JP3395170B2 - Apparatus and method for controlling thrust magnetic bearing - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thrust magnetic bearing control device and method for supporting a rotating shaft in a non-contact manner.

[0002]

2. Description of the Related Art Since a magnetic bearing supports a rotating body in the air in a non-contact manner by a magnetic force, there is no problem of lubrication, it can be used in a special environment, and there are problems of friction, wear and noise. Since it has almost no characteristics and is capable of ultra-high speed rotation, research is being actively conducted toward its practical application. Among the magnetic bearings, those that utilize magnetic attraction force have a larger supporting force than those that utilize repulsive force, and because they are easy to apply damping, they have been widely developed and are being applied to many rotary machines. When this bearing is applied, the supporting friction is very small and it is possible to increase the speed of turbomachines and machine tool spindles. However, since the magnetic attractive force itself forms an unstable system, it is necessary to control at least the levitation of the magnetic bearing using the electromagnet. Here, a general description of the thrust bearing of the control type magnetic bearings utilizing the magnetic attraction force is as follows. That is, two fixed members, which are a rotating shaft, a disc-shaped rotor fixed to the rotating shaft and made of a magnetic material, and electromagnets provided on the fixed side so as to face each other on both axial sides of the rotor via an air gap. A child, two current amplifiers for supplying current to the two stators, a displacement detector provided near the stator for detecting axial displacement of the rotor with respect to the stator, a displacement command value and signals of the displacement detector , A phase compensator that operates by receiving the signal of the comparator, a signal of the displacement detector and a signal of the phase compensator, and sends a signal to the current amplifier, and the signal of the phase compensator and the rotation axis Is composed of a compensator that compensates so as to have a proportional relationship with the axial supporting force received from the stator, and is controlled so that the axial position of the rotating shaft is at a predetermined position and is supported in a non-contact manner. The structure of the compensator and the compensation method are described in detail in Japanese Examined Patent Publication No. 63-19734, Japanese Examined Patent Publication No. 3-43766 and the like. If this compensator is used, the magnetic attraction force acting between the electromagnet of the stator and the rotor of the rotating shaft has a non-linear characteristic, and the whole control system of the bearing becomes a non-linear system, which makes stable control difficult. The overall system is a linear system, and a stable levitation control system can be obtained. The above-mentioned displacement detector used for the magnetic bearing needs to be a non-contact type, and in general, an eddy current type displacement detector is often used from the viewpoint of detection accuracy and price. This displacement detector is detected by utilizing the fact that when a high-frequency current is supplied to the detection coil, an eddy current is generated in the rotor that is the detection target, and this is affected by the air gap. If this eddy current displacement detector is used, the detector itself is expensive, which increases the overall cost, and the displacement detector is installed near the stator electromagnet, and the bearing support point and displacement detection point are Differently, there is a problem that the stability margin of the control system is limited. Therefore, recently, a method of detecting a gap value as a change in inductance from the current ripple by utilizing the fact that a PWM type using a switching element for a current amplifier for supplying a current to an electromagnet has come to be used. Has been proposed (for example, JP-A-5-118329).

[0003]

However, if the general structure of such a magnetic bearing is applied to, for example, a thrust bearing of a machine tool spindle, the following problems occur. When a magnetic bearing is applied to a machine tool spindle, a radial bearing is generally provided on the tip side of the spindle provided with a tool, and a thrust bearing is generally provided between two radial bearings or near the rear end of the spindle. Therefore, when the main spindle expands due to a temperature rise when the main spindle operates, the tip of the main spindle is displaced and the machining accuracy is reduced by the amount of thermal expansion. The degree of this is greater when the thrust bearing is provided near the rear end of the main shaft than when the thrust bearing is provided between the two radial bearings. The effect of thermal expansion of the main shaft when the thrust bearing is installed between the two radial bearings is smaller than the thermal expansion when the thrust bearing is installed near the rear end of the main shaft, but the installation space for the displacement detector is necessary. It has drawbacks such as disassembly and assembly are not easy. In order to avoid the influence of thermal expansion of the spindle, a method of providing a displacement detector at the tip of the spindle was also proposed, but the machining accuracy is maintained high because the position of the tip of the spindle is controlled, but the compensator operates normally. However, there was a problem that the stability of the control system was gradually impaired, and eventually it became impossible to ascend. On the other hand, the above-mentioned method of detecting the displacement by calculating from the ripple of the electromagnet current without providing the displacement detector can be detected in principle, but the electromagnet is originally used as the displacement detector. Since it is not used, the detection accuracy and other characteristics are very poor compared to the displacement detector using eddy current. Therefore, it is not suitable for bearings that require high accuracy, such as machine tool spindles. Therefore, it is an object of the present invention to solve the above-mentioned problems relating to thermal expansion of a rotary shaft between a displacement detector and an electromagnet by using an existing displacement detector.

