Servo control method

Details Servo control method Servo control method

2002-04-16

2004-01-13

Nappi, Robert E. (Department: 2837)

Electricity: motive power systems

Positional servo systems

With compensating features

C318S623000, C318S572000, C318S600000, C700S188000, C700S193000, C700S045000

Reexamination Certificate

active

06677722

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a servo control method to synchronously control multi axes of machines. Especially, this invention relates to the servo control method about a Numerical Control machine tool (hereinafter, Numerical Control is abbreviated to NC.), a robot and the like to control a contour.
2. Description of the Related Art
Recent machine tools have been demanded to drive the machine at a high velocity or a high acceleration/deceleration in order to improve working efficiency. Further, a high-precision contouring control is demanded. The contouring control is to relate motions about two axes or more multiple axes at the same time with a linear interpolation, a circular interpolation and the like, thereby an aisle of tool is always controlled.
There are following problems at a high acceleration/deceleration operation and a high velocity operation.
(1) A ball screw is extended (due to an elastic deformation) by an influence of an inertial force caused by the high acceleration/deceleration, whereby generating a distance between a commanded position and a present position.
(2) A radius reduction amount is increased by the high-velocity operation.
(3) Ordinarily, a table is mounted on a saddle, and then a mass ratio between a spindle and a column is twice or more in a vertical type machine. Also in a horizontal type machine, the mass ratio between the spindle and the column is twice or more if a column drive and the like are employed. If the two axes are driven at the same time under the above-described condition, an error due to the inertial force is increased twice or more, so that an elliptic error is generated in the circular interpolation.
(4) At high velocity, a viscous friction increases, and then the velocity cannot be followed in a transient state. Thus, a phase shift occurs, thereby causing an error that an axis of a circular interpolation is inclined.
Following countermeasures and the like against the above problems have been taken conventionally.
(1) As for the velocity, a velocity feedforward corrects a velocity follow-up error. Also as for the acceleration, an acceleration feedforward improves a follow-up characteristics of the acceleration.
(2) A linear scale detects and corrects a mechanical position error.
As some technologies for correcting a mechanical elastic deformation in a semi-closed loop control type,
(1) a technology for correcting the elastic deflection by the inertial force of the machine with the acceleration feedforward (Japanese Patent Application Laid-Open No. H7-78031)
(2) a technology for correcting the elastic deflection by the inertial force of the machine with the feedforward by estimating a rigidity (Japanese Patent Application Laid-Open No. H4-271290)
(3) a technology for correcting the elastic deflection of the machine considering a load weight and a viscous resistance in the semi-closed loop control type (Japanese Patent Application Laid-Open No. 2000-172341)
(4) a technology for correcting the elastic deflection of the machine with feedforward considering the load weight and the viscous resistance (Japanese Patent Application Laid-Open No. H11-184529) have been well known.
The countermeasure (1) is taken with the semi-closed loop control type, thereby cannot correcting a lead error of the ball screw, an error due to a thermal dislocation and the like. Therefore, a high-precision feed drive cannot be executed. The countermeasure (2) is executed, and then the full closed loop control type generates a time lag corresponding to at least a sampling time, thereby causing a follow-up error. Thus, the lead error of the ball screw, the error due to the thermal dislocation and the like cannot be corrected completely. Further, in terms of stability, a gain is not increased in a largely loaded machine, so that a contouring precision is deteriorated. Moreover, if the countermeasure (2) is carried out under hybrid control type, a time lag corresponding to a hybrid time constant is generated. In this case, the lead error of the ball screw, the error due to the thermal dislocation and the like cannot be corrected. Accordingly, in any case, an oblique elliptic error is generated in the circular interpolation.
In any of the above-described cases, the hybrid control type in which a scale feedback is added to an encoder feedback as a first-order lag element has not been described.
The elastic deformation of machine is corrected according to the above-described method in the hybrid control type, and then a difference in the mechanical characteristics is reflected on a transfer function by the scale feedback. Therefore, the difference in the mechanical characteristics is reflected on the transfer function in interpolation of multiple axes, thereby causing a contouring motion error. As a result, a motion precision of the machine is deteriorated.
However, the conventional semi-closed loop control type cannot suppress a mechanical error such as the lead error of the ball screw and the thermal dislocation. As a method for suppressing such the mechanical error, it is considered that the hybrid control type and full closed loop control type are major in future.
The control system of the hybrid control type will be described with reference to an approximate block diagram shown in FIG.
1
.
In
FIG. 1
, a reference numeral
11
denotes a servomotor system. A reference numeral
12
denotes a mechanical system including a load such as a feed screw and a table. A reference numeral
13
denotes a low-pass filter. A reference numeral
14
denotes a pre-compensation part. Further, &ohgr;
o
is a position loop gain, &ohgr;
h
is a first-order lag frequency, s is a Laplace transform operator and G
m
(s) is a transfer function of the mechanical system.
The mechanical transfer function G
m
(s) is described as follows:
G
m
(
s
)=
s
2
+2&zgr;&ohgr;
n
·s+&ohgr;
n
2
where &zgr; is a damping ratio and &ohgr;
n
is a proper angular frequency.
A total transfer function G
h
(s) in this case equals
G
h
⁡
(
s
)
=
ω
0
s
1
+
ω
0
s
⁡
[
ω
h
s
+
ω
h
⁢
{
G
m
⁢
(
s
)
-
1
}
+
1
]
Substituting s=j&ohgr; into the above equation, a total frequency transfer function G
h
(j &ohgr;) equals
G
h
⁡
(
j
⁢
 
