Title: Method for controlling a continuously variable transmission
Abstract: The transmission ratio of a stepless transmission is adjusted in accordance with a selectable closed loop control strategy, whereby the transmission ratio is infinitely variable between two transmission ratio end values by actuator forces effective on axially adjustable bevel discs forming part of the stepless transmission. High end position forces on the bevel discs in their end positions are limited, yet the entire control range of the bevel discs may be used. For this purpose the transmission ratio is modulated in an end position operating mode by modulating the actuator forces that axially shift the bevel discs. The actuator forces are controlled in a closed loop manner.
Patent Number: 6,939,264 Issued on 09/06/2005 to Hommes, et al.
| Inventors:
|
Hommes; Georg (Ingolstadt, DE);
Ludwig; Rainer (Nuremberg, DE);
Ojamies; Ari (Georgensgmuend, DE)
|
| Assignee:
|
Conti Temic Microelectronic GmbH (Nuremberg, DE);
Audi AG (Ingolstadt, DE)
|
| Appl. No.:
|
471534 |
| Filed:
|
March 5, 2002 |
| PCT Filed:
|
March 5, 2002
|
| PCT NO:
|
PCT/EP02/02362
|
| 371 Date:
|
September 9, 2003
|
| 102(e) Date:
|
September 9, 2003
|
| PCT PUB.NO.:
|
WO02/07307 |
| PCT PUB. Date:
|
September 19, 2002 |
Foreign Application Priority Data
| Mar 10, 2001[DE] | 101 11 607 |
| Current U.S. Class: |
477/45; 701/61 |
| Intern'l Class: |
B60K 041/12; G06F 017/00 |
| Field of Search: |
477/45,46,49
701/61,64
|
References Cited [Referenced By]
U.S. Patent Documents
| 4355547 | Oct., 1982 | Poole et al.
| |
| 4466521 | Aug., 1984 | Hattori et al.
| |
| 4663990 | May., 1987 | Itoh et al.
| |
| 4827803 | May., 1989 | Miyawaki.
| |
| 4867732 | Sep., 1989 | Soga et al.
| |
| 5131297 | Jul., 1992 | Yamashita et al.
| |
| 5368530 | Nov., 1994 | Sanematsu et al.
| |
| 5983152 | Nov., 1999 | Genzel et al.
| |
| 6042501 | Mar., 2000 | Yamamoto.
| |
| 6165101 | Dec., 2000 | Takizawa et al.
| |
| 6173227 | Jan., 2001 | Speicher et al.
| |
| 6533702 | Mar., 2003 | Asyama et al.
| |
| Foreign Patent Documents |
| 4106471 | Aug., 1991 | DE.
| |
| 0785381 | Jul., 1997 | EP.
| |
Primary Examiner: Pang; Roger
Attorney, Agent or Firm: Fasse; W. F., Fasse; W. G.
Claims
1. A method for closed loop controlling a stepless transmission (G) having a
transmission ratio that is continuously variable between two transmission ratio
end values, said method comprising the following steps:
a) generating at least one transmission ratio shifting actuator force that is
effective on position shiftable transmission members (P
2, S
2) for
shifting said transmission members and thus on the transmission ratio between two
transmission end positions corresponding to two transmission ratio end values,
b) defining an end position operating mode as an operating mode by performing
the further steps of
c) generating a power oscillation,
d) first modulating said at least one actuator force with said power oscillation
to vary a mean proportion of said at least one actuator force to provide a closed
loop control signal,
e) second modulating said transmission ratio in response to said first modulating
so that said transmission ratio oscillates about a transmission ratio mean value,
f) ascertaining a measured value (a
M) which depends on said second
modulating of said transmission ratio (i) and represents a mean spacing between
each of said position shiftable transmission members (S
2, P
2) and
the respective end position, and
g) controlling said measured value (a
M) in response to said closed
loop control signal so that said measured value (a
M) corresponds to
a predetermined rated value (a
S), whereby said at least one actuator
force is limited when said position shiftable transmission members (P, S) move
into a respective end position.
2. The method of claim 1, further comprising predetermining said rated value
(a
S) in such a way that at least one of said position shiftable transmission
members (P
2, S
2) bears against a stop with a predetermined mean force
provided by said limited actuator force.