[0004]

In order to solve the above problems, the present invention is directed to a rotary shaft, a disc-shaped rotor fixed to the rotary shaft and made of a magnetic material, and to both axial sides of the rotor. Two stators composed of electromagnets provided on the fixed side so as to face each other via an air gap, two PWM-type current amplifiers for supplying current to the stators, and axial displacement of the rotor with respect to the stators. , A first comparator for comparing a displacement command value with a signal of the displacement detector, a phase compensator that operates by receiving the signal of the first comparator, and the displacement detector. Signal and the signal from the phase compensator are sent to the two current amplifiers to compensate for the signal from the phase compensator and the axial supporting force received by the rotating shaft from the two stators. Composed of a compensator and supports the axial direction of the rotating shaft in a non-contact manner. In a thrust magnetic bearing control device, two current detectors for detecting a current supplied to the stator by the current amplifier, and a displacement for detecting an axial displacement of a rotary shaft tip portion provided near the rotary shaft tip portion. A detector and a correction calculator for calculating the thermal expansion amount of the rotating shaft between the stator and the displacement detector from the signal of the current detector and the signal of the displacement detector and correcting the gap command value on both sides. Thus, the axial gap and the supporting force are maintained regardless of the thermal expansion of the rotating shaft.

[0005]

With the above means, the ripple of the current supplied to the electromagnet is proportional to the air gap between the stator and the rotor. Therefore, the amount of expansion between the tip of the main shaft and the bearing is calculated by the correction calculator, and the displacement detector is detected. That is, the displacement signal used for the compensator is corrected by adding or subtracting to the signal of (1), and the initial stable levitation system is maintained even if the rotating shaft thermally expands.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a control device of a thrust magnetic bearing according to the present invention, and FIG. 2 is a block diagram showing a correction calculator. In FIG. 1, 1 is a spindle of a machine tool, 2 is a tool fixed to the tip of the spindle 1 via a holder (not shown),
Reference numeral 3 is a thrust magnetic bearing, 31 is a disk-shaped rotor fixed to the main shaft 1 and made of a magnetic material, and 32 and 33 are fixed to a frame or the like not shown so as to face both sides of the rotor 31 in the axial direction with an air gap. It is an annular stator composed of an electromagnet provided on the side. Reference numeral 4 is a control device for the thrust magnetic bearing 3, and reference numeral 41 is a displacement detector which is provided in the vicinity of the tip of the main shaft 1 on a frame, a bracket or the like, and which detects the axial displacement of the tip of the main shaft 1. In 42 and 43, the PWM type current amplifiers 71 and 72 using switching elements are the stator 3
This is a current detector that detects the currents I _L and I _R supplied to 2, 33. Reference numeral 44 is a first comparator for comparing the displacement command value Xs with the signal X of the displacement detector 41, and 5 is a first comparator 44.
Is a phase compensator including a PID compensator and the like which operates by receiving the output signal Xs-X. 8 receives a detection current I _L, I _R and signal X of the displacement detector 41 of the current detector 42, 43,
The correction calculator outputs signals X ₁ and X ₂ corresponding to the air gap between the rotor 31 and the stators 32 and 33. 6
Is the signal of the phase compensator 5 and the signals X ₁ and X of the correction calculator 8.
_It is a compensator that receives ₂ and sends a command to the current amplifiers 71 and 72, and corrects the signal of the phase compensator 5 and the axial supporting force received by the rotating shaft from the two stators in a proportional relationship. The main shaft 1 is non-contact supported by a thrust magnetic bearing 3 and two radial magnetic bearings (not shown) provided on the main shaft 1, and can be rotated by a motor (not shown) provided on the main shaft 1. The displacement detector 41 and the thrust magnetic bearing 3 are arranged apart from each other. In FIG.
81 and 81 are current signals I _L and I of the current detectors 42 and 43.
Two high pass filters 82, 82 which receive _R and pass the carrier frequency of the PWM type current amplifiers 71, 72 and remove other low frequency signals are high pass filters 81, 8
Two amplifiers for amplifying the signal of No. 1, 83, 83 are two full-wave rectifiers that receive the signal of the amplifier 82 and perform full-wave rectification, respectively, and 84 is a second comparator for comparing the signals of the two full-wave rectifiers 83. , 85 is a first adder for adding the bias X ₀ to the signal of the comparator 84, and 86 is a signal X of the displacement detector 41.
A second adder for adding the signal of the first adder 85 to
Reference numeral 7 denotes a third subtracter that subtracts the signal of the first adder 85 from the signal of the displacement detector 41, and these constitute a correction calculator 8. In the above configuration, the levitation control of the thrust magnetic bearing is performed as follows. Displacement detector 4
1 detects the axial displacement X of the spindle 1, the first comparator 44 compares the displacement command value Xs with the deviation signal (Xs-
X) is output. Phase compensator 5 which receives the deviation signal (Xs-X), the left and right of the gap [delta] _L, so that [delta] _R is equal, and outputs a signal X _L, X _R to compensate for the phase. Compensator 6, the signal X _L, receives the signals X _1, X ₂ of the correction arithmetic unit 8 to be described later X _R, the current amplifier 71, 72 ₁ Two current command i _L, giving a i _R. Current amplifier 71, 72
