Machine tools abstract
A magnetic bearing spindle device for machine tools comprises a
spindle for mounting a tool element as changeably attached to a
forward end thereof, a plurality of controllable magnetic bearings
for contactlessly supporting the spindle, an electric motor for
rotating the spindle as contactlessly supported by the magnetic
bearings, a draw bar inserted through the spindle and axially movable
for unclamping and clamping the tool element, state sensor switches
for detecting the axial position of the draw bar to detect a state
of having no tool element, an unclamping state or a clamping state,
and a control unit for controlling the support of the spindle by
the magnetic bearings and the rotation of the spindle by the motor
based on the result of detection by the switches.
Machine tools claims
What is claimed is:
1. A magnetic bearing spindle device for machine tools which comprises
a spindle for mounting a tool element as changeably attached to
a forward end thereof, a plurality of controllable magnetic bearings
for contactlessly supporting the spindle, rotating drive means for
rotating the spindle as contactlessly supported by the magnetic
bearings, and a draw bar inserted through the spindle and axially
movable for unclamping and clamping the tool element, the spindle
device being characterized in that the device comprises a tool state
sensor means for detecting the axial position of the draw bar to
detect a state of having no tool element, an unclamping state or
a clamping state, and control means for controlling the support
of the spindle by the magnetic bearings and the rotation of the
spindle by the drive means based on the result of detection by the
sensor means.
2. A magnetic bearing spindle device for machine tools as defined
in claim 1 which is characterized in that the control means makes
it impossible for the magnetic bearings to support the spindle and
for the drive means to rotate the spindle when the result of detection
by the sensor means is the state of having no tool element or the
unclamping state, or permits the magnetic bearings to support the
spindle and the drive means to rotate the spindle when the result
of detection by the sensor means is the clamping state.
3. A magnetic bearing spindle device for machine tools as defined
in claim 1 which is characterized in that the draw bar has a flange
for use in detecting the position thereof, and the tool state sensor
means comprises three state sensors for detecting the state of the
draw bar by detecting the flange of the draw bar.
4. A magnetic bearing spindle device for machine tools as defined
in claim 3 which is characterized in that the state sensors are
each a proximity switch.
Machine tools description
BACKGROUND OF THE INVENTION
The present invention relates to magnetic bearing spindle devices
for machine tools, and more particularly to a magnetic bearing spindle
device wherein a spindle for mounting a tool element as changeably
attached to its forward end is rotatable by rotating drive means,
as contactlessly supported by a plurality of controllable magnetic
bearings.
Machine tool spindle devices such as machining centers are already
known in which a spindle for carrying a tool element as changeably
attached to its forward end is rotatably supported by antifriction
bearings. With such a spindle device, the tool element is automatically
changed by an automatic tool changer provided on the machine tool.
The spindle has inserted therethrough a draw bar movable axially
of the spindle for clamping the tool element to be attached to the
forward end and unclamping the tool element. The draw bar clamps
the tool element by being moved to a rearward clamping position
by a spring and unclamps the tool element by being moved to a forward
unclamping position by a pusher against the force of the spring.
The draw bar is in an unclamping state when moved to the unclamping
position, in a clamping state in which the bar as moved to the clamping
position holds the tool element actually clamped, or in a state
in which the bar clamps no tool element although moved toward the
clamping position. The tool element is changed by moving the draw
bar to the unclamping position with the spindle held out of rotation
to unclamp the tool element which is so far clamped, replacing the
element by an alternative tool element and thereafter moving the
draw bar to the clamping position to clamp the alternative tool
element. On completion of change of the tool element, a command
for rotation is given by the machine tool to rotate the spindle.
For use with machine tools, magnetic bearing spindle devices have
been proposed in recent years in which a spindle is contactlessly
supported by controllable magnetic bearings and thereby made rotatable
at a higher speed. Such devices usually comprise a spindle for mounting
a tool element thereon, a plurality of controllable magnetic bearings
(e.g., an axial magnetic bearing and two radial magnetic bearings)
for contactlessly supporting the spindle axially and radially thereof,
an electric motor for rotating the spindle, a controller for controlling
the magnetic bearings and the motor, and touchdown bearings (protective
bearings) for restricting the axial and radial movable ranges of
the spindle and mechanically supporting the spindle when the spindle
is not supported by the magnetic bearings.
Although the automatic tool changer comprising a draw bar and described
above has not been used in magnetic bearing spindle devices for
machine tools, the spindle device encounters the following problem
if incorporating the tool changer.
