Method and system for adaptive control of turning operations

Description

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 ΔM developed by the spindle drive. Depending on an unpredictable variation of cutting conditions B, the cutting torque ΔM may have different current values ΔM

c

, in accordance with which the torque sensor generates current signals U

c

proportional to ΔM

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 ΔM

c

as close as possible to its maximal value ΔM

max

, required for the maximal metal-working productivity.

The maximal value of the cutting torque ΔMmax 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}^{'} = \frac{1}{U_{\max}},

where U

max

is a signal from the torque sensor corresponding to the maximal torque ΔM

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:

\begin{matrix} \frac{F_{c}}{F_{o}} = A - k_{o}^{'} U_{c}, & (1) \end{matrix}

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 ΔM

c

, the relationship (1) may be presented, for the purpose of explaining the physical model of the adaptive controller, as follows:

\begin{matrix} \frac{F_{c}}{F_{o}} = A - K_{o}^{'} Δ M_{c} = a_{c}, & (2) \end{matrix}

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}^{'} = \frac{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 ΔM

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 ΔM

c

is proportional to the depth of cut h

c

as follows:

ΔM

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 ΔM

c

may be expressed as:

\begin{matrix} Δ M_{c} = \frac{A c F_{o} h_{c}}{1 + c F_{o} h_{c} K_{o}^{'}} & (4) \end{matrix}

If in the equation (4), the coefficient A=2 and h

c

=h

max

, the maximal cutting torque ΔM

c

may be expressed as:

\begin{matrix} Δ M_{\max} = \frac{2 c F_{o} h_{\max}}{1 + {cF}_{o} h_{\max} K_{o}^{'}} & (5) \end{matrix}

Similarly, when the depth of cut is of a very small value h

min

such that h

min

/h

max

<<1, the cutting torque ΔM

min

will also be very small:

ΔM

min

≈2cF

o

h

min

<<ΔM

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 ΔM

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 ΔM substantially at a predetermined value ΔM

o

and thereby to substantially compensate the variation of said output operation parameter ΔM 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 ΔM

c

of the output parameter ΔM,

(b) estimating the relation between ΔM

c

and ΔM

o

by multiplying ΔM

c

by a correction coefficient K which comprises an invariant correction coefficient component K

o

inversely proportional to ΔM

o

, and

(c) determining a value F

c

to which the input operation parameter F should be adjusted, as a function of KΔM

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=K

o

−K

c

.

The operation input parameter F is preferably a feed rate of the turning tool and the operation output parameter ΔM is preferably a cutting torque developed by a drive rotating the workpiece. However, the operation output parameter may also be a cutting force applied by the tool to the workpiece or a power consumed by the drive.

The predetermined value ΔM

o

of the output parameter is preferably a maximal value ΔM

max

which this parameter may have when the varying operation condition B differs to a maximal extent from its original or nominal value.

In accordance with preferred embodiments of the present invention, the invariant correction coefficient component K

o

is defined as

K_{o} = \frac{A}{Δ M_{\max}},

where

A = \frac{F_{id}}{F_{o}},

with F

id

being an idle feed and F

o

being a pre-programmed basic feed rate.

The varying operation condition B may be a real physical parameter such as a depth of cut h

c

=h

c

(t), hardness of the workpiece material, etc., whereby current values of the varying coefficient component K

c

may then be obtained based on sensing current values of this parameter. Alternatively, the varying operation condition B may be a mathematical equivalent of one or more physical parameters of the cutting process.

In accordance with another aspect of the present invention, there is provided an adaptive control system for adaptively controlling a turning operation performed at a workpiece by a turning tool, by adjusting a controlled input operation parameter F to maintain an output operation parameter ΔM substantially at a predetermined value ΔM

o

and thereby to substantially compensate variation of said output operation parameter ΔM caused by the variation of at least one operation condition B=B(t), the system comprising:

a sensor of the output operation parameter ΔM for providing a signal U

c

proportional to a current value ΔM

c

;

an adaptive controller for determining a value F

c

to which the input operation parameter F should be adjusted, as a function of kU

c

, where k is a signal transmission coefficient which comprises an invariant signal transmission coefficient component k

o

inversely proportional to ΔM

o

, said controller including an amplifier capable of transforming the signal U

c

into kU

c

; and

an input parameter override unit capable of being controlled by said adaptive controller to adjust the controlled operation input parameter to F

c

;

characterized in that

said controller further comprises a correction processing means for calculating k

c

Uc

c

, where k

c

is a varying signal transmission coefficient component whose current values depend on the variation of said operation condition B=B(t), the controller being capable of calculating k=ƒ(k

o

,K

c

).

