HAFFNER, Ken-Yves (Ländliweg 21, Baden, CH-5400, CH)
LOBO NETO, Julio Danin (Hasselstrasse 19b, Baden, CH-5400, CH)
RYBING, Peter (Klockartorpsgatan 29C, Västerås, S-723 44, SE)
DETLEF, Pape (General-Guisan-Str. 49, Nussbaumen, CH-5415, CH)
HAFFNER, Ken-Yves (Ländliweg 21, Baden, CH-5400, CH)
LOBO NETO, Julio Danin (Hasselstrasse 19b, Baden, CH-5400, CH)
RYBING, Peter (Klockartorpsgatan 29C, Västerås, S-723 44, SE)
| CLAIMS 1 . A device for measuring a physical property of a magnetic bearing associated with a distance, the device comprising at least one sensor, characterized in that said at least one sensor comprises at least one acoustic transducer (4a, 4b, 4c, 4d) configured to emit acoustic signals and receive such signals reflected by a part (2, 3) of said bearing and means (5) configured to analyze said reflected signals so as to determine a value of said physical property. 2. A device according to claim 1 , characterized in that it is configured to measure a said physical property of a bearing being electro-magnetically driven. 3. A device according to claim 1 or 2, characterized in that it is configured to measure a distance, and that said means (5) is configured to establish the time elapsed between a said signal is emitted and then received after having been reflected so as to calculate the distance between the position of said emission and the position of said reflection in said bearing. 4. A device according to claim 1 or 2, characterized in that it is configured to measure a distance between parts (2, 3) of said bearing, and that said means (5) is configured to establish the difference of time elapsed between a said signal is emitted and then received after having been reflected by two surfaces of such parts of the bearing at different distances from the position of said emission and use this time difference for calculating the distance between these two surfaces. 5. A device according to claim 3 or 4, characterized in that said transducer (4a, 4b, 4c, 4d) is configured to emit acoustic signals substantially radially with respect to a rotating (3) part of said bearing, and that said means (5) is configured to calculate a value of a distance in said radial direction of said rotating part to a stationary part (2) of the bearing surrounding said rotating part. 6. A device according to claim 4, characterized in that it is con- figured to measure a said distance in the form of a thickness of a bearing part, such as a stationary part of the bearing surrounding a rotating part (2) of the bearing , and that the transducer (4a) is configured to emit acoustic signals in the direction of said thickness and said means (5) is configured to analyze acoustic signals reflected by two opposite surfaces (7, 1 7) of said bearing part defining the thickness of the bearing part. 7. A device according to claim 5, characterized in that said means (5) is configured to calculate a said distance between said rotating part (3) and said stationary part (2) several times during a full rotation of said rotating part and compare these distance values for checking the cross section shape of said rotating part. 8. A device according to any of the preceding claims, characterized in that it comprises at least one said acoustic transducer (4a, 4b, 4c, 4d) configured to emit acoustic signals from a position on a rotating part (3) or a stationary part (2) of the bearing towards the other of said two parts of said bearing, and that said means (5) is configured to establish a value of the frequency f of acoustic signals reflected by said other part and calculate a value of the velocity v of said rotating part with respect to said stationary part in the direction of said reflection by means of the value of the frequency f and of the frequency of the acoustic signals emitted and of the speed of sou n d between sa id emission position and the reflection position on said other part. 9. A device according to any of the preceding claims, characterized in that it comprises at least one said acoustic transducer (4a) configured to emit acoustic signals from a position on a rotating part or a stationary part of the bearing in a direction making an angle differing from 90° with respect to the axis of rotation of said rotating part (3), and that said means is configured to analyze such acoustic signals emitted and reflected by a surface (8) on the other of said two parts directed substantially perpendicularly to the direction of said emission so as to calculate a value of the distance or velocity of said rotating part (3) with respect to said stationary part (2) in the direction of said axis of rotation of the rotating part. 10. A device according to any of the preceding claims, characterized in that it comprises at least one said acoustic transducer (4a, 4b, 4c, 4d) which is configured to emit acoustic signals from a location on a stationary part (2) of said bearing. 1 1 . A device according to any of the preceding claims, characterized in that it comprises at least one said acoustic transducer (4a) which is configured to emit acoustic signals from a location on a rotating part (3) of said bearing. 