ISIS designs, manufactures, assemblies and develops rotating machines for “neutron selection" called neutron choppers. By inserting these rotating discs with an opening slit in the neutron beam after the moderator, neutrons can only pass through when the open slit is at the position of the beam. By tuning the rotation speed of the disc, it's possible to chop up the beam pulse emitted from the moderator into one or several bunches of neutrons with a well-defined starting point and time.
ISIS is currently designing and building two instruments for the European Spallation Source (ESS) in Sweden. For the FREIA instrument, which is in the build phase, the choppers are 1.3m wide carbon fibre discs mounted on an overhung rotor, rotating on magnetic bearings, all run in vacuum. They will be grouped on a quickly removeable pallets, so they can be easily and quickly removed from the high radiation area for maintenance in a clean area.
“This means we have to consider not just the vibration level of the rotating chopper, but also the effect on the structure, and particularly the excitement of any natural frequencies of the pallet system," explains Mike Brind, an ISIS chopper technician. “Transmitted vibration also effects the other machines on the same structure."
On each rotating chopper, there is an accelerometer measuring the casing vibration in three different directions, along the x, y and z axes. This gives the level of vibration, but it's more difficult to visualise the effect on the structure and, more importantly, for the team to know whether to stiffen the structure to reduce the magnitude of any vibration.
“I saw a video of Motion Amplification at a trade show and thought it would be useful tool in our chopper development," says Mike. “I arranged the UK agent for the technique to come to site and demonstrate on our test bed."
Motion Amplification (MA) is a fantastic technique to allow engineers to visualise and see the effect of vibration on the surrounding structure. Using a high-speed camera to film the structure in detail, any movement is then magnified 250 times bigger to show an exaggerated version of what is happening. The technique allows the technicians to run a machine up and visually access the different types of vibration present.
Sometimes vibrations occur at the natural frequency of the surrounding structure, causing large movements, but if the machine does not have to operate at that speed, then it can be acceptable to drive through that speed to the operating speed where the vibration is much lower. If the vibration is derived by unbalance forces, these forces will be proportional to speed and should be balanced out.
The maximum movement that the team observed using the MA agreed with the measurement from conventional accelerometers, confirming it was an accurate technique. “We learned that we could improve the construction method for these bases in future systems by including solid joints," says Mike. The video below shows the amplified chopper movements when it's vibrating.
“We learned that for one pallet it will be necessary to run one chopper up to operational speed before starting the second chopper," he adds. “This will prevent the first chopper's peak vibration during run up tripping the second chopper by transmitted vibration."