Depending on the current commands i _L and i _R
To supply currents I _L and I _R. The current I _L, the magnetic attraction force is generated between the rotor 31 and the stator 32, 33 by I _R. When the rotor 31 is displaced from the position designated by the command value Xs, the deviation signal (Xs-X) of the first comparator 44 is not zero, so the phase corrector 5 works to output a signal,
In response to this signal, a supporting force is generated in the opposite direction to the displaced direction, and an attempt is made to restore the displaced position of the spindle. In this way, the spindle 1 is axially supported in a non-contact manner. Further, during such an operation, the signals X ₁ and X _{2 of} the correction calculation 8 correspond to the air gaps between the rotor 31 and the stators 32 and 33, respectively. The supporting force of the directional magnetic bearing 3 has a proportional relationship, and the initially stable floating system is maintained regardless of the floating position of the rotor 31 and thermal expansion.
The currents I _L and I _{R of the} stators 32 and 33 are the current detector 42,
43 detected by two high-pass filters 8
The low frequency signals are blocked by 1 and 81, respectively, and only the signals of the carrier frequencies of the current amplifiers 71 and 72 are output. Here, this output signal, the rotor 31 and the stator 3
The relationship with the air gap between the two and 33 will be described below. When supplying current to the stator composed of electromagnets in the PWM type current amplifier, the voltage of the main circuit power supply of the current amplifiers 71 and 72 is high and the resistance of the stator is small in order to reduce the loss of the current amplifier and the stator. To be done. Therefore, the stators 32 and 33 composed of electromagnets as seen from the current amplifiers 71 and 72 can be electrically regarded as equivalent to the inductance, and the main circuit voltage V of the current amplifier, the stator inductance L composed of the electromagnets, and the electromagnets. There is approximately the following relationship with the current I of the stator consisting of V = L (dI / dt) Further, the inductance L and the air gap X between the stator composed of the electromagnet and the rotor have the following relationship.
L∝1 / X From these two equations, the following equation is obtained. dI∝VXdt The dt in this equation is the reciprocal of the carrier frequency of the current amplifier and is constant and equal to the time interval for switching the switching element. Since the main circuit voltage V is also constant, the above equation becomes the following equation, and the current ripple generated by switching is approximately proportional to the air gap. dI∝X Therefore, the output signals of the two high-pass filters 81, 81 are triangular waves having an amplitude proportional to their respective air gaps. Since the current ripple is small when the inductance L is large, the output signals of the high pass filters 81 and 81 are amplified by the amplifiers 82 and 82, respectively. Further, since the full-wave rectifiers 83 and 83 perform full-wave rectification, the output signals of the full-wave rectifiers 83 and 83 become DC signals having a magnitude corresponding to the amplitude of the current ripple, and are proportional to the respective air gaps. The two signals are compared by the second comparator 84, and the bias signal X ₀ is added by the first adder 85.
The bias signal X ₀ is set so that the left and right gaps are equal in the initial state. If the output signal of the first adder 85 is adjusted to zero before the thrust magnetic bearing 3 shown in FIG. Axial magnetic bearing 3 of main shaft 1 after expansion
When the rotor 31 of the part (1) is displaced in the axial direction, a signal proportional to the displacement is output from the first adder 85. Here, if the amplification factors of the two amplifiers 82, 82 are appropriately selected,
When the signal of the first adder 85 is added by the second adder 86 to the displacement signal X of the displacement of the tip of the spindle detected by the displacement detector 41, the air gap between the rotor 31 and the stator 32 becomes When the signal of the first adder 85 is subtracted by the third subtractor 87 from the displacement signal X of the spindle tip portion obtained and detected by the displacement detector 41, an air gap between the rotor 31 and the stator 33 is obtained. Is obtained. Adder / subtractor 8 at the final stage of the correction calculator 8
Of the two signals used in 6 and 87, one has high accuracy because it is the displacement signal X detected by the displacement detector 41, while the other detects the output current of the PWM type current amplifier, Since the signal corresponds to the thermal expansion calculated by the circuit from the high pass filter 81 to the first adder 85,
As mentioned above, it does not have sufficient accuracy. However, the signals X ₁ and X ₂ corresponding to the air gap between the rotor 31 and the stators 32 and 33 calculated by the correction calculator 8 are obtained by applying the correction corresponding to the thermal expansion to the displacement signal X. Therefore, even if it is not as accurate as the displacement detector 41, it has considerably higher accuracy than that obtained by calculating only the currents I _L and I _R without using the signal of the displacement detector 41. There is.
Therefore, the compensator 6 using these two signals makes the thrust bearing control system sufficiently stable. In this way, even if the signal of the compensator 6 is calculated using the correction calculator 8, the axial position of the spindle 1 is controlled using the signal of the displacement detector 41, so that it is held with high accuracy as described above. To be done. In this way, the effect of thermal expansion of the main shaft can be corrected.

