Silence as strength: Magnetic bearings for computed tomography
22.09.2020 I Güllü Beydilli
Computed tomography today generate background noise that many patients find very unpleasant. A new magnetic bearing from thyssenkrupp is providing relief: an X-Ray camera can move on it almost in silence.
The examination starts. ”Please lie as still as possible.” The radiographer closes the door and the patient is alone. Accompanied by whirring and humming sounds, the examination table moves millimeter by milli meter through a tunnel, around which the X-ray camera rotates. Although computed tomography (CT) involves absolutely no contact with the patient, many find the examination stressful. While they are alone and surrounded by technology, seconds seem an eternity. It is hard to hold still, and the whirring and humming eat at their nerves – which are already on edge if they are afraid of the impending diagnosis.
Silence as a competitive advantage
“CT manufacturers are very interested in bringing scanners that are as quiet as possible onto the market. Lower noise is a measurable quality criterion that can be experienced by the patient,” explains Bernd Lüneburg, group leader for research and development at thyssenkrupp rothe erde. The same applies for speed: The faster a camera rotates, the more it reduces the length of the examination. Additionally, speed reduces the amount of exposure to radiation and as well as the costs.
“The magnetic bearing technology as a “digital bearing solution” opens up extensive possibilities, both for CT development and for the operation and service of CT scanners,” says Lüneburg. Thanks to the controllability of the bearing forces, for example, the camera can be precisely positioned. Further advantages lie in a possible integration of the drive unit and use of the sensor information for servicing the systems.
This is why Lüneburg’s team at thyssenkrupp is developing the “digital bearing solution” for the CT scanners of the future. The prototype is rotating on the wall of the test building in Lippstadt. There is no whirring or humming. The rotor weighing almost 300 kilo – grams moves silently round an inner ring the size of a CT tunnel. The “silent bearing” is stabilized by the invisible force of dozens of electromagnets.
Computed tomography: magnetic bearings beats rolling-contact bearings
The magnetic bearing in Lippstadt is currently one of the biggest and most powerful in the world. It is set to open up new possibilities for the makers of computed tomography equipment. Up to now, equipment manufacturers have been dependent on large-diameter rolling-contact bearings. In these huge bearings, balls or rollers reduce the friction when the stationary and rotating rings move against each other. In CT scanners, for example, the bearings en – sure that a heavy X-ray camera attached to the rotating ring circles at high speed round the patient and can perform the X-ray examination on them. On the opposite side of the ring, detectors capture the X-rays that have not been absorbed by the patient’s tissue or bones. On the basis of these signals, the computer generates three-dimensional cross-sectional images, with the help of which the radiologist can diagnose bone fractures, joint disease, or tumor-like changes to the organs.
Project “Silent Bearing”
To overcome the noise problem, thyssenkrupp rothe launched the “Silent Bearing” project some years ago. “We had one goal: to develop a bearing with maximum quietness and super-low friction that is as wear-free as possible,” Lüneburg reports. The idea was that, in this innovative bearing, the stationary stator ring and the rotating rotor ring would be kept apart not by rolling elements but by magnetic forces. In itself, this idea is not new. In centrifuges, machine tools, pumps, and compressors, electromagnetic bearings have long ensured that rotating components moving at high speeds operate without wear. However, for large-scale applications in which large-diameter bearings have been used so far, complex control technology and the high standards required for production have meant that these bearings have been suitable only on a very limited scale.
Maglev technology for the future of computed tomography
For this reason, the thyssenkrupp engineers developed a new concept. The inspiration was the Transrapid. This high-speed train hovers on a traveling magnetic field that counteracts the Earth’s force of gravity. This field is created by electromagnets. It ensures that the undercarriage does not touch the track but glides smoothly above it.
This principle is now being used by Lüneburg’s team to make medical technology hover, too. Dozens of electromagnets integrated into the stator create a field that attracts the rotor, which is made from magnetizable steel. A thin air gap of approximately 1.5 millimeters is created between the stator and the rotor by intelligent control. “It was a big challenge to control the magnetic fields so that this air gap is constantly open – even when the rotor is carrying the weight of a heavy X-ray camera just on one side,” Lüneburg recalls. A total of 48 electromagnets – axial and radial, meaning along the direction of rotation and at right angles to it – had to be integrated into the stator. In addition, there are sensors that constantly measure the size of the air gap during operation.
A connected computer evaluates the data and transmits commands to the individual electromagnets. These have to align the rotor with micrometer precision – whether it is rotating or has been stopped, or whether or not it is bearing a load. The engineers of thyssenkrupp developed the control systems for the silent bearing in collaboration with the Zittau University of Applied Sciences.
Safety bearing for emergencies
The fact that they actually work is demonstrated by test on the prototype that is rotating silently on the wall of the thyssenkrupp rothe erde test building. It can support a load on one side of up to 750 kilograms, the theoretical maximum capacity of a CT scanner – the electromagnets keep it constantly in the correct position. And what happens if the power fails? A rotor in a bearing of this size that was kept in balance by electromagnetic forces alone would suddenly fall onto the stator and cause considerable damage. To cope with emergencies, therefore, the engineers have included a safety bearing in the stator – a bronze ring that slows down the movement if the electromagnets are switched off accidentally.
thyssenkrupp has already patented the new magnetic bearing. “We are now at the point where the technology is so mature that we are able to test it in all kinds of applications now,” Bernd Lüneburg says. This is good news for the makers of computed tomography equipment: in the future they will be able to build silent machines that are less of a strain on patients’ nerves.
The engineers at thyssenkrupp also aim to speed up the silent bearings in the future. The prototype currently achieves 150 rotations per minute, and the goal is to reach 300 and later perhaps even more by making further improvements to the control systems. That would make the magnetic bearings faster than present high-end devices that use antifriction bearings. Increasing the rotation speed would have many advantages. A computer tomography examination would be quicker, the amount of exposure to radiation for the patient would be lower, and the radiologist would be able to monitor dynamic processes, such as the opening and closing of heart valves, in real time.
And the best: Not only medicine can benefit from the new technology, as Lüneburg explains. “Wherever noise and lubricants must be avoided while maintaining maximum precision, contactless magnetic bearings are a promising alternative to traditional bearings. Good examples for that are controlling telescopes in observatories, radar antennae on ships or the assembly of satellites in clean rooms.”