# Patent

## Title
Method for reaching a neutral position of a gearshift fork in a shift transmission

## Abstract

A method for reaching a neutral position N located between two end positions of a gearshift fork in a shift transmission in a motor vehicle from a starting position of the gearshift fork. The method allows finding the neutral position of the gearshift fork without a sensor determining the position of the gearshift fork or in the alternative in case such the sensor is defect.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This is a continuation application of application Ser. No. 12/253,801 filed Oct. 17, 2008, now U.S. Pat. No. 8,346,445 and claiming the priority right from the European patent application EP 07119124.1 that was filed on Oct. 23, 2007, the content of which is herewith incorporated in its entirety by reference.


BACKGROUND OF THE INVENTION
The invention relates to a method for reaching a neutral position located between two end positions of a gearshift fork in a shift transmission of a motor vehicle.
From the EP 1 300 614 A2 a method for finding a neutral position of a gearshift fork is known. The gearshift fork can be moved between two end positions in two directions. If the gearshift fork is positioned in one of its end positions, a gear of the shift transmission is engaged. For engaging a gear, starting from the neutral position, the gearshift fork passes through a synchronization point related to the particular gear, wherein the rotational speed of an input shaft of the shift transmission is synchronized with the rotational speed of an output shaft. After passing beyond the synchronization point, a positive interlocking is performed between a loose wheel that relates to the respective gear and a shaft (input shaft or output shaft) supporting the loose wheel. By means of this positive interlocking between the loose wheel and the respective shaft the gear is engaged.
According to the EP 1 300 614 A2 a latching force acts upon the gearshift fork or the shifting rod that is part of the gearshift fork, said latching force urging the gearshift fork in the direction of the neutral position when closer to the neutral position, and away from the neutral position when the gearshift fork is more remote from the neutral position. The method according to the EP 1 300 614 A2 focuses on finding the neutral position when the gearshift fork is already close to the neutral position and the latching force pushes the gearshift fork in the direction of the neutral position. For finding this approximate neutral position the EP 1 300 614 A2 suggests to determine the two end positions of the gearshift fork or the two end positions of a shifting sleeve that are moved by means of the gearshift fork and to determine by calculating the arithmetic mean value of the distance between the end positions the neutral position that is typically provided in the middle between the end positions. This, however, requires at least one sensor for determining the position or the position change of the gearshift fork.
In case of failure of the sensor S4 in determining the position, the method according to the EP 1 300 614 A2 can no longer be performed. It is no longer possible to find the neutral position reliably.
SUMMARY OF THE INVENTION
It is an object of the invention to create a method allowing measures that are as simple as possible to find the neutral position reliably.
This is object is achieved, in a motor vehicle, by a method for failsafe disengaging of a gear and reaching a neutral position located between a first end position and a second end position of a gearshift fork in a shift transmission from a starting position of the gearshift fork where a first gear is engaged, said transmission being adapted to move the gearshift fork between the first end position where a first gear is engaged and a second end position where a second gear is engaged in a first direction and in a second direction; and synchronize a rotational speed of an input shaft of the shift transmission with a rotational speed of an output shaft of the shift transmission at a respective first synchronization point that is related to the first gear and a second synchronization point that is related to the second gear; said method comprising the steps of:

    
    
        applying a latching force to the gearshift fork that pushes the gearshift fork in a direction of the neutral position when located in a first position, and away from the neutral position when the gearshift fork is located in a second position that is more remote from the neutral position than the first position, wherein the latching force preferably reaches a maximum in the neutral position;
        determining a first time period Δt1 that is high enough so that the gearshift fork is pressed from the engaged gear into a section where the latching force starts urging the gearshift fork into the neutral position;
        applying in a step S1 from a starting position of the gearshift fork where the first gear is engaged over the first time period Δt1 a first actuating force to the gearshift fork that is higher than the maximum latching force, said first actuating force acting in one of the first and second directions and pushing the gear shift fork from the starting position where the first gear is engaged within the first time period Δt1 into a section where the latching force starts urging the gearshift fork into the neutral position; and
        terminating in a step S2 the first actuating force after the first time period Δt1 has expired to allow the latching force to urge the gearshift fork into the neutral position.
    
    


DETAILED DESCRIPTION OF THE INVENTION
The method according to claim 1 comprises two steps. At first, in a step S1, the gearshift fork is subjected to a first actuating force F1 acting in one of two possible directions, wherein the actuating force F1 is higher than the maximum latching force. In step S2 the step S1 is terminated when a certain first time period Δt1 is exceeded.