3. The method of claim 1, wherein a normal operation mode is defined as a further
operation mode, and controlling the transmission ratio (i) of the transmission
(G) or the drive r.p.m. (n
P) of the transmission (G) in a closed loop
manner in accordance with a predetermined control strategy, operating the transmission
(G) in said end position operating mode when values of the transmission ratio (i)
are in a range of said transmission ratio end values, and operating said transmission
in said normal or further operation mode outside said range of said transmission
ratio end values.
4. The method of claim 2, wherein a normal operation mode is defined as a further
operation mode, and controlling the transmission ratio (i) of the transmission
(G) or the drive r.p.m. (n
P) of the transmission (G) in a closed loop
manner in accordance with a predetermined control strategy, operating the transmission
(G) in said end position operating mode when values of the transmission ratio (i)
are in a range of said transmission ratio end values, and operating said transmission
(G) in said normal or further operation mode outside said range of said transmission
ratio end values.
5. Controlling a drive unit of a motor vehicle having a stepless transmission
by performing the following steps:
a) generating at least one transmission ratio shifting actuator force that is
effective on position shiftable transmission members (P
2, S
2) for
shifting said transmission members and thus on the transmission ratio between two
transmission end positions corresponding to two transmission ratio end values,
b) defining an end position operating mode as an operating mode by performing
the further steps of
c) generating a power oscillation,
d) first modulating said at least one actuator force with said power oscillation
to vary a mean proportion of said at least one actuator force to provide a closed
loop control signal,
e) second modulating said transmission ratio in response to said first modulating
so that said transmission ratio oscillates about a transmission ratio mean value,
f) ascertaining a measured value (a
M) which depends on said second
modulating of said transmission ratio (i) and represents a mean spacing between
each of said position shiftable transmission members (S
2, P
2) and
the respective end position, and
g) controlling said measured value (a
M) in response to said closed
loop control signal so that said measured value (a
M) corresponds to
a predetermined rated value (a
S), whereby said at least one actuator
force is limited when said position shiftable transmission members (P, S) are in
a respective end position.
Description
FIELD OF THE INVENTION
The invention relates to a method for controlling a stepless transmission. Such
transmissions are used, for example, in motor vehicles.
BACKGROUND INFORMATION
A method for controlling a stepless transmission used in a motor vehicle is known
from German Patent Publication DE 41 06 471 A1. The transmission is thereby constructed
as a looping transmission with two bevel disc gears and a looping belt, whereby
the bevel disc wheels are coupled to one another through the looping belt. Each
bevel gear disc comprises an axially displaceable bevel disk functioning as an
adjustment means. The transmission ratio of the transmission can be adjusted in
a stepless manner by a shifting of the adjustment means within a range that is
limited by two transmission ratio end values. Hereby it is disadvantageous that
a transmission ratio end values are depending on manufacturing tolerances and on
wear and tear phenomena. Additionally, the transmission ratio end values can vary
dynamically depending on the slip between the bevel disc gears and the looping
belt so that the adjustment means are moved into end positions that are determined
by mechanical displacement stop when the transmission is operating with a maximal
or minimal transmission ratio. High end position forces are effective on the adjustment
means in the end positions which have an adverse influence on the efficiency and
the useful life of the transmission.
It is known from European Patent Publication EP 785,381 A1 that the running of
the adjustment means into their mechanical end position is undesirable. Such an
undesirable operation can be avoided in that a safety clearance to the transmission
ratio end values is maintained, which, however has proven itself to be disadvantageous
because an optimal operation with a maximum or minimum permissible transmission
ratio is not possible for certain driving strategies.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method for controlling a continuously
variable transmission so that the entire control range of the transmission can
be used. This object has been achieved by limiting the end position forces that
are effective on the transmission ratio shifting or adjustment members in their
end positions. More specifically, the end position forces are controlled or limited
according to the invention by the following steps:
- a) generating at least one transmission ratio shifting actuator force
that is effective on position shiftable transmission members (P2, S2) for shifting
said transmission members and thus on the transmission ratio between two transmission
end positions corresponding to two transmission ratio end values,
- b) defining an end position operating mode as an operating mode by performing
the further steps of
- c) generating a power oscillation,
- d) first modulating said at least one actuator force with said power
oscillation to vary a mean proportion of said at least one actuator force to provide
a closed loop control signal,
- e) second modulating said transmission ratio in response to said first
modulating so that said transmission ratio oscillates about a transmission ratio
mean value,
- f) ascertaining a measured value (aM) which depends on said
second modulating of said transmission ratio (i), and represents a mean spacing
between each of said position shiftable transmission members (S2, P2) and the respective
end position, and
- g) controlling said measured value (aM) in response to said
closed loop control signal so that said measured value (aM) corresponds
to a predetermined rated value (aS) whereby said at least one actuator
force is limited when said position shiftable transmission members (S2, P1) move
into a respective end Position.