12. A device according to any of the preceding claims, characterized in that it comprises at least one said acoustic transducer (4a) which is configured to be arranged outside said bearing (1 ), and that the device comprises a wave guide (10) configured to connect said transducer acoustically to the bearing. 13. A device according to any of the preceding claims, characterized in that it comprises at least two said acoustic transducers (4a, 4b) configured to be directed radially towards a rotating part (3) of said bearing and to be arranged in the same cross- section plane of the rotating part with a mutual angle α of the emission directions thereof, for which 0 < α < 180°, especially 5° < α < 175°. 14. A device according to any of the preceding claims, charac- terized in that it comprises at least two said acoustic transducers (4a, 4c) configured to be arranged in the same radial cross- section plane of a rotating part of the bearing and to emit acoustic signals radially towards said rotating part in opposite directions for obtaining redundancy for the determination of said value by said means. 15. A device according to any of the preceding claims, characterized in that it comprises a said at least one acoustic transducer (4a, 4b, 4c, 4d) of piezoelectric type. 16. A device according to any of the preceding claims, characterized in that it comprises at least one said acoustic transducer (4a, 4b, 4c, 4d) being able to withstand temperatures of at least 1500C, preferably of at least 3000C. 17. A magnetic bearing, characterized in that it is provided with a device for measuring a physical property thereof associated with a distance according to any of claims 1 -16. 18. A magnetic bearing according to claim 17, characterized in that it comprises means in the form of electrically driven magnets configured to create magnetic forces for journalling a rotating part (3), means ( 15, 1 7) configured to supply electric power to said magnets (12, 13) and a control arrangement (14) configured to control the supply of electric power to said mag- nets and to utilize measurement results of said device for this control. 19. A method for measuring a physical property of a magnetic bearing associated with a d istance , characterized in th at acoustic signals are emitted and brought to be reflected by a part of said bearing, and that the reflected signals are analyzed for determining a value of said physical property. 20. Use of a device according to any of claims 1 -16 in a mag- netic bearing (1 ) according to claim 1 7 or 18 immersed into a molten metal bath (16), such as a molten zinc bath, for measuring a said physical property of the magnetic bearing immersed. |
TECHNICAL FIELD OF THE INVENTION AND BACKGROUND ART
The present invention relates to a device for measuring a physical property of a magnetic bearing associated with a distance, in which the device comprising at least one sensor, a magnetic bearing provided with such a device as well as a method for measuring such a physical property of a magnetic bearing.
Magnetic bearings are used for seating rotating axes, where low friction between a rotating part and stationary parts of the bearing surrounding said rotating part is needed or where a contact between the rotating part and such stationary parts should be avoided . Due to their contactless operation a very low friction can be gained and no slip agent, such as lubrification, is needed, which is of advantage for hygienic or vacuum applications. Additionally, for harsh environments, where aggressive fluids or high temperature will result in a fast corrosion or ero- sion for other types of bearings with touching parts, an expansion of the life time can be gained for the system with a magnetic bearing.
In such a magnetic bearing the rotating part is held by magnetic forces produced by permanent magnets or solenoid coils, i .e. electromagnets, or a combination of these two types of magnets.
Physical properties of such magnetic bearings associated with a distance have to be measured for ensuring proper operation of the bearing. Such a physical property may be a distance within the bearing , such as between a rotating part and a stationary part of the bearing for checking proper location of said rotating part, and such measurement result may then in an electro-mag- netically driven bearing be used for the control of the electromagnets and by that the position of the axis of rotation of said rotating part thereof. Said physical property may also be a thickness of a bearing part, and the measurement result may then be used to determine the condition of that bearing part and by that of the bearing and a possible need of maintenance of the bearing, replacement of the bearing or of a bearing part etc. The velocity of a rotating part in a vibrating movement, such as mainly in a direction perpendicular to the axis of rotation thereof is another such physical property of interest, and measurement results thereof may also be used in electro-magnetically driven bearings for the control of the electromagnets.
