Spindle apparatus

Abstract: A spindle apparatus for preventing a bending vibration of a rotary shaft. A magnetic bearing composite motor having a magnetic bearing function for controlling a position of a rotary shaft by magnetic forces and a motor function for imparting a rotational torque to the rotary shaft is provided between a front electromagnets and a rear electromagnet. A control circuit for controlling the magnetic bearing function of said magnetic bearing composite motor is commonly used with a control circuit for the magnetic bearing and is so constructed as to perform the function of the positional control only in the resonance frequency band. Accordingly, even if the rotary shaft is subjected to a bending vibration by using the position of the electromagnets as nodes of vibration, the bending vibration may be suppressed by the magnetic bearing function of the magnetic bearing composite motor located at a position of the middle portion of the vibration.

Description: BACKGROUND OF THE INVENTION The present invention relates to a spindle apparatus and more particularly to a spindle apparatus using a magnetic bearing. In some cases, in a spindle apparatus, in order to realize a super high speed rotation, or a long term non-maintenance or the like, a rotary shaft (i.e., rotary body) is floated without any contact with a magnetic bearing by a magnetic force. The magnetic bearing is so constructed that an output of a positional shift sensor for detecting a position of the rotary shaft is fed back to control an excited current ol an electromagnet and to control the floating position of the rotary shaft. In this feedback operation, if an imbalance (offset between an axial center and a gravitational center) is present in the rotary shaft, a magnetic force which is in synchronism with a rotary frequency (i.e., rpm) is generated in order to suppress a vibratory rotation of the rotary shaft which is generated due to the imbalance. Accordingly, when the rotary frequency is equal to a natural frequency (resonant frequency), the rotary shaft is resonated by the magnetic force of the magnetic bearing, as a result of which, in particular, in a high speed rotational region, the rotary shaft is deformed to cause a bending vibration. Fig. 2 shows a state in which the rotary shaft 10 that is floatingly held by the magnetic bearing is subjected to the bending vibration as indicated by dotted lines with nodes at points A and B. As shown in Fig. 2, the rotary shaft 10 is supported at both ends thereof by four electromagnets 12, 14, 16 and 18. The positional shift in a radial direction of the rotary shaft 10 is detected by positional shift sensors 20, 22, 24 and 26. In general, in the bending vibration, the nodes of the vibration are generated in the vicinity of both ends of the shaft, and also, the support positions of the magnetic bearing, i.e., the electromagnets are located in the vicinity of both ends of the shaft. Accordingly, as shown in Fig. 2, the nodes of the vibration (points A and B) are located in the vicinity of the electromagnets. Usually, a circuitry or the like for phase compensation on the basis of a PID control (proportional-integral-derivative control) is incorporated into a control circuit for controlling the excited current of the electromagnets in the magnetic bearing. In order to suppress the resonant vibration, an electric damping is applied by using the magnetic force of the electromagnets and the current in the vicinity of the resonant frequency is interrupted by using a filter. Also, in the prior art, a mechanical damper made of rubber material has been used at a portion C which is a middle portion in the vibration of the rotary shaft 10, thereby suppressing the generation of the bending vibration. However, as shown in Fig. 2. if the points A and B that are the nodes of the vibration are located in the vicinity of mount positions of the respective electromagnets 12, 14, 16 and 18, the magnetic force of the electromagnets is not applied to the rotary shaft 10 as a force for suppressing the vibration. Accordingly, in the electric damping control using the above-described compensation circuit and the like, it is impossible to suppress the generation of the resonance. Also, in the case where the current in the vicinity of the resonance frequency is interrupted by using the filter, a rigidity of the magnetic bearing is degraded so that the rotary shaft 10 per se is likely to be vibrated by disturbances. With the mechanical damper, it is possible to suppress the vibration of the middle portion C, but in this case, it is impossible to actively suppress the bending vibration unlike the electric damping control. Also, the mechanical damper also suffers from a problem in durability. SUMMARY OF THE INVENTION It is an object of the present invention to provide a spindle apparatus which