[0007]

As described above, according to the present invention, the displacement of the portion of the thrust magnetic bearing due to thermal expansion is calculated from the current supplied to the stator and corrected, and the air gap between the rotor and the stator is corrected. Therefore, without adding a displacement detector in addition to the displacement detector for levitation, destabilization of the magnetic bearing control system due to thermal expansion can be avoided and the initially stable levitation system can be maintained. . Therefore, since it is only necessary to add the control device to the existing machine, it is possible to easily improve the performance of the machine.

[Brief description of drawings]

FIG. 1 is a block diagram of a thrust magnetic bearing control device according to an embodiment of the present invention.

FIG. 2 is a block diagram showing the correction calculator of FIG.

[Explanation of symbols]

1 spindle 2 tools 3 Thrust magnetic bearing 31 rotor 32, 33 Stator 41 Displacement detector 4 Magnetic bearing control device 42,43 Current detector 44 First Comparator 5 Phase compensator 6 compensator 71, 72 Current amplifier 8 Correction calculator 81 High-pass filter 82 amplifier 83 Full wave rectifier 84 Second comparator 85 First adder 86 Second adder 87 Third Comparator

Claims (3)

(57) [Claims] 1. A rotary shaft, a disk-shaped rotor fixed to the rotary shaft and made of a magnetic material, and electromagnets provided on the fixed side so as to face each other on both axial sides of the rotor via an air gap. , Two PWM type current amplifiers for supplying current to the stator, a displacement detector for detecting axial displacement of the rotor with respect to the stator, a displacement command value and the displacement detection. Comparator for comparing the signals of the detector, a phase compensator that operates by receiving the signal of the first comparator, and the two currents that receive the signal of the displacement detector and the signal of the phase compensator A compensator that sends a signal to an amplifier and compensates the signal of the phase compensator so that the rotational shaft receives a supporting force in the axial direction from the two stators in a proportional relationship. In the control device of the thrust magnetic bearing to be supported, the current increase Current detectors that detect the current supplied to the stator by the device, a displacement detector that is provided near the tip of the rotary shaft and that detects axial displacement of the rotary shaft tip, and a signal of the current detector. And a displacement detector signal to calculate the amount of thermal expansion of the rotating shaft between the stator and the displacement detector, and a correction calculator that corrects the gap command values on both sides. . 2. The correction arithmetic unit receives two signals from a current detector to pass a carrier frequency of a current amplifier, and two amplifiers each receive a signal from the high pass filter to adjust a signal level. And two full-wave rectifiers for full-wave rectifying the signals of the amplifier, a second comparator for comparing the signals of the two full-wave rectifiers, and a signal of the second comparator A first adder for adding a reference signal, a second adder for receiving a signal from the first adder and adding a signal from the displacement detector, and a signal for receiving the signal from the first adder 2. The thrust magnetic bearing control device according to claim 1, comprising a third subtractor for subtracting from the signal of the displacement detector. 3. The thrust magnetic bearing control device according to claim 1, wherein a displacement obtained from a current ripple supplied to a stator by a PWM type current amplifier and a displacement obtained from a displacement detector are added / subtracted, and A method for controlling a thrust magnetic bearing, which is characterized by compensating for a change in displacement due to thermal expansion.