Preferably, the adaptive controller is capable of calculating k=k

o

−k

c

, and calculating k

o

as

k_{o} = \frac{A}{U_{o}},

where U

o

is a signal from the sensor of the operation output parameter corresponding to the value ΔM

o

. Preferably, ΔM

o

=ΔM

max

and U

o

=U

max

.

Preferably, the sensor of the output operation parameter ΔM is a sensor of a cutting torque developed by a drive rotating the workpiece and the input parameter override unit is a feed rate override unit.

The correction processing means may comprise a sensor or a calculator for, respectively, sensing or calculating current values of the operation condition B, to be subsequently used in the calculation of k

c

.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, preferred embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which:

FIGS. 1A and 1B

are block diagrams of adaptive control systems having adaptive controllers in accordance with two different embodiments of the present invention;

FIGS. 2A and 2B

illustrate physical models of the adaptive controllers shown, respectively, in

FIGS. 1A and 1B

;

FIG. 3

is a block diagram of a control system having a known adaptive controller;

FIG. 4

illustrates a physical model of the known adaptive controller shown in

FIG. 3

;

FIG. 5

illustrates the dependence of the cutting torque ΔM

c

on the cutting depth h

c

in systems having a known adaptive controller as shown in

FIGS. 3 and 4

(curve I), and having an adaptive controller according to the present invention (curve II).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1A and 1B

illustrate two different embodiments of an adaptive control system according to the present invention, for use with a CNC-operated lathe for adaptively controlling a turning operation performed on a workpiece by a cutting tool (not shown).

The control systems designated as

1

a

and

1

b

in respective

FIGS. 1A and 1B

, each have a feeding means

2

connected to the cutting tool and a spindle drive

4

associated with the workpiece, that are instructed by a program of a CNC unit

6

to establish the relative movement between the cutting tool and the workpiece with pre-programmed values of respective basic feed rate F

o

of the cutting tool and basic rotational speed S

o

of the spindle.

Each control system,

1

a

and

1

b

further comprises a torque sensor

8

for measuring a cutting torque ΔM

c

developed by the spindle drive and varying in time depending on a cutting depth h

c

=h

c

(t) and generating a signal U

c

proportional to the cutting torque ΔM

c

. It also has a feed rate override unit

9

for adjusting the feed rate F

c

so as to maintain the cutting torque ΔM

c

as close as possible to its maximal value ΔM

max

, required for the maximal metal-working productivity. The feed rate override unit

9

is controlled by an adaptive controller

10

operating on the signal U

c

from the torque sensor

8

to determine the extent F

c

/F

o

to which the override unit

9

should adjust the feed rate F

c

.

In accordance with the equation (1) presented in the Background of the Invention, the known adaptive controller of turning operations described therein determines F

c

/F

o

as:

\frac{F_{c}}{F_{o}} = A - k_{o}^{'} U_{c},

where k

o

′ is a signal transmission coefficient of the known adaptive controller.

It will now be explained how in the adaptive controller

10

of the present invention, the signal transmission coefficient k, or its physical equivalent—the correction coefficient K− is calculated in a manner that takes into account the variation of the depth of cut h

c

.

As explained in the Background of the Invention, the cutting torque ΔM

c

in turning operations may be expressed in accordance with the equation (4), in which, for the purpose of the present explanation, A is a coefficient characterizing the extent to which the feed rate F

c

may be increased relative to the pre-programmed value F

o

.

It follows from the equation (4) that, for ensuring the condition ΔM

c

=ΔM

max

, the correction coefficient K should be:

\begin{matrix} K = \frac{A}{Δ M_{\max}} - \frac{1}{c F_{o} h_{c}}, & (7) \end{matrix}

where in accordance with the present invention, A/ΔM

max

=AK

o

′ constitutes a first correction coefficient component K

o

which is invariant in time and 1/cF

o

h

c

constitutes a second correction coefficient component K

c

which varies in accordance with the variation of the depth of cut h

c

.

