Control system for an infinitely variable change-speed gear with a torque converter with a lock-up clutch

A control system for a continuously variable transmission (17), especially for control of a lock-up clutch (10) of a torque converter (2). The lock-up clutch (10) is controlled according to a strategy whereby the torque converter (2) is considered as being serially mounted with the continuously variable transmission (17). Engagement and disengagement of the lock-up clutch (10) is exclusively a function of the control of the theoretical engine speed.

The invention relates to a control system for control of a lock-up clutch 
in a continuously variable transmission. 
BACKGROUND OF THE INVENTION 
Hydrodynamic torque converters in automatic transmissions chiefly perform 
the task of making possible a comfortable starting operation with an 
increased start-up torque and to uncouple the torsional vibrations of the 
engine from the drive line. With the constantly increasing demands for 
reducing both fuel consumption and gas emission, it has become necessary 
further to reduce the losses in the hydrodynamic torque converter. In the 
first place, this is achieved by optimizing the torque converter itself 
and by using a (slip regulated) lock-up clutch. The latter objective is to 
reduce the share of the comfortable but dissipative hydrodynamic power 
transmission and thus to spare fuel without giving up a sufficient 
uncoupling of vibrations. 
The control of the lock-up clutch essentially consists of a strategy and 
pressure modulation. In the strategy established is which state the torque 
converter lock-up clutch assumes, by taking into consideration an optimal 
consumption and comfort characteristic. At least two basic states (open 
and closed) have been defined. Together with that, a slipping operation of 
the torque converter lock-up clutch can be implemented. 
DE-A 41 04 542 has disclosed a control system for control of a lock-Lip 
clutch in a continuously variable transmission. For control a ratio of the 
speeds of rotation of the input engine and of the primary pulley of the 
continuously variable transmission is formed. If this value is one, the 
torque converter lock-Lip clutch is closed. By comparing an actual ratio 
between the primary and secondary pulleys of the continuously variable 
transmission with the computed theoretical ratio, it is determined that 
the torque converter must be activated. In this case, a disengagement 
signal is generated to disengage the torque converter lock-up clutch. With 
the engagement and disengagement, time steps become effective in order to 
ensure that the secondary pressure is adapted to the torques to be 
transmitted--in accordance with the operating state of the torque 
converter lock-up clutch. The strategy takes into consideration the basic, 
functions for control of a torque converter lock-up clutch. 
SUMMARY OF THE INVENTION 
The problem on which the invention is based is to provide a strategy for 
control of a lock-up clutch which is a component part of an operating 
strategy for a continuously variable transmission. A control system for a 
continuously variable transmission (17) of a motor vehicle having a torque 
converter (2) with a lock-up clutch (10), especially for control of the 
lock-up clutch, for locking up the torque converter and means (31 to 42) 
for detecting input signals which are derived from a driver-vehicle system 
and processed to an engagement and disengagement signal in accordance with 
the detected states, characterized in that said lock-up clutch (10) is 
controlled following a strategy in which the hydrodynamic torque converter 
(2) is considered as hydrodynamic variator serially mounted with the 
continuously mounted with the continuously variable transmission (17) and 
an en disengagement of the lock-up clutch (10) is exclusively function of 
the control of the engine theoretical speed of rotation (n.sub.-- 
Mot.sub.-- soll).

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
FIG. 1 diagrammatically shows a continuously variable transmission: an 
input unit 1, preferably an internal combustion engine, drives a start-up 
unit, preferably a hydrodynamic torque converter 2. The hydrodynamic 
torque converter is constructed in a manner known already: a converter 
housing 3 is non-rotatably connected with an impeller 4. A turbine wheel 5 
is associated with the impeller 4. The arrangement is completed by a 
stator 6. Simultaneously with the impeller is driven an oil pump 7 to 
supply pressurized oil to the system. 