According to the invention an end position operation mode is defined as
an operating mode for the control of a stepless transmission. In the operating
mode the transmission ratio of the transmission is modulated by the modulation
of at least one actuator force that causes the adjustment of the transmission ratio.
Further, a measured value is ascertained that depends on the modulation of the
transmission ratio of the transmission, and the ascertained measured value is adjusted
in a closed loop manner to a predetermined rated value by varying the mean actuator
force or the mean actuator forces. More specifically, the measured value is adjusted
by varying the equal proportion of the actuator force or of the equal proportion
of the actuator forces.
The predetermined rated value (a
S) is advantageously selected so that
at least one shiftable adjustment means, the displacement of which causes the adjustment
of the transmission ratio of the transmission, is effective on a mechanical displacement
stop with a predetermined mean force when the measured value is adjusted in closed
loop fashion to the predetermined rated value.
In an advantageous further development of the present method a normal operation
mode is defined in addition to the end position operation mode as a further operating
mode of the transmission. In the normal operation mode the transmission ratio of
the transmission is adjusted in a conventional manner to a value that depends on
a selected closed loop control strategy. This adjustment can be accomplished either
through the closed loop control of the transmission ratio, or of the r.p.m. of
the motor that drives the transmission. The selection of the operating mode takes
place dependent on the transmission ratio in such a way that the drive unit is
operated in accordance with the end position operation mode for values within the
range of the transmission ratio end values of the transmission and otherwise it
is operated in the normal operating mode.
The advantages of the method according to the invention are seen particularly
in the increasing of the efficiency of the transmission by the utilization of the
full control range and in the avoidance of high end position forces that were effective
in the end positions of the adjustment means. The high operational life of the
transmission which results from the avoidance of high end position forces and from
the resulting reduction of the mechanical loading of the components of the transmission
is a further advantage. The method is best suitable for use in a motor vehicle
with a stepless transmission.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be clearly understood, it will now be described
in connection with example embodiments thereof, with reference to the accompanying
drawings, wherein:
FIG. 1 shows a basic illustration of a stepless transmission;
FIG. 2 shows a function block diagram of a closed loop control circuit by means
of which the transmission ratio of the transmission according to FIG. 1 is controlled
in closed loop fashion in an end position operation mode; and
FIG. 3 shows a diagram of a measured value ascertained with the closed loop
control circuit of FIG. 2 as a function of the transmission ratio of the transmission.
DETAILED DESCRIPTION OF A PREFERRED EXAMPLE EMBODIMENT AND OF THE BEST MODE OF
THE INVENTION
The stepless transmission G shown in FIG. 1 is a component of a drive unit including
the transmission G and an engine of a motor vehicle not shown. The transmission
G comprises a primary shaft WP driven by the engine of the motor vehicle, a primary
bevel disc wheel P connected with the primary shaft WP, a secondary shaft WS, a
secondary bevel disc wheel S connected with the secondary shaft WS, and a looping
belt U for example constructed as a thrust link belt. The looping belt loops around
the two bevel disc wheels P and S and transmits the rotational motion of the primary
bevel disc wheel P to the secondary bevel disc wheel S. The primary bevel disc
wheel P comprises two bevel discs P
1 and P
2, whereby one of the bevel
discs P
1 is rigidly connected to the primary shaft WP and the other bevel
disc P
2 is axially shiftable on the primary shaft WP by the actuator force
F
AP of a primary actuator AP. The axial motion of the bevel disc P
2
is limited by a symbolically indicated mechanical displacement stop LP.
When the shiftable bevel disc P
2 is moved into its end position as indicated
by dashed lines and as determined by the displacement limit stop LP, then the limit
stop LP is effective on the bevel disc P
2 with an end position force F
LP.
