Maglev train

Feel the force: active magnetic bearings roll out

Active magnetic bearings under control.

To many people, magnetic levitation conjures up images of trains like the Shanghai Transrapid, the world's first commercial high-speed maglev service which shuttles about 7,500 passengers a day at up to 270mph between the city centre and Pudong International Airport.

Its propulsion system is based around a synchronous linear motor, which works like a rotating electric motor whose stator has been 'unrolled' along the track to produce a force along its length instead of torque. The 'rotor' is formed by electromagnets in the underside of the carriages to provide lift.

Increasingly, however, industry is finding applications for a version of the technology known as active magnetic bearings (AMBs). In an AMB the stator has been rolled back around as on an electric motor and, unlike conventional rolling bearings, the shaft or rotor in an AMB is suspended by stationary electromagnets, which act as the stator. The rotor spins within the stator's magnetic field and, typically, two radial bearings support and position the shaft in lateral directions while a thrust bearing does the same longitudinally.

This 'contactless' arrangement gives AMBs a number of advantages. With almost zero friction, there is no need for lubrication systems, making them suitable for use in remote locations or, say, in a vacuum, where maintenance would be difficult and costly, and conferring eco-friendly credentials on them. It also means they are capable of very high rotational speeds - up to 50,000 rpm or so, with no known maximum - so they're now finding their way into applications such as compact power-generators.

AMB technology

The technology itself is not new. Patents for AMBs date back to the early 1940s, while French company S2M was the first on the market with them in the 1970s. In the early years their use was highly specialised - for example in turbomachinery such as turbines and compressors, and high-speed centrifuges for purifying radioactive isotopes. One major reason for this, apart from cost, has been the control systems and their evolution from analogue to digital technology.

The control system for a modern AMB consists of three main components: shaft position sensors on the stator; a digital controller; and a set of power amplifiers that supply bias current to opposing pairs of electromagnets on the rotor.

The sensors detect any deviation by the shaft from its centre position as a voltage. When the shaft is in the centre, the sensors produce a null voltage. If the shaft moves above this position, a positive voltage is produced; if it moves below, a negative voltage results.

The controller consists of anti-aliasing filters, A/D converters, a digital signal processor (DSP) and pulse-width modulation (PWM) generators. Sensor signals are passed through the filters to eliminate high-frequency noise, which can cause inaccurate readings of the position of the shaft. Also, because the controller periodically samples the signals, some of the high-frequency information can 'fold over' into false low-frequency information, aliasing the information received by the controller.

After the high-frequency content is removed, the position signal is sampled by the A/D converter for processing by the DSP, which produces an output proportional to the amount of current needed to correct the position error in the shaft.

This current is compared to the actual current in the bearing, which is also sensed, filtered and sampled using an A/D converter. The error between the actual and requested current is then used to characterise the PWM signal sent to the amplifiers. This information is sent to the PWM generators, which create the PWM waveform sent to the amplifiers.

Each bearing axis has a pair of amplifiers to provide current to the bearing coils and E F provide an attractive force to correct the position of the rotor along that particular axis. The amplifiers are simply high-voltage switches that are turned on and off at a high frequency, as instructed by the PWM signal from the controller.

The whole process is typically repeated at a frequency of 10-15kHz, and this improvement in the precision of their control systems has been a key development in AMB technology.

At first glance, AMBs might be seen to offer a direct replacement for their conventional, rolling-bearing counterparts. Not so, according to Professor Stathis Ioannides, visiting professor in mechanical engineering at Imperial College London. 'At the moment there are three issues with AMBs compared with their rolling-bearing counterparts,' he says.

'The main issue is their load-bearing capabilities, which are much lower than the equivalent size rolling bearings, so they need to be larger for a given load.

'Secondly, the control electronics needs to be housed in an external cabinet, which has to be sited nearby in a 'friendly' environment, and they need to be protected from loss of electrical power. They therefore have to be fitted with auxiliary 'landing' rolling bearings to prevent their destruction, and in some applications require a UPS back-up.

'They also cost more than their rolling-bearing equivalents.'

