Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Reexamination Certificate
2001-06-11
2002-11-05
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
C318S571000, C318S637000, C318S568220, C409S080000, C409S186000, C409S188000, C408S008000, C408S009000, C408S010000, C408S013000
Reexamination Certificate
active
06476575
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to adaptive control of cutting operations on CNC-operated machine tools in which a controlled input parameter characterizing the movement of a cutting tool relative to a workpiece, is continuously adjusted during a cutting operation in response to a measured output operation parameter defining the productivity of the operation. The present invention particularly concerns the adaptive control of turning operations performed on lathes, where the controlled input parameter is a feed rate of the cutting tool and the output parameter is a cutting torque, cutting force or consumed power of the lathe's spindle drive.
BACKGROUND OF THE INVENTION
In a CNC-operated lathe, a program instructs a feeding means on a feed rate with which a tuning tool should cut a workpiece and instructs the lathe's spindle drive on a speed with which a workpiece associated therewith should be rotated. The feed rate and the selected speed are controlled input parameters that are normally fixed by the program for each cutting operation based on pre-programmed cutting conditions such as depth of cut, diameter of the workpiece, material of the workpiece to be machined, type of the cutting tool, etc.
However, the efficiency of CNC programs is limited by their incapability to take into account unpredictable real-time changes of some of the cutting conditions, namely the changes of the depth of cut, non-uniformity of a workpiece material, tool wear, etc.
Optimization of cutting operations on CNC-operated lathes, as well as on most other machine tools, is usually associated with the adaptive control of the movement of a cutting tool relative to a workpiece and, particularly, with the adjustment of the cutting tool's feed rate as a function of a measured cutting torque developed by the machine tool, to compensate the change in cutting conditions.
FIG. 3
illustrates a known control system for adaptively controlling a turning operation, for use with a CNC-operated lathe having a feeding means and a spindle drive that are instructed by a CNC program to establish the movement of, respectively, a cutting tool and a workpiece attached to the spindle, with pre-programmed values of respective controlled input parameters F
o
that is a basic feed of the cutting tool and S
o
that is a basic rotational speed of the spindle (the cutting tool and the workpiece are not shown).
As seen in
FIG. 3
, the control system comprises a torque sensor for measuring a cutting torque &Dgr;M developed by the spindle drive. Depending on an unpredictable variation of cutting conditions B, the cutting torque &Dgr;M may have different current values &Dgr;M
c
, in accordance with which the torque sensor generates current signals U
c
proportional to &Dgr;M
c
. The control system also comprises a known adaptive controller including an amplifier with a signal transmission coefficient k
o
′, transforming the signal U
c
into k
o
′U
c
and subsequently determining a value F
o
/F
o
=ƒ(k
o
′U
c
) to which the feed rate F
c
should be adjusted, by a feed rate override unit, in order to compensate the variation of the cutting conditions B and to, thereby, maintain the cutting torque &Dgr;M
c
as close as possible to its maximal value &Dgr;M
max
, required for the maximal metal-working productivity.
The maximal value of the cutting torque &Dgr;Mmax is a predetermined cutting torque developed by the spindle drive during cutting with a maximal depth of cut, and the signal transmission coefficient of the amplifier is defined as
k
o
′
=
1
U
max
,
where U
max
is a signal from the torque sensor corresponding to the maximal torque &Dgr;M
max
.
The current value F
o
/F
o
is defined by the adaptive controller based on its signal transmission coefficient k
o
′, pre-programmed basic feed rate F
o
and signal U
c
, in accordance with the following relationship:
F
c
F
o
=
A
-
k
o
′
U
c
,
(
1
)
where A=F
id
/F
o
, and F
id
is an idle feed (feed without cutting).
The coefficient A characterizes the extent to which the feed rate F
c
may be increased relative to its pre-programmed value F
o
, and it usually does not exceed 2.
Since, as mentioned above, the signal U
c
is proportional to the cutting torque &Dgr;M
c
, the relationship (1) may be presented, for the purpose of explaining the physical model of the adaptive controller, as follows:
F
c
F
o
=
A
-
K
o
′
Δ
M
c
=
a
c
,
(
2
)
where K
o
′ is a correction coefficient corresponding to the signal transmission coefficient k
o
′ of the adaptive controller and it is accordingly calculated as
K
o
′
=
1
Δ
M
max
.
The physical model of the adaptive controller is illustrated in FIG.
4
. As seen, the change of the cutting conditions B influences the current value &Dgr;M
c
of the cutting torque which is used by the adaptive controller to determine the coefficient a
c
characterizing the current value F
c
to which the feed rate should be adjusted to compensate the changed cutting conditions B.
It is known that, in a turning operation, the cutting condition that changes unpredictably in time and that is mostly responsible for the variation of the cutting torque is the depth of cut h
c
=h
c
(t). When turning a workpiece of a given diameter, the cutting torque &Dgr;M
c
is proportional to the depth of cut h
c
as follows:
&Dgr;M
c
=cF
c
h
c
=cF
o
a
c
h
c
, (3)
where c is a static coefficient established for turning operations and a
c
is defined in the equation (2).
Based on the equations (3) and (2), the cutting torque &Dgr;M
c
may be expressed as:
Δ
M
c
=
A
c
F
o
h
c
1
+
c
F
o
h
c
K
o
′
(
4
)
If in the equation (4), the coefficient A=2 and h
c
=h
max
, the maximal cutting torque &Dgr;M
c
may be expressed as:
Δ
M
max
=
2
c
F
o
h
max
1
+
cF
o
h
max
K
o
′
(
5
)
Similarly, when the depth of cut is of a very small value h
min
such that h
min
/h
max
<<1, the cutting torque &Dgr;M
min
will also be very small:
&Dgr;M
min
≈2cF
o
h
min
<<&Dgr;M
max
(6)
It follows from the above that, with Me adaptive controller as described, there still may be a significant variation of the cutting torque &Dgr;M
c
during cutting with the depth of cut varying in a wide range, as illustrated in
FIG. 5
, curve I.
It is the object of the present invention to provide a new method and system for the adaptive control of a turning operation.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention there is provided a method of adaptively controlling a turning operation performed on a workpiece by a turning tool, by controlling an adjustable input operation parameter F of the movement of the turning tool relative to the workpiece, to maintain an output operation parameter &Dgr;M substantially at a predetermined value &Dgr;M
o
and thereby to substantially compensate the variation of said output operation parameter &Dgr;M caused by the variation of at least one operation condition B=B(t) varying in time, the method comprising the steps of:
(a) measuring a current value &Dgr;M
c
of the output parameter &Dgr;M,
(b) estimating the relation between &Dgr;M
c
and &Dgr;M
o
by multiplying &Dgr;M
c
by a correction coefficient K which comprises an invariant correction coefficient component K
o
inversely proportional to &Dgr;M
o
, and
(c) determining a value F
c
to which the input operation parameter F should be adjusted, as a function of K&Dgr;M
c
; characterized in that
(d) said correction coefficient K comprises a varying correction coefficient component whose current value K
c
changes in accordance with the variation of said operation condition B=B(t), the step (b) further comprising calculating the current value K
c
and calculating K=ƒ(K
o
, K
c
).
Preferably, K
Fainstein Boris
Rubashkin Igor
Takachnik Eduard
Zuckekrman Mark
Browdy and Neimark PLLC
Duda Rina I.
Nappi Robert E.
Omat Ltd.
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