DE 102005032675 discloses the use of optical sensors for measuring the position of a rotating part, accordingly distance sensors, in an electro-magnetically driven bearing, in which the measurement results are used by a control arrangement for controlling the electromagnets of the bearing for obtain ing proper location of said rotating part. Such optical distance sensors can very precisely measure the position of the rotating part, but their use requires an optical access to the rotating part and a reflective surface of this part. If the bearing casing is made from an optically non-transparent material , an optical window has to be provided, which in many applications makes it difficult to integrate such a device in the bearing and leads to additional costs. Also an opaque fluid in the gap between the rotating part and stationary parts of the bearing inhibit the use of an optical sensor.
US 5 763 972 discloses a device measuring the velocity of a rotating part of an electro-magnetically driven bearing in the vibrating sense. The inductivity change in a coil of an electromagnet occurring due to changes in the gap between that coil on the stationary part of the bearing and the ferromagnetic counterpart on the rotating part of the bearing is measured. Another type of electromagnetic sensor is disclosed in EP 1 679 492, which is based on eddy current measurements for obtaining a value of a distance within the bearing. There still exist also other electromagnetic variations for a distance/velocity sensor, as for example in using the actuator coil of an electromagnet as the sensor itself.
All of these electromagnetic sensor principals have the disad- vantage of a possible coupling between the strong actuator and the sensor field . A cross talk or a saturation of the sensor can occur. Thus, the sensor has to be shielded from the magnetic field, which would also be the case for a magnetic bearing only provided with permanent mag nets , a nd its sensitivity will decrease. Especially for large magnetic bearings with very strong magnetic fields the sensitivity of the sensor is low and an accurate measurement is difficult to obtain. Also electrical conductive and magnetic fluids, such as acids or liquid metals, will reduce the sensitivity of the sensor and a precise measurement of said physical property cannot be done for such applications.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a device of the type defined in the introduction being improved in at least some aspect with respect to such devices already known.
This object is according to the invention obtained by providing such a device, in which said at least one sensor comprises at least one acoustic transducer configured to emit acoustic signals and receive such signals reflected by a part of said bearing and means configured to analyze said reflected signals so as to determine a value of said physical property.
Acoustic signals are not affected by magnetic fields of the magnetic bearing, so that influence thereof on the measurement re- suit does not have to be considered, which simplifies the achievement of accurate measurement results. Additionally, these signals can travel through a large amount of different media (solid, liquid or gaseous) and thus overcome the limitations of optical and electromagnetic sensors. Especially, the acoustic signals can be sent through most of the materials used for the casing or a stationary part of a magnetic bearing and through-the-wa l l measu rement i s possi bl e , so that no additional protection or windows are necessary for the sensor. Measurement of both a distance and a velocity simultaneously may through this technique easily be obtained, which in the case of an electro-magnetically driven bearing results in a possibility to improve the control algorithm for the electromagnets of the bearing resulting in a better operation and prolonged lifetime thereof.
"Acoustic transducer configured to emit acoustic signals and receive such signals reflected" as used in this disclosure is to be interpreted broadly. This covers any type of means emitting acoustic signals and receiving such signals reflected , in which the receipt of the reflected signals may even be accomplished by a member bei ng physically separated from the emitti ng member of this means.
The use of an ultrasound transmitter and receiver to measure displacements between the outer ring and inner ring in wheel bearings of a vehicle is known through US 2002/0157 470 A1 . However, such beari ngs are q u ite d ifferent than mag netic bearings and problems typical for magnetic bearings do not have to be considered when selecting measuring method.
According to an embodiment of the invention the device is configured to measure a distance, and said means is configured to establish the time elapsed between a said signal is emitted and then received after having been reflected so as to calculate the distance between the position of said emission and the position of said reflection in said bearing. Only a correct value of the speed of sound in the media between said two positions is required for obtaining an accurate distance value.
According to another embodiment of the invention the device is configured to measure a distance between parts of said bearing, and said means is configured to establish the difference of time elapsed between a said signal is emitted and then received after having been reflected by two surfaces of such parts of the bearing at different distances from the position of said emission and use this time difference for calculating the distance between these two surfaces. The distance between different parts of said bearing may be accurately measured in this way, and the measurement result will be independent of the position of said emis- sion, so that possible uncertainties of the exact mounting location of said transducer will not influence the measurement result.
