Parallel mechanism for multi-machining type machining center

A six-degree-of-freedom parallel mechanism is provided for expanding a grade of a spindle. The parallel mechanism has a spindle that turns round a workpiece at a tilting angle of 90.degree. in a workspace, to thereby allow a machining for both vertical and horizontal planes of the workpiece, and a vertical turning process, by a single machining center. A six-axis multi-machining type machining center embodying the parallel mechanism of the present invention is also disclosed. An over-actuated multi-machining type machining center further including over-actuated actuators is presented to solve a problem of driving joints' singularity caused by the parallel mechanism.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a machining center, and more particularly, 
to a parallel mechanism for allowing a pentahedral machining by a single 
set-up, and for allowing a multi-machining including turning, boring, 
drilling, grinding as well as milling. 
2. Description of the Related Art 
It is necessary to control a three-dimensional position and orientation of 
a tool, in machining using a machining center, so as to shape and form raw 
materials into useful, desired products. A serial mechanism shown in FIG. 
1, is the most fundamental mechanism, used for manipulating the position 
and orientation control. The serial mechanism has a construction which 
allows every shaft between a base 1000 and a spindle 1200 to be at right 
angles with one another. This mechanism has the advantages of a relatively 
large workspace, and a simplified operation software and controller. 
In recent years, there has been made a study of a parallel mechanism 
capable of motion in six degrees of freedom, employed for a machining 
center. The parallel mechanism features a design that can connect a base 
and a spindle parallel with each other, by using a plurality of links. 
FIG. 2 shows a typical parallel mechanism, explaining a hexapod structure 
which allows connecting a base 2100 and a spindle 2200 by means of six 
links. These links are expanded to be capable of motion in six degrees of 
freedom. 
In general, the machining center with the hexapod structure is divided into 
two kinds according to a position of the spindle: vertical machining 
center and horizontal machining center. The former is to machine only a 
vertical plane of a workpiece. The latter is to machine only a horizontal 
plane of a workpiece. Therefore, a machining for both vertical and 
horizontal planes of a workpiece isn't achieved by a single machining 
center. In addition, a turning process is impossible by the machining 
center with the typical parallel mechanism, requiring an additional 
process on a lathe. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention is directed to a parallel mechanism for 
multi-machining type machining center that substantially obviates one or 
more of the limitations and disadvantages of the related art. 
An object of the present invention is to provide a six-degree-of-freedom 
parallel mechanism employed for expanding a spindle tilting angle. 
Another object of the present invention is to provide a parallel mechanism 
having a spindle that turns round a workpiece at a tilting angle of 
90.degree. in a workspace, to thereby allow a machining for both vertical 
and horizontal planes of the workpiece, and a vertical turning process, by 
a single machining center. 
Still another object of the present invention is to provide a six-axis 
machining center with a six-degree-of-freedom parallel mechanism. 
Still another object of the present invention is to provide an 
over-actuated machining center designed for increasing robustness of a 
six-axis machining center. 
Additional features and advantages of the invention will be set forth in 
the description which follows, and in part will be apparent from the 
description, or may be learned by practice of the invention. The 
objectives and other advantages of the invention will be realized and 
attained by the structure as illustrated in the written description and 
claims hereof, as well as the appended drawings. 
To achieve these and other advantages, and in accordance with the purpose 
of the present invention as embodied and broadly described, a parallel 
mechanism includes: three fixed length links connected to a spindle; three 
rectilinear, vertical guides for moving the three links in a vertical 
direction; a circular, horizontal guide for allowing the vertical guides 
to move on its circular arc; spherical joints for connecting the spindle 
to the three fixed length links; revolute joints for connecting the fixed 
length links to the rectilinear, vertical guides; prismatic joints on a 
rectilinear line, allowing vertical movement of the fixed length links on 
the rectilinear, vertical guides; and prismatic joints on a circular arc, 
allowing horizontal movement of the three rectilinear, vertical guides on 
the circular, horizontal guide. 
A multi-machining type machining center allowing a pentahedral machining by 
a single set-up, and a multi-machining including vertical turning, boring, 
drilling, grinding, etc. is attained by using the parallel mechanism of 
the present invention. 
