, some electronic noise
problems were encountered which interfered with the operation of
the 74121. It was thought that the noise was entering through
the power leads to the stirring motor controller. The timing
circuit was isolated from this noise by placing the relays in a
separate metal box and driving them through 6N136 opto-iso1ators.
The 7406 is used to provide the proper inputs for the 6N136
isolators. With these modifications in place the electronic
timer controller operated properly and provided us with the
needed flexibility to carry out growth of crystals under a
variety of conditions.
References:
i) A.H. Holden, Bell Lab. Record, 1947, 2j>, 458-461.
£) A.H. Holden, Discuss. Farady Soc., 1949, 5, 313-315.
3) A.C. Walker, Bell Lab. Record, 1947, 2_5, 357-368.
4) A.C. Walker and G.T, Kohman, Trans. Am. Inst. Elec. Engrs.,
1948, .67, 565-570.
5) W.G. Jung, 1C Timer Cookbook, Howard W. Sams & Co., Inc.,
Indianapolis, 1977.
List of figure captions:
Fig.l. Reciprocating Rotary Crystallizer
Fig.2. Block Diagram of Controller System
Fig.3. Schematic of the Timer Circuit
TEMPERATURE
READOUT
TEMPERATURE
PROBE
BATH
REVERSIBLE
MOTOR
TEMPERATURE
CONTROLLER
'GROWTH
CELL
RECIPROCATING ROTARY CRYSTALL5ZER
CRYSTAL
MOTOR
CONTROLLER
r ~i
110V AC
POWER
SUPPLY
+5V dc
+18V dc
+1EV dc
TIMER
CIRCUIT
SDLID STATE
SPEED CONTROLLER
LATCHING RELAY
NDN-LATCHING RELAY
110V AC-'
I
_l
Figure 2
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. 7.


Electronic controller for reciprocating rotary crystallizer

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