JPH0640953Y2 - Turbo molecular pump - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a turbo molecular pump.

(Prior Art) There is a magnetic bearing using an electromagnet as a means for floatingly holding a rotating body. This magnetic bearing has less loss than a fluid lubricated bearing, can dry the bearing, and can clean the atmosphere, and is a useful bearing especially in a vacuum state.

A turbo molecular pump is one of the most promising applications for this magnetic bearing. A conventional example of this turbo molecular pump will be described with reference to FIG. 6. (5) is a rotating body, and the rotating body (5) has one end fixed to the disk (9) and the center of the disk (9). The shaft body (8) and the circular plate (9) are fixed to the peripheral portion of the coaxial body (8).
And an outer cylinder (7) surrounding the outer peripheral surface and having a plurality of exhaust gas blades (10) attached to the outer peripheral surface thereof. Also (14)
Is a support cylinder of the rotating body (5) inserted in the space between the shaft body (8) and the outer cylinder (7), and (12) is the inner peripheral surface of the support cylinder (14). Attached motor for rotating body, (6
A) and (6B) are two sets of journal magnetic bearings mounted on the inner peripheral surface of the support cylinder (14), and (11) is a thrust magnetic bearing fixed to the end of the shaft (8) on the side opposite to the disc. The magnetic disk (13) is a set of thrust magnetic bearings mounted on the inner peripheral surface of the supporting cylinder (14) with the magnetic disk (11) sandwiched therebetween, and two sets of journal magnetic bearings (6A). The rotating body (5) is levitated and held by (6B) and a pair of thrust magnetic bearings (13).

In this magnetic bearing, the levitation position of the levitation body (rotating body (5)) is measured by the next levitation position setting means, that is, the levitation position of the levitation body is measured, and the electromagnet of the magnetic bearing is determined based on the measurement signal obtained as a result. The current value to be flown is determined and set by the levitation position setting means for controlling the magnitude of the magnetic force generated in the electromagnet. An example of this floating position setting means is shown in FIG.
(1) is a position sensor (for example, an eddy current displacement meter) that measures the floating position of the floating body, and (2) is for proportionally multiplying the magnitude of the measurement signal from the position sensor (1) to the required magnitude. The position feed back gain, (3) is a controller that determines the current value to flow to the electromagnet (4) of the magnetic bearing based on the signal from the same position feed back gain (2) and outputs it to the electromagnet (4). A coil is wound around the electromagnet (4),
The magnetic force for levitation is generated according to the current from the controller (3). The controller (3) has a PID (proportional,
Integrating, differentiating) circuits, phase compensation circuits, or circuits combining these circuits are used. However, in the case of the simplest position feedback system in which the controller (3) is composed of only proportional elements (P elements), the input I to the electromagnet (4) and the magnetic force F which is the output from the electromagnet (4) The transfer function between and becomes the following first-order lag system due to the resistance and inductance of the coil, iron core, etc.

F / I = K _M / (I + T _M・ S) ・ where K _M is the gain of the electromagnet (4) and T _M is the electromagnet (4)
, S is the Laplace operator. Therefore, the transfer function from the measured displacement D of the position feedback system to the magnetic force F to the rotating body (5) is as follows.

F / D = K _F・ K _P・ K _M / (I + T _M・ S) ・ ・ ・ where K _F is the position feedback back gain 2 and K _P is the proportional gain of the controller (3) Is.

In order to see the frequency characteristics of (magnetic force F / displacement D) of the position feedback system, the Laplace operator S = j2πf is set and substituted in the above equation. Where f is the frequency (H ₂ ), Is. (Magnetic force F / displacement D) is a complex number and is set as follows.

F / D = K _M (f) + j · K _I (f) · The real part of (Magnetic force F / Displacement D) in the equation means the rigidity depending on frequency f, and (Magnetic force F / Displacement D) The imaginary part means attenuation depending on the frequency f. 1 as in the above formula
In the second-order lag, the imaginary part is always negative, and the levitation body becomes an unstable force opposite to the damping.