The method according to the invention can be performed without any sensor for determining the position of the gearshift fork. This means, when the position sensor in a shift transmission fails, the neutral position can still be found reliably.
As already mentioned above, depending on the position of the gearshift fork the latching force acts in two different directions. Close to the neutral position latching is provided, urging the gearshift fork in the direction of the neutral position. The latching mechanism comprises preferably a latching curve and a latching body, for instance in the form of a spring biased ball or roller, wherein the latching curve is preferably designed so as to provide a maximum latching force in the neutral position.
During the latching action, the latching body is pressed with a particular force against the latching curve. The latching force is therefore the force resulting from the interaction between the latching body and the latching curve in the moving direction of the gearshift fork and onto that gearshift fork.
According to a preferred embodiment the method starts from a starting position in which one gear of the shift transmission is engaged. The step S1 is preceded by a step S0. In step S0, a clutch of the motor vehicle is opened separating the transmission or a partial transmission that relates to the engaged gear from the power of the motor vehicle. Further, it is determined in step S0 that the actuating force F1 acts in the direction of the neutral position. Whether a gear is engaged and if so what gear is engaged can be determined by measuring the rotational speeds of the input shaft and the output shaft. If these rotational speeds (during a certain period of time) are in a constant ratio, knowing the transmission ratios for each gear in the shift transmission, the engaged gear can be identified reliably. If the engaged gear is identified, also the direction can be identified clearly into which gearshift fork has to be moved for reaching the neutral position starting from this engaged gear.
In a preferred embodiment in step 2 only the termination criterion a) is considered. The other two termination criteria b) and c) are not checked. According to the termination criterion a) the gearshift fork is only subjected over a time period Δt1 by the force F1. The time period Δt1 is preferably chosen to be high enough so that the gearshift fork is pressed from the engaged gear into a section where the latching force starts urging the gearshift fork into the neutral position. If the latching force is higher than the frictional force that has to be overcome, the gearshift fork reaches the neutral position by means of the latching action.
After step S2 the gearshift fork can be subjected over a second time period Δt2 in the same direction with a second actuating force F2 that is smaller than the maximum latching force. By limiting the force F2 it is achieved that the gearshift fork, starting from the engaged gear, is not moved beyond the neutral position. This is achieved when the latching force assumes a maximum value in the neutral position, wherein a movement of the gearshift fork from the neutral position is counteracted by the latching force.
In the alternative, in step S2, the gearshift fork is subjected in the same direction over a third time period Δt3 to a third actuating force F3 that is lower than the actuating force F1 and higher than the maximum latching force. The time period Δt3 has to be limited. The time period Δt3 is shorter than the time period that is required for a synchronization of the rotational speeds of the input and output shafts, namely for the gear that is located opposite the gear that was engaged at the beginning of the method. According to this alternative method, if the frictional forces are not too high, due to the force F3, the gearshift fork is moved starting from the engaged gear beyond the neutral position and is stopped at the synchronization point of the gear on the opposite side due to the synchronization starting at this point. By means of limiting the time period Δt3 it is achieved that the gearshift fork is not moved beyond the synchronization point of the opposite gear, but, as far as the latching force in that synchronization point is directed in the direction of the neutral position, pushed back into the neutral position by that latching force. In this connection it is pointed out that the latching forces in the area of the neutral position and in the area of the synchronization point are substantially higher than the occurring frictional forces, wherein it has to be considered that the frictional forces are strongly dependent on the temperature in the transmission.
Further, it is possible that in step S2 only the termination criterion b) is taken into consideration. This is based on the determination that a strong change in the rotational speed of the input shaft in relation to the rotational speed of the output shaft occurs only when the gearshift fork is moved starting from an engaged gear beyond the neutral position to the synchronization point of the opposite gear and at this point there is an acceleration or deceleration due to the starting synchronization of the input shaft.
Accordingly, in step S2 the gearshift fork is subjected over a fourth time period Δt4 in the opposite direction with a fourth actuating force F4 that is smaller than the maximum latching force. The force F4 is therein directed towards the neutral position. Limiting the maximum latching force achieves that the force F4 of the gearshift fork does not push said gearshift fork beyond the neutral position, since the maximum of the latching force is reached in the neutral position.