According to another embodiment of the invention said trans- ducer is configured to emit acoustic signals substantially radially with respect to a rotating part of said bearing, and said means is configu red to calculate a value of a distance i n said radial direction of said rotating part to a stationary part of the bearing surrounding said rotating part.
According to another embodiment of the invention the device is configured to measure a said distance in the form of a thickness of a bearing part, such as a stationary part of the bearing surrounding a rotating part of the bearing , and the transducer is configu red to emit acoustic sig nals in the d irection of said thickness and said means is configured to analyze acoustic signals reflected by two opposite surfaces of said bearing part defining the thickness of the bearing part. This makes it possible to carry out a diagnostic monitoring of the bearing quality, such as the thickness of a said stationary part, which may have been reduced as a consequence of an arrangement of the bearing in a harsh environment due to for instance corrosion, erosion, fouling or high temperatures.
According to another embodiment of the invention said means is configured to calculate a said distance between said rotating part and said stationary part several times during a full rotation of said rotating part and compare these distance values for checking the cross-section shape of said rotating part, which makes it possible to discover possibly occurring irregularities of the rotating part, which may impair the proper function of the bearing.
According to another embodiment of the invention the device comprises at least one said acoustic transducer configured to emit acoustic signals from a position on a rotating part or a stationary part of the bearing towards the other of said two parts of said bearing, and said means is configured to establish a value of the frequency f of acoustic signals reflected by said other part and calculate a value of the velocity v of said rotating part with respect to said stationary part in the d irection of said reflection by means of the value of the frequency f and of the frequency of the acoustic signals emitted and of the speed of sound between said emission position and the reflection position on said other part. An advantage of utilizing a measurement of a frequency shift of the reflected signal with respect to the emitted signal in this way for obtaining said velocity is that the velocity value may be obtained without any delay, which would be the case if instead the velocity would be obtained by only forming the time derivative of the position signal. Accordingly, it is by this possi ble to simultaneously obtain a position and a velocity value improving the possibilities of a high accuracy of the control of electromagnets of said bearing.
According to another embodiment of the invention the device comprises at least one said acoustic transducer configured to emit acoustic signals from a position on a rotating part or a sta- tionary part of the bearing in a direction making an angle differing from 90° with respect to the axis of rotation of said rotating part, and said means is configured to analyze such acoustic signals emitted and reflected by a surface on the other of said two parts directed substantially perpendicularly to the direction of said emission so as to calculate a value of the distance or velocity of said rotating part with respect to said stationary part in the direction of said axis of rotation of the rotating part. This means that a displacement of said rotating part in the direction of said axis of rotation with respect to a stationary part of the bearing may be determined enabling taking control actions for counteracting such a displacement, such as controlling electromagnets accordingly in a bearing being electro-magnetically driven.
According to another embodiment of the invention the device comprises at least one said acoustic transducer which is configured to emit acoustic signals from a location in a stationary part of said bearing, which is one possible location of said emission, and according to another embodiment of the invention the device comprises at least one said acoustic transducer which is configured to emit acoustic signals from a location in a rotating part of said bearing constituting another possible such location.
According to another embodiment of the invention the device comprises at least one said acoustic transducer which is configured to be arranged outside said bearing, and the device comprises a wave g uide configured to connect said transd ucer acoustically to the bearing. It is pointed out that the emission position of the transducer may be regarded to be the position in which the acoustic signals leave said wave guide, and this may be arranged inside the bearing on a said stationary part or on a said rotating part. By adding a wave guide acoustic signals produced by the transducer will be less disturbing when receiving and analyzing reflected such signals, which is of particular importance when the transducer is of piezoelectric type, as ac- cording to another embodiment of the invention. When using a transducer as transmitter and receiver, the decay of the piezo- oscillation due to the transducer excitation may in such a case overlap with possible reflections from a bearing part. For short distances between the transducer and the reflection position the amplitude of the reflection will be smaller than the amplitude of the piezo-oscillation due to the excitation, and the reflection cannot be detected or only with a low accuracy, so that no measurement will be possible for short distances from the trans- ducer. By adding a wave guide in front of the transducer all reflections are delayed in time corresponding to the length of the wave guide. By choosing an appropriate length of the wave guide the reflection will arrive after the decay of the excitation and also measurements of short distances are possible. Another advantage of using a wave guide is that the transducer may be given such a location that magnetic or electromagnetic interferences from the magnetic bearing may be avoided.