is capable of preventing the bending vibration of the rotary shaft. From "Lagerung schneller Spindeln, Tagungsband, Zürich (CH), 11. Januar 1994, S. 112- 125; R. Schöb: "Der lagerlose Induktion-smotor - ein neues Konstruktionselement zum Antrieb und zur Lagerung schneller Spindeln" there is already known a basic concept for a spindle apparatus according to the preamble of independent claim 1. It is an object of the present invention to provide a spindle apparatus which - based on these known basic concepts - is capable of preventing the bending vibration of the rotary shaft in a spindle apparatus. This object is achieved by the spindle apparatus according to claim 1 of the present invention. The dependent claims relate to further advantageous embodiments of the spindle apparatus according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a structural view showing a primary part of a spindle apparatus according to a first embodiment of the invention: Fig. 2 is an illustration of an example of a vibration condition in case where the rotary shaft is subjected to the bending vibration in the conventional spindle apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the spindle apparatus according to the present invention will now be described in more detail with reference to Figs. 1 and 2. Fig. 1 is a structural view representing a primary structure of a spindle apparatus 30 according to a first embodiment. Incidentally, in Fig. 1, the left side is referred to as the front and the right side is referred to as the rear. The mechanical structural part of the spindle apparatus 30 will first be described. The spindle apparatus 30 includes a rotary shaft 32 having a disc-like thrust bearing rotor portion 32a, front electromagnets 34 and 35 and rear electromagnets 36 and 37 which confront each other and clamp the rotary shaft 32. The respective electromagnets 34, 35, 36 and 37 are so arranged as to apply magnetic forces (attractive forces) to the rotary shaft 32 in a radial direction. A pair of upper and lower front positional sensors 40 and 41 for detecting the positional shift in the radial direction of the rotary shaft 32 are arranged in front of the front electromagnets 34 and 35. Also, rear positional sensors 42 and 43 are arranged in the same manner behind the rear electromagnets 36 and 37. Although not shown, two pairs of electromagnets and positional sensors are provided in the same manner in the direction perpendicular to the paper surface of the Fig. 1, respectively. Incidentally, in this embodiment, inductance convertor type positional sensors are used as the positional sensors 40, 41, 42 and 43. It is however possible to use any other positional sensors such as a differential transmission type positional sensor, a static capacitance type positional sensor, a hole element positional sensor, an eddy current positional sensor and so on. The mechanical structural portion of the radial magnetic bearing for floatingly holding the rotary shaft 32 by the radial magnetic forces is composed of the respective electromagnets 34, 35, 36 and 37, the positional sensors 40, 41, 42 and 43 and the electromagnets (not shown) and the positional sensors (not shown). Also, the spindle apparatus 30 is provided with axial electromagnets 46 and 47 embracing the thrust bearing rotor portion 32a of the rotary shaft 32 in the front and rear sides. Each of the axial electromagnets 46 and 47 is a coil wound in the form of the annular shape around an outer circumference of the rotary shaft 32 so as to apply the axial electromagnetic forces to the thrust bearing rotor portion 32a. An axial positional sensor 50 for detecting the axial positional shift of the rotary shaft 32 is arranged to face the rear end portion of the rotary shaft 32 at a rear portion of a frame 48 of the spindle apparatus 30. The mechanical structure of the thrust magnetic bearing for holding the rotary shaft 32 in the axial direction is composed of the axial positional sensor 50 and the axial electromagnets 46 and 47. In the foregoing embodiment, a magnetic bearing composite motor 52 is interposed between the front electromagnets 34 and 35 and the axial electromagnets 46 and 47. The magnetic bearing composite motor 52 is one which is composed of the motor function for imparting the rotational force to the rotary shaft 32 and the magnetic bearing function for magnetically floating and positionally controlling the rotary shaft 32. In this embodiment, in the magnetic bearing composite motor 52, a motor winding for generating a rotary magnetic field relative to the rotary shaft 32 and a magnetic bearing winding for generating magnetic force for performing the positional control of the rotary shaft 32 are wound around a ferric core independently of each other. The magnetic bearing winding is composed of independent four circuits whose windings are