The turbine wheel 5 is connected with an output shaft 8 of the hydrodynamic 
torque converter. Also with the turbine wheel connected non-rotatably but 
axially movably (not shown in the drawing) is a piston 9 of a lock-up 
clutch 10. On the periphery of the piston 9 is placed a friction lining 
11. 
To close the lock-up clutch 10, the oil space lying to the right of the 
piston 9--according to the drawing--is loaded with pressurized oil from a 
hydraulic line 12. The piston 9 moves to the left and the friction lining 
comes to abut on the torque converter housing 3. In this case, a through 
drive exists from the input unit 1 to the output shaft 8. 
If the frictional engagement connection should diminish, occurring because 
the oil pressure prevalent in the pressure space, lying to the right, is 
either lowered or, under mutual control, moved over to another hydraulic 
line 13, in the pressure space to the left of the piston 9, according to 
the drawing, a pressure is built up which moves the piston 9 to the right 
in an opening direction. The lock-up clutch 10 is then opened so that a 
differential speed (slip) prevails, between the impeller 4 and the turbine 
wheel 5. 
The hydraulic lines 12 and 13 are attached to a hydraulic control 14. 
Connected downstream of the hydrodynamic converter 2 is a reversing set in 
the form of a planetary gear 15. With the latter, the forward or reverse 
drive directions is switched by adequate switching components (clutch and 
brake). 
An output shaft 16 drives a continuously variable transmission 17 
(variator). The continuously variable transmission essentially comprises a 
primary pulley S1 and a secondary pulley S2, which are formed from tapered 
pulleys disposed in pairs and lodged between them is a belt drive member 
18. A primary cylinder 19 and a secondary cylinder 20 are attached, via 
hydraulic lines 21 and 22, to the hydraulic control 14. 
An output shaft 23 drives, via reduction steps 24 and 25 and a differential 
26, axle half shafts 27 and 28 of driven wheels 29 of a vehicle itself not 
shown. 
The continuously variable transmission 17 is controlled by an electronic 
transmission control 30 (EGS). The function of the transmission control is 
to adjust the ratio according to a preset operation strategy. To this end, 
the processing of a multiplicity of operation parameters is required. For 
example, a sensor 31 detects the input variable: actual throttle valve 
position .varies..sub.-- DK. With a sensor 32 the actual engine speed 
n.sub.-- Mot is detected. A sensor 33 detects the position of a control 
member 34 with which the driver of the vehicle communicates a power need 
(accelerator pedal wish FPW). A sensor 35 signals a manual driver contact. 
The enumeration of the possible operation parameters (input variables) is 
incomplete. The temperature of the hydraulic fluid, for example, is among 
said variables. This is detected by means of a sensor 36. For certain 
driving situations additional parameters have to be processed. To detect 
cornering, it is convenient to utilize the cross acceleration and/or wheel 
speed differences. This is done with another sensor 37. A sensor 38 
detects rises and drops. 
The speeds of the impeller 4, of the turbine wheel 5 and of the output 
shaft 8 are transmitted by sensors 39 and 40 to the electronic 
transmission control 30. 
With the aid of another sensor 41 the speed of rotation n.sub.13 S1 of the 
primary pulley is monitored. Another sensor 42 delivers the speed of 
rotation n.sub.-- S2 of the secondary pulley. The input variables are 
processed by the electronic transmission control to output variables 
having different purposes (information for indication of system states, 
control signals for actuators, etc.). A few output variables abut as input 
variables on the hydraulic control 14 in order to trigger the actuation of 
electromagnetic valves, for example, for adjustment of the primary and 
secondary pulleys or also for control of the lock-up clutch. 
Point of departure of the strategy for control of the lock-up clutch is the 
idea of regarding the hydrodynamic torque converter as a hydrodynamic 
variator serially mounted with the continuously variable transmission. 