A basic six-axis multi-machining type machining center allows a tool to be 
capable of motion in six degrees of freedom by six actuators including 
three actuators for actuating the prismatic joints on a rectilinear line, 
allowing vertical movement of the three fixed length links on the 
rectilinear, vertical guides, and three actuators for actuating the 
prismatic joints on a circular arc, allowing horizontal movement of the 
three rectilinear, vertical guides on the circular, horizontal guide. 
To solve an actuator's singularity of the parallel mechanism, an 
over-actuating machining center is provided by the present invention. A 
seven-axis over-actuated machining center includes an over-actuating 
actuator for over-actuating one of the three revolute joints, in addition 
to the six basic actuators of six-axis machining center, in such a way 
that a tool is capable of motion in six degrees of freedom. However, the 
problem of actuator's singularity isn't solved completely. 
In this connection, the present invention provides an eight-axis 
over-actuated machining center, in order to completely solve the 
above-mentioned problem. This machining center includes two over-actuating 
actuators for over-actuating two of the three revolute joints, in addition 
to the six basic actuators of six-axis machining center. Therefore, a tool 
is capable of motion in six degrees of freedom by a total of eight 
actuators. 
To consider symmetry of over-actuated machining center and increase 
robustness of machining center, moreover, a nine-axis over-actuated 
machining center provided by the present invention, includes three 
over-actuating actuators for over-actuating all of the three revolute 
joints. 
The multi-machining type machining center with the parallel mechanism 
according to the present invention, allows a pentahedral machining by a 
single set-up, particularly a vertical turning process. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory and are 
intended to provide further explanation of the invention as claimed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Reference will now be made in detail to the preferred embodiments of the 
present invention, examples of which are illustrated in the accompanying 
drawings. 
Referring to FIG. 3 showing a parallel mechanism according to the present 
invention, a spindle 72 is connected to three fixed length links 73, 74, 
and 75, that may be equal or different in length. The links 73, 74, and 75 
shown in FIG. 3 have the same lengths. These three fixed length links 73, 
74, and 75 move along three corresponding rectilinear, vertical guides 83, 
84, and 85 in a vertical direction. A circular, horizontal guide 76 allows 
the rectilinear, vertical guides 83, 84, and 85 to move horizontally on 
its circular arc. 
The following description will now relate to a connection between the 
spindle 72, three fixed length links 73, 74, and 75, three vertical guides 
83, 84, and 85, and circular, horizontal guide 76. The spindle 72 and 
three fixed length links 73, 74, and 75 are connected by spherical joints 
S. Revolute joints R connect the three fixed length links 73, 74, and 75 
with the corresponding rectilinear, vertical guides 83, 84, and 85. They 
are connected to the rectilinear, vertical guides by prismatic joints P on 
a rectilinear line, in order to vertically move the fixed length links 73, 
74, and 75 on the corresponding rectilinear, vertical guides 83, 84, and 
85. Prismatic joints P' on a circular arc connect the three rectilinear, 
vertical guides 83, 84, and 85 and circular, horizontal guide 76, for the 
purpose of horizontally moving these guides 83, 84, and 85 on the guide 
76. 
The basic active joints of the parallel mechanism are the prismatic joints 
P on rectilinear line, for allowing the three fixed length links 73, 74, 
and 75 to move vertically on three rectilinear, vertical guides 83, 84, 
and 85, and prismatic joints P' on circular arc, for allowing the three 
rectilinear, vertical guides 83, 84, and 85 to move horizontally on 
circular, horizontal guide 76. These six joints are driven to make the 
spindle capable of motion in six degrees of freedom. 
To prevent a workspace from getting smaller in area, two 83 and 84 of three 
rectilinear, vertical guides 83, 84, and 85 are located above the 
circular, horizontal guide 76, and the rest 85 is positioned under the 
guide 76. 
