## Machine tools abstract
A control structure for the active damping of low-frequency oscillations
in numerically-controlled machine tools. The control structure includes
an rpm regulator having a proportional component and an integral
component. The control structure further includes an active damping
element that forms a low-frequency correction signal, which is phase-shifted
with respect to an interfering low-frequency oscillation and free
of d.c. components, and a summing point that is upstream or downstream
of the integral component and receives the low-frequency correction
signal.
## Machine tools claims
We claim:
1. A control structure for the active damping of low-frequency
oscillations in numerically-controlled machine tools, comprising:
a speed regulator for generating a nominal current based on a difference
between a nominal speed and an actual speed, said speed regulator
comprising: a proportional component; and an integral component;
an active damping element that forms a low-frequency correction
signal, which is phase-shifted with respect to an interfering low-frequency
oscillation and free of d.c. components; and a summing point that
is upstream or downstream of said integral component and receives
said low-frequency correction signal.
2. The control structure in accordance with claim 1 further comprising:
a second integral component that corresponds to said integral component
of said speed regulator, wherein said low frequency correction signal
is applied to an input of said second integral component and said
second integral component generates a signal at its output that
is applied to a summing station located downstream of said integral
component.
3. A control structure for the active damping of low-frequency
oscillations in numerically-controlled machine tools, comprising:
a speed regulator comprising: a proportional component; and an integral
component; an active damping element that forms a low-frequency
correction signal, which is phase-shifted with respect to an interfering
low-frequency oscillation and free of d.c. components; a summing
point that is upstream or downstream of said integral component
and receives said low-frequency correction signal; and a second
summing point that determines a second deviation of an actual speed
from a nominal speed and said second deviation is directed to said
proportional component; and wherein a first deviation of said actual
speed from said a nominal speed is determined at said summing point
and is directed to said integral component, and said low-frequency
correction signal is applied at said summing point upstream of said
integral component.
4. The control structure in accordance with claim 3 further comprising:
a position regulator that generates a nominal speed signal; a third
summing point within said damping element that receives said nominal
speed signal and a derived speed signal that is derived from a nominal
position value, said third summing point generates said correction
signal based on a difference of said nominal speed signal and said
speed derived signal.
5. The control structure in accordance with claim 4 further comprising
a first order delay time member within said damping element that
receives said difference between said nominal speed signal and said
derived speed signal.
6. The control structure in accordance with claim 5 wherein a
signal from an output of said first order delay time member is supplied
to a second order delay time member.
7. The control structure in accordance with claim 6 wherein said
output of said first order delay time member is supplied via a delay
member to a branch of said nominal speed conducted on said integral
component of said speed regulator.
8. The control structure in accordance with claim 6 wherein a
damping time constant of said second order delay time member corresponds
to a resonance frequency to be damped.
9. The control structure in accordance with claim 5 wherein an
output of said first order delay time member is supplied via a delay
member to a branch of said nominal speed conducted on said integral
component of said speed regulator.
10. The control structure in accordance with claim 4 wherein said
difference between said nominal speed and said derived speed is
multiplied by an amplification factor.
11. The control structure in accordance with claim 3 wherein said
nominal speed is conducted over a reference model of a control track
prior to said determining said second deviation with said actual
speed at said second summing point.
12. The control structure in accordance with claim 11 wherein
said reference model of said control track is embodied as a second
order delay time element, which simulates said control track and
acts in a counter-phase manner.
13. A control structure for the active damping of low-frequency
oscillations in numerically-controlled machine tools, comprising:
a speed regulator comprising: a proportional component; and an integral
component; an active damping element that forms a low-frequency
correction signal, which is phase-shifted with respect to an interfering
low-frequency oscillation and free of d.c. components; a summing
point that is upstream or downstream of said integral component
and receives said low-frequency correction signal; a second integral
component that corresponds to said integral component of said speed
regulator, wherein said low frequency correction signal is applied
to an input of said second integral component and said second integral
component generates a signal at its output that is applied to a
summing station located downstream of said integral component; a
position regulator that generates a nominal speed signal; and a
second summing point within said damping element that receives said
nominal speed signal and a derived speed signal that is derived
from a nominal position value, said second summing point generates
said correction signal based on a difference of said nominal speed
signal and said derived speed signal.