Correspondingly, the secondary bevel disc wheel S comprises two bevel discs S
1
and S
2 of which one bevel disc S
1 is rigidly connected to the secondary
shaft WS while the other bevel disc S
2 is shiftable along the secondary
shaft WS by the actuator force F
AS of a secondary actuator AS. The axial
motion of the displaceable bevel disc S
2 is thereby limited by the symbolically
shown mechanical displacement stop SP When the displaceable bevel disc S
2
is moved into its end position as determined by the displacement stop SP and as
indicated by dashed lines, then the displacement stop SP is effective on the bevel
disc S
2 with an end position force F
LS. The spacing between the
bevel discs of the respective bevel disc wheel P or S and thus the effective diameter
of the looping belt U at the respective bevel disc wheel P or S is varied in a
stepless manner by the stepless axial displacement of the bevel discs P
2,
S
2. This stepless varying causes an infinite variation of the transmission
ratio i=n
P/n
S of the transmission G. That means, the ratio
of the drive r.p.m. n
S of the primary shaft WP to the output r.p.m.
n
S of the secondary shaft WS is infinitely variable. The axially displaceable
bevel discs P
2 and S
2 thus constitute an adjustment means for controlling
the transmission ratio i of the transmission G.
The actuator forces F
AP, F
AS are transformed into forces
which tension the looping belt U. The axial motions of the displaceable bevel discs
P
2 and S
2 are coupled through these forces. These forces compensate
one another at a constant transmission ratio i because the transmission G is then
in a force equilibrium. When the transmission ratio i is constant, one thus obtains
the following Equations for the forces that are effective in an axial direction
on the displaceable bevel discs P
2 and S
2:
whereby the forces F
P, and F
S are the pressing forces
which act on the displaceable bevel discs P
2 or S
2 and the factor
K is a support force ratio which is a function of work point parameters, particularly
a function of the transmitted torque moment, of the transmission ratio i, of the
drive r.p.m. n
P, and of the pressing force. Further, a shift force F
V
is defined as follows:
The shift force F
V is composed of an actuator proportion F
VA=K·(F
AS-F
AP)
and an end position force proportion F
VL=K·(F
LS-F
LP).
This shift force F
V is zero when the transmission ratio is constant.
Otherwise it is not zero and determines the speed of the transmission ratio shifting.
During the transmission shifting the actuator forces F
AP and F
AS
are always adjusted in such a manner that certain required values are maintained
for the pressing forces F
P and F
S while also maintaining
a certain required value for the shift force F
V whereby F
V depends
on the desired transmission shifting speed.
Since the axial displacement of the shiftable bevel discs P
2, S
2
is limited, the transmission ratio i can be varied only between a lower transmission
ratio end value—namely the initial transmission ratio—and an upper
transmission ratio value—namely the end transmission ratio—. The exact
transmission ratio values, however, are not known because they are dependent on
geometric characteristics of the transmission G, for example manufacturing tolerances
or wear and tear occurring during operation and on the expansion of the looping
belt U as well as on dynamic characteristics, for example the slip between the
bevel gear wheels P and S and the looping belt U.
During the driving operation the driving r.p.m. n
P, that is the
r.p.m. of the engine that drives the primary shaft WP, and the transmission ratio
i of the transmission are adjusted in a closed loop manner to certain rated values
that depend on the desire of the driver, on the driving situation and on a selected
closed loop control strategy. In a certain driving situation it is desired to drive
with a transmission ratio i that is as high as possible or as low as possible.
Thus, it is possible to optimize with a small transmission ratio i the fuel consumption
and the generation of noise of the motor vehicle while also optimizing the acceleration
and climbing characteristics of the motor vehicle with a high transmission ratio.
With a high or a low transmission ratio i, however, there is the danger that the
transmission G is over-controlled, that means the displaceable bevel discs P
2
or S
2 are pressed with a high force into their end positions, whereby high
end position forces F
LP or F
LS may occur. High end position
forces F
LP, F
LS, however can lead to a reduction of the efficiency
and of the useful life of the transmission G.
In order to avoid these disadvantages, a normal operation mode and an end position
operation mode are defined as operating modes of the transmission, whereby during
the normal operation mode a movement of the displaceable bevel discs P
2,
S
2 into their end positions is undesirable and a motion of the displaceable
bevel discs P
2, S
2 into their end position is permitted during the
end position operation mode.
The operation modes are selected depending on the instantaneous transmission
ratio i. For this purpose, advantageously, two transmission ratio threshold values
are defined which are positioned between the transmission ratio end values. These
two threshold values are the limits of a normal transmission ratio range. These
transmission ratio threshold values are so defined that the displaceable bevel
discs P
2, S
2 are certainly not moved into their end positions when
the transmission ratio values i are within the normal transmission ratio range.
