Electricity: motive power systems – Positional servo systems – With compensating features
Reexamination Certificate
1998-05-15
2001-02-06
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Positional servo systems
With compensating features
C318S624000, C318S626000, C318S630000
Reexamination Certificate
active
06184644
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a position control apparatus for a ball screw drive system.
2. Description of the Prior Art
FIG. 1
of the accompanying drawings shows a general mechanism of a ball screw drive system about one axis. When a servomotor
100
rotates, gear train
101
a,
101
b
having a gear ratio of 1:n and serving as a reduction gear connected to the servomotor
100
also rotates. The larger gear
101
b
rotates a threaded portion of a ball screw (hereinafter also called the screw)
102
, and this rotational motion is converted into a linear motion by a ball screw nut
103
(hereinafter simply called the nut). As the servomotor
100
rotates, both a table
104
connected to the nut
103
and a work
105
supported on the table
104
are moved axially of the ball screw
102
(hereinafter simply called “axially” or “longitudinally” by this conversion. The balls crew
102
is supported at opposite ends by a pair of brackets
107
a,
107
b
via a motor-side bearing
106
a
and a counter-motor-side bearing
106
b,
the brackets
107
a,
107
b
being rigidly connected to a machine frame. A motor position detector
108
is connected to the servomotor
100
to detect the motor position in rotation and, at the same time, to detect the position of an object (hereinafter also called the table position) indirectly.
In
FIG. 1
, L represents an entire length of the ball screw
102
and x represents a distance between the motor-side end of the screw
102
and the table position. Each of distances between the motor-side end of the screw
102
, the gear shafts and the motor-side bearing
106
a
is negligibly small as compared to distance x on the motor-side leg of the stroke of the table
104
. The distance between the gear shaft and the table position and the distance between the motor-side bearing
106
a
and the table position are designated by x. In a ball screw drive system which drives the table via this ball screw, the position, speed, and propulsive force of the table are indirectly controlled by controlling the rotational position, speed, and torque of the servomotor.
FIG. 2
is a block diagram of a conventional semi-closed position control apparatus employing a ball screw system as an object system. To this conventional position control apparatus, a position command value X from a non-illustrated host apparatus is given. In the semi-closed position control, the motor position xm is defined as a position feedback xf. A subtracter
50
calculates a position deviation (X−xf), and an amplifier
51
multiplies this calculated position deviation by a position loop gain Kv. A speed command value V, which is obtained by differentiating the position command value X by a differentiator
57
, is added to the output of the amplifier
51
by an adder
52
, whose output is to be used as a speed command input Vma. Another subtracter
54
subtracts a motor speed obtained by differentiating the motor position xm by a differentiator
53
, from Vma to thereby calculate a speed deviation (V−vm). This speed deviation (V−vm) is amplified in proportional integration by a speed amplifier section
55
. Simultaneously, a differentiator
58
differentiates the speed command value V to thereby calculate an acceleration command value A. Further, a value obtained by multiplying the acceleration command value A by a torque command conversion factor K
1
is added to the amplified speed deviation as a torque command value &tgr;ca by an adder
56
.
A power amplifier section
60
composed of a non-illustrated power amplifier and a non-illustrated servomotor amplifies the torque command value &tgr;ca into a motor output torque &tgr;; its amplification factor is represented by a torque conversion constant Ct. An object system
61
is the ball screw drive system of
FIG. 1
, and the motor feed position xm indirectly the table position. These differentiators
57
,
58
,
59
constitute a feedforward system to realize an improved response of position control. S of the individual differentiator
57
,
58
,
59
stands for a Laplace operator defining a differential function.
The manner in which the torque is transmitted in the ball screw drive system will now be described using FIG.
3
and contrasted to
FIGS. 1 and 2
.
FIG. 3
schematically shows a model of the ball screw drive system as converted with respect to the motor axis. The motor output torque &tgr; is transmitted to the table via the motor gear train and the screw. Km and Kc are a torsional rigidity of the motor shaft and a torsional rigidity of the gear shaft, respectively, while Kb(x) is a torsional rigidity of the screw at a distance x from the motor-side end of the gear shaft to the table position. Km, Kc and Kb(x) collectively stand for an integrated torsional rigidity Kt existing between the motor and the table. A table transmission torque &tgr;r is a torque to be transmitted to the table, and the screw is subject to this torque reaction longitudinally, i.e., axially.
Kn is a thrust rigidity of the nut, and Kbl(x) and Kbr(x) are a motor-side thrust rigidity and a counter-motor-side thrust rigidity, respectively, at the distance x from the motor-side bearing on the screw to the table position. Likewise, Kgl and Kgr represent a motor-side thrust rigidity and a counter-motor-side-bearing thrust rigidity, respectively. Likewise, Krl and Krr each represent a thrust rigidity as substituted for the flexural rigidity when a beam is supported as a cantilever by the motor-side bracket or the counter-motor-side bracket. Ksl is a thrust rigidity on the motor side which is an integrated value of Krl, Kgl and Kbl(x); and Kisr is a thrust rigidity on the counter-motor-side which is an integrated value of Krr, Kgr and Kbr(x); and Ks is an integrated thrust rigidity existing between the table and the machine frame which rigidity is obtained by collecting Krl, Kgl, Kbl(x), Ksr, Krr, Kgr, Kbr(x) all together.
&tgr;d represents a turbulence torque (such as cutting torque or gravitational torque) acting outwardly from the table. Xm represents a feed position by the motor, xL represents a table movement position in the direction of rotation of the ball screw, and xo represents a starting position of the table in the direction axial of the ball screw; as a result, a real position (not shown) xi of the table is represented by xi=xL+xo. IL represents a load-side inertial moment (composite inertial moment of table plus work), and Is represents a motor-side inertial moment (composite inertial moment of motor plus gear plus screw). Torque, rigidity, position and inertial moment are all regarded as having the same converted value on the motor shaft.
The manner in which the ball screw drive system of
FIG. 3
is operated at an adjustable speed in the conventional semi-closed position control apparatus will now be described. During the adjustable driving, a table transmission torque &tgr;r can be expressed by the following equation (1):
&tgr;
r=IL·L+&tgr;d
(1)
wherein aL is a twice-differential of the movement position xL indicating an acceleration of the table movement. At the same time, a deflection expressed by the following equation (2):
xm−xL=&tgr;r/Kt
xo=−&tgr;r/Ks
(2)
occurs in the ball screw drive system. Namely, between the feed position xm by the motor and the real position xi of the table, a position deviation expressed by the following equation (3), which is obtained from equation (2), occurs.
xm
-
xi
=
xm
-
(
xL
+
xo
)
=
xm
-
xL
-
xo
=
(
1
/
Ks
+
1
/
Kt
)
τ
r
(
3
)
Thus, during the adjustable driving, the feed position xm by the motor and the real position xi of the table do not coincide. As a consequence, in the conventional semi-closed position control apparatus with a position feedback xf defined by the feed position xm, controlling the real position of the table in accordance with the position command value X much precisely, which is the objective of this conventional concept.
FI
Okuma Corporation
Oliff & Berridg,e PLC
Ro Bentsu
LandOfFree
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