14. The control structure in accordance with claim 13 further
comprising a first order delay time member within said damping element
that receives said difference between said nominal speed signal
and said derived speed signal.
15. The control structure in accordance with claim 14 wherein
a signal from an output of said first order delay time member is
supplied to a second order delay time member.
16. The control structure in accordance with claim 15 wherein
said output of said first order delay time member is supplied via
a delay member to a branch of said nominal speed conducted on said
integral component of said speed regulator.
17. The control structure in accordance with claim 15 wherein
a damping time constant of said second order delay time member corresponds
to a resonance frequency to be damped.
18. The control structure in accordance with claim 14 wherein
an output of said first order delay time member is supplied via
a delay member to a branch of said nominal speed conducted on said
integral component of said speed regulator.
## Machine tools description
Applicants claim, under 35 U.S.C. .sctn.119 the benefit of priority
of the filing date of Jan. 18 2003 of a German patent application,
copy attached, Serial Number 103 01 765.8 filed on the aforementioned
date, the entire contents of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control structure for the active
damping of low-frequency oscillations in numerically-controlled
machine tools, having an rpm regulator with a proportional component
and an integral component.
2. Discussion of Related Art
Such oscillations considerably affect the surface quality of a
treated work piece.
The numerical control of a machine tool controls the processing
of a work piece by a parts program, in which an exact treatment
process is fixed in the most different treatment cycles. In the
course of this, a tool is required to follow a predetermined track
as exactly as possible, so that the shape of the finished work piece
corresponds to the preset conditions. To this end it is necessary
to appropriately control the various shafts of the machine tool
with their respective rotary or linear drive mechanisms. In order
to be able to maintain a predetermined treatment track, control
structures are employed which, in a position regulating device,
calculate a nominal speed (for linear drive mechanisms) or nominal
rpm (for rotary drive mechanisms) from the respective predetermined
nominal position and the actual position of the tool, by which it
is intended to correct a possible position deviation. The difference
between the nominal rpm and the actual rpm is converted in an rpm
regulator into a nominal current for the drive mechanism which,
via the motor constant of the drive mechanism, also corresponds
to a nominal torque. After a comparison with the actual current,
a nominal voltage is determined from this nominal current by regulation
in a current regulator and is converted in the drive mechanism amplifier
and applied to the phases of the motor. Suitable measuring systems
check the actual position of the work piece, from which the actual
rpm can be derived. Current sensors in the supply lines to the motor
detect the actual current.
The connection between the drive mechanism and the tool is never
completely rigid, instead it contains elastic components, which
are therefore capable of oscillation. Thus, mechanical resonance
frequencies occur, which can lead to undesired oscillations in case
of an adverse parameterization of the control structure and/or reduced
internal damping of the elastic components. Because of the demand
for increasingly greater bandwidths of the control structures, primarily
realized by high amplifier factors in the position control circuit,
such low-frequency resonance frequencies are also amplified and
are superimposed on the tool track. Low-frequency oscillations in
the range of up to 50 Hz are clearly visible in the form of an undesired
surface waviness of the treated work piece.
A negative phase angle rotation has particularly negative effects
in the formation of such resonance oscillations, such as is created
in particular by the delays in the control system during the cooperation
with the integral component of the rpm regulator. The integral portion
can be reduced by reducing the corresponding amplification factor,
and the resonance oscillation weakened in the process, but at the
same time the rigidity of the machine tool and the quality of the
interference removal are also reduced.
