Method and System for Electronic Simulation of a Freewheel

The present invention relates to automatic transmissions and in particular to automatic transmissions wherein friction coupling means are provided for transmitting torque from either the engine or a torque converter to the differential and the wheels. In particular, this invention relates to automatic transmissions wherein engine braking, that is transmission of torque in the direction from the wheels to the engine, is to be prevented.

Automatic transmissions operative in a plurality of gear ratios are known in which at a particular gear ratio it is desired to transmit torque in one direction only. For example, when the lowest gear is engaged, engine braking is prevented in some known transmissions since this would cause a jolt if the driver removed his foot from the gas pedal. The freewheel operation improves the quality of the gear shift for particular types of transmissions and increases the driving comfort.

It is a disadvantage of the known freewheel transmissions that the cost of the transmissions is greatly increased.

It is an object of the present invention to furnish an automatic transmission which does not have a mechanical freewheel but wherein the freewheel operation is simulated electronically. The cost of the equipment is to be reduced substantially thereby.

In the automatic transmissions in accordance with the present invention the torque transmitted by a friction coupling means is continuously computed and a "release" signal is furnished when the torque is substantially equal to zero. The friction coupling means is released in response to the "release" signal.

In preferred embodiments, the computation of the torque transmitted by the friction coupling means requires only the sensing of parameters which are normally sensed in any case in a motor vehicle having an automatic transmission.

FIG. 1 is a schematic diagram illustrating a system in accordance with the present invention; and

FIG. 2 shows a flow chart for the microprocessor of FIG. 1,

FIGS. 3A and 3B show the transfer characteristic of a torque conver and friction element, respectively.

In order to simulate the operation of a freewheel electronically, it is necessary to determine the torque in the friction elements of the transmission, for example in the clutches or in the brake bands, continually and to release the corresponding friction element when the torque transmitted thereby passes through zero. The difficulty in solving this problem lies in the fact that the torque transmitted by a fully engaged friction element during gear shift cannot be measured because another friction element is slipping at the same time. To use additional measurement instruments to measure the torque transmitted by a friction element would increase the cost of the automatic transmission to such an extent that the economical advantages of the electronic simulation would be nullified. The method and system in accordance with the present invention thus utilize only operating parameters of the drive system which can be easily sensed and which are, in general, present in the motor vehicle in any case. The operating parameters concerned are the input and output speeds n.sub.1, n.sub.2 of the hydraulic torque converter which precedes the automatic transmission and the output speed n.sub.3 of the automatic transmission. Further, it is assumed that the torque transmitted by a slipping friction element can be determined from its control current.

In FIG. 1, an engine 10 is coupled to an output shaft 11 which also constitutes the input shaft of a torque converter 12. The output shaft 13 of torque converter 12 is connected to an automatic transmission 14. The output shaft of automatic transmission 15 is connected to the driven portion, namely a differential 17 and wheels 18, possibly through a torsiometer 16. A speed sensor 19 senses the speed of shaft 11. Speed sensors 20 and 21 respectively sense the rotary speeds of shafts 13 and 15. The outputs of speed sensors 19, 20, and 21, as well as the output of torsiometer 16 if present, are connected to inputs of a computer 22 which, in a preferred embodiment, is a microprocessor. The microprocessor also includes a read only memory 23. Further, computer stage 22 furnishes control signals for friction elements K.sub.x, k.sub.y of automatic transmission 14. Gear ratios 1:u.sub.x and 1:u.sub.y are associated with friction elements K.sub.x and K.sub.y, respectively. The gear stages are designated by reference numerals 141 and 142 respectively.

Input shaft 11 transmits a torque M.sub.1 at a speed n.sub.1 ; output shaft 13 of torque converter 12 transmits a torque M.sub.2 at a rotary speed n.sub.2. The torque M.sub.2 is split into two partial torques M.sub.x and M.sub.y in automatic transmission 14. Torques M.sub.x and M.sub.y are transmitted by friction elements k.sub.x and k.sub.y, respectively. The currents controlling friction elements k.sub.x and k.sub.y are denoted by i.sub.x and i.sub.y respectively. Finally, output shaft 15 of automatic transmission 14 transmits an output torque M.sub.3 at a rotary speed n.sub.3, causing wheels 18 of the motor vehicle to have an angular velocity .omega..sub.R.