used to form four pole electromagnets for generating magnetic forces in the up-and-down direction and the direction perpendicular to the paper surface of Fig. 1 in the same manner as in the respective electromagnets 34, 35, 36 and 37 and the electromagnets (not shown). A control system for the spindle apparatus will now be described. The front positional sensors 40 and 41 are connected to a position detector 54 for obtaining positional signals corresponding to the positional shift of the rotary shaft 32 out of the respective positional sensors 40 and 41. The position detector 54 is connected to an arithmetic operator 56 to which a reference signal S1 for commanding the floating position of the rotary shaft 32 is fed. The arithmetic operator 56 subtracts the positional signal to be fed by the positional detector 54 from the reference signal S1 to output to the PID compensator 58. The PID compensator 58 performs a process for advancing the phase of the signal fed from the arithmetic operator 56 on the basis of the PID control or the like and feeds the output to a phase inverter 60. The phase inverter 60 performs a process for inverting the phase or the like to feed the output to power amplifiers 62 and 63. The power amplifiers 62 and 63 amplify the excited currents of the front electromagnets 34 and 35 corresponding to the signal out of the phase inverter 60. The feedback control of excited magnetic current of each electromagnet 34, 35, 36 and 37 is carried out by the respective control circuits(i.e., the positional detector 54 and the arithmetic operator 56 and the like) so as to generate the electromagnetic forces for floatingly holding the rotary shaft 32 at a position to be designated by the reference signal S1. Incidentally, although not shown in Fig. 1, a control circuit for performing a feedback control on the basis of the output of the rear positional sensors 42 and 43 in the same manner is independently provided for the rear electromagnets 36 and 37. A similar control circuit is provided for the electromagnets (not shown) and the like for controlling the rotary shaft 32 in the vertical direction to the paper surface in Fig. 1. Also, a control circuit for performing a similar feedback control on the basis of the output of the axial positional sensor 50 is provided for the axial electromagnets 46 and 47. With the above-described respective electromagnets, control circuits and so on, the five axis control type magnetic bearing for floatingly holding the rotary shaft 32 in the five directions (5-axes) is provided. On the other hand, the motor windings of the magnetic bearing composite motor 52 is connected to a motor driver 66 for feeding a drive electric power. Also, the magnetic bearing windings of the magnetic bearing composite motor 52 is connected, respectively, to power amplifiers 64 and 65 for feeding excited current. In this embodiment, the PID compensator 58 is connected to a motor magnetic bearing adjusting circuit 68 for feeding control signals to the power amplifiers 64 and 65 to use the control circuits of the magnetic bearing and so on as a part of the control system for the magnetic bearing windings. The motor magnetic bearing adjusting circuit 68 has a filter function for passing only the resonant frequency component generated when the rotary shaft 32 is subjected to the bending vibration, relative to the signals fed from the PID compensator 58. Incidentally, the resonant frequency which causes the bending vibration in the rotary shaft 32 depends upon the shape thereof (ratio between longitudinal and lateral dimensions). Namely, since the resonance frequency is varied in accordance with a weight or the like of a tool or the like to be fixed to the rotary shaft 32, the system is constructed so that the operator may set the frequency component to be filtered. Also, the motor magnetic bearing adjusting circuit 68 is so structured as to invert the phase of the signal to be fed to the power amplifiers 64 and 65, corresponding to the vibration mode when the rotary shaft 32 is vibrated. Namely, as indicated by hatching in Fig. 2, the direction of the vibration (i.e., upper portions in Fig. 2) at the measurement positions obtained by the positional sensors 20, 22 and so on is opposite to the direction of the vibration (i.e., lower portions in Fig. 2) at the portion C in the middle portion of the vibration. As shown in Fig. 1, since the magnetic bearing composite motor 52 is located at the middle portion of the vibration shown in Fig. 2, in the case where the directions of vibration are different between the position of the magnetic bearing composite motor 52 and the measurement positions of the front positional sensors 40 and 41, the motor magnetic bearing adjusting circuit 68 inverts the phase of the signal. Also, since the amplitudes of the vibration are different between the