Therefore WO 01/23967 A1 describes the parameterization of a regulator
system, in which the feedback of the actual rpm to the nominal rpm
upstream of the rpm regulator is split onto two summing points,
and wherein a reference model in the form of a proportional component
with second order delay (PT2 member) is switched into the branch
upstream of the integrating element of the rpm regulator. This reference
model is matched to the behavior of the closed control circuit without
an integral component in the rpm regulator, so that the disadvantageous
influence of the integral component does not appear in the guidance
behavior of the rpm regulator. This disadvantageous influence is
particularly strong because of the negative phase shift by 90 degrees
occurring in the integral component. However, only resonance frequencies
above approximately 20 Hz can be damped with this arrangement, and
in connection with large machines with several resonances this circuit
can only positively act on the respectively highest resonance frequency,
while lower resonance frequencies possibly are even negatively affected.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to create a
control structure which is able in particular to actively damp low
resonances in numerically-controlled machine tools.
This object is attained by a control structure for the active damping
of low-frequency oscillations in numerically-controlled machine
tools. The control structure includes an rpm regulator having a
proportional component and an integral component. The control structure
further includes an active damping element that forms a low-frequency
correction signal, which is phase-shifted with respect to an interfering
low-frequency oscillation and free of d.c. components, and a summing
point that is upstream or downstream of the integral component and
receives the low-frequency correction signal.
A control structure for the active damping of low-frequency oscillations
in numerically-controlled machine tools is proposed, wherein in
a control structure with an rpm regulator with a proportional component
and integral component a correction signal, which is phase-shifted
with respect to the interfering low-frequency oscillation and is
free of d.c. components, is switched to a summing point upstream
or downstream of the integral component. This correction signal
is formed in an active damping element.
Here, a possible embodiment of the present invention assumes that
the interfering low-frequency oscillation is also superimposed on
the nominal rpm at the output of the position regulator, since the
position regulator is supplied with the difference between nominal
position value and actual position value, and the interfering oscillation
in fact finds expression in an oscillating actual position value.
If therefore the nominal rpm is freed of its d.c. components and
the phase relation is correctly set, the correction signal obtained
in this way is suitable, when switched to the integral component
of the rpm regulator, for the active damping or cancellation of
the interfering low-frequency oscillation.
An advantage of the present invention lies in that, once a damping
element has been parameterized, it operates very stably, even if
a shift of the interfering resonance frequency results, for example
by load changes. This of course also means that the parameterization
itself can be simply performed.
Further advantages, as well as details, of the present invention
result from the following description of preferred embodiments by
the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an embodiment of a control structure with active damping
of low-frequency oscillations in accordance with the present invention;
FIG. 2 shows a modification of the control structure in FIG. 1
which is equivalent in accordance with the block circuit algebra
and in accordance with the present invention; and
FIG. 3 shows a third embodiment of a control structure with active
damping of low-frequency oscillations in accordance with the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a portion of the control structure of a numerically-controlled
machine tool. An interpolator 1 calculates short segment portions
from the data of a parts program, whose end points are put out as
the nominal position value p_nominal. The actual position value
p_actual is subtracted from this nominal position value p_nominal
at a summing point 4.1. The position difference obtained in this
way is supplied to the position regulator 2 which, as a function
of the position difference and the set position regulator amplification,
forms a nominal rpm n_nominal, with which the position difference
is intended to be compensated. For example, the position regulator
2 can be embodied as a simple proportional regulator, which multiplies
the position difference by a factor and puts it out as the nominal
rpm n_nominal.