According to the invention, the electronic simulation of a freewheel operation is carried out by first determining the variation with respect to time of the torques M.sub.x and M.sub.y transmitted by the friction elements and then releasing the respective friction elements at the moment at which the transmitted torque passes through zero.

For a slipping friction elements K.sub.x, K.sub.y, the time at which the transmitted torque passes through zero is determined by comparing the input and output speeds of the friction element to each other and furnishing a "release" signal when the two are equal. The input speed of friction elements K.sub.x, K.sub.y is equal to n.sub.2, the output speed is computed by measuring speed n.sub.3 and adjusting it by the respective gear ratios u.sub.x, u.sub.y.

When one of the friction elements is slipping while the other is fully engaged, it is difficult to determine the torque transmitted by the engaged element as a function of time, since no corresponding measuring apparatus is normally present in automatic transmission 14. It is, however, in accordance with the present invention, possible to determine the torque transmitted in the fully engaged friction element from other available operating parameters. The following equation holds for the output torque M.sub.2 of hydraulic torque converter 12:

The output torque M.sub.3 is given by the equation:

Since the ratios u.sub.x and u.sub.y are known, the torque transmitted by the fully engaged friction element can be determined if two of the three torques M.sub.2, M.sub.3 and the torque transmitted by the slipping friction element are known.

The torque transmitted by the slipping friction element can be computed in a very simple fashion from the control current i and the characteristic curve of the friction element, that is the curve of M=f(i), shown in FIG. 3B. The output torque of torque converter 12, namely torque M.sub.2, can be determined from the converter input and output speeds n.sub.1, n.sub.2. First, the input torque M.sub.1 is computed according to the equation:

where C1 is a constant and .phi.=f(n.sub.2 /n.sub.1) is a characteristic function of the torque converter (FIG. 3A). Once the input torque M.sub.1 is known, the output torque M.sub.2 in a first region

(where P is a fraction less than 1 whose exact value depends upon the particular system) can be determined in accordance with the relationship:

In a second speed region

the output torque M.sub.2 is computed according to the relationship:

It is particularly advantageous to store the characteristic curves such as the characteristic curve of the torque converter mentioned above and the torque transmission characteristic curves of the friction elements in a storage means which, preferably, is a read only memory.

The output torque M.sub.3 can be determined by means of torsiometer 16. A measuring instrument suitable for use as such a torsiometer is described in German DE-OS 2727556. Such a torsiometer converts torque M.sub.3 into a mechanical rotation and thence into an electrical signal.

A further possibility to determine torque M.sub.3 is to first calculate torque M.sub.3 from torque M.sub.2 and the applicable gear ratio at the moment at which the gear shift is initiated. This can be done because at the time the gear shift is initiated only one of the clutches transmits torque. The road resistance R.sub.W relative to wheels 18 at the beginning of a gear shift then is calculated according to the equation:

where .THETA..sub.F is the moment of inertia of the vehicle relative to wheels 18 while .omega..sub.R is the first derivative with respect to time of the circumferential velocity of wheels 18. The latter can of course be determined simply from the rotational velocity n.sub.3. It is then assumed that the road resistance R.sub.w of the vehicle relative to wheels 18 does not change in the short period of time involved in the gear shift (approximately one-half second). With this assumption, the torque M.sub.3 during gear shift can be derived for changing .omega..sub.R from the equation:

The required computations in accordance with the above equations are carried out by microprocessor 22 operating in conjunction with read only memory 23. As mentioned above, the computer stage or microprocessor 22 furnishes the release signals which cause friction elements K.sub.x, K.sub.y to be disengaged when the torque transmitted by them passes through zero.

The following values apply for a preferred embodiment:

u.sub.x =1,478

u.sub.y =1

C1=4.10.sup.-5 Nm min.sup.2

Phi=f(n.sub.2 /n.sub.1)

p=0.88

.THETA..sub.F =12.64 Nm s.sup.2

K.sub.1 =1

K.sub.2 =2

K.sub.3 =1,136

The flow chart for microprocessor 22, which describes the program stored in storage means 23 in addition to the various characteristic curves is shown on FIG. 2. It is self-explanatory.

Various changes and modifications may be made within the scope of the inventive concept.