FIG. 4 illustrates a state in which the spindle is at a tilting angle of 
90.degree. in the parallel mechanism of FIG. 3. As shown in FIG. 4, the 
spindle is capable of motion at a tilting angle of 90.degree.. It is hard 
to raise a tilting angle of the spindle up to 90.degree. in a typical 
parallel mechanism. This is because there is a limit to a rotation angle 
of spherical joints for connecting the spindle and plural links. 
FIG. 5 is a plan view of a state in which the spindle is at a tilting angle 
of 90.degree. while a tool turns round a workpiece, in the parallel 
mechanism of FIG. 3. Even though the spindle at a tilting angle of 
90.degree. turns from a first position to a second position, three fixed 
length links maintain their relative positions, and three rectilinear, 
vertical guides move on circular, horizontal guide, so that there is no 
change in the angle of spherical joints S for connecting the spindle and 
three fixed length links. 
The following, detailed description relates to a multi-machining type 
machining center embodied by the parallel mechanism of the present 
invention, with reference to FIGS. 6 to 11. 
FIG. 6 is a perspective view of a six-axis multi-machining type machining 
center embodied by the parallel mechanism of the present invention, FIG. 7 
is a plan view of FIG. 6, and FIG. 8 is a cross-sectional view taken along 
the line A--A of FIG. 7. 
The multi-machining type machining center comprises: a spindle system 
including a tool 1 and a spindle motor 2 for allowing rotation of the 
tool; and a movement system for determining a position and orientation of 
the tool. 
Three fixed length links 401, 402, and 403 are connected to the spindle 
system by means of spherical joints 301, 302, and 303. The three fixed 
length links 401, 402, and 403 are equal or different in length. The links 
401, 402, and 403 shown in FIG. 6 have the same lengths. The spherical 
joints 301, 302, and 303 have the advantage of a relatively large, 
allowable angle of inclination more than 50.degree., in comparison to 
typical spherical joints with a tilting angle of 40.degree.. 
Rectilinear, vertical guides 601, 602, and 603 and the three fixed length 
links 401, 402, and 403 are connected by corresponding revolute joints 
501, 502, and 503. Three fixed length links 401, 402, and 403 connected to 
the revolute joints 501, 502, and 503 are driven by respective actuators 
for vertical movement 701, 702, and 703, to move vertically. The 
rectilinear, vertical guides 601, 602, and 603 include two upward 
rectilinear, vertical guides 602 and 603 that are disposed above a 
circular, horizontal guide 8, a downward rectilinear, vertical guide 601 
that is positioned under the guide 8. As another embodiment, there may be 
provided an upward rectilinear, vertical guide and two downward 
rectilinear, vertical guides. 
FIG. 9 is a cross-sectional view taken along the line B--B of FIG. 8, that 
is a cross-section of an upward vertical guide, and FIG. 10 is a 
cross-sectional view taken along the line B--B of FIG. 8, that is a 
cross-section of a downward vertical guide. 
The rectilinear, vertical guides 601, 602, and 603 comprise: ball screws 
621, 622, and 623 for allowing the respective, fixed length links 401, 
402, and 403 to vertically move by the respective actuators for vertical 
movement 701, 702, and 703; ball nuts 611, 612, and 613 for connecting the 
ball screws 621, 622, and 623 to the revolute joints 501, 502, and 503 
connected with the respective, fixed length links 401, 402, and 403; and 
couplings 631, 632, and 633 for connecting the ball screws 621, 622, and 
623 to the respective actuators for vertical movement 701, 702, and 703. 
A rotational driving force of the actuators for vertical movement 701, 702, 
and 703 are transmitted to the ball nuts 611, 612, and 613 through the 
ball screws 621, 622, and 623, before transmitted to the fixed length 
links 401, 402, and 403 through the revolute joints 501, 502, and 503 
connected to the ball nuts 611, 612, and 613. As a result, the fixed 
length links 401, 402, and 403 move along a vertical movement passage 
provided by the ball screws 621, 622, and 623. 