The difference between the nominal rpm n_nominal and the actual
rpm n.sub.13 actual is formed at separate summation points 4.2 and
4.3 and this deviation of the rpm is put out to the proportional
component 3.1 or the integral component 3.2 of the rpm or speed
regulator 3. A nominal current i_nominal is available at the output
of the rpm regulator 3 and includes the sum of the outputs of the
proportional component 3.1 and the integral component 3.2 of the
rpm regulator 3. Multiplied by the motor constant the nominal current_nominal
corresponds to a nominal torque converted into a nominal voltage
in a current regulator (from here on out the control circuit is
no longer represented in the drawing figure). A control circuit
is also used for this, which is supplied with an actual current
value picked up by current sensors at the motor. An output amplifier
generates the requested voltage, for example by controlling the
motor phases by pulse width modulation (PWM). The resultant movement
is then detected by position measuring systems, which provide the
actual position value p actual and, derived from this, also the
actual rpm n_actual.
If now a low-frequency mechanical resonance frequency is excited
in the machine tool, the machine tool begins to oscillate. This
oscillation is propagated via the actual position value p_actual
into the control circuit. In the course of this, frequency-dependent
different phase shifts and dampings or amplifications occur in the
various elements of the control circuit. Here, the integral component
3.2 of the rpm regulator 3 provides a particularly large contribution
to the negative phase shift. Therefore an active damping of an interfering
low-frequency oscillation acts particularly effectively there. The
idea on which the present invention is based rests on providing
the integral component 3.2 of the rpm regulator 3 with a correction
signal n_correction, which damps the undesired oscillation or even
eliminates it. Represented in a simplified way, a signal free of
d.c. components with the frequency of the undesired oscillation,
but which is phase-shifted by approximately 180 degrees with respect
to the latter, is required at the input of the integral component
3.2 of the rpm regulator 3. In this case the phase shift relates
to the phase relation of the oscillation, which is coupled in via
the nominal rpm n_nominal at the summation point 4.3 of the integral
component 3.2 of the rpm regulator 3. But the actual phase shift
required for optimal damping or elimination will slightly diverge
from 180 degrees, since an amplification of the undesired oscillation
also takes place via the proportional component 3.1. This and further
interactions in the control circuit must be taken into consideration
in the course of parameterization of the active damping element
5.
The task of the active damping element 5 not further represented
in FIG. 1 initially includes making a suitable correction signal
n_correction available, by which the described negative effects
of the integral component 3.2 on the oscillation behavior can be
combated. The distribution of the formation of the difference between
the nominal rpm n_nominal and the actual rpm n_actual to the summation
points 4.2 and 4.3 permits the specific influencing of the integral
component 3.2 in an elegant manner.
An alternative to the circuit in accordance with FIG. 1 which is
equivalent in accordance with regulation technology, but more expensive,
is represented in FIG. 2. Now the correction signal n_correction
is coupled in at the summation point 4.4 not upstream of the integral
component 3.2 but only downstream of the integral component 3.2.
In accordance with the rules of block circuit algebra it is completely
equivalent to the arrangement in accordance with FIG. 1 if an additional
integral component 3.2' is inserted between the active damping element
5 and the summation point 4.4. In this case the integral component
3.2 and the additional integral component 3.2' must completely match
each other which, at least in case of an analog realization, means
a considerable outlay. Now the formation of the difference between
the nominal rpm n_nominal and the actual rpm n_actual can take place
at only a single summation point 4.7. Thus, there are two completely
equivalent (and in the sense of the block circuit algebra actually
identical) options of applying the correction signal n_correction.
FIG. 3 shows a possible embodiment of the present invention. Corresponding
elements are identified the same as in FIG. 1 or FIG. 2 so that
they and their linkages need not be explained again. The present
embodiment is based on the realization that interfering low-frequency
oscillations are also contained in the nominal rpm n_nominal of
the position regulator 2. This fact can be used for producing the
correction signal n_correction.
For eliminating the d.c. components of the nominal rpm n_nominal,
the difference between a derived rpm n_derived and the nominal rpm
n_nominal is produced at a summation point 4.5. In this case the
derived rpm n_derived was produced in a differentiator 6 which
differentiates the nominal position values p_nominal of the interpolator
1. This derived rpm n_derived does not contain any interfering resonance
oscillation and can also be used for rpm pre-control. If the difference
is formed at the summation point 4.5 in such a way that the derived
rpm n_derived with a negative sign is switched in, a signal which
is free to a great extent of d.c. components is obtained, whose
phase relation corresponds to the oscillation of the nominal rpm.
