Model 310CD0 pages

Optical Chopper – High Speed
Model 310CD

↑ 310CD with blade protector

Features
→ 100 Hz – 120 kHz standard frequency range,
depending on the disc used.
→ Compact chopper head
→ Photochemically etched non-magnetic discs
→ Variable aperture
→ Range of accessories
→ Scitec Instruments Model 310CD is a high
speed variable frequency optical chopper.
The basic system consists of control unit,
chopping head, a blade protector and a
chemically blacked photo etched disc. This
system provides operation over the frequency
range 100 Hz to 120 kHz, depending on the
disc(s) purchased. A wide selection of
additional discs and accessories is available
to extend the frequency range and to satisfy
individual requirements.

→ The Model 310CD is a high speed optical
chopper based on our standard 102 mm diameter
discs. By spinning the discs many times faster
than our standard system, chopping speeds up to
120 kHz can be achieved.
→ The high speed is reached by using a 50W
electrically commutated motor and drive unit. The
high efficiency of the motor means that a large
proportion of the energy is transferred to the disc.
Wind resistance is the limiting factor on the
speed discs can be spun at. The greater the
number of slots, the higher the wind resistance
produced by the edge of the slots and hence the
slower the maximum speed. To minimise losses,
the chopping disc is enclosed in a carefully
designed blade protector which guides the air
flow around the disc.

Model_310CD_issue1.0

→ Fast chopping speeds come at a cost however. The
50W of energy put into the motor has to be released
somewhere. The action of the disc moving through
the air causes the air to heat significantly. This
coupled with the high speed of the disc causes jets of
warm air to be released through any open apertures
in the blade protector. Vibration is also an issue and it
is therefore necessary to ensure that the chopping
head is securely bolted to a secure surface at all
times. Finally, the motion of the 2 slot disc, in
particular, through the air causes a siren effect. This,
at maximum speed, is deafening to the extent that ear
defenders are considered necessary. Operation with
the 445 slot disc is considered loud but only to the
point where you would leave the room to make a
phone call.
→ Safety is an obvious concern with a system that has
parts moving at 130m/s (290mph). Some protection is
achieved by enclosing the chopping disc in a blade
protector. However, apertures in the blade protector
remain a cause for safety concern. To help reduce the
dangers to a minimum, blanking plates are provided
to cover all unused apertures. Plates are also
supplied to reduce the size of apertures in use.
However, in some experiments, due to the size of the
optical beam to be chopped, it is not possible to
completely remove the danger of finger ingress. It is
therefore necessary for the user to ensure that it is
not possible for fingers to come into contact with a
moving disc through the use of guards etc.
Disc
300D2
(2 slots)
300D5
(5 slots)
300D10
(10 slots)
300D30
(30 slots)
300D445
(445 slots)

Revs Per
Second
50 - 400
50 – 370
50 - 340
50 - 300
50 - 270

Revs Per
Minute
3000 24000
3000 22000
3000 20000
3000 18000
3000 16000

Chopping
Frequency Range
100 - 800 Hz
250 - 1850 Hz
500 - 3400 Hz
1500 - 9000 Hz
22 - 120 kHz

Note: The 300D200 (200 slot disc) is not suitable for use with the Model
310 system as it is not strong enough for the speeds involved.

→ When switched on and set to minimum speed the
motor will rotate at 7 to 10 rps. For the disc to be
stopped it is necessary to power off the control unit.

→

The speed of the chopping disc can be controlled via
a 10 turn potentiometer on the front of the control unit
or an external DC voltage can be applied to a front
panel BNC input. The motor speed is then set using
the equation:
Chopping = external voltage (V) x slots in disc x 82.7
speed (Hz)

e.g. an external voltage of 3.26V with a 445 slot disc
will give a chopping rate of 120kHz.

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