Based on the equation (3)

\frac{1}{c F_{o} h_{c}} = \frac{a_{c}}{Δ M_{c}},

wherefrom the correction coefficient K may also be expressed as:

\begin{matrix} K = \frac{A}{Δ M_{\max}} - \frac{a_{c}}{Δ M_{c}} . & (8) \end{matrix}

It follows from the above that the second coefficient component K

c

may be expressed either as 1/cF

o

h

c

or as a

c

/ΔM

c

.

The determination of the correction coefficient K should be performed under the logical conditions that K should not be less than a zero and should not exceed 1/ΔM

max

.

FIGS. 2

a

and

2

b

represent physical models of the determination of the coefficient K, based on the above equations (7) and (8).

In the control systems

1

a

and

1

b

of the present invention, the physical models presented in

FIGS. 2A and 2B

are implemented by the adaptive controller

10

constructed to determine kU

c

=k

o

U

c

−k

c

U

c

, where k

o

is a predetermined invariant sign transmission coefficient component and k

c

is a varying signal transmission coefficient component dependent on the depth of cut h

c

.

The coefficient components K

o

and K

c

are determined in the same manner as the correction coefficients K

o

and K

c

. Namely, the invariant coefficient component k

o

is determined as

k_{o} = \frac{A}{U_{\max}},

where U

max

is a signal from the torque sensor

8

corresponding to the maximal torque ΔM

max

. The varying coefficient component k

c

is determined either as

\begin{matrix} k_{c} = \frac{1}{c F_{o} h_{c}}, & (9) \end{matrix}

or, based on the equation (3), as

\begin{matrix} k_{c} = \frac{a_{c}}{U_{c}} . & (10) \end{matrix}

To determine kU

c

, the adaptive controller

10

comprises an amplifier

14

with the invariant signal transmission coefficient k

o

and a correction processing means

16

with the varying signal transmission coefficient k

c

. Depending on the manner in which the varying signal transmission coefficient component k

c

is determined (according to either the equation

9

or the equation

10

), the correction processing means

16

may have either a depth of cut sensor

20

a

(

FIG. 1

a

) or a variation calculator of cutting conditions

20

b

(

FIG. 1

b

), and a computing element

22

for determining current values of k

c

U

c

respectively based on either equation (9) or equation (10) in accordance with the respective physical models in

FIGS. 2A and 2B

.

By virtue of the adaptive control provided by the control system of the present invention, the feed rate of turning tools may be adjusted, taking into account the variation of the depth of cut h

c

, so as to maintain the cutting torque ΔM

c

as close as possible to its maximal value ΔM

max

, in a substantially wide range of the depth of cut, whereby the productivity of the metal-working is increased. This is illustrated in

FIG. 5

as well as in the following table showing experimental results obtained with a known adaptive control system and with an adaptive control system according to the present invention:

h

c

/h

max

:

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Known

a

I

1.0

1.0

1.1

1.1

1.2

1.3

1.4

1.5

1.6

1.8

2.0

adaptive

ΔM

I

1.0

0.90

0.80

0.72

0.70

0.62

0.50

0.43

0.30

0.20

0

Control

ΔM

max

system (I)

Adaptive

a

II

1.0

1.1

1.3

1.6

1.9

2.0

2.0

2.0

2.0

2.0

2.0

control

ΔM

II

1.0

0.98

0.98

0.98

0.98

0.90

0.82

0.70

0.43

0.25

0

system

ΔM

max

of the

present

invention (II)

Comparative

(a

II

/a

I

-1)x

0

10

18

41

58

54

43

33

25

11

0

Productivity

100%

The above-described embodiments of the adaptive control system according to the present invention present non-limiting examples thereof, and it should be clear to a skilled person that, within the scope of the claims, this system may have features different from those described, and shown in the drawings.