In the alternative, in step. S2 the gearshift fork can be subjected in the opposite direction over a fifth time period Δt5 to a fifth actuating force F5 that is lower than the actuating force F1 and higher than the maximum latching force. Subjecting the force F5 ends after the time period Δt5 that is shorter than the time period that is required for a synchronization of the rotational speeds of the input and output shafts. By limiting the time period of subjection to force F5 it is achieved that after reversing the direction of the gearshift fork it is not positioned for too long a time period at the synchronization point of the gear that had just been disengaged. Also in this case appropriately dimensioned latching action accomplishes that the latching force slides the gearshift fork from the synchronization point of the initially engaged gear into the neutral position.
Further, it can be provided that in the step S2 only the termination criteria c) is considered wherein it is checked whether the input shaft and the output shaft are rotating in synchronization with each other. If such a synchronized rotation is no longer present, since due to the missing positive interlocking between the output shaft and the input shaft of the rotational speed of the input shaft decreases, after step S2 the gearshift fork is subjected over a sixth time period Δt6 in the same direction with a sixth actuating force F6 that is smaller than the maximum latching force. In this case, it is taken advantage that the non-synchronized rotation between the output shaft and input shaft is an indicator for the fact that the gear is no longer engaged and therefore the gearshift fork has already moved away from the end position that is related to the gear. The force F6 is therein sufficient to move the gearshift fork further in the direction of the neutral position.
After expiration of the time period Δt6 for the actuating force F6 the direction of that actuating force F6 is reversed likewise only to act over the time period Δt6. This reversing of the direction can be repeated several times, wherein the absolute value of the force F6 and/or the time period Δt6 can be reduced successively. In this embodiment the gearshift fork is moved in the area of the neutral position back and forth until the method finally ends and the gearshift fork is finally positioned safely in the neutral position.
According to a preferred embodiment, all three termination criteria are checked, and depending on the criterion that has been met, different procedural steps are started. Preceding the step S1 the step S0′ can be performed. In this step S0′ it is first identified whether a gear is engaged, wherein in case of a successful identification the actuating force F1 in the direction of the neutral position is established, and after an unsuccessful identification the actuating force F1 acts in the direction of a higher gear and wherein the clutch of the motor vehicle is opened, separating the transmission or a transmission part that relates to the identified or a higher gear from the power of the motor vehicle.
After the steps S0′, S1 and S2 further steps S3 and S4 can follow. In step S3, after start of the termination criteria b) is met, the actuating force F1 is turned off, while when meeting the termination criteria a) or c) the gearshift fork is subjected in the same direction to a seventh actuating force F7 that is lower than the actuating force F1, wherein subjecting to the actuating force F7 is turned off after a seventh time period Δt7 or ends when a fast change of the rotational speed of the input shaft occurs. In the step S4, if the actuating force F7 has not been turned off due to a fast change of the rotational speed of the input shaft, the method is repeated in the opposite direction of movement with the steps S1-S4 if the rotational speed ratios remain the same. If the rotational speed remains the same, apparently the gearshift fork was pushed in the direction of the engaged gear so that the gear has not been disengaged. In this case, the chosen direction of the forces F1 and F7 was wrong and is reversed in step S4. According to a preferred embodiment, the moving speed of the gearshift fork is limited. In case of hydraulic actuators, this can for instance be achieved by flow resistance allowing a particular maximum volume flow rate. In case of electric actuators only a maximum voltage or actuating frequency can be provided.
The values for the time periods Δt1 to Δt7, the rotational speed changes, further rotational speed values as well as the values for the actuating forces F1 to F7 can be determined depending on the temperature and the presumably engaged gear.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described in more detail by referring to the embodiments shown in the drawings. The drawings show in:
FIG. 1 schematically a latching curve and a latching body;
FIG. 2 the diagram of the latching force over a gearshift fork path;
FIG. 3 the gearshift fork path and the actuating force over time in a first embodiment;
FIG. 4 the gearshift fork path, actuating force and rotational speed of an input shaft over time in a second embodiment;
FIG. 5 the gearshift fork path, actuating force and rotational speed of an input shaft and an output shaft over time in a third embodiment;
FIG. 6 a flow diagram of a fourth embodiment;
FIG. 7 the gearshift fork path, actuating force and rotational speed of an input shaft over time in a fifth embodiment;
FIG. 8 the gearshift fork path, actuating force and rotational speed of the input shaft over time in a sixth embodiment; and
FIG. 9 the gearshift fork path, actuating force and rotational speed of the input shaft over time in a seventh embodiment.



DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a latching mechanism 1, comprising a ball-shaped latching body 2 and a latching curve 3. The latching curve 3 is part of a gearshift fork 4 as part of a shift transmission in a motor vehicle. In particular, the latching curve 3 can be provided on an enclosing outer surface of a shifting rod that is part of the gearshift fork 4 and can be disposed in relation to a transmission that is not shown so that it can be moved into opposing directions L, R. The directions L, R are linear in FIG. 1, but can also be of a circular shape, i.e. result from a rotational movement.
The latching body 2 is pressed by a force against the latching curve 3 as depicted by the arrow demoted 5. The force pressing the latching body 2 against the latching curve 3 can for example be a spring force. The latching curve 3 should be symmetrical in relation to a neutral position N.
If the gearshift fork 4 and therefore the latching curve 3 are shifted in direction L or in direction R, a latching force FR counteracts this movement due to the latching body being biased by a spring force. The diagram shows the latching force FR over the shifting force path s as is shown in FIG. 2 for a range reaching from the neutral position N up to one of the end positions of the shifting force, in which the gear G is engaged. In FIG. 1, 2 gears G1 and G2 are disposed on opposite ends.
By means of the shifting rod 4 a shifting sleeve can be moved along a shaft comprising two loose wheels that are disposed in an axial distance with respect to each other. Each of the loose wheels comprises teeth that can be brought into engagement with the shifting sleeve that rotates together with the shaft if the loose wheel and the shaft rotate with the same rotational speed. This synchronization of the shaft and the loose wheel is achieved by means of a synchronizing unit. Each loose wheel can be provided for one gear. If the gear is in engagement, the teeth of the shifting sleeve and of the respective loose wheel are in engagement with each other.
In the neutral position N, i.e. in that position of the latching curve 3 in relation to the latching body 2 as shown in FIG. 1, the latching force FR comprises a maximum that is denoted with FR, max. This force FR, max keeps the gearshift fork 4 in the neutral position N as long as no higher force acts into one of the directions L or R. In the neutral position N the latching body stands at two points 6, 7 in contact with the latching curve 3 (FIG. 1). In all other positions, a one-point contact between the latching body 2 and the latching curve 3 is established.
As shown in FIG. 2, the latching force FR, starting from the neutral position N, up to a reverse point, is positive. This means that a movement of the gearshift fork 4 from its neutral position is counteracted by the force FR. Due to this force, a positive latching force FR presses the gearshift fork 4 in direction of the neutral position when it is in the range between the neutral position N and the reverse point U.
Beyond the reverse point U the latching force FR is negative. This results in that beyond the reverse point U the gearshift fork is forced away from its neutral position. The reverse point U in FIG. 2 corresponds with the reverse point U depicted in FIG. 1 at the local maximum of the latching curve 3.
The range of movement of the shifting rod 4 is limited by two end positions in which one of the gears G1, G2 are engaged, respectively. If the gearshift fork 4 is moved in direction L into the left end position, a gear G1 should be engaged. Correspondingly, in the opposite direction R, an opposite gear G2 should be engaged when the other end position is reached.
When moving in the direction of a gear G, at first a synchronizing point Syn is reached. In this synchronizing point the rotational speed of the loose wheel of the gear that is to be getting engaged is synchronized with the rotational speed of the shaft. After synchronization, a positive interlocking between the teeth of the shifting sleeve and of the loose wheel is established, as denoted in FIG. 2 with “Dock-in”. After the teeth have come into engagement, the shifting rod is moved further so as to finally reaching its end position in which the gear is fully engaged.
In the following it is assumed that the two loose wheels are supported on an output shaft of the transmission and stand in engagement with fixed wheels on an input shaft of the transmission. For establishing a positive interlocking between the shifting sleeve and one of the loose wheels, the rotational speeds of the input shaft and the output shaft have to be synchronized.