⁢
ω
)
=
A
+
j
⁢
 
⁢
B
C
+
j
⁢
 
⁢
D
where A=&ohgr;
h
&ohgr;
o
&ohgr;
n
2
B=&ohgr;
o
&ohgr;
n
2
&ohgr;
C=&ohgr;
4
−{2&zgr;&ohgr;
n
(&ohgr;
h
+&ohgr;
o
)+&ohgr;
n
2
}&ohgr;
2
+&ohgr;
h
&ohgr;
o
&ohgr;
n
2
D=−(&ohgr;
h
+&ohgr;
o
+2&zgr;&ohgr;
n
)&ohgr;
3
+(&ohgr;
h
+&ohgr;
o
)&ohgr;
n
2
&ohgr;
As a result of the G
h
(j&ohgr;), a gain |G
h
(j&ohgr;)| is given as follows:
&LeftBracketingBar;
G
h
⁡
(
j
⁢
 
⁢
ω
)
&RightBracketingBar;
=
A
2
+
B
2
C
2
+
D
2
and a phase ∠G
h
(j&ohgr;) is
∠G
h
⁢
 
⁢
(
j
⁢
 
⁢
ω
)
=
tan
-
1
⁡
(
AC
+
BD
BC
-
AD
)
where the proper angular frequency &ohgr;
n
is defined as
ω
n
=
k
M
and the damping ratio &zgr; is defined as
ζ
=
c
2
⁢
kM
Therefore, the gain G
h
(j&ohgr;) and the phase ∠G
h
(j&ohgr;) of the hybrid control type are functions of a mass M, a spring constant k, a viscous damping coefficient c, an angular velocity &ohgr;, the first-order lag frequency (hybrid frequency) &ohgr;
h
and the position loop gain &ohgr;
o
.
Ordinarily, according to the hybrid control type, the hybrid frequency &ohgr;
h
and the position loop gain &ohgr;
o
are set to have the same value in all the axes, so that no gain shift occurs.
However, in most cases, the mass M, the spring coefficient k, and the viscous damping coefficient c are different between one axis and another axis in the mechanical system. In other words, the elastic dislocation about each axis differs on each axis and a difference in the mass M, the spring coefficient k and the viscous damping coefficient c generates shift of the gain and the phase about each axis. Hence, the shift of

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