The tool element to be automatically changed is attached to a holder
having a contour in common with other holders for tool elements,
and is changed as attached to the holder. Accordingly, the spindle
differs considerably in overall weight when equipped with the tool
element (clamping state) and when equipped with no tool element
(unclamping state or in a state having no tool element). This varies
the natural frequency of the entire spindle. On the other hand,
the control characteristics of the magnetic bearings of the spindle
device are so adjusted as to be optimum with the tool element attached
to the spindle, so that if it is attempted to rotate the spindle
as contactlessly supported by the magnetic bearings with no tool
element attached to the spindle, the spindle will vibrate, for example,
with the natural frequency of the entire spindle and will not always
be supportable with good stability. Further if an attempt is made
to rotate the spindle with the tool element attached thereto improperly,
the tool element is likely to slip off.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnetic bearing
spindle device for machine tools which is adapted to preclude the
vibration of the spindle or the removal of the tool element that
would occur when the spindle is rotatable as contactlessly supported
by magnetic bearings with no tool element attached to the spindle
or with the tool element attached thereto incompletely.
The prevent invention provide a magnetic bearing spindle device
for machine tools which comprises a spindle for mounting a tool
element as changeably attached to a forward end thereof, a plurality
of controllable magnetic bearings for contactlessly supporting the
spindle, rotating drive means for rotating the spindle as contactlessly
supported by the magnetic bearings, and a draw bar inserted through
the spindle and axially movable for unclamping and clamping the
tool element, the spindle device being characterized in that the
device comprises a tool state sensor means for detecting the axial
position of the draw bar to detect a state of having no tool element,
an unclamping state or a clamping state, and control means for controlling
the support of the spindle by the magnetic bearings and the rotation
of the spindle by the drive means based on the result of detection
by the sensor means.
The position of the draw bar varies with the unclamping state,
the clamping state and the state of having no tool element. Accordingly,
the particular state of the draw bar, i.e., one of these states,
can be detected by detecting the axial position of the draw bar.
The spindle can therefore be prevented from vibrating by controlling
the support of the spindle by the magnetic bearings and the rotation
of the spindle by the drive means based on the result of detection,
for example, by not effecting the support by the magnetic bearings
and not rotating the spindle in the unclamping state in which no
tool element is mounted on the spindle or in the state of having
no tool element. Unless the tool element is mounted properly even
if clamped, the draw bar is in the unclamping state or in the state
of having no tool element, so that when mounted incompletely, the
tool element can be precluded from slipping off similarly by not
effecting the support by the magnetic bearings and not rotating
the spindle in the unclamping state or in the absence of tool element.
Preferably, the control means makes it impossible for the magnetic
bearings to support the spindle and for the drive means to rotate
the spindle when the result of detection by the sensor means is
the state of having no tool element or the unclamping state, or
permits the magnetic bearings to support the spindle and the drive
means to rotate the spindle when the result of detection by the
sensor means is the clamping state.
This eliminates the likelihood that the spindle will be supported
contactlessly or rotated with no tool element attached to the spindle
or with a tool element incompletely mounted thereon, consequently
precluding the spindle from vibrating while no tool element is mounted
on the spindle or preventing the tool element as mounted incompletely
from slipping off.
For example, the draw bar has a flange for use in detecting the
position thereof, and the tool state sensor means comprises three
state sensors for detecting the state of the draw bar by detecting
the flange of the draw bar.
The draw bar in the state having no tool element, in the unclamping
state or in the clamping state is detectable reliably by the three
sensors.
Each of the state sensors is, for example, a proximity switch.
The flange of the draw bar is reliably detectable by the proximity
switch to detect the state of the draw bar reliably.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram schematically showing the construction of a
magnetic bearing spindle device embodying the invention for use
in machine tools; and
FIG. 2 is an enlarged fragmentary view of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
An embodiment of the invention for use in machine tools, for example,
as a machining center will be described below with reference to
the drawings.
FIG. 1 schematically shows a magnetic bearing spindle device 1
a numerical control unit (hereinafter referred to as the "NC
unit") 2 serving as means for controlling the working operation
of a machine tool, and an automatic tool changer 3 serving as automatic
tool changing means. FIG. 2 shows a portion of the spindle device
1 on an enlarged scale.