The rectilinear, vertical guides 601, 602, and 603 are driven by respective 
actuators for horizontal movement 901, 902, and 903, to move horizontally 
on the circular, horizontal guide 8. For this horizontal movement, a ring 
gear 81 is provided outside the circular, horizontal guide 8, and pinion 
gears 821, 822, and 823 that are driven by the actuators for horizontal 
movement 901, 902, and 903, are provided at the rectilinear, vertical 
guides 601, 602, and 603. The pinion gears 821, 822, and 823 that are 
driven by the actuators for horizontal movement 901, 902, and 903, engage 
with the ring gear 81, so that the rectilinear, vertical guides 601, 602, 
and 603 perform their horizontal movements. 
Rollers 83 are supplied to endure vertical load of the fixed length links 
401, 402, and 403, a horizontal centrifugal force of the rectilinear, 
vertical guides 601, 602, and 603, and a horizontal or vertical internal 
force between robust bodies. 
As mentioned above, by vertical movement of three fixed length links 401, 
402, and 403 on the rectilinear, vertical guides 601, 602, and 603, and 
horizontal movement of three rectilinear, vertical guides 601, 602, and 
603 on the circular, horizontal guide 8, the spindle is capable of motion 
in six degrees of freedom, to determine its position and orientation. As 
shown in FIG. 6, the machining center with six actuators including three 
actuators for vertical movement and three actuators for horizontal 
movement is called six-axis machining center. 
The position and orientation of the tool for six-axis machining center are 
determined by rotation of the actuators for vertical and horizontal 
movements. In case of a computer numerical control machining center, input 
values of respective movement actuators that are determined by mechanical 
interpretation, are controlled numerically, such that the relative 
position and orientation of the tool with respect to a workpiece are 
controlled to shape and form useful, desired products. 
As illustrated in FIG. 11, in the six-axis multi-machining type machining 
center, the spindle is at a right angle with the workpiece, to perform a 
workpiece side machining by a single set-up. 
A rotation angle of spherical joints 301, 302, and 303 for connecting the 
fixed length links 401, 402, and 403 to the spindle system, is the 
decisive element in determining the position and orientation of the tool. 
The range of vertical movement of the fixed length links 401, 402, and 403 
on the rectilinear, vertical guides 601, 602, and 603, is set up, not to 
apply a burden to the spherical joints 301, 302, and 303. For this, a 
limit switch for fixed length link vertical movement limit 11 is formed at 
each of the rectilinear, vertical guides 601, 602, and 603. 
Three rectilinear, vertical guides 601, 602, and 603 have their respective 
collision prevention limit switches 10 and 10' for preventing them from 
colliding with one another when they are driven by the respective 
actuators for horizontal movement 901, 902, and 903, to move horizontally 
on the circular, horizontal guide 8. 
In general, the operation of the computer numerical control multi-machining 
type machining center is controlled by the variation in initial value 
determined by initial position and orientation of tool. Therefore, it is 
necessary to set up a zero point for determining an initial value. The 
present invention is constructed of a horizontal movement recovery limit 
switch 12 and a vertical movement recovery limit switch 13. 
The six-axis multi-machining type machining center further includes a 
workpiece rotation system for turning a workpiece itself, in addition to 
the spindle system and movement system. The workpiece rotation system 
comprises: a fixing member 31 for fixing a workpiece 20; a chuck 32 for 
supporting the fixing member 31; an air compression cylinder 35 connected 
to the chuck 32; a pulley 33 for rotating the chuck 32; and a pulley 
driving motor 36 for operating the pulley 33. If the pulley driving motor 
36 isn't a direct-coupled motor, a V-belt 34 is further included to 
transmit a rotational driving force of the pulley driving motor 36 to the 
pulley 33. 
As shown in FIG. 11, the workpiece rotation system allows a vertical 
turning process, with the spindle at a right angle with the workpiece. 
In the meantime, the parallel mechanism of the present invention may have 
the problem of actuator's singularity, that will be discussed hereinbelow. 
A constraint equation of parallel mechanism is expressed by; 
EQU g(u,v)=0, u.epsilon.R.sup.n, v.epsilon.R.sup.m, g:R.sup.n .times.R.sup.m 
.fwdarw.R.sup.m, 
wherein u is indicative of a driving joint, and v is indicative of a 
passive joint. 