FIG. 8 shows (magnetic force F / displacement D), that is, the relationship between the value of the imaginary part of the above equation and the frequency f. Dotted line A in FIG. 8
Indicates the above-mentioned state corresponding to the above equation. Depending on the damping of the natural frequency f _C consisting of the levitation body and the position feedback system, especially the damping of the levitation body, the frequency f =
If the value at f _C is large, the levitation body oscillates divergently and operation becomes impossible.

Therefore, in order to have a damping effect on the (magnetic force F / displacement D) of the position feedback system, a derivative element (D element) or phase compensation element is provided in parallel with the proportional element (P element) in the controller (3). Here, the differential element is taken as an example to represent these. When the differential element (D element) is inserted into the controller (3) as a circuit, the following first-order lag system is obtained.

Derivative element = K _D S / (I + T _D S) -where K _D is the gain of the differential element and T _D is the time constant. Position feedback system with only differential element (Magnetic force F / Displacement D)
Is as follows:

Since the numerator of the equation is the first order of S and the denominator is the second order of S, the imaginary part of the equation is as shown by the chain line B in FIG.
That is, it has an unstable effect in the low frequency region. In order to maintain the position of the floating body, the controller (3) needs to have a proportional element and a derivative element. The position feedback system (Magnetic force F / Displacement D) of the controller (3) is And becomes a solid line C in FIG. 8 and has the same characteristics as described above. If the natural frequency f _C consisting of the levitation body and the position feedback system is placed in the low frequency region that has the damping effect, stability can be secured and operation can be performed without generating vibration.

When the magnetic bearing having such characteristics is applied to the rotating body (5) of the double structure shown in FIG. 6 and the rotating body (5) of the same double structure is levitated and supported, the following phenomenon occurs. Present. Two
The rotating body (5) having a heavy structure is shown in FIGS. 9 (a) (b) (c).
(D) As shown in (e) to, it has an infinite number of natural frequencies. The damping of the material constituting the rotating body (5) acts instable at natural frequencies below the rotational speed, and acts so as to attenuate at natural frequencies above the rotational speed. Therefore, it is necessary to bring the natural frequency lower than the rotational frequency to the frequency range having the damping effect of (magnetic force F / displacement D) of the position feedback system of the magnetic bearing. However, the natural frequency of the rotating body (5) is infinite as shown in FIGS. 9 (a), (b), (c), (d), and (e). There is a natural frequency in the frequency domain having an unstable effect. Therefore, if the non-guidance action of the position feed back system of the magnetic bearing becomes larger than the damping of the natural frequency by the rotating body (5) itself, it becomes unstable, and the vibration becomes divergently large and it may be rotated. Disappear.

In particular, the damping ability having a third natural frequency higher than the maximum rotation speed of the rotating body (5) is small, and it tends to divergely vibrate due to the instability of the position feed back system of the magnetic bearing. As a countermeasure, even if the damping action region is extended to the third natural frequency by the magnetic bearing, the destabilizing action is larger in the frequency region lower than that, and the fourth natural frequency becomes divergent. Vibration will occur.

(Problems to be solved by the invention) As described above, in the conventional magnetic bearing, the position of the floating object is measured, and the measurement signal obtained at that time is fed back,
The magnetic force from the electromagnet (4) acts on the levitated object to hold the position of the levitated object, but the magnetic force from the electromagnet (4) becomes a destabilizing force that vibrates the levitated object. Even if the controller (3) performs processing such as PID and phase compensation, it still has a large destabilizing force in the low frequency region.
In particular, in a levitating body having an infinite number of natural frequencies such as a rotating body (5) with a double structure, there is always a natural frequency in the region that becomes the unstable force, and the levitating body is divergent by the magnetic bearing. It will vibrate. As a countermeasure against this, the damping region by the magnetic bearing is extended to the bending primary natural frequency (third order) of the shaft body (8), which has the least damping for the rotating body (5) having the double structure. However, the bending second natural frequency (4
With respect to (2), there was a problem that the destabilizing force increased further and the bending secondary frequency changed to divergent vibration.