Within the scope of a driving strategy, an operation point is preset. A 
preferred process for determining an operation point in a dynamic drive 
range is explained in the applicant's German patent application 196 00 
915.4 of Jan. 12, 1996. To that extent, reference will supplementarily be 
made to the statements contained there. Within the scope of this operation 
strategy, a reduction ratio iv.sub.-- soll or a speed of rotation of the 
primary pulley n.sub.-- S1.sub.-- soll is preset. To said presetting 
corresponds a presetting of the engine speed n.sub.-- Mot.sub.-- soll. 
When the lock-up clutch is opened, the engine operation point n.sub.-- Mot 
(regulating variable) directs itself to said drive strategy. Let it be 
supplementarily observed that the driving range, on one hand, is defined 
by the driving performance characteristic line FL.sub.-- characteristic 
line up to the characteristic line of the smallest ratio iv.sub.-- 
min.sub.-- characteristic line. 
With this general standard an operation of the lock-up clutch becomes 
possible with the following advantages: 
In the starting range, when the engine during gas emission runs into the 
range of the torque converter, the lock-up clutch becomes closed whereby 
are prevented in an overshoot of the engine above the value n.sub.-- 
S1.sub.-- soll of the speed of rotation of the primary pulley, preset by 
the driving strategy, and thus the negative gradient of speed of rotation 
of the engine resulting during the closing process. 
The theoretical value of the speed of rotation of the engine n.sub.-- 
Mot.sub.-- soll is controlled in the whole driving range by the standard 
of an engine speed of rotation n.sub.-- Mot. The regulating characteristic 
of the lock-up clutch regulator, especially its transition to the closed 
state (closing quality), is thus a function of the control of the engine 
theoretical speed of rotation n.sub.-- Mot.sub.-- soll. 
When the lock-up clutch is opened, a drop of the engine to coasting in the 
torque converter range (undershoot) is prevented. The drop would be felt 
as a disturbance by the driver because the operation point standard 
n.sub.-- S1.sub.-- soll or iv.sub.-- soll is abandoned by the sudden drop 
of the engine and the engine is substantially affected by the coasting 
characteristic of the torque converter. 
Because the engine speed of rotation n.sub.-- Mot is linked directly with 
the standard of an operation point, a flow rate of the oil 
pump--corresponding to the n.sub.-- S1.sub.-- soll speed of rotation--is 
produced when braking. Also, clearly assisted is the downshift in 
direction to LOW, whereby a substantial problem in connection with a 
continuously variable transmission is effectively obviated. In addition, 
too early an uncontrolled drop to idling speed is prevented. 
In the technical implementation two cases are to be basically 
differentiated: 
Case 1: 
The vehicle moves on flat ground or uphill; and 
Case 2: 
The vehicle moves downhill. 
Supplementarily the variogram (FIG. 3) outside range 1 delimited by the 
iv.sub.-- max--, iv.sub.-- min-- and LOW characteristic line and the range 
delimited by the LOW-- and OD--guide beam (overdrive) (designated here as 
range 2) must be differently treated. 
Depending on said cases and ranges 1 and 2, the standard of the operation 
point is implemented in different ways. The following definitions apply 
here: 
Regulating variable: 
n.sub.-- Mot 
Control variable: 
Range 1: n.sub.-- Mot.sub.-- soll 
Range 2: n.sub.-- Mot.sub.-- soll corresponding to the theoretical speed of 
rotation of the primary pulley n.sub.-- S1.sub.-- soll. 
The value n.sub.-- S1.sub.-- Soll is generated here from the product of the 
theoretical ratio iv.sub.-- soll and the speed of rotation of the 
secondary pulley n.sub.-- S2. 
Correcting variable: 
for the condition lock-up clutch-close: i.sub.-- drwk=p.sub.-- wk 
(correcting variable torque converter clutch pressure corresponding to 
p.sub.-- wk); and for the condition lock-up clutch open: i.sub.-- driv 
corresponding to the pressure in the primary cylinder P.sub.-- S1. 