positions of the front electromagnets 34 and 35 and the position of the magnetic bearing composite motor 52, the motor magnetic bearing adjusting circuit 68 is so structured as to adjust the amplitude values of the signals in view of the distance between the measurement positions of the front positional sensors 40 and 41 and the position of the magnetic bearing composite motor 52. The operation of the thus constructed embodiment will now be described. First of all, the rotary shaft 32 is magnetically floated to a position to be designated by the reference signal S1 by the respective control circuits and the respective electromagnets 34, 35, 36, 37, 46 and 47. Then, the drive power is fed from the motor driver 66 to the magnetic bearing composite motor 52 whereby the motor windings of the magnetic bearing composite motor 52 cause the rotary magnetic field to be generated to thereby rotate the rotary shaft 32 kept under the floating condition. At this time, although the signal process in the PID compensator 58 is fed also to the motor magnetic bearing adjusting circuit 68, the frequency component other than the resonant frequency of the bending vibration of the current to be fed to the magnetic bearing windings of the magnetic bearing composite motor 52 is interrupted by the filter function of the motor magnetic bearing adjusting circuit 68. Accordingly, until the rpm of the rotary shaft 32 reaches the resonant frequency, the positional control of the rotary shaft 32 is not carried out by the magnetic bearing function of the magnetic bearing composite motor 52. When the rpm is increased to reach the resonant frequency, the signal containing the resonant frequency component is fed from the PID compensator 58 to the motor magnetic bearing adjusting circuit 68. The motor magnetic bearing adjusting circuit 68 passes the resonant frequency component of the fed signal, and at the same time, outputs the processed signal to the power amplifiers 64 and 65 through the processes such as a phase inverse, an amplitude adjustment and so on. As a result, the feedback controlled current is supplied to the magnetic bearing windings of the magnetic bearing composite motor 52 on the basis of the output of the front positional sensors 40 and 41 to thereby generate the magnetic forces for positionally controlling the rotary shaft 32. Namely, the rotary shaft 32 is subjected not only to the rotational torque but also the magnetic forces in the radial direction for the positional control from the magnetic bearing composite motor 52. Accordingly, even if the nodes are present at the positions of the respective electromagnets 34, 35, 36 and 37, it is possible to suppress the bending vibration by the magnetic bearing function of the magnetic bearin g composite motor 52 positioned at the middle portion of the vibration. According to this embodiment, the magnetic bearing composite motor 52 is located between the front electromagnets 34 and 35 and the rear electromagnets 36 and 37. So, even if the positions of the front electromagnets or the rear electromagnets or the thrust bearing rotor portion 32a are regarded as the vibration nodes, it is possible to effectively suppress the bending vibration using the nodes therebetween. Also, since the positional control of the rotary shaft 32 is carried out only by the resonant frequency band, it is possible to reduce the power consumption to a lower level. Also, since the magnetic bearing composite motor 52 having the magnetic bearing function and the motor function together is used, it is possible to suppress the bending vibration of the rotary shaft 32 without increasing the axial length of the rotary shaft 32 and the enlargement of the spindle apparatus 30. Also, since the bending vibration is actively suppressed by the electric damping by the magnetic bearing composite motor 52 using the PID compensator 58, it is possible to effectively carry out the damping in comparison with the mechanical damper. In addition, since it is of the non-contact type, the electric damping is superior also in durability. Furthermore, since the control circuit of the magnetic bearing composite motor 52 is used commonly for the control circuits(i.e., 54, 56, 58 and so on) of the magnetic bearing, it is possible to suppress the increase of the manufacturing cost of the spindle apparatus 30 in comparison with the case that the positional sensor or the control circuit is provided specially for the magnetic bearing composite motor 52. Incidentally, in the foregoing embodiment, the control circuits of the front electromagnets are commonly used in order to control the current of the magnetic bearing windings of the magnetic bearing composite motor 52. However, in the spindle apparatus in which the thrust magnetic bearing is provided adjacent to the front electromagnets, it is possible that the control