1. Field of the Invention
The present invention relates to a thrust magnetic bearing system, and more particularly to a thrust magnetic bearing system wherein magnetic circuits of electromagnets are separated from those of permanent magnets so that each permanent magnet produces a bias magnetic field while each electromagnet functions only to control the position of a rotating body, thereby making it possible to achieve desired displacement and current stiffness without flowing a bias current through the electromagnet.
2. Description of the Related Art
Recently, a magnetic bearing is widely used in various precision mechanical devices.
The magnetic bearing floats and supports a rotating body by a magnetic force generated by an electromagnet. Precision mechanical devices with a magnetic bearing prevent the generation of dust by abrasion of its shaft and bearing and also use no lubricant, so that they have various advantages such as low maintenance costs, a high rotation rate, and low noise.
Due to these advantages, the magnetic bearing is widely used in mechanical devices, which are employed in extremely clean environments such as clean rooms for semiconductor fabrication, and in aerospace fields in which it is difficult to use mechanical bearings since the coefficient of friction is very high in vacuum.
The magnetic bearing controls the supply of current to an electromagnet according to the position of a rotating body to generate a magnetic force, thereby floating and supporting the rotating body and controlling the movement of the floated rotating body.
The magnetic bearing stably supports a floating body by increasing and decreasing a magnetic force, which each electromagnet produces, depending on changes in the position of the floating body while each pair of opposing magnetic poles of the electromagnets attract the floating body. The magnetic bearing requires a contactless displacement sensor to detect the displacement of the floating body.
That is, the magnetic bearing increases and decreases the magnetic force produced by each electromagnet by controlling current flowing through the electromagnet according to changes in the position of the floating body. The magnetic bearing must also previously apply a bias magnetic force to the floating body according to the weight of the floating body and then actively increase and decrease the bias magnetic force according to changes in the position of the floating body.
However, this conventional magnetic bearing reduces the available operation range of electromagnet drivers since it previously applies a bias magnetic force to the floating body. The magnetic bearing requires larger electromagnets to compensate for the reduction in the operating range. The magnetic bearing also requires a pair of electromagnet drivers since it uses the electromagnets while flowing current through the electromagnets in only one direction.
However, there are limitations to using the magnetic bearing in devices such as a turbo compressor for vehicles having a limited space for mounting a rotating shaft and a bearing since the size of each electromagnet and the sectional area of each pole, from which a magnetic force is produced, must be minimized so that the magnetic bearing cannot afford to supply a bias current using the electromagnet drivers.
A permanent magnet bias type magnetic bearing has been suggested to overcome the problem that the bias current limits the use of the magnetic bearing. As shown in
Since it is not necessary to form a bias magnetic force, the permanent magnet bias type magnetic bearing has a wide variation range of magnetic forces produced by the electromagnets, thereby making it possible to mount the magnetic bearing even in a narrow space and to reduce the amount of generated heat as the power consumption is reduced.
However, when the permanent magnet bias type magnetic bearing increases and decreases the magnetic forces produced by the electromagnets by controlling current applied to the electromagnets, the magnetic fields produced by the electromagnets pass through the permanent magnets so that the magnetic circuits of the electromagnets interfere with those of the permanent magnets, thereby reducing displacement and current stiffness characteristics of the magnetic bearing.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a thrust magnetic bearing system wherein magnetic circuits of electromagnets are separated from those of permanent magnets so that each permanent magnet produces a basic magnetic field while each electromagnet functions only to control a magnetic force to control the position of a rotating body, thereby making it possible to float the rotating body without flowing a bias current through the electromagnet.
In accordance with the present invention, the above and other objects can be accomplished by the provision of a thrust magnetic bearing system including a thrust displacement sensor and a thrust magnetic bearing, the system floating a disk floating body based on displacement information detected through the thrust displacement sensor, the thrust magnetic bearing including a donut permanent magnet; a pair of electromagnets connected in series to form an inductor at both sides of the donut permanent magnet; and a pair of magnetic poles provided opposite each other outside the pair of electromagnets, wherein the thrust magnetic bearing floats the disk floating body through a bias magnetic flux generated by the donut permanent magnet and a control magnetic flux generated by the electromagnets.