According to another embodiment of the invention the device comprises at least two said acoustic transducers configured to be directed radially towards a rotating part of said bearing and to be arranged in the same cross-section plane of the rotating part with a mutual angle α of the emission direction thereof, for which 0 < α < 180°, especially 5° < α < 175°. This means that values of said physical property, such as distance and velocity of said rotating part, in two dimensions may be obtained, which is of importance for the control of electromagnets in an electro- magnetically driven bearing.
According to another embodiment of the invention the device comprises at least two said acoustic transducers configured to be arranged in the same radial cross-section plane of a rotating part of the bearing and to emit acoustic signals radially towards said rotating part in opposite directions for obtaining redundancy for the determination of said value by said means. This results in an increased reliability of the device. According to another embodiment of the invention the device comprises at least one said acoustic transducer being able to withstand temperatures of at least 150 0 C, preferably of at least 300 0 C. This makes the device suitable to be used in bearings located in high temperature regions, such as immersed in a molten metal bath, in which the use of acoustic signals for said measurement is particularly advantageous.
The invention also relates to a magnetic bearing provided with a device for measuring a physical property thereof associated with a distance according to the present invention. The features and advantages of such a magnetic bearing appear clearly from the above discussion of such a device.
According to an embodi ment of the invention said magnetic bearing comprises means in the form of electrically driven magnets configured to create magnet forces for journalling a rotating part, means configured to supply electric power to said magnets and a control arrangement configured to control the supply with electric power to said magnets and to utilize measurement results of said device for this control. The preferred features and advantages of such a magnetic bearing appear from above.
The invention also relates to a method for measuring a physical property of a magnetic bearing associated with a distance, in which acoustic signals are emitted and brought to be reflected by a part of said bearing, and the reflected signals are analyzed for determining a value of said physical property. This consti- tutes a simple and reliable way of obtaining excellent measurement results relating to a distance in different types of magnetic bearings.
Finally, the invention also relates to a use of a device according to the invention in a magnetic bearing according to the invention immersed into a molten metal bath, such as a molten zinc bath, for measuring a said physical property of the magnetic bearing immersed. The advantages of such a use appear from the above discussion of a device according to the present invention.
Further advantages and advantageous features of the invention appear from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
With reference to the appended drawings, below follows a specific description of embodiments of the invention cited as examples.
In the drawings:
Fig 1 is a very simplified view schematically illustrating a magnetic bearing provided with a device according to a first embodiment of the invention,
Fig 2 is a view similar to Fig 1 of a magnetic bearing and a device according to a second embodiment of the invention,
Fig 3 is a view schematically illustrating a th i rd a n d a fourth possible embodiment of a device according to the invention,
Fig 4 illustrates a magnetic bearing and a device according a fifth embodiment of the invention,
Fig 5 is a graph illustrating amplitude of acoustic signals emitted by a transducer and received after having been reflected in a device according to the present invention versus time, Fig 6 is a simplified view schematically illustrating a magnetic bearing and a device according to a sixth embodiment of the invention, and
Fig 7 schematically illustrates a magnetic bearing and a device according to the present invention immersed into a molten metal bath.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVEN- TION
It is pointed out that the appended Figures are not drawn at scale and are very si mpl ified with the ai m to only illustrate different principal ways of carrying out the invention.