circuits for the magnetic forces of the rear electromagnets on the basis of the rear positional sensor are commonly used in order to control the current of the magnetic bearing windings of the magnetic bearing composite motor 52. The other structure is the same as that of the first embodiment. In this embodiment, since the magnetic bearing composite motor 52 is functioned also as the radial magnetic bearing, it is possible to enhance a rigidity of the magnetic bearing. Incidentally, in the type of the respective foregoing embodiments, the magnetic bearing composite motor 52 independently has the magnetic bearing windings and the motor windings, but it is possible to use a magnetic bearing motor in which the current for the magnetic bearing and the current for the motor are provided to a single winding. In the spindle apparatus according to the present invention, since not only the first and second magnetic bearings provided at both ends but also the electromagnetic bearing composite motor interposed between the first and second magnetic bearings are used to control the position of the rotary shaft, it is possible to prevent the bending vibration of the rotary shaft from being generated. Various details of the invention may be changed without departing from its scope as defined in the claims. Furthermore, the foregoing description of the embodiments according to the present invention is provided for the purpose of illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

Claims: A spindle apparatus (30) comprising: a rotary shaft (32); a first magnetic bearing (34, 35) for floatingly holding one end side of said rotary shaft (32) by magnetic forces; a second magnetic bearing (36, 37) for floatingly holding the other end side of said rotary shaft (32) by magnetic forces; a magnetic bearing composite motor (52) interposed between said first magnetic bearing (34, 35) and said second magnetic bearing (36, 37), said magnetic bearing composite motor (52) having a magnetic bearing function for positionally controlling said rotary shaft (32) by magnetic forces and a motor function for imparting a rotational torque to said rotary shaft (32) by magnetic forces; characterized by a first positional sensor (40, 41) for detecting a position of said rotary shaft (32) for feeding back to said first magnetic bearing (34, 35) for positional control of said rotary shaft (32); a second positional sensor (42, 43) for detecting a position of said rotary shaft (32) for feeding back to said second magnetic bearing (36, 37) for positional control of said rotary shaft (32); and wherein said magnetic bearing composite motor (52) carries out the positional control of said rotary shaft (32) by feed back from a detected value of just one of said first (40, 41) and second (42, 43) positional sensors, and a motor magnetic bearing adjusting circuit (68) having a filter function for passing only a resonant frequency component generated when said rotary shaft (32) rotates in the vicinity of a frequency at which said rotary shaft (32) is subjected to a bending vibration, said motor magnetic bearing adjusting circuit (68) feeding control signals to said magnetic bearing composite motor (52). The spindle apparatus according to claim 1, wherein the magnetic bearing composite motor (52) has a motor winding for generating a rotary magnetic field relative to said rotary shaft (32) and a magnetic bearing winding for generating magnetic forces for performing the positional control of said rotary shaft (32), a motor driver (66) connected to said motor winding for feeding a drive electric power, a power amplifier (64, 65) connected to said magnetic bearing winding for feeding excited current. The spindle apparatus according to claim 1, wherein the positional control is performed by a position detector (54) for outputting a positional signal on the basis of an output of a positional sensor (40, 41;42, 43 ), an arithmetic operator (56) for subtracting a reference signal (S1) from said positional signal, a compensator (58) for varying the phase of the output signal of said arithmetic operator (56), a phase inverter (60) for inverting the phase of the output signal of said compensator (58), and a power amplifier (62, 63) for amplifying an excited current of electromagnets in response to the output of said phase inverter (60). The spindle apparatus according to claim 1, wherein said motor magnetic bearing adjusting circuit (68) is so structured as to invert the phase of the signal to be fed to said power amplifier (64, 65), corresponding to a vibration mode when said rotary shaft (32) is vibrated.

Inventors: ANDO SHIGENORI, ANDO, SHIGENORI, ANDO, SHIGENORI, C/O SEIKO SEIKI KABUSHIKI KAISHA
Applicants: SEIKO SEIKI KK, SEIKO SEIKI KABUSHIKI KAISHA
Application year: 1995
Main classification: F16C39
IPC codes: B23B19, F16C32, B23Q11, H02K7, F16C39