Closing of the lock-up clutch: 
The standard of an engine theoretical speed of rotation n.sub.-- Mot.sub.-- 
soll is generated during a throttle valve position &lt;1.5% (not zero) in the 
ranges 1 and 2 as follows: 
Range 1: 
The applied characteristic lines iv.sub.-- min-- and iv.sub.-- max-- are 
converted by the speed of rotation of the secondary pulley n.sub.-- S2 to 
minimum engine speeds of rotation n.sub.-- Mot.sub.-- min or maximum 
engine speeds of rotation n.sub.-- Mot.sub.-- max and laid down in the 
electronic transmission control as characteristic lines. The ratios 
calculated to the left of the LOW guide beam n.sub.-- S2 thus assume 
values which are higher than the mechanical LOW ratio. In the actual drive 
range, for example, via fuzzy control equipment, an engine theoretical 
value of speed of rotation n.sub.-- Mot.sub.-- soll is generated which 
serves as the control of the engine speed of rotation n.sub.-- Mot. The 
standard of the engine theoretical speed of rotation n.sub.-- Mot.sub.-- 
soll must result from the parked state of the vehicle. The standard of the 
operation point n.sub.-- Mot.sub.-- soll cannot have discontinuity in the 
transition from the range 1 to the range 2 and is preferably combined with 
one another. 
Range 2: 
The standard of the ratio iv.sub.-- soll within the scope of the operation 
strategy is converted via the speed of rotation of the primary pulley 
n.sub.-- S1 directly to a theoretical engine speed of rotation n.sub.-- 
Mot.sub.-- soll and used as a control variable. n.sub.-- Mot.sub.-- soll 
follows out from the stationary state of the LOW characteristic line up to 
the intersection point with the iv.sub.-- min=n.sub.-- Mot.sub.-- min 
characteristic line. 
In a position of the throttle valve &lt;1.5% (zero) there apply, when driving 
on flat ground or uphill, the modes of operation described above are for 
the ranges 1 and 2. 
For driving downhill, the theoretical engine speed of rotation is preset 
according to the process explained above in relation to range 2. The 
lock-up clutch is closed at the synchronization point. By synchronization 
point it is understood that the point at which the value engine 
theoretical speed of rotation n.sub.-- Mot.sub.-- soll, formed from the 
product of the actual ratio and of the speed of rotation of the secondary 
pulley n.sub.-- S2, is higher or equal to the idling speed of rotation of 
the engine. Thereby is ensuring that the engine is rigidly coupled as 
early as possible with the adjustment of the continuously variable 
transmission in order to achieve the best possible engine braking effect. 
For opening the lock-up clutch the next process applies: 
When opening the lock-up clutch an overlapping gearshift takes place 
between the lock-up clutch and the continuously variable transmission. 
With the signal for opening the lock-up clutch, the ratio of the 
continuously variable transmission is used as a correcting variable in 
order further to keep the engine on the actual operation point which so 
far had been used for control of the ratio of the continuously variable 
transmission. This means: the standard of the ratio iv.sub.-- soll of the 
speed of rotation of the primary pulley n.sub.-- S1.sub.-- soll becomes 
the theoretical standard for the engine speed of rotation n.sub.-- 
Mot.sub.-- soll. The objective is to control, in an ideal way, the 
interference level of the torque converter by the continuously variable 
transmission. Here the opening of the lock-up clutch can be especially 
adapted in order to assist the regulator. As to the process of opening the 
lock-up clutch, no distinction is made regarding a dependence on the 
tractional resistance. 
The basic functions implemented in the electronic transmission control have 
top priority in relation to the demands on the operation state of the 
lock-up clutch independently of the operating strategy used. 
The main conditions are defined, according to the engagement and 
disengagement criteria, by the basic functions exclusively by the 
electronic transmission control independently of the operating strategy 
chosen. The engagement and disengagement criteria must necessarily be 
satisfied so that the lock-up clutch can be controlled by a strategy. The 
information of whether an engagement or disengagement of the lock-up 
clutch is desired is communicated to the basic functions of the electronic 
transmission control via an adequate flag Z.sub.-- WK. 