When the transmission ratio values i are positioned in the normal transmission
ratio range, the normal operation mode is selected while otherwise the end position
operation mode is selected as the operating mode. It is, however, also possible
to define transmission ratio threshold bands instead of transmission ratio threshold
values. In that case a switching from one of the operation modes into the other
takes place only when, in response to a change of the transmission ratio i one
of the transmission ratio threshold bands has been fully passed through. Thus,
one obtains a hysteresis shaped switching characteristic which prevents a continuous
switch over of the operation modes when the transmission ratio values i are within
the range of the transmission ratio threshold values.
During the normal operation mode the drive unit is controlled in a conventional
manner. More specifically, the transmission ratio i of the transmission G is intentionally
controlled in closed loop fashion or the control is achieved by the closed loop
control of the r.p.m. n
P, whereby the respective closed loop control
is performed depending on the driving situation, the selected driving strategy,
and on the driver's wishes as expressed by the driver through the operation of
the gas pedal. Contrary thereto, in the end position operation mode the actuator
forces F
AP, F
AS are controlled in closed loop fashion in
such a manner that just still permissible end position forces F
LP, F
LS occur.
For this purpose a closed loop control during the end position operation mode
is performed according to the functional circuit block diagram shown in FIG.
2.
The function block diagram shows a closed loop control circuit with the transmission
G forming the controlled system and including a measuring circuit or device
2,
an adjustment or transmission shifting member
1, and a rated value comparator
3. In the closed loop control performed by this closed loop control circuit,
the drive r.p.m n
R and the output r.p.m. n
S of the transmission
G are measured with suitable sensors and evaluated in the measuring circuit or
device
2. The measuring circuit or device
2 delivers a measured value
a
M as the result of the evaluation. The measured value a
M is
a measure for the mean end position forces F
LP, F
LS that
are effective on the displaceable bevel discs P
2, S
2. Then the measured
value a
M is compared in the rated value comparator
3 with a rated
value a
S which is, for example, provided by a control device. The result
of this comparing, namely the closed loop deviation a
E, is supplied
to the adjustment or transmission shifting member
1 which controls the actuator
forces F
AP, F
AS that are effective on the displaceable bevel
discs P
2, S
2 in accordance with the closed loop deviation a
E.
The adjustment member
1 comprises a first closed loop controller
10,
a second closed loop controller
11 and an actuator unit
12. The actuator
unit is constructed, for example as an electrohydraulic transducer including actuators
AP and AS. The first adjustment member
10 produces a first adjustment signal
S
10 based on the closed loop control deviation a
E. The first
adjustment signal S
10 corresponds to the rated value of the actuator
proportion F
VA of the desired shift force F
V. Optionally,
a feed-forward signal x(i) and/or a feed-forward signal x(di/dt) can be superimposed
on the first adjustment signal S
10 for example through a summing circuit
13. The feed-forward signal x(i) is dependent on the transmission ratio
i. The feed-forward signal x(di/dt) is dependent on the time variation di/dt of
the transmission ratio i. The superposition of these feed-forward signals x(i),
x(di/dt) is performed, however only for certain time durations and only when a
large closed loop control deviation a
E exceeding a certain value occurs.
Such large closed loop control deviations a
E may, for example, be caused
by a rapid actuation of the gas pedal.
The first adjustment signal S
10 and, if applicable, the feed-forward
signals x(i) and/or x(di/dt) as well as an oscillator signal o periodically generated
by an oscillator
4 are supplied as input signals to the second closed loop
controller
11. The second closed loop controller
11 produces from
these signals at least one further control signal S
11 which signal or
signals is used to control the actuators AP, AS of the actuator unit
12
and thus for controlling the actuator forces F
AP, F
AS, which
are effective on the displaceable bevel discs P
2, S
2. The oscillator
4 thereby causes a modulation of the actuator forces F
AP, F
AS,
more specifically, a force oscillation is imposed by the oscillation signal o on
the actuator forces F
AP, F
AS. Thus, the actuator proportion
F
VA of the shift force F
V is composed of a mean value and
an oscillation proportion having an oscillation amplitude A
o caused
by the oscillator signal o. The oscillation proportion causes thereby an oscillation
shift of the shiftable bevel discs P
2, S
2 and thus a respective modulation
of the transmission ratio i. The modulation of the actuator forces F
AP,
F
AS is accomplished by the selection of the oscillation amplitude A
o
and the oscillator frequency in such a manner that the resulting modulation
of the transmission ratio i is not noticed by the driver or at least it is not
noticed as a disturbance.