It is found out from the equation that a joint space forms n-dimension 
manifold placed on a (n+m)-dimension space. If 
.differential.g/.differential.v is invertible by implicit function 
theorem, v can be expressed as a function of u. If a rank of 
.differential.g/.differential.v falls, not to express v by a function of 
u, all the driving joints are not driven independently. We call it 
actuator's singularity. No reaction force occurs with respect to an 
external force, actuators shake partially. 
The following description will relate to the actuator's singularity of the 
six-axis machining center according to the present invention. 
FIG. 12 depicts a Jacobian state in which a constraint equation is 
differentiated with respect to a passive joint variable when a tilting 
angle of the spindle changes from 0.degree. to 90.degree., in the six-axis 
machining center shown FIGS. 6 to 11. A horizontal shaft is indicative of 
an angle of inclination of a spindle, and a vertical shaft is indicative 
of a rotation angle .gamma. of a spindle itself. 
A light part of FIG. 12 means that the spindle slopes smoothly, and a dark 
part of FIG. 12 means that there is an actuator's singularity, not to 
drive all the driving joints independently. 
When a rotation angle .gamma. of a spindle is 0.degree., a singularity 
occurs near the spindles of 36.degree. and 57.degree.. The dark parts in 
FIG. 12 never fail to meet each other no matter what rotation angle 
.gamma. of spindle is set up, when an angle of inclination of spindle 
changes from 0.degree. to 90.degree.. The use of rotation angle .gamma. of 
spindle, that is an excessive degree of freedom attained by rotating the 
spindle itself, can't avoid an actuator's singularity. 
To solve the above-mentioned problem of actuator's singularity, the present 
invention provides an over-actuated machining center having an additional 
actuator for over-actuating passive joints, in addition to six actuators 
for actuating six driving joints. The additional actuator is at least more 
than one actuator. 
A seven-axis machining center provided with an over-actuating actuator will 
now be described hereinbelow as a first embodiment of the present 
invention. 
FIG. 13 shows passive joints where over-actuating actuators can be mounted. 
Revolute joints 51, 52, and 53 of rectilinear, vertical guides 331, 332, 
and 333 of six-axis machining center are passive joints. The seven-axis 
machining center has a construction which mounts an over-actuating 
actuator at any one of the revolute joints 51, 52, and 53, to actuate 
them. For symmetrical structure, the over-actuating actuator is preferably 
mounted at the revolute joint 51 of the downward rectilinear, vertical 
guide 331. 
FIG. 14 depicts a Jacobian state in which a constraint equation is 
differentiated with respect to a passive joint variable when a tilting 
angle of the spindle changes from 0.degree. to 90.degree., in the 
seven-axis machining center where over-actuating actuators are mounted at 
revolute joints of the downward vertical guides. A horizontal shaft is 
indicative of an angle of inclination of a spindle, and a vertical shaft 
is indicative of a rotation angle .gamma. of a spindle itself. In case of 
seven-axis machining center, a singularity barrier disappears, and instead 
the singularity exists in the form of a point. And the singularity isn't a 
barrier near 36.degree. of incline of spindle, but still exists badly. 
This problem may be solved by adding another over-actuated actuator. 
An eight-axis machining center with two over-actuating actuators is 
presented by the present invention. A sensitivity test to the revolute 
joints 51, 52, and 53, shown in FIG. 13 is performed to select a passive 
joint for adding an over-actuating actuator. 
Among the respective passive joints 51, 52, and 53 shown in FIG. 13, the 
revolute joint 51 of downward rectilinear, vertical guide 331 is referred 
to as a first passive joint, the revolute joint 52 of upward rectilinear, 
vertical guide 332 as a second passive joint, and the revolute joint 53 of 
another upward rectilinear, vertical guide 333 as a third passive joint. 
FIG. 15 shows that sensitivity of respective passive joints varies with a 
left singularity barrier (near 36.degree. of incline of spindle) of FIG. 
12. FIG. 16 shows that sensitivity of respective passive joints varies 
with a right singularity barrier (near 55.degree. of incline of spindle) 
of FIG. 12. 