Fig 1 is a simplified cross-section view of a magnetic bearing 1 , in which magnets not shown are arranged in a stationary part 2 surrounding a rotating part 3 for applying magnetic forces to this part and keeping it centred with respect to said stationary part. Only the inner side 7 of the stationary part is shown in the Figu re , a n d s e n s o rs 4 a a n d 4 b i n th e fo rm of acoustic transducers are mounted at the inner side of the stationary part 2 and configured to emit acoustic signals towards the rotating part 3 and receive such signals reflected by that part. The receiving of the reflected signals may either be done by the same transducer or by a second sensor nearby acting as a receiver. The sensors 4a and 4b are connected to means 5 configured to analyze the reflected signals so as to determine a value of a physical property i n the form of a d istance or a velocity. The time t needed for the signal to travel from the transducer to the rotating part is determined by the speed of the sound c of the media 6 in the gap between the rotating part and the stationary part and can be calculated by:
t = 2 L/c, where L is the gap distance. Thus, by measuring the time t between the emitted and the received signal, the gap distance can be determined. This time is indicated by U in Fig 5, in which E is the original acoustic signal emitted and R 2 is the signal reflected by the rotating part 3 in the case of mounting the transducers at a distance to said inner side 7 of the stationary part. R 1 will then be the signal received after reflection at the interface between the gap and the stationary part, i.e. at said inner side 7. Such a mounting of the transducers means that the distance between the inner side 7 and the rotating part 3 may be calculated by the means 5 from the difference in time t 2 of the receipt of the two reflected signals Ri and R 2 making the positioning of the transducers not that critical. Furthermore, delays of the acoustic signal, which can occur due to the mounting of the transducer or inside the transducer and which will influence the measurement, can thus be eliminated. Alternatively, the travel time of the signal inside the gap can be measured by a resonance method, where the resonance shift of an at least partly continuous signal is used as measurement value, or any other method for an acoustical travel time measurement.
Since the acoustic sensor will measure an absolute position signal a radial control of the position of the rotating part 3 inside the stationary part 2 needs at least two sensors 4a, 4b for con- trolling the x and the y component of a movement of the rotating member 3 in the radial direction thereof. These two sensors have to be mounted at an angle α, for which 0 < α < 180°, and this angle is preferably 90° as shown in Fig 1 . It is also shown in Fig 1 how additional sensors 4c and 4d may be mounted to emit and receive reflective signals in opposite directions to the sensors 4a and 4b for obtaining redundancy. Furthermore, by using two additional sensors opposed to each other the difference signal between these sensors can be determined . By using this difference signal uncertainties and changes in the geometry can be eliminated as well as changes in the sound velocity, which otherwise has to be determined separately. With further additional sensors the accuracy can be further improved.
If the uncertainties and changes in the geometry can be as- sumed as small, an additional sensor configuration with only three sensors is possible. Such a configuration is shown in Fig
2. The sensors 4a , 4b and 4c are here oriented at angles of about 120° to each other. By taking the given geometry into account, the uncertainties can be eliminated and also the sound velocity be determined by the use of only three sensors.
Two possible configurations of the device for measurement of a displacement of the rotating part 3 with respect to the stationary part in the z direction, i.e. in the direction of the axis of rotation of the rotating part, is shown i n Fig 3. It is shown how the acoustic transducer 4a configured to emit acoustic signals from a position on the stationary part of the bearing towards the rotating part is tilted so as to make an angle differing from 90° with respect to the axis of rotation of the rotating part. Further- more, said means 5 is configured to analyze acoustic signals emitted and reflected by a surface 8 on the rotating part directed substantially perpendicularly to the direction of said emission so as to calculate a value of the distance or velocity of the rotating part with respect to the stationary part in the z direction. To the right in Fig 3 it is shown how the acoustic transducer 4a may alternatively be arranged at the end of the rotating member 3 for emitting acoustic signals in the direction of the axis of rotation of the rotating part towards an end surface 9 of the rotating part for determining movements in the z direction of the rotating part.
Moreover, the acoustic transducers are preferably piezoelectric transducers. A piezoelectric transducer can be used as transmitter as well as receiver and thus a very simple sensor can be built. Also a piezoelectric transducer is not influenced by the magnetic field and operates normally at frequencies above 100 kHz, which is to be compared with operating frequencies of electromagnetic bearings normally in the Hz or lower kHz ranges. This means that disturbances in the measurement signal due to the controlling of the magnets in such a bearing can be easily filtered out by electronic devices, such as a filter. For high temperature applications, like liquid metal baths, such as baths of molten zinc or aluminium, high temperature transducer with high temperature piezoceramic materials like bismuth titan- ate, lithium niobate, gallium phosphate, or others can be used. Also magnetostrictive materials or a microphone loud speaker combination could be used.