The present invention is proposed in view of the above problems, and the purpose thereof is to stabilize the frequency region by causing phase reversal also for the secondary natural frequency of the bending of the rotating body outside the phase compensation range. The point is to provide a turbo molecular pump that can be converted into force (damping force), can prevent divergent vibrations, and can stably support the levitation of a rotating body.

(Means for Solving the Problems) In order to achieve the above-mentioned object, the present invention provides a disc and a shaft body, one end of which is fixed to the center of the disc, and a shaft which is fixed to the periphery of the disc. A rotating body composed of an outer cylinder surrounding the coaxial body and having a plurality of exhaust gas blades mounted on the outer peripheral surface thereof; and a supporting body for the rotating body fitted in a space between the shaft body and the outer cylinder. ，
A turbo having a rotary body drive motor mounted on the inner peripheral surface of the support cylinder, two sets of journal magnetic bearings and one set of thrust magnetic bearings interposed between the support cylinder and the shaft. In the molecular pump, the journal magnetic bearings are axially spaced from each other and are interposed between the support cylinder and the shaft, and the thrust magnetic bearing is an anti-disc of the support cylinder and the shaft. A position sensor installed between the side magnetic head and the central part of the magnetic disk of the thrust magnetic bearing is extended to the side opposite to the disc side, and a position sensor for measuring the radial position of the extension is provided for each of the journal magnetic bearings. Of these, it is connected to the control system of the journal magnetic bearing on the anti-disc side.

(Operation) The turbo-molecular pump of the present invention is configured as described above, and includes two sets of journal magnetic bearings and one set of thrust magnetic bearings interposed between the supporting cylindrical body and the shaft body, thereby providing a rotating body. Phase compensation (stabilization) is performed for frequencies including the 1st natural frequency, 2nd natural frequency, and bending 1st natural frequency, but the free-free 2nd natural frequency of the rotor Considering the mode, the center part of the magnetic disk of the thrust magnetic bearing is extended to the side opposite to the disc, and a position sensor that measures the radial position of the extension is arranged around the same extension. The sensor is connected to the control system of the journal magnetic bearing on the anti-disc side, and phase inversion occurs even for the secondary natural frequency of the bending of the rotating body outside the phase compensation range, and its frequency range stabilizes (damping force). ), Divergent vibration is prevented, and rotation It is floatingly supported stably.

(Embodiment) Next, a turbo molecular pump of the present invention will be described with reference to an embodiment shown in FIGS. 1 to 4. (5) in FIG. 1 is a rotating body, and the rotating body (5) is a disk. A shaft body (8) whose one end is fixed to the center part of the same circular plate (9) as the (9) and a peripheral part of the same circular plate (9) are fixed to surround the coaxial body (8) and a plurality of them are provided on the outer peripheral surface. And an outer cylinder (7) to which the exhaust gas vane (10) is attached. Further, (14) is a support cylinder of the rotating body (5) fitted in the space between the shaft body (8) and the outer cylinder (7), and (12) is a support cylinder of the support cylinder (14). A rotating body drive motor mounted on the inner peripheral surface, two sets of journal magnetic bearings (6A) and (6B) mounted on the inner peripheral surface of the supporting cylindrical body (14),
Reference numeral (11) is a magnetic disk of a thrust magnetic bearing fixed to the end of the shaft (8) on the side opposite to the disk, and the center of the magnetic disk (11) is extended in the axial direction. See also (13)
And the supporting cylindrical body (1
A set of thrust magnetic bearings mounted on the inner peripheral surface of 4) holds the rotor (5) in a floating state by two sets of journal magnetic bearings (6A) (6B) and one set of thrust magnetic bearings (13). ing.