The modulation of the transmission ratio i, that is the amplitude with which
the transmission ratio i oscillates about its mean value in response to the modulation
of the actuator forces F
AP, F
AS, depends on whether the displaceable
bevel discs P
2, S
2 are moved into their end positions, which causes
a damping of the oscillation. The modulation of the transmission ratio i is thus
a measure for the mean spacing of the displaceable bevel discs P
2, S
2
from their respective end positions and thus a measure for the mean end position
forces F
LP, F
LV that are effective on the displaceable bevel
discs P
2, S
2 or a measure of the mean end position force proportion
F
VL of the shift force F
V. As stated above, the measured
value a
M is a measure for the mean end positioning forces F
LP and
F
LS. It follows, that the measured value a
M is also a measure
for the mean spacing of the displaceable bevel discs P
2, S
2 from
their respective end positions.
This measure is ascertained as the above mentioned measured value a
M
through the measuring device
2 from the drive r.p.m. n
P and the
output r.p.m. n
S of the transmission G. The measuring device
2
comprises for this purpose a transmission ratio ascertaining device
20 which
ascertains the transmission ratio i from the drive r.p.m. n
P and from
the output r.p.m. n
S. The measuring device
2 further comprises
a transmission ratio normalization unit
21 which normalizes the transmission
ratio i to a transmission ratio normalization value i
o, a bandpass device
22 which ascertains from the normalized transmission ratio signal i/i
o
the signal components that originate from the oscillator signal o. The measuring
device
2 further comprises an amplitude ascertaining device
23 which
ascertains the oscillation amplitude A
N of the transmission ratio i
from the signal f provided by a bandpass device
22, for example by rectifying
and then filtering through a low pass filter. The measuring device
2 further
comprises an amplitude normalization device
24 which forms the measured
value a
M as a result of a normalization of the oscillation amplitude
A
M of the transmission ratio i to an oscillation amplitude A
O of
the actuator proportion F
VA of the shift force F
V.
In cases in which the oscillation proportion of the transmission ratio i, stemming
from the oscillator signal o, shows a plurality of signal components having different
frequencies, the bandpass device
22 may comprise a path range for each of
these signal components. These signal components may, for example, correspond to
a signal component of the base harmonic frequency of the oscillator frequency and
to further signal components corresponding to harmonic frequencies of the base
harmonic frequency. The amplitude of each signal component that has been passed
by the band pass device
22 is then weighted with a respective defined factor
in the amplitude ascertaining device. Subsequently, the weighted signals are summed
to a signal which is supplied to the amplitude normalization device
24 as
an oscillation amplitude A
M of the transmission ratio i.
It is possible not to use the transmission ratio normalization device
21
and the amplitude normalization device
24. However, not using these devices
results in a substantial effort and expense when the closed loop control circuit
is to be adapted to different types of transmissions.
The closed loop control circuit varies the mean proportion of the actuator forces
in such a way that the measured value a
M is controlled in a closed loop
manner to the rated value a
S. The rated value a
S is thereby
selected in such a way that the damping of the oscillation of the transmission
ratio i is recognized with certainty when the measured value a
M has
been adjusted. The rated value a
AS is further selected so that the then
effective mean end position force F
LP or F
LS is as small
as possible when the measured value a
M has been adjusted. The measured
value a
M is thus controlled in closed loop fashion to a value at which
a predetermined and still permissible value is obtained for the mean end position
force F
LP or F
LS.
FIG. 3 shows the curve of the measured value a
M as a function of
the mean proportion of the transmission ratio i. The entire transmission ratio
range is divided into three sections B
1, B
2, B
3 by the two
transmission ratio threshold values i
SL, i
SH. The section
B
1 between the transmission ratio threshold values i
SL, i
SH
thereby represents the normal transmission ratio range in which the transmission
G is operated in the normal operation mode. The section B
2 below the normal
transmission ratio section B
1 represents a lower end position transmission
ratio zone. The section B
3 above the normal transmission ratio zone B
1
represents an upper end position transmission ratio zone. The drive unit is driven
in the end position operation mode in these lower and upper end position zones.
