Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-07-16
2004-10-26
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S012060, C310S323170
Reexamination Certificate
active
06809460
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to improvements in a position control method and apparatus using a table driven by a rotative driving device such as a motor.
2. Description of the Related Art
FIG. 5
shows a conventional position control apparatus for a table driven by a stepping motor.
Reference numeral
31
designates a table slidably moved on a base
32
via a guide section extending along an axial direction of the motor. The table
31
and the base
32
move relative to each other along an X direction in FIG.
5
. The table
31
has a nut
33
fixed thereto such that it is moved together with the table
31
. Fitted through the nut
33
is a threaded shaft
35
which is rotated by a stepping motor
34
. The nut
33
has a threaded structure formed in an inner peripheral portion thereof, whereas the threaded shaft
35
has a threaded structure formed in an outer peripheral portion thereof. The nut
33
and the threaded shaft
35
are thus fitted together. Accordingly, the table
31
with the nut
33
fixed thereto is moved along an X axis in a direction according to the rotating direction of the threaded shaft
35
. One end of the threaded shaft
35
is coupled to a rotating shaft of the stepping motor
34
by a coupling
36
. A bearing
37
is fixed to the base
32
at a location intermediate between the coupling
36
and a driving part of the threaded shaft
35
fitted in the nut
33
. The bearing
37
supports the threaded shaft so as to prevent the shaft from shaking in a direction orthogonal to the X direction. Thus, the position of the table is regulated in the direction orthogonal to the X axis. An index scale, not shown, provided on the base
32
is arranged opposite a main scale, not shown, provided on the table
31
. A floodlight and a light receiving sensor are provided opposite the main scale via the index scale. Light emitted by the floodlight is transmitted through the index scale and then reflected by the main scale. The light is transmitted through the index scale again and then enters the light receiving sensor. By analyzing a photoelectric conversion signal from the light receiving sensor, the amount of displacement of the table can be accurately detected with high resolution.
The above described positioning apparatus for the table can surely achieve position control that meets an accuracy of the order of 0.1 &mgr;m. However, it is difficult to control positioning of the table with an increased accuracy, for example, control the positioning of the table with an accuracy of the order of 10 nm. It has been ascertained that even if the table can be stopped at a desired position, even a disturbance caused by minute vibration or the like may result in a positional deviation of 10 nm or more.
Possible causes of this phenomenon will be described below.
The apparatus that controls positioning of a table constructed as above often uses a ball screw. As shown in
FIG. 5
, one end of the ball screw is coupled to the stepping motor, whereas the other end is fitted in the nut integrally joined to the table. If the stepping motor is rotated through a very small angle, a response characteristic as shown in
FIG. 6
is observed between a rotating force F acting upon the ball screw and rotational displacement D of the ball screw.
In
FIG. 6
, a point O indicates a state where both the stepping motor and the ball screw are stopped. When the stepping motor is driven in this state, the relationship between the force F and the rotational displacement D moves from the point O toward a point A along a curved characteristic line. This is because if the motor is rotated through a very small angle, then during an initial phase of rotation, the ball screw is finely twisted and thus elastically deformed, so that the rotating force is absorbed by the elastic deformation. In this state, the stepping motor having been rotated through a very small angle, one end of the ball screw coupled to the stepping motor is also rotated through a very small angle, but the part of the ball screw which is fitted in the nut is not rotated. That is, the rotational displacement D before the point A is caused by the torsional elastic deformation of the ball screw, and at this time the ball screw itself has not yet started rolling (rotation relative to the nut). When the motor is further rotated so that the force F reaches the point A, a resistance force against the elastic deformation of the ball screw increases above a rolling frictional force F
0
of the ball screw so that the rotating force F amounts to the frictional force F
0
. When the motor is further rotated in this state, the ball screw starts to slide on the surface of the nut without being further elastically deformed, and thus the ball screw starts to roll. At this time, the ball screw rolls while remaining elastically deformed.
Since the ball screw has started rolling, the motor is further rotated until a point B is reached. Once the point B has been reached, the rotation of the motor is stopped. Then, in the ball screw, which has been elastically deformed, a force is generated which acts to restore the ball screw from the deformation. Even if holding current then flows to the rotor of the stepping motor, it is difficult for this current to completely resist the restoring force. Thus, the restoring force slightly rotates the rotor of the stepping motor. The rotation of the rotor causes the ball screw to be restored from the elastic deformation. The restoration gives the ball screw a tolerance for elastic deformation, that is, allows the ball screw to be elastically deformed again by the amount of restoration. If the table is subjected to a disturbance such as vibration when the ball screw has a tolerance for elastic deformation, then elastic deformation occurs within the range of tolerance so that the table is moved by the amount of deformation. If the motor is stopped before the point A is reached, the ball screw has not rolled yet but has only been elastically deformed. Thus, after the stoppage of the motor, a force to restore the ball screw from its elastic deformation is exerted. Then, the restoration gives the ball screw a tolerance for elastic deformation. This tolerance amounts to 10 nm or more. Therefore, it is difficult to control the positioning of the table with the accuracy of the order of 10 nm.
That is, if an attempt is made to control the positioning of the table with the accuracy of the order of 10 nm using the stepping motor as a driving source, the problem of hysteresis between the rotating force F acting upon the ball screw and the rotational displacement D of the ball screw is encountered.
Thus, this hysteresis, i.e. a nonlinear response must be eliminated, and screws for this purpose have been proposed. One of them is a hydrostatic screw. In this screw, a nut has a recess formed in a threaded surface thereof, and oil is filled between the nut and the screw body. With this screw, the friction between the nut and the screw body is generated only by oil viscosity resistance, and hence the frictional resistance is small, and the screw can be constructed to have high rigidity in a feed direction (thrust direction), while having no rigidity in a direction (radial direction) perpendicular to the feed direction. This prevents vertical vibration caused by bending or waviness of the screw. This hydrostatic screw may be used for all mechanical elements of the feed system. In addition to the hydrostatic screw, an aerostatic nut and the like have been proposed. However, these screws have special constructions and thus require much time and labor to fabricate, leading to high costs.
Further, an apparatus has been proposed, which has a stepping motor, a ball screw, and a piezo actuator which are coaxially arranged, as described in Japanese Laid-Open Patent Publication (Kokai) No. 10-58267. In this apparatus, the stepping motor is used to rotate the screw to drive a nut, which is fitted on the screw, in the axial direction, thereby achieving coarse adjustment driving. Further, the piezo
Nishimoto Yoshifumi
Okamoto Takuji
Suzuki Masaharu
Yanagi Eiichi
Cowan Liebowitz & Latman
Dougherty Thomas M.
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