By referring to the FIGS. 3-9 embodiments of the invention are discussed as to how the gearshift fork 4 can be shifted into a desired position. The gearshift fork 4 is operated by an actuator that has not been shown here, and exerts an actuating force on the gearshift fork 4. The actuating force over time is shown according to a first embodiment in the lower part of FIG. 3. Starting point of the process is assumed to be the engaged gear G1 (see upper part of FIG. 3). For moving from the engaged gear G1 into the neutral position N, for a predetermined first period of time Δt1 a first actuating force F1 is exerted on to the gearshift fork, pressing the gearshift fork 4 in direction of the neutral position N. Subjecting with the force F1 results in that the gearshift fork passes a synchronization point Syn1 that relates to gear 1. Thereafter, the gearshift fork is subjected to a second actuating force F2 acting into the same direction as the force F1, but is smaller than the maximum latching force FR, max. At this point in time, two forces act on the gearshift fork 4, namely on the one hand the latching force FR according to the diagram shown in FIG. 2, as well as the actuating force F2. Both of these forces are counteracted by frictional forces. The sum of the actuating force F2 and the latching force FR accomplishes that the gearshift fork is brought into the neutral position N. After expiration of the second time period Δt2 the actuating force F2 is turned off. When the gearshift fork has reached its neutral position and the latching body 2 contacts at the points 6, 7 the latching curve 3, a further movement of the gearshift fork 4 in direction of the actuating force F2 is now counteracted by the latching force FR, namely by the maximum latching force FR, max. Since the actuating force F2 is smaller than the maximum latching force FR,max, the gearshift fork is not moved beyond the neutral position N. In the upper part of FIG. 3, additional, generally reachable positions for the shifting rod 4 are shown. This is a gear G2 that is located on the opposite end in relation to gear G1, and its respective synchronizing point Syn2. However, these two positions are not established in the process according to FIG. 3.
The process according to FIG. 1 also assumes the engaged gear G1 as the starting point. Shown are the diagrams of the shifting rod path s, the actuating force F and the rotational speed of the input shaft.
For shifting the gearshift fork 4 from this point into the neutral position N, the gearshift fork is subjected to a fourth actuating force F4 in direction of the neutral position N that is higher than the maximum latching force FR, max. The turning off criteria for the actuating force F4 is exceeding a limit value for the gradient of the rotational speed nE of the input shaft. A fast decrease in the rotational speed nE (denoted by the reference numeral 8) when starting from the gear G1 indicates that the synchronization point Syn2 of the opposite gear G2 is reached (see point 9), wherein the gear G2 is a higher gear compared to gear G1. As it can be gathered from the diagram of the gearshift fork path s over time (upper part of FIG. 4), the gearshift fork passes through the neutral position and is moved up to the synchronization point Syn2 at which due to the start of the synchronization action a strong reduction of the rotational speed nE of the input shaft results. After reaching the respective turning off criteria, the gearshift fork is subjected in opposite direction to a fifth actuating force F5 that is smaller than the maximum latching force FR, max. After a certain fifth time period Δt5 the actuating force F5 is turned off. Due to the actuating force F5 and the latching force FR the gearshift fork is pushed safely back into the neutral position N.
The diagram shown in FIG. 5 that is similar to FIG. 4 demonstrates a further embodiment. Also here, the starting point is the engaged gear G1. The gearshift fork 4 is pushed by a force F1 in direction of the neutral position N. This opens the positive interlocking between the teeth of the shifting sleeve and a loose wheel, so that the rotational speed nE of the input shaft slightly decreases due to friction. If the relation between the rotational speeds of the input shaft and the output shaft exceeds a certain threshold of value, the actuating force F1 is turned off and a new actuating force, namely the sixth actuating force F6 is established. The actuating force F6, however, is lower than the maximum latching force FR, max. The actuating force F6 accomplishes that the gearshift fork is pushed further into the neutral position N. Due to dynamic effects it might happen that the gearshift fork is moved beyond the neutral position N, as this is demonstrated by the diagram showing the shifting rod path over time.
After expiration of a sixth time period Δt6 the actuating force F6 is reversed and is also established for a limited time period. This reverse of the direction of the force happens several times so that the neutral position is reached safely.
FIG. 6 shows a flow diagram of a further embodiment. After the start 50 it is determined at 51 whether the input shaft and the output shaft are rotating. If so, by the knowledge about the gear transmission ratio of each gear the particular gear that is currently engaged can be determined. This is achieved in the step denoted 52. In addition, the direction of movement is determined for the gearshift fork. Following the step 52 is a step 53, in which the clutch is opened and wherein the transmission or the partial transmission that relates to the respective gear is separated from the power of the motor vehicle. Thereafter, a shifting rod is moved by a force F1 that is higher than the maximum latching force (compare step 54). When subjected to the force F1, continuously, three determinations 55 to 57 are made.
In determinations 55 it is checked whether the gradient of the rotational speed of the input shaft exceeds a certain threshold of value or whether the rotational speed of the input shaft comes close to a rotational speed the input shaft assumes after synchronization with the opposite gear. If a high gradient of the rotational speed nE is determined, the shifting rod was moved starting from the engaged gear beyond the neutral position N and is at the synchronization point of the opposite gear. The force F1 is then turned off at a step 58 and the process is terminated (compare step 59). Since the shifting rod is a position in the synchronization point of the opposite gear, the shifting rod, according to the latching force the diagram according to FIG. 2, is pushed into the neutral position N.