An amplifier with an adjustable amplification factor M is located
inside the active damping element 5. This is represented in FIG.
3 by a multiplication point 5.4 where the difference between the
derived rpm n_derived and the nominal rpm n_nominal is multiplied
by the factor M. This allows the continuous regulation of the active
damping of the low-frequency oscillation. At M=0 the active damping
element 5 is completely deactivated.
It is then additionally recommended to further reduce the low-frequency
portion of the difference between the nominal rpm n_nominal and
the derived rpm n_derived. This can be done with a differential
element with a first order delay time (DT1) member 5.1 through
which the difference between the derived rpm n_derived and the nominal
rpm n_nominal is conducted. The signal, now free of d.c. components,
is finally conducted to a second order delay time (PT2) member 5.2
which is tuned to the interfering resonance frequency. For stabilization,
the output of the DT1 member 5.1 is supplied in a transverse branch
via a delay element (PT1 member) 5.3 and a summing point 4.6 also
to the branch of the nominal rpm n_nominal conducted to the integral
component 3.2 of the rpm regulator 3.
The correction signal n_correction generated in the active damping
element 5 therefore is a signal free of d.c. components and phase-shifted
with respect to the interfering low-frequency oscillation and is
additionally applied at the summing point 4.3 upstream of the integral
component 3.2 of the rpm regulator. In this way the interfering
resonance can be effectively suppressed.
For parameterizing the active damping element 5 the damping time
constant T2 of the PT2 member 5.2 is selected to correspond to the
resonance frequency Fres to be damped: T2=k/(2*.pi.*Fres) with a
time constant displacement factor k, which in actual use swings
between 0.8 and 1.0. The time constant displacement factor k permits
the detuning of the PT2 element 5.2 which can bring advantages
when parameterizing the control structure. Thus, for a resonance
frequency of 10 Hz, a time constant T2 of 0.016 s (for k=1), for
example, is obtained. For optimal damping of D=0.35 the result
then is (from the equation D=T2/(2*T1) applicable to PT2 elements)
for example a T1 time constant for the PT2 member 5.2 of T1=T2/0.7
The time constant of the DT1 member 5.3 then should be clearly greater,
for example approximately ten times that of T2 the time constant
of the PT1 member should be less, for example approximately one-quarter
of T2. The stated number values of course only provide an approximate
order of magnitude for a special application case, the exact parameterization
of the active damping element 5 will differ from one case to the
next.
A particularly stable control structure can be obtained if the
active damping element 5 is combined with a reference model 7 described
in the preamble which, although of little use for particularly low
resonances, does have an application for higher frequency resonances.
To this end, the reference model 7 is connected directly upstream
of the summing point 4.3 upstream of the integral component 3.2
of the rpm regulator 3. This reference model 7 is matched to the
behavior of the closed control circuit with the deactivated integral
component 3.2 in the rpm regulator, so that the undesirable influence
of the integral component 3.2 on the guidance behavior of the rpm
regulator 3 can be eliminated, or minimized. The reference model
7 and the active damping element 5 do not interfere with each other,
but complement each other advantageously.
This description was based on a rotary drive mechanism. In actual
use, linear drive mechanisms are also increasingly employed, for
which speed is a more suitable definition than rpm and force is
a more suitable definition than torque. The control structure of
the present invention can of course be employed in the same way
with linear drives, because of which the terms rpm and torque are
synonymous with speed and force. With machine tools with several
shafts, the invention can also be employed separately for each shaft.
The control structure can be realized analog as well as digitally,
the present invention is of course not dependent on the type of
realization.
Besides the exemplary embodiments described, it is understood that
alternative variants also exist within the scope of the present
invention. |