In the following a distinction is made in the end position operation mode between
transmission ratio values i in the lower end position transmission section B
2
and end position transmission values i in the upper end position transmission section
B
3. An overdrive operation is involved in the lower section B
2 and
an underdrive operation is involved in the upper section B
3.
In the normal transmission ratio section B
1 the measured value a
M
assumes its maximum value a
MMAX because due to the modulation of the
actuator forces F
AP, F
AS none of the displaceable bevel discs
P
2, S
2 reach their respective end position. Thus, the oscillation
of the transmission ratio i is not damped. The end position force proportion F
VL
of the shift force F
V is then equal to zero.
In the lower end position transmission ratio section B
2, that is during
overdrive operation, the end position force proportion F
VL of the shift
force F
V is not equal to zero because the displaceable bevel disc S
2
of the secondary bevel disc wheel S is moved into its end position by the modulation
of the actuator forces F
AO, F
AS. This modulation entails
a damping of the vibration component of the transmission ratio i. The damping expresses
itself in a reduction of the measured value a
M while the transmission
ratio i decreases.
Correspondingly, the end position force proportion F
VL
of the shift force F
V is also not zero for transmission ratio values
in the upper end position section B
3, that is, during an underdrive operation.
This is so, because now in this case the displaceable bevel disc P
2 of the
primary bevel disc wheel P is moved into its end position by the modulation of
the actuator forces F
AP, F
AS. In turn, such modulation entails
a damping of the oscillation proportion of the transmission ratio i. The damping
expresses itself in a reduction of the measured value a
M while the transmission
ratio i increases.
It has been found to be advantageous to adjust the measured value a
M
in a closed loop control in the end position operation mode to the rated value
a
S=a
MMAX/2. With such a rated value a
S the steepness
of the a
S (i) curve is high so that small fluctuations of the transmission
ratio i can be rapidly recognized and controlled in closed loop fashion. Additionally,
the mean end position force F
LP or F
LS is so small that it
can be regarded as permissible because its negative effect on the efficiency and
on the useful life of the transmission G is of an acceptable size.
The transmission ratio values I
OD and I
UD, which one obtains
in the end position operation mode when the measured value a
M is controlled
in closed loop fashion to the predetermined rated value a
S, represent
the maximally or minimally permissible transmission ratio values, that is, they
represent the control limits of the transmission G. These control limits I
OD,
I
UD can be ascertained with the described method whereby these limits
can be adapted to the varying operational conditions. For this purpose a test is
made, when the measured value a
M is achieved by the closed loop control,
whether certain operational conditions are present. For example, a test is made
whether the motor moment supplied by the engine to the transmission G, is within
a certain range that is selected depending on the transmission ratio i. Thus, advantageously
a test is made whether the motor moment is within the range of -10 Nm to 10 Nm
during an underdrive operation and whether it is larger than 50 Nm during an overdrive
operation. If this is so, the mean value of the actually ascertained transmission
ratio i is stored as an upper control limit i
UD in the case of an underdrive
operation, and as a lower control limit I
OD in the case of an overdrive
operation. Advantageously, these method steps are repeated several times and the
control limits valid at respective different points of time are put into an intermediate
storage. The upper or lower control limit i
UD or i
OD valid
for the actual point of time is then obtained by averaging the respective intermediately
stored control limits. This averaging increases the measuring accuracy since noise
components are being suppressed.
The control limits i
UD and i
OD can be used for defining
the normal transmission ratio section B
1. Thus, it is possible to define
the normal transmission ratio section B
1 as a range positioned between the
control limits i
UD, i
OD, whereby the limits of the section
B
1, namely i
SH, i
SL are spaced respectively by a defined
value from the upper or lower control limits i
UD or i
OD.
The control limits i
UD, i
OD are advantageously used in
the closed loop control circuit of FIG. 2 as transmission ratio normalization value
i
o to which the transmission ratio i is normalized in the transmission
normalization device
21. Thereby the transmission normalization value i
o
is first set to a suitably selected initiation value when the closed loop
control circuit is put into operation. After the actual control limits iU
UD,
i
OD have been ascertained, the lower control limit i
UD is
used as a transmission normalization value i
o when overdrive operation
prevails. The upper control limit i
UD is used as a transmission ratio
normalization value i
U when underdrive operation prevails.
*