Preferably, the thrust displacement sensor includes a shaft having different diameters; a pair of ring electrodes provided outside the shaft; and a guard electrode surrounding the pair of ring electrodes, wherein the thrust displacement sensor detects changes in a capacitance formed by the ring electrodes by amplifying the capacitance changes using a differential amplifier.
The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
In
The thrust magnetic bearing 10 floats the disk floating body 30 through a bias magnetic flux (or circuit) generated by the donut permanent magnet 11 and a control magnetic flux generated by the electromagnets 12.
As shown in
Changes in the position of the disk floating body 30 are detected through the thrust displacement sensor 20 and the disk floating body 30 is moved horizontally by controlling the direction and amount of current applied to the electromagnets 12 based on the detection.
For example, if the floating body 30 is moved to the left, the direction and amount of current applied to the electromagnets 12 is controlled to allow the electromagnets 12 to generate a control magnetic flux in a clockwise direction, thereby moving the floating body 30 to the right, as shown in
More specifically, the bias magnetic flux generated by the permanent magnet 11 sequentially passes through a housing, gaps, and the disk floating body 30 and then returns to the permanent magnet and the control magnetic flux generated by the electromagnets increases and decreases the bias magnetic flux generated by the permanent magnet 11.
If the control magnetic flux is generated in a clockwise direction, the control magnetic flux increases the intensity of magnetic field at the right gap while decreasing the intensity of magnetic field at the left gap, thereby moving the floating body to the right.
On the other hand, if the floating body 30 is moved to the right, the direction and amount of current applied to the electromagnets 12 is controlled to allow the electromagnets 12 to generate a control magnetic flux in a counterclockwise direction, thereby moving the floating body 30 to the left, as shown in
That is, if the control magnetic flux generated by the electromagnets 12 is in a counterclockwise direction, the control magnetic flux decreases the intensity of magnetic field at the right gap while increasing the intensity of magnetic field at the left gap, thereby moving the floating body to the left.
While the conventional magnetic bearing requires two pairs of current drive circuits since it is implemented to be differential, the magnetic bearing using the bias magnetic flux of the permanent magnet according to the invention has an advantage in that it can operate with one current drive circuit since the same current flows through the coils.
The thrust displacement sensor 20 for detecting changes in the position of the floating body 30 includes a shaft 21 having different diameters, a pair of ring electrodes 22 provided outside the shaft 21, and a guard electrode 23 surrounding the pair of ring electrodes 22 as shown in
As shown in
A switch circuit as shown in
The following is a more detailed description with reference to
Then, the switches S1 and S3 are opened simultaneously as shown in
Then, as shown in
A current integration circuit, which includes an OP amp, a feedback resistor Rf, and a feedback capacitor Cf as shown in
Since the input impedance of the OP amp varies depending on circumstances, a discharge current may flow into an input of the OP amp, causing an abrupt voltage increase. To prevent this, it is preferable that a capacitor C with a capacitance much higher than the unknown capacitance Cx be provided between the input of the OP amp and ground to absorb the instantaneous current, thereby keeping the input grounded.
As is apparent from the above description, the present invention provides a thrust magnetic bearing system with a variety of advantages. For example, magnetic circuits of electromagnets are separated from those of permanent magnets in the thrust magnetic bearing system so that each permanent magnet produces a basic magnetic field while each electromagnet functions only to control a magnetic force to control the position of a rotating body. This makes it possible to control the displacement of the rotating body while achieving displacement and current stiffness levels similar to those of the electromagnet type magnetic bearing without flowing a bias current through the electromagnet.
In addition, while the conventional magnetic bearing requires two pairs of current drive circuits since it is implemented to be differential, the magnetic bearing using the bias magnetic flux of the permanent magnet according to the invention can operate with one current drive circuit since the same current flows through coils.
Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.
Number | Date | Country | Kind |
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10-2007-0093836 | Sep 2007 | KR | national |