Fig. 2 shows the positional relationship between the rotating body (5) and the two sets of journal magnetic bearings (6A) and (6B), and the secondary natural frequency when the bearing portion of the rotating body (5) is free ( Hereinafter, it is shown corresponding to the vibration mode F _O of the free-free secondary natural frequency). As shown in FIG. 2, two sets of journal magnetic bearings (6A) and (6B) are separately arranged on the left and right sides (upper and lower sides in FIG. 1) of the center of gravity G of the rotating body (5). Of these, the left side (anti-disc side) of the journal magnetic bearing (6A) is
Position sensor (1L), position feed back gain (2L), controller (3L), electromagnet (4L), right side (disc side)
The journal magnetic bearing (6B) has a position sensor (1R), a position feed back gain (2R), a controller (3R) and an electromagnet (4R). And journal magnetic bearings (6
The position sensor (1L) and the electromagnet (4L) in (A) are arranged vertically above and below the node point P of the vibration mode F _O of the free-free secondary natural frequency of the rotating body (5). In the vibration mode F _O , the rotating body (5) has a double structure, the shaft body (8) resembles a cantilever, and at the free-free secondary natural frequency, the tip of the shaft body (8) is This is a vibration mode having one node point P at the position of the magnetic disk (11) provided near (the end of the shaft (8) on the side opposite to the disc). On the other hand, since the magnetic disk (11) is heavy and is provided on the side of the shaft (8) opposite to the disk, it is difficult to move in the secondary bending.
That is, it becomes a node point. Further, the position sensor (1L) is arranged so as to sandwich the magnetic disk (11) with the electromagnet (4L) (so as to sandwich the node point P of the vibration mode F _O ), and the position of the magnetic disk (11) is reduced. It is designed to measure the radial position.

FIG. 3 shows vibration modes F1, F2, F3, F4 of the first, second, third, and fourth natural frequencies of the rotating body (5) and the journal magnetic bearings (6A) (6B). Shows the relationship with. Same third
As shown in the figure, since the rigidity of the rotating body (5) is higher than that of the magnetic bearing (6), the shape of the vibration mode of the fourth natural frequency is the free-free second order shown in FIG. It has the same shape as the vibration mode of the natural frequency. As can be seen from Fig. 3, the position sensor (1L) of the journal magnetic bearing (6A)
The position sensor (1R) of the electromagnet (4L) and the journal magnetic bearing (6B) and the electromagnet (4R) are the primary, secondary, and tertiary
It is located in a vibration mode that sways in the same direction with respect to the next natural frequency. However, regarding the fourth natural frequency,
Since the vibration mode F4 of the fourth natural frequency has the same waveform as the vibration mode F _{O of the} free free second natural frequency,
The position sensor (1L) of the journal magnetic bearing (6A) and the electromagnet (4L) are located in a vibration mode in which they swing in opposite directions, and the position sensor (1R) and electromagnet (4) of the journal magnetic bearing (6B) are located.
R) and are in a vibration mode that does not shake together. The controllers (3L) and (3R) have the function of providing attenuation in the low frequency region as shown by the solid line C in FIG. Thus, the first-order, second-order, and third-order natural frequencies of the rotating body (5) are placed in the damping frequency range, and the fourth-order natural frequency of the rotating body (5) is the destabilizing force. Is placed in the frequency domain that gives

By the way, of the journal magnetic bearing (6A) (magnetic force F / displacement D)
Is expressed by the above equation, the first and second order of the rotating body (5)
Regarding the second and third natural frequencies, the magnetic bearing characteristics do not change and remain as the formulas, but the fourth natural frequency of the rotating body (5) is reversed from the formula and F / D = -K _R (fc ₄ ) -j · K _I (fc ₄ ), and the destabilizing force changes to a damping force. Note that fc ₄ is the 4th natural frequency. Therefore, the characteristic for the rotating body (5) has damping for the first, second, third and fourth natural frequencies as shown by the solid line D _C in FIG. Further, the journal magnetic bearing (6B) is placed in a place where the vibration mode does not oscillate at all with respect to the fourth natural frequency, and therefore does not give any destabilizing force to the fourth natural frequency.