The spindle device 1 is of the vertical type and has a spindle
5 vertically disposed within a casing 4. Further provided inside
the casing 4 are controllable magnetic bearings for contactlessly
supporting the spindle 5 i.e. an axial magnetic bearing 6 and upper
and lower two radial magnetic bearings 7 8 an axial position sensor
9 for detecting the axial displacement of the spindle 5 upper and
lower two sets of radial position sensors 10 11 for detecting the
radial displacement of the spindle 5 a high-frequency electric
motor 12 serving as rotating drive means for rotating the spindle
5 at a high speed, and upper and lower two touchdown bearings 13
14 serving as means for restricting the axial and radial movable
ranges of the spindle 5 and mechanically supporting the spindle
5 in contact therewith when the spindle 5 is not supported by the
magnetic bearings 6 7 8.
Usually, the axial magnetic bearing 6 comprises a pair of electromagnets
6a opposed to each other axially of the spindle, and each of the
radial magnetic bearings 7 8 comprises electromagnets 7a (8a) in
two pairs in two respective radial directions orthogonal to each
other. In the following description, the two radial control axes
(horizontal axes) orthogonal to each other will be referred to respectively
as X-axis and Y-axis, and the axial control axis (vertical axis)
orthogonal to these axes as Z-axis. Of the electromagnets 7a, 8a
of the radial magnetic bearings 7 8 and the radial position sensors
10 11 those arranged along X-axis only are shown in FIG. 1. The
magnetic bearings 6 7 8 and the position sensors 9 10 11 are
known and therefore will not be described in detail.
The axial position sensor 9 and the radial position sensors 10
11 are driven by a sensor drive circuit 15. The sensor drive circuit
15 detects the displacements of the spindle 5 in the axial direction
(Z-axis direction) and in the radial directions (X-axis direction
and Y-axis direction) based on the outputs of the sensors 9 10
11 and outputs analog position detection signals with respect to
the respective control axis directions. The position sensors 9
10 11 and the sensor drive circuit 15 provide means for detecting
the axial and radial displacements of the spindle 5.
The spindle device 1 further comprises a main controller 16 and
a rotation controller 17 as control means for controlling the magnetic
bearings 6 7 8 and the electric motor 12. The main controller
17 controls the magnitude of the exciting currents to be supplied
to the electromagnets 6a, 7a, 8a of the magnetic bearings 6 7
8 based on the axial and radial displacements of the spindle 5
causes the rotation controller 17 to control the rotation of the
motor 12 and consists mainly of a digital signal processor. The
term digital signal processor refers to specific hardware adapted
to receive digital signals and deliver digital signals, is programmable
by software and processes digital signals at a high speed in real
time. The rotation controller 17 controls the rotation of the motor
12 in accordance with a command from the main controller 16 and
comprises a known inverter.
The analog position detection signals from the sensor drive circuit
15 are converted by an A/D converter 18 into digital position detection
signals, which are then fed to the main controller 16. The main
controller 16 controls the magnitude of the exciting currents to
be supplied to the electromagnets 6a, 7a, 8a of the magnetic bearings
6 7 8 based on the digital position detection signals, i.e., based
on the axial and radial displacements of the spindle 5. Thus, the
main controller 16 delivers digital control signals, which are converted
by a D/A converter 19 into analog control signals. A power amplifier
20 supplies exciting currents to the respective electromagnets 6a,
7a, 8a based on the analog control signals, with the result that
the spindle 5 is contactlessly supported in position with respect
to the axial and radial directions by being attracted by the electromagnets
6a, 7a, 8a. The exciting currents supplied to each pair of electromagnets
6a, 7a, 8a of the bearings 6 7 8 along the control axis are each
a constant steady-state current equal to the other and plus a control
current controlled according to the position of the spindle 5.
A tool element 21 joined to a holder is attached to the forward
end (lower end) of the spindle 5. Inserted through the spindle 5
is a draw bar 22 serving as tool clamp means for unclamping and
clamping the tool element 21 with the axial movement of the spindle
5. The draw bar 22 has a rear end (upper end) projecting upward
beyond the rear end of the spindle 5 and integrally formed with
a flange 22a for use in detecting the position of the bar. Although
not shown, a forward end portion of the draw bar 22 inside the spindle
5 is provided with a clamp for holding the tool element 21 and
the draw bar 22 is biased rearward (upward) by a disk spring disposed
inside the spindle 5. The clamp of the draw bar 22 is opened by
the forward movement of the draw bar 22 and closed by the rearward
movement of the bar 22. Disposed at the rear end of the casing 4
is a pusher 23 serving as means for moving the draw bar 22 forward
against the force of the spring. When the pusher 23 is advanced
to a forward limit position, the draw bar 22 is moved to a forward
limit position, i.e., an unclamping position indicated at A in FIG.