To further avoid interferences from the electromagnetic field the transducer can also be situated outside of the magnetic bearing as shown in Fig 4 and the acoustic signals are guided by an acoustic line 10 or an acoustic wave guide into the bearing to the measurement location, in which the emission position here is located at the end 1 1 of the wave guide. The transducer 4a is located outside of the bearing in a secure distance from the solenoid coils (electromagnets) 12, 13 of the bearing shown in this Figure. This electromagnetic bearing has of course further such electromagnets mounted in the stationary part around the rotating part. The acoustic signal is guided via the acoustic line 10 to the measurement position 1 1 at the gap. Thus, electromagnetic interferences of the transducer are avoided and the line 10 also makes it possible to measure very small distances for the reasons explained further above. It is shown in Fig 4 how this electromagnetic bearing has a control arrangement 14 configured to control means 1 5 configured to supply electric power to the electromagnets 12, 13 of the bearing. The control arrangement 14 is configured to utilize measurement results arriving from the means 5 for the control of the electromagnets.
Fig 6 illustrates an alternative way of arranging the sensor 4a, which here is arranged on the rotating part 3 instead of on the stationary part 2. The transducer can be better protected or constructed in a way to survive in a harsh environment (corrosive environment) or high/low temperatures. In some applications due to the corrosive nature of the solution the bearing will likely degrade over time. In this case the acoustic sensor can also be used in a monitoring mode. This is illustrated in Fig 7, which shows how the magnetic bearing 1 is immersed into a bath 16 of molten metal, such as molten zinc. Such a bath may have a temperature exceeding 500 0 C and be used for for example coating steel sheets in the automo bi le i n d u stry with a zi nc layer in which case the temperature is normally about 465°C . In this cylindrical bearing the reflection of the internal surface 7 of the bearing can be measured to monitor the thickness of the metal at that point and thus measure the corrosion damage to the bearing , which is illustrated by the arrows showing reflection on the inner surface 7 and on the outer surface 17 of the stationary part. It is shown how a display 18 may be arranged outside the metal bath for displaying the thickness value measured.
Furthermore, for monitoring purposes it is possible to measure and calculate a distance between the rotating part and the stationary part several times during a full rotation of the rotating part and compare these distance values for checking the cross- section shape of the rotating part.
Up to now only the position measurement by means of an acoustic sensor was taken into account. Additionally to the position measurement also a velocity measurement is interesting for the control of a magnetic bearing. By using also the velocity information the control algorithm for a control arrangement 14 may be optimized. Thus, an additional velocity measurement with the acoustic sensor is here proposed. Besides the time measurement of the acoustic signal also the frequency shift of the reflective signal is analyzed . When the acoustic signal is reflected at a surface, such as at the rotating part or the stationary part, the frequency of the signal will be shifted by a velocity component of the rotating part movement parallel to the travel path of the acoustic signal according to the Doppler Shift. If the transmitting signal has a frequency f, the reflective signal will have a frequency f , which is shifted compared to f and may under some circumstances be expressed by f = f (1 +v/c),
where v is the velocity of the rotating part parallel to the travel path of the acoustic signal. However, the formula to calculate f may be slightly d ifferent i n case of d ifferent g eometries/ applications. By analyzing the frequency of the receiving signal the velocity instead of the position can be measured . Also a combined position and velocity measurement can be done, by measuring the time as shown in Fig 5 and by measuring the frequency shift of the reflected signal . As already stated , this measurement method has the advantage that the position and velocity signal are measured at the same time and there is no delay in the measurement as occurring by only differencing the position signal for deriving the velocity.
The invention is of course not in any way restricted to the embodiments described above, but many possibilities to modifications thereof will be apparent to a person with skill in the art without departing from the scope of the invention as defined in the appended claims.
The different embodiments described above may of course be combined with each other.
The part of the bearing reflecting said acoustic signals may in the direction of the axis of rotation of the rotating part extend beyond the overlap of stationary parts and the rotating part and the transducer(s) be arranged outside the real bearing.
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