Claims

1. 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 ΔM substantially at a predetermined value ΔMo and thereby to substantially compensate the variation of said output operation parameter ΔM 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 ΔMc of the output parameter ΔM, (b) estimating the relation between ΔMc and ΔMo by multiplying ΔMc by a correction coefficient K which comprises an invariant correction coefficient component Ko inversely proportional to ΔMo, and (c) determining a value Fc to which the input operation parameter F should be adjusted, as a function of KΔMc; characterized in that (d) said correction coefficient K comprises a varying correction coefficient component whose current value Kc changes in accordance with the variation of said operation condition B=B(t), the step (b) further comprising calculating the current value Kc and calculating K=ƒ(Ko,Kc).
2. A method according to claim 1, wherein K=Ko−Kc.
3. A method according to claim 1, wherein the operation input parameter F is a feed rate of the turning tool.
4. A method according to claim 1, wherein the operation output parameter ΔM is a cutting torque developed by a drive rotating the workpiece.
5. A method according to claim 1, wherein the predetermined value ΔMo of the output parameter is a maximal value ΔMmax which this parameter may have when the varying operation condition B differs to a maximal extent from its original or nominal value.
6. A method according to claim 5, wherein the invariant correction coefficient component Ko is defined as Ko=AΔ⁢ ⁢Mmax,where A=FidFo,with Fid being an idle feed and Fo being a pre-programmed basic feed rate.
7. A method according to claim 1, wherein the varying operation condition B is a real physical parameter.
8. A method according to claim 7, wherein said parameter is the depth of cut hc=hc(t).
9. A method according to claim 7, wherein current values of the varying coefficient component Kc are obtained based on sensing current values of said parameter.
10. A method according to claim 1, wherein the varying operation condition B is a mathematical equivalent of one or more physical parameters of the cutting process.
11. An adaptive control system for adaptively controlling a turning operation performed at a workpiece by a turning tool, by adjusting a controlled input operation parameter F to maintain an output operation parameter ΔM substantially at a predetermined value ΔMo and thereby to substantially compensate variation of said output operation parameter ΔM caused by the variation of at least one operation condition B=B(t), the system comprising:a sensor of the output operation parameter ΔM for providing a signal Uc proportional to a current value ΔMc; an adaptive controller for determining a value Fc to which the input operation parameter F should be adjusted, as a function of kUc, where k is a signal transmission coefficient which comprises an invariant signal transmission coefficient component ko inversely proportional to ΔMo, said controller including an amplifier capable of transforming the signal Uc into koUc; and an input parameter override unit capable of being controlled by said adaptive controller to adjust the controlled operation input parameter to Fc; characterized in that said controller further comprises a correction processing means for calculating kcUc, where kc is a varying signal transmission coefficient component whose current values depend on the variation of said operation condition B=B(t), the controller being capable of calculating k=ƒ(ko,kc).
12. An adaptive controller according to claim 11, further capable of calculating k=ko−kc, and calculating ko as ko=AUo,where Uo is a signal from the sensor of the operation output parameter corresponding to the value ΔMo.
13. An adaptive controller according to claim 12, wherein ΔMo=ΔMmax and Uo=Umax.
14. An adaptive controller according to claim 11, wherein said sensor of the output operation parameter ΔM is a sensor of a cutting torque developed by a drive rotating the workpiece.
15. An adaptive controller according to claim 11, wherein said input parameter override unit is a feed rate override unit.
16. An adaptive controller according to claim 11, wherein said correction processing means comprises a sensor or for sensing current values of the operation condition B, to be subsequently used in the calculation of kc.
17. An adaptive controller according to claim 11, wherein said correction processing means comprises a calculator for calculating current values of the operation condition B, to be subsequently used in the calculation of kc.

Priority Claims (1)

Number	Date	Country	Kind
126033	Sep 1998	IL

PCT Information

Filing Document	Filing Date	Country	Kind
PCT/IL99/00477		WO	00

Publishing Document	Publishing Date	Country	Kind
WO00/14612	3/16/2000	WO	A

US Referenced Citations (11)

Number	Name	Date	Kind
3728595	Adams	Apr 1973	A
4150327	Camera et al.	Apr 1979	A
4237408	Frecka	Dec 1980	A
4509126	Olig et al.	Apr 1985	A
4547847	Olig et al.	Oct 1985	A
4698773	Jeppsson	Oct 1987	A
5083280	Yamamoto et al.	Jan 1992	A
5083485	Link et al.	Jan 1992	A
5315789	Takashi	May 1994	A
5727912	Rubashkin et al.	Mar 1998	A
6202002	Fainstein et al.	Mar 2001	B1

Method and system for adaptive control of turning operations

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (1)

PCT Information

US Referenced Citations (11)