Sorvall RT/T6000D - Page 109

System
Descriptions:
SORVALL®
Centrifuges
Electrical
6-9.
Zero Speed
Detection Circuit
(refer to figure
6-7)
At
the
beginning
of
a
run,
the
zero
speed
circuit
de-energizes
the
door
solenoid
(LI).
preventing
the
user
from
unlocking
the
door
during
a run.
It
also immediately
extinguishes
the
zero
speed
Indicator
because
the timer (SW2)
and
speed
(SW6)
control switches
close.
When
the
centrifuge is at
zero
speed,
the
motor
tachometer's
output
is not
producing
any
pulses.
Without
any
pulses being generated,
the
Frequency
To Voltage
Converter
circuit's
output
goes
to
zero
and
the effective
tachometer
converted voltage
level
(at
zero
rpm)
is
+2.5Vdc (RT/T6000DandRT/T6000B. SNs
9101800
and
above)
orO.OVdc (RT/TCOOOB,
SNs
below
9101800.
and
RT/T6000). This
+2.5
Vdc
level,
when
compared
to
the
Zero
Speed Detector,
produces
a
HIGH
output
signal.
This
HIGH
output
is
then inverted three
times through
the
Hex Inverter Drivers
(ULN2004s)
until
the
resulting
LOW
signal
is
used
to
sink the
+24
Vdc
current
that
is
passing
through
the Door Solenoid
LI.
This
energizes
the
Door Solenoid, allowing
the
user
to unlock
the
chamber
door.
At the
same time,
the
LOW
signal
is
also sinking
the
current through
the
Zero
Speed
Indicator located
on
the
front
panel.
This
Illuminates the Zero
Speed
Indicator.
When the
rotor starts
turning at
the
beginning
of
a run.
the
motor
tachometer
begins
producing pulses
that
are
fed
into
the
Frequency
To
Voltage Converter.
The
converter
changes
the
input
frequency
into
an equivalent
dc
voltage
level
proportional to
the
speed
of the
motor;
it
then
applies
it
to
Zero Speed Detector.
When
the
voltage
is
compared to
the
+2.5
Vdc
reference,
the threshold
is
exceeded;
this
changes
the
output
state
of the
Zero
Speed Detector
from
a
HIGH
(while
at
zero speed)
to
a
LOW.
This LOW
signal
is
then
inverted three more times
by
the
ULN2004
Hex Inverters before finally
becoming
a
HIGH
at
the last
output.
This
HIGH
output signal
effectively
removes
the
ground
potential
from
the +24
Vdc
supply
and
de-energizes
the Door Solenoid
LI.
This
in turn mechanically
"locks"
the
door
shut, preventing
the
user
from
accidentiy
unlocking
the
door
while the
rotor is
spinning.
When the
ground potential
is
removed,
the
Zero
Speed
Indicator
lamp
also
is extinguished.
The
Zero
Speed
Detection
circuit
prevents
the
door
from
being opened
when
the
rotor
speed
is
greater
than
100
rpm,
even
if
the
timer
or
speed
control
knob is
then
turned
OFF
(which would otherwise
cause
the
solenoid to
re-energize).
If
incorrect drive
belt
tension is
set (see
Calibration
paragraphs
in
Sections
7,
8.
and
9)
or
if
the belt
breaks,
the
drive
belt
switch (SW5)
located
on
the
moveable base
plate
of the
motor
assembly
will
be
tripped.
This
causes
the
+24 Vdc from
the
door
solenoid
to
be
Immediately
removed,
thereby locking
the
door
(if
already
latched) and
preventing
access
to
the
chamber,
even
if
the
rotor is at zero
speed.
Access
to
the
chamber
can
only
be
made
using
the
EMERGENCY
SAMPLE
RECOVERY
procedure
as
in
paragraph
3-3
6-10