Thus, according to the present embodiment, although the controllers (3L) (3R) that give damping only to the first, second, and third natural frequencies are used, the fourth natural vibration is used. The region that acts as the most destabilizing force in the number can be changed to the region where the damping force (stabilizing force) can be obtained. Therefore, in addition to the problem of the medium and high frequency handing of the rotating body (5) being reduced, it is possible to operate up to the bending secondary critical velocity (the bending secondary critical velocity corresponding to the fourth natural frequency). For the natural frequencies above the fifth natural frequency, there is almost no problem because the destabilizing force is in the frequency region where it is small.

FIG. 5 shows another turbo molecular pump of the present invention in which the magnetic disk (11) is composed of an annular body and a cylindrical body in order to reduce the weight of the magnetic disk (11) of the thrust magnetic bearing (13). In this embodiment, the same operation as described above can be achieved in this embodiment.

(Effects of the Invention) As described above, the turbo molecular pump of the present invention uses the two sets of journal magnetic bearings and one set of thrust magnetic bearings interposed between the support cylinder and the shaft to make the primary of the rotating body. Phase compensation (stabilization) is performed for frequencies including natural frequency, secondary natural frequency, and bending primary natural frequency, but considers free-free secondary natural frequency vibration modes of the rotating body. Then, the center of the magnetic disk of the thrust magnetic bearing is extended to the side opposite to the disc, and a position sensor for measuring the radial position of the extension is arranged around the extension and the same position sensor is removed. It is connected to the control system of the journal magnetic bearing on the disk side.
Phase inversion also occurs for the secondary natural frequency of bending of a rotating body outside the phase compensation range, and its frequency range can be converted into a stabilizing force (damping force), divergent vibrations can be prevented, and a rotating body can be stabilized. There is an effect that it can support levitating.

[Brief description of drawings]

FIG. 1 is a longitudinal side view showing an embodiment of a turbo molecular pump according to the present invention, and FIG. 2 is an explanatory view showing the arrangement of magnetic bearings for the vibration modes of the free-free secondary natural frequency of a rotating body. Fig. 4 is an explanatory diagram showing the arrangement of magnetic bearings for the vibration modes of the 1st, 2nd, 3rd, and 4th natural frequencies, Fig. 4 is an explanatory diagram showing the damping characteristics of the magnetic bearings with respect to the rotor, and Fig. 5 FIG. 6 is a vertical sectional side view showing a conventional turbo molecular pump, FIG. 7 is a block diagram showing a control system of a magnetic bearing, and FIG. 8 is an explanatory diagram showing damping characteristics of the magnetic bearing. , (A) to (e) of FIG. 9 are explanatory views showing types of natural frequencies of the rotating body. (1L) (1R) …… Position sensor, (2L) (2R) …… Position feed back gain, (3L) (3R) …… Controller, (4L)
(4R) ... electromagnet, (5) ... rotating body, (6L) (6R) ...
… Jeanal magnetic bearing, (7) …… Outer cylinder, (8) …… Shaft, (9) …… Disk, (10) …… Exhaust gas blade, (11)
…… Magnetic disk, (12) …… Rotor driving motor,
(13) …… Thrust magnetic bearing, (14) …… Supporting cylinder.

Claims (1)

[Scope of utility model registration request] 1. A disc and a shaft having one end fixed to the center of the disc, and a shaft fixed to the peripheral edge of the disc to surround the coaxial body and a plurality of exhaust gas blades mounted on the outer peripheral surface. A rotating body composed of an outer cylinder, a supporting cylinder of the rotating body fitted in a space between the shaft body and the outer cylinder, and a rotating body driving motor attached to an inner peripheral surface of the supporting cylinder. And a turbo molecular pump having two sets of journal magnetic bearings and one set of thrust magnetic bearings interposed between the support cylinder and the shaft, wherein the journal magnetic bearings are axially spaced from each other. The thrust magnetic bearing is interposed between the support cylinder and the shaft, and the thrust magnetic bearing is interposed between the support cylinder and the end of the shaft opposite to the disc side. A position sensor that extends the center of a magnetic disk in the axial direction and measures the radial position of the extension. Wherein among the journal magnetic bearing turbomolecular pump, characterized in that connected to the control system of the journal magnetic bearing counter-disc side.