2. This opens the clamp, unclamping the tool element 21 so far clamped
and making the tool element 21 removable. The draw bar 22 is moved
to an intermediate position, i.e., a clamping position indicated
at B in FIG. 2 by fitting the tool element 21 to the clamp in the
unclamping state and retracting the pusher 23. This closes the clamp,
causing the clamp to hold the tool element 21 to mount the element
21 on the spindle 5. If the pusher 23 is retracted with no tool
element fitted to the clamp in the unclamping state, the draw bar
22 moves to a rearward limit position, i.e., a tool absent position
indicated at C in FIG. 2 to close the clamp with no tool element.
The tool element 21 is changed by moving the draw bar 22 to the
unclamping position as described above, with the spindle 5 held
out of rotation and supported by the touchdown bearings 13 14 and
with the magnetic bearings 6 7 8 held unenergized, replacing the
tool element 21 by an alternative tool element and moving the draw
bar 22 to the clamping position. The tool is thus changed automatically
by the automatic tool changer 3 in response to a tool number command
from the NC unit 2. The automatic tool changer 3 is known and therefore
will not be described in detail. The tool number command from the
NC unit 2 is given also to the main controller 16.
Provided inside the casing 4 are three state sensors for detecting
the state of the draw bar 22 by detecting the flange 22a of the
draw bar 22 i.e., a first sensor 24 for detecting the bar 22 in
the unclamping state, a second sensor 25 for detecting the bar in
the clamping state and a third sensor 26 for detecting the absence
of the tool element. These sensors 24 25 26 each comprise, for
example, a proximity switch. The first sensor 24 is turned on only
when the draw bar 22 is in the unclamping position A, the second
sensor 25 is on only when the draw bar 22 is in the clamping position
B, and the third sensor 26 is on only when the draw bar 22 is in
the tool absent position C. The sensors 24 to 26 provide tool state
sensor means.
The output of the sensor 24 25 or 26 is fed to the main controller
16 which in turn controls the support of the spindle 5 by the magnetic
bearings 6 7 8 and the rotation of the spindle 5 by the motor
12 based on the output of the sensor 24 25 or 26 i.e., based on
the state of the draw bar 22 detected. The main controller 16 renders
the spindle 5 supportable by the magnetic bearings 6 7 8 and rotatable
by the motor 12 when the draw bar 22 in the clamping state while
making it impossible for the magnetic bearings 6 7 8 to support
the spindle 5 and for the motor 12 to rotate the spindle 5 when
the draw bar 22 is in the unclamping state or in the state of having
no tool element. For example, when the draw bar 22 is in the clamping
state on completion of tool element changing, the magnetic bearings
6 7 8 are immediately energized to contactlessly support the spindle
5. Alternatively, the bearings 6 7 8 are energized for the contactless
support of the spindle 5 when a rotation command is given by the
NC unit 2 first after the draw bar 22 is brought into the clamping
state upon completion of tool element changing. The spindle 5 is
rotated by driving the motor 12 when a rotation command is output
from the NC unit 2 while the draw bar 22 is in the clamping state.
The characteristics of the magnetic bearings 6 7 8 under the control
of the main controller 16 are so adjusted as to be optimum when
the spindle 5 is equipped with the tool element 21. While no tool
element is mounted on the spindle 5 i.e., while the draw bar 22
is in the unclamping state or in the state of having no tool element,
the magnetic bearings 6 7 8 are held unenergized so as not to
support the spindle 5 contactlessly despite a command from the NC
unit 2 and the motor 12 is held out of operation to hold the spindle
5 at rest even if a rotation command is given by the NC unit 2.
Consequently, the spindle 5 is held out of rotation as supported
by the touchdown bearings 13 14. With no tool element mounted on
the spindle 5 the difference due to the weight of the tool element
21 alters the overall weight and natural frequency of the spindle
5. Since the control characteristics of the magnetic bearings 6
7 8 are so adjusted as to be optimum when the spindle 5 is equipped
with the tool element 21 as described above, the control characteristics
of the bearings 6 7 8 are not always optimum when no tool element
is mounted on the spindle 5. If the spindle 5 is rotated as contactlessly
supported by the bearings 6 7 8 in this state, the spindle 5 is
likely to vibrate. However, the spindle device 1 described is free
of such a problem since the device is so adapted that the spindle
5 is not supported contactlessly, nor is it rotated when equipped
with no tool element. Further if the tool element 21 is attached
to the spindle 5 incompletely despite a clamping movement, the draw
bar 22 assumes the unclamping state or the state of having no tool
element, and the spindle is not supported contactlessly and is held
out of rotation in this state as described above. This eliminates
the likelihood of the tool element slipping off. |