Engagement criteria: 
The lock-up clutch is engaged--Z.sub.-- WK engaged--, when: 
n.sub.-- mot.sub.-- soll&gt;1 200 l/min AND 
v.sub.-- fzg &gt;3 km/h OR 
n.sub.-- S2&gt;150 l/min AND 
C.sub.-- getr&gt;5.degree. C. AND 
brake=0 (that is, the brake is not actuated) AND 
Direction of rotation of secondary pulley n.sub.-- S2=positive. 
Wherein v.sub.-- fzg means the vehicle speed and C.sub.-- getr, the 
transmission temperature. 
Disengagement criteria: 
For the disengagement of the lock-up clutch (open)--Z.sub.-- WK=disengage, 
there apply: 
Z.sub.-- WK=disengage when n.sub.-- Mot&lt;1 200 l/min OR 
Z.sub.-- blr=1 (blocking wheels detected or ABS active). 
The closing point for closing the lock-up clutch is established by taking 
into account particular drive states. In determining the closing point in 
time, avoided must be drive states in which a closed lock-up clutch: 
does not seem necessary (for example, in case of small loads or low vehicle 
speeds like shunting) or 
not convenient for reasons of comfort (for example, low driving speed and 
high ratio or city traffic at the low speed range where very often 
traction and coasting changes are to be expected). 
For determining the point in time, a simple algorithm can serve in the 
simplest case. The state flag Z.sub.-- WK is set to engage as a function 
of the accelerator pedal wish (FPW) and of the vehicle speed (v.sub.-- 
fzg) when the operation point is above a characteristic line. From FIG. 2 
tentatively seen is the curve of said characteristic line. At a vehicle 
speed v.sub.-- fzg of zero or near zero said ranges are separated from 
each other by about 20% in an accelerator pedal wish. At a vehicle speed 
of v.sub.-- fzg&gt;about 50 km/h, the signal of lock-up clutch engage is 
always set. 
Instead of a simple algorithm, an expanded algorithm can be used. Suitable 
for this in the first place is a control system according to fuzzy 
equipment by which the closing point in time can be determined according 
to several parameters. Preferably, adequate here are the variables: 
accelerator pedal wish, vehicle speed, ratio and start-up wish. 
The point in time for opening the lock-up clutch can also be determined 
according to a simple algorithm. Here the lock-up clutch is opened, via a 
characteristic line, by which the vehicle speed and the vehicle 
deceleration are related to each other. Z.sub.-- WKA=f(v.sub.-- fzg,a) 
The time for opening the lock-up clutch can also be determine within the 
scope of an expanded algorithm. The lock-up clutch here is opened 
according to a stopping and braking wish. The braking and stopping wish, 
in turn, are generated by fuzzy control equipments. An adaptation for 
finding out the adequate opening time can be carrier out by field tests. 
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Reference numerals 
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1 input unit 
2 torque converter 
3 torque converter housing 
4 impeller 
5 turbine wheel 
6 stator 
7 oil pump 
8 output shaft 
9 piston 
10 lock-up clutch 
11 friction lining 
12 hydraulic line 
13 hydraulic line 
14 hydraulic control 
15 planetary gear 
16 output shaft 
17 continuously variable transmission 
18 belt drive member 
19 primary cylinder 
20 secondary cylinder 
21 hydraulic line 
22 hydraulic line 
23 output shaft 
24 reduction step 
25 reduction step 
26 differential 
27 axle half shaft 
28 axle half shaft 
29 wheels 
30 transmission control 
31 sensors 
32 sensors 
33 sensors 
34 control member 
35 sensors 
36 sensors 
37 sensors 
38 sensors 
39 sensors 
40 sensors 
41 sensors 
42 sensors 
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