Instrumentation and measurement

310CD High Speed Rotating Optical Chopper

August 9, 2018

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 110 kHz can be achieved.

The high speed is reached by using a 50 W 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.

Fast chopping speeds come at a cost however. The 50 W 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). This is achieved by enclosing the chopping disc in a blade protector. However, apertures in the blade protector remain a cause for safety concerns. 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.

(please 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). A special 200 slot disc, part number 300D200HS, is available.

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 speed (Hz) = external voltage(V) x slots in disc x 82.7. e.g. an external voltage of 3.26V with a 445 slot disc will give a chopping rate of 120kHz.

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