Abstract:
A spindle apparatus comprises a rotary body and at least a pair of magnetic bearings for levitating the rotary body by magnetic forces. A combined motor and magnetic bearing device is disposed between the magnetic bearings for imparting a rotational torque to the rotary body and for positionally controlling the rotary body by magnetic forces. A control circuit outputs and exciting current to the magnetic bearings and the combined motor and magnetic bearing device and for makes an adjustment of the exciting current of the magnetic bearing and the combined motor and magnetic bearing device to hold the rotary body afloat in a predetermined position.

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 spindle apparatus, in order to realize a super high speed rotation, or a long term non-maintenance or the like, a magnetic bearing is leviated floated without any contact with a rotary shaft (i.e., rotary body) 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 of 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. 3 shows a state in which a rotary shaft  10  that is floatingly held by a magnetic bearing is subjected to the bending vibration as indicated by dotted lines with nodes at points A and B. 
     As shown in FIG. 3, 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. 3, 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. 3, if the points A and B that are the nodes of the vibration are located at 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 disturbance. 
     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 with 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 a rotary shaft. 
     In order to attain the foregoing object, according to the present invention, there is provided a spindle apparatus comprising: a rotary shaft; a first magnetic bearing for floatingly holding one end end of the rotary shaft by magnetic forces; a second magnetic bearing for floatingly holding another end side of the rotary shaft by magnetic forces; and a magnetic bearing composite motor interposed between the first magnetic bearing and the second magnetic bearing, the magnetic bearing composite motor having a magnetic bearing function for positionally controlling the rotary shaft by the magnetic forces and a motor function for imparting a rotational torque to the rotary shaft by the magnetic forces. 
     In such a spindle apparatus, the rotary shaft is rotated by the magnetic bearing composite motor and simultaneously the magnetic forces are applied to a portion of the rotary shaft between the first and second magnetic bearings to perform the positional control of the rotary shaft. 
    
    
     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 a structural view showing a primary part of a spindle apparatus according to a second embodiment of the invention; and 
     FIG. 3 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 
     To attain the above-noted and other objects, according to a first aspect of the present invention, there is provided a spindle apparatus comprising: a rotary shaft; a first magnetic bearing for floatingly holding one end of the rotary shaft by magnetic forces; a second magnetic bearing for floatingly holding another end of the rotary shaft by magnetic forces; and a magnetic bearing composite motor interposed between the first magnetic bearing and the second magnetic bearing, the magnetic bearing composite motor having a magnetic bearing function for positionally controlling the rotary shaft by the magnetic forces and a motor function for imparting a rotational torque to the rotary shaft by the magnetic forces. 
     According to a second aspect of the invention, there is provided a spindle apparatus, wherein the first and second magnetic bearings are provided with first and second positional sensors for detecting a position of the rotary shaft for positional control of the rotary shaft; and wherein the magnetic bearing composite motor carries out the positional control of the rotary shaft on the basis of a detected value of one of the first and second positional sensors. 
     According to a third aspect of the invention, there is provided a spindle apparatus comprising a composite motor positional sensor for detecting the position of the rotary shaft in the vicinity of the magnetic bearing composite motor for controlling the position of the rotary shaft. 
     According to a fourth aspect of the invention, there is provided a spindle apparatus wherein the magnetic bearing composite motor is used to control the position of the rotary shaft in the vicinity of a frequency at which the rotary shaft is subjected to a bending vibration. 
     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 body or shaft  32  having a disc-like thrust bearing rotor portion  32   a , 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 leviating 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  32   a  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 an 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  32   a . 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 combined motor and magnetic bearing device, hereafter referred to as 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 four independent 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 S 1  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 S 1  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 S 1 . 
     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 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, a five axis control type magnetic bearing for floatingly holding the rotary shaft  32  in 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 a shape (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 filtrated. 
     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. 3, the direction of the vibration (i.e., upper portions in FIG. 3) 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. 3) 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. 3, 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 Si 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 bearing 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  32   a  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 . 
     FIG. 2 shows a spindle apparatus  31  in accordance with a second embodiment. Incidentally, the same reference numerals are used to designate the similar components used in the first embodiment and detailed explanation therefor will be omitted properly. 
     In this embodiment, positional sensors  70  and  71  for detecting an up-and-down positional shift of a rotary shaft  32  in FIG.  2  and two positional sensors (not shown) for detecting a positional shift in the vertical direction to the paper surface are provided adjacent to the magnetic bearing composite motor  52 . Control circuits for controlling, on the basis of the outputs from the positional sensors  70  and  71 , a power of the magnetic bearing windings of the magnetic bearing composite motor  52  are provided individually of and separately from the magnetic bearing composed of the electromagnets  34 ,  35 ,  36  and  37 , respectively. 
     Namely, the positional sensors  70  and  71  are connected to a position detector  73  which in turn is connected to an arithmetic operator  74  to which a standard signal S 1  is fed. Then, the arithmetic operator  74  is connected to a PID compensator  76  which in turn is connected to a phase invertor  78 . Power amplifiers  80  and  81  for feeding the excited magnetic current to the magnetic bearing windings of the magnetic bearing composite motor  52  are connected to the phase invertor  78 . 
     The foregoing control circuits are used to perform the feedback control of the excited magnetic current of the magnetic bearing windings of the magnetic bearing composite motor  52  on the basis of the positional sensors  71  and  72 . Namely, the structure of the control circuits is the same as that of the control circuits for the magnetic bearing. 
     Accordingly, the control circuit system may attain the positional control of the rotary shaft  32  not only in the resonant frequency band but also over the full range. Namely, the spindle apparatus  31  according to this embodiment is provided with another radial magnetic bearing in addition to the radial bearing which is constituted by the electromagnets  34 ,  35 ,  36  and  37  and so on, and the rotary shaft  32  is supported at three positions in the radial direction. 
     The other structure is the same as that of the first embodiment. 
     In this embodiment, since the magnetic bearing composite motor  52  also functions 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 spirit nor its scope. 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.