But, he says, they do have advantages in addition to the speed and lubrication factors. 'AMBs offer the facility for additional control, for example through positional self-correction of the rotor and variable stiffness to maintain a required shaft position with varying loads, as well as the facility to use the power supply for diagnostics, as in monitoring the current to different parts of the coil, for example.'

To bolster these advantages, AMB manufacturers are improving the technology to address the issues Professor Ioannides cites. The most notable development in this respect is the Fusion AMB from US company Synchrony, which won an R&D award last year.

Integrated control

The main innovation behind the Fusion is to integrate the control system into the bearing, doing away with the need for a separate control cabinet - the only connections are a set of wires for a 48V dc power supply and an optional Ethernet cable for communications.

The size of the unit itself has also been reduced by raising the bearing pressure, a governing factor in its load capacity, by increasing the amount of electrical steel at the bore of the stator where the force is created. Splitting the flux and isolating the electromagnets, and integrating the position sensors into the electromagnets, have cut size further.

Synchrony says cost competitiveness is enhanced by using standardised bearing sizes and commercial off-the-shelf components in the control hardware. These components are smaller than previous devices and are further reduced in size as a result of using control algorithms that allow stable operation at lower voltages.

Developments like these are helping AMBs to evolve from a bespoke technology, where they usually have to be customised for an application, to an off-the-shelf technology - especially since, as their physical size falls, retrofitting into existing plant becomes more feasible. As that trend continues, the advantages of AMBs for many applications begin to hold sway.

As Pat Risen, director of business development at AMB supplier Synchrony, explains, 'Modern control systems benefit the technology in several ways. First is the ability to analyse how the bearings can best operate with rotating shaft characteristics such as vibration nodes [points at certain rotation speeds at which rotor vibrations peak] then optimise the control parameters to allow the bearings to operate through these 'critical' areas while maintaining control. This rotordynamic analysis can be performed with or without the shaft spinning.

'Another aspect is the bearing's ability to absorb shocks and transient loads while maintaining stability,' he says. 'Previously, AMB systems would lose stability under these extreme conditions and require complete 'rebooting' in order to return to stable operation.

'AMBs also have numerous sensing capabilities of critical parameters such as speed, forces, temperature, and vibration, allowing machine health monitoring to be performed so that steps can be taken to prevent machine failure before a failure occurs,' he adds. 'This data can be monitored remotely since it can be accessed via the Internet, and alarm systems designed to ring cell phones, for example.'

Cost benefits

Risen says AMBs these days are competitive on a first-cost basis with conventional bearings, which require a complex lubrication system with pumps, filters, piping, cooling systems and so on. By eliminating these expensive and toxic lubrication systems - as well as the more frequent and expensive service intervals conventional bearings need - operating costs are lower too.

This trend towards standardisation, simplification and lower cost is broadening the uptake of AMB technology, but there are other trends as well. According to Igor Derylo, research analyst for consultancy Frost & Sullivan's Technical Insights group, 'The trend towards business centricity, or vertical business integration, has made real-time plant-floor data important for strategic decision-makers. Advanced software automation solutions collect huge amounts of data from a variety of sensors, and process the data to make it digestible for non-engineers at higher company levels.

'Moreover, huge amounts of this plant-floor data have recently started to be used for maintenance purposes, for instance for predictive maintenance to keep high uptime,' he says. 'Signals used to control the bearings could be important in this trend. Something that used to be a technology restraint - the need for control systems for bearings - now becomes a clear advantage.'

Wireless control of AMBs now becomes a distinct possibility as well - something the oil and gas sector, with its widespread adoption of wire-free protocols at the moment, should be eyeing with interest. This facility appears to be some way off though. As Risen says, 'We're not looking at wireless yet but it's definitely achieveable. For one thing, the Ethernet option in the Fusion bearing makes wireless an easy next step.'

But, while AMBs have a growing part to play in the grand scheme of things, the general view appears to be that they'll always be for particular applications. Risen says, 'It is unlikely that magnetic bearings will ever be directly cost equivalent to standard rolling element bearings for all applications, but for many applications magnetic bearings provide superior value.'

Professor Ioannides adds, 'I can't at this stage foresee them ever taking over completely from roller bearings, which are smaller and cheaper than their AMB equivalents and which themselves are advancing too. But never say never.'

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