Electricity: motive power systems – Positional servo systems
Reexamination Certificate
2002-06-26
2004-06-01
Duda, Rina I (Department: 2837)
Electricity: motive power systems
Positional servo systems
C318S567000, C318S569000, C318S572000, C318S600000, C318S609000, C318S610000
Reexamination Certificate
active
06744233
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method and an apparatus for controlling a servomotor for driving a CNC machine tool, etc.
BACKGROUND OF THE INVENTION
In a CNC machine tool, usually a servomotor rotates a ball screw for driving, thereby moving a table with a workpiece fixed. Thus, for example, to execute circular cutting, as well known, there is a problem of degrading work accuracy as the table move direction is not instantaneously switched due to backlash of the ball screw, the friction of each part of a machine, etc., and a projection called quadrantal projection occurs in the vicinity of the quadrant boundary on the cut face of the workpiece.
FIG. 9
is a block diagram of a servo control apparatus in a related art intended for preventing such a quadrantal projection from occurring, etc. In
FIG. 9
, numeral
1
denotes a position command generation section, numeral
2
denotes a position control section, numeral
3
denotes a speed control section, numeral
4
denotes a current control section, numeral
5
denotes a power amplification circuit, numeral
6
denotes a servomotor for driving a machine system
16
, numeral
7
decodes an encoder or detecting the rotation position of the servomotor
6
, and numeral
8
denotes differentiating means for differentiating a position detection signal
10
output by the encoder
7
to calculate speed. The encoder
7
and the differentiating means
8
make up motor speed detection means. Numeral
9
denotes a position command output from the position command generation section
1
, numeral
10
denotes position feedback of the position detection signal output from the encoder
7
, numeral
11
denotes a speed command output from the position control section
2
, numeral
12
denotes speed feedback of a speed detection signal output from the differentiating means
8
, numeral
13
denotes a speed deviation signal of the difference between the speed command
11
and the speed feedback
12
, numeral
14
denotes a current command output from the speed control section
3
, numeral
15
denotes a current feedback signal indicating a current flowing into the servomotor
6
, numeral
16
denotes the machine system of a CNC machine tool, etc., driven by the servomotor
6
, numeral
17
denotes load torque produced by the reaction force applied from the machine system
16
to the servomotor
6
or friction, numeral
18
denotes a speed proportional control section in the speed control section
3
, numeral
19
denotes a speed integration control section in the speed control section
3
, numeral
20
denotes a proportional term command output by the speed proportional control section
18
, and numeral
21
denotes an integration term command output by the speed integration control section
19
, the integration term command being added to the proportional term command
20
to generate the current command
14
.
Numeral
22
denotes a correction signal generation section for suppressing an error relative to the command position occurring when the direction of the servomotor
6
or the machine is reversed by the effect of friction, etc., and preventing a quadrantal projection from occurring, etc., when circular cutting is executed, and numeral
23
denotes a current command correction signal (correction value) output by the correction signal generation section
22
.
In the apparatus in the related art, the correction value
23
corresponding to the frictional amount is added when the direction of the servomotor
6
is reversed, and as the correction amount, the value preset as a parameter is used or the optimum value for each condition of the feed rate and acceleration stored in memory is used. The correction amount is added as a time function or a travel distance or feed rate function in some cases.
However, in the servo control apparatus in the related art, as the above-mentioned correction amount, the optimum value under a predetermined condition needs to be previously determined at the machine adjusting time, and the frictional amount, etc., of the error cause at the direction reversing time changes largely due to a secular variation and the difference in the condition of the machine position, etc., and it is difficult to determine the optimum correction amount.
Even if the correction amount is determined, the optimum effect becomes hard to provide with the passage of time; this is a problem.
Further, in the machine system with large elastic change in torsion of the ball screw, a seal material (being provided slidably in the surrounding of the shaft of the servomotor, etc., so as to prevent oil, etc., from entering the servomotor side, the outer peripheral part of the seal material being fixed to base section), etc., a correction can be made to an error caused by a follow-up delay (quadrantal projection) occurring at the direction reversing time, but undercut caused by later torsion restoration, etc., cannot be suppressed.
FIGS. 10A and 10B
show simulation of behavior at the direction reversing time before correction in a machine system with large elastic change in torsion of a ball screw, a seal material, etc.;
FIG. 10A
shows roundness accuracy and
FIG. 10B
shows speed and current waveform at the direction reversing time. The result of making correction in the related art shown in
FIG. 9
in such a machine system is shown in
FIGS. 11A and 11B
. In
FIGS. 11A and 11B
,
FIG. 11A
shows roundness accuracy and
FIG. 11B
shows speed and current waveform at the direction reversing time.
In the correction in the related art, the correction value corresponding to the frictional amount is added at the direction reversing time in such a manner that the correction amount is gradually increased in response to the distance from the direction reversing. To apply to a system having elasticity of torsion, etc., in machine system, the correction value may result in over correction instantaneously as shown in
FIGS. 11A and 11B
and even in such a case, means for changing the correction value does not exist and thus undercut occurs.
An apparatus shown in
FIG. 12
exits as a servo control apparatus in another related art.
Shown in
FIG. 12
is the invention disclosed in JP-A-1-276315. In the figure, Xc denotes a position command, numeral
101
denotes a subtracter for comparing the position command Xc with an output signal X of a position detector
106
for performing subtraction and outputting deviation E, numeral
102
denotes an amplifier for amplifying the deviation E and outputting a speed command V, numeral
103
denotes a speed controller for controlling drive output to a servomotor
104
in response to the input speed command V, numeral
105
denotes a working machine wherein, for example, a working tool is moved or a working table on which a workpiece is placed is moved by drive of the servomotor
104
, numeral
106
denotes a position detector for detecting the position of the above-mentioned mobile unit in the working machine
105
, and numeral
107
denotes an ideal position calculator. This ideal position calculator
107
is made up of a subtracter
108
for outputting deviation Ei between the position command Xc and an ideal position Xi, an amplifier
109
for amplifying the deviation Ei and outputting a speed signal Vi, and an integrator
110
for performing time quadrature of the speed signal Vi (speed command) and outputting an ideal position signal Xi corresponding to the ideal position.
Numeral
111
denotes a subtracter for outputting the deviation between the deviation Ei and the deviation E, numeral
112
denotes an amplifier for multiplying the deviation output by he subtracter
111
by a correction gain, and numeral
113
denotes an adder for adding the deviation multiplied by the correction gain to the speed command V.
The servo control apparatus adds the result of multiplying the error between the ideal position and the actual position by the gain and amplifying the multiplication result to the speed command V so as to decrease the speed command V when the position detected by the position detector
106
is ahead o
Duda Rina I
Mitsubishi Denki & Kabushiki Kaisha
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