As depicted in FIG. 15, the sensitivity of first passive joint 51 is still 
smaller than that of other passive joints 52 and 53 with respect to left 
singularity barrier, but gets larger with increasing rotation angle of 
spindle. As shown in FIG. 16, however, the sensitivity of second and third 
passive joints 52 and 53 is smaller than that of first passive joint 51 
with respect to right singularity barrier. 
As FIGS. 15 and 16 show, the seven-axis machining center having one 
over-actuating actuator mounted at first passive joint 51 can't completely 
solve the problem of actuator's singularity near 37.degree. of incline of 
spindle. 
In the eight-axis machining center according to the present invention, a 
first over-actuated actuator is mounted at the first passive joint 51 that 
is a revolute joint of a downward rectilinear, vertical guide, and a 
second over-actuating actuator at either of the second and third passive 
joints 52 and 53 that are revolute joints of upward rectilinear, vertical 
guides. 
The actuator's singularity is removed by the eight-axis over-actuated 
machining center of the present invention, and the spindle inclines 
smoothly with angles of 0.degree. to 90.degree.. 
Referring to FIGS. 17 to 21, the eight-axis over-actuated machining center 
of the present invention will now be described in detail. 
FIG. 17 is a perspective view of the eight-axis over-actuated machining 
center of the present invention. FIG. 18 is a plan view of the eight-axis 
over-actuated machining center of FIG. 17. FIG. 19 is a cross-section as 
taken along the line A--A of FIG. 18. FIG. 20 in detail shows an upward 
rectilinear, vertical guide of the eight-axis over-actuated machining 
center of the present invention. FIG. 21 in detail shows a downward 
rectilinear, vertical guide of the eight-axis over-actuated machining 
center of the present invention. 
The eight-axis over-actuated machining center has the same construction as 
a general machining center. That is, the eight-axis over-actuated 
machining center comprises: a spindle system having a tool and a spindle 
motor 38 for allowing rotation of the tool; a movement system for 
determining a position and orientation of the tool; and a workpiece 
rotation system for turning a workpiece. 
The movement system has eight actuators including three actuators for 
vertical movement 111, 112, and 113, and three actuators for horizontal 
movement 121, 122, and 123 plus two over-actuating actuators 130 and 131. 
The three actuators for vertical movement 111, 112, and 113 allow fixed 
length links 351, 352, and 353 to move on the rectilinear, vertical 
guides. A rotational driving force of three actuators for vertical 
movement 111, 112, and 113 are transmitted to ball nuts 91 through 
respective ball screws 92, before transmitted to the fixed length links 
351, 352, and 353 through revolute joints 361, 362, and 363 connected to 
the ball nuts 91. As a result, the fixed length links 351, 352, and 353 
move along a vertical movement passage provided by the ball screws 92. 
The respective rectilinear, vertical guides move along a circumference 
passage provided by a circular, horizontal guide 31 by means of three 
actuators for horizontal movement 121, 122, and 123. 
The revolute joint of downward rectilinear, vertical guide 331 is 
over-actuated by first over-actuating actuator 130, and the revolute joint 
of upward rectilinear, vertical guide 332 is over-actuating by second 
over-actuated actuator 131. 
Two additional over-actuating actuators 130 and 131 remove the problem of 
actuator's singularity completely, and there is no shaking when an angle 
of inclination of spindle is changed from 0.degree. to 90.degree.. 
In order to consider symmetry of over-actuated machining center and 
increase robustness of machining center, a nine-axis over-actuated 
machining center provided by the present invention, has a construction 
that mounts over-actuating actuators at all of revolute joints of three 
rectilinear, vertical guides. A position and orientation of a tool are 
controlled by nine actuators of the nine-axis machining center. 
As described above, the over-actuated machining center of the present 
invention solves the problem of actuator's singularity that may be 
included in the parallel mechanism. Also, a turning process is possible 
with a tilting angle 90.degree. of the spindle. 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in a parallel mechanism for multi-machining 
type machining center of the present invention without deviating from the 
spirit or scope of the invention. Thus, it is intended that the present 
invention cover the modifications and variations of this invention 
provided they come within the scope of the appended claims and their 
equivalents.