If in determination 55 the established threshold value for the gradient of the rotational speed of the input shaft is not exceeded, in the subsequent determinations 56 it is checked whether the input shaft is still synchronized with the output shaft. If such a synchronized rotation is no longer established, a new seventh actuating force F7 is established in step 60 that is smaller than the maximum latching force FR, max.
Step 60 is also reached when a predetermined period of time Δt1 expired during which the shifting rod is subjected to force F1 (compare determinations 57). While subjected to the actuating force F7 it is checked by the determinations 61 and 62 whether the rotational speed of the input shaft comes close to a certain extent to the rotational speed that would be established after synchronization of the opposite gear G2. If this is the case, the gearshift fork was moved beyond the neutral position N up to the opposite synchronization point and the actuating force is turned off in step 58.
If during the determination 62 a seventh time period Δt7 has expired, it is checked in determination 63 whether the rotational speed ratio between input shaft and output shaft as determined during step 52 has changed. If that is the case, likewise the actuating force is turned off (compare steps 58, 59).
If determination of the rotational speeds of the input shaft and the output shaft in determination 51 was not possible and therefore it was impossible to determine the engaged gear, a direction for movement of the shifting rod in direction of the higher gear is determined (step 64). With this direction of movement that had been determined in step 64 the process is run starting with a step 53 up to the end denoted 59. In this case, the chosen direction of movement of the shifting rod might be wrong. For example, the shifting rod could be pushed in direction of a gear that has already been engaged in the process shown in FIG. 6, without the prospect of reaching the neutral position N. Determination 63 is provided for identifying such a case and for making sure in step 65 that the direction of movement for the shifting rod is reversed. Therefore, it is possible by running through the steps 53 up to the end 59 to reach the neutral position N.
The embodiment shown in FIG. 7 provides a method by means of which in case of a non-moving motor vehicle the neutral position can be reached safely. For accomplishing this, the clutch is slightly closed while the vehicle break is activated, so that the input shaft is subjected to a low torque. Thereafter, the shifting rod is subjected to a force F1 acting into one of the directions L, R. If no change in the rotational speed of the input shaft results after a certain time, the direction of the force F1 is reversed. In this case, the input shaft could not receive any rotational speed, since the engaged gear established a positive interlocking with the non-rotating output shaft. Since now the actuating force is reversed and does now act into a different direction, i.e. into the direction of the neutral position N, the gear that had been engaged so far is disengaged, resulting in that due to the still slightly closed clutch the input shaft rotates increasingly faster. If the change in the rotational speed nE of the input shaft is determined, the force F1 is reduced to an eighth force F8 that is smaller than the maximum latching force FR, max. If then, as shown in the diagram of the shifting rod path over time, the opposite synchronization point Syn2 is reached, the force F8 is reversed. The reaching of the synchronization point Syn2 is determined by a decrease in the rotational speed nE of the input shaft. Finally, after a certain period of time, the force F8 can be turned off that was chosen such so that until then the shifting rod was able to safely reach the neutral position N.
In contrast to the embodiments discussed above the embodiment shown in FIG. 8 assumes that no gear has been engaged. After opening the clutch the shifting rod is moved into any direction by force F1 that is higher than the maximum latching force FR,max. If the ratio of the rotational speeds of the input shaft and the output shaft comes close quickly to the synchronization rotational speed nSyn1 of the respective gear G1, the synchronization point Syn1 of the gear G1 was reached or exceeded. If after a certain period of time the input shaft still rotates with the synchronization rotational speed nSyn1, the direction of force is reversed since then the gearshift fork was pushed into the direction of gear that has already been engaged. If now a fast change in the rotational speed nE of the input shaft towards the synchronization rotational speed nSyn2 of the opposite gear occurs the synchronization point Syn2 of the respective gear has been reached. Thereafter, the shifting rod is subjected to a lower ninth actuating force F9 acting into the opposite direction that is smaller than the maximum latching force FR, max. This allows reaching the neutral position N safely. At the end of the process the rotational speed nE of the input shaft decreases further due to friction while the clutch is opened.
FIG. 9 shows a process by means of which starting from the neutral position N a gear G1 can be engaged. At first, the clutch is opened, so that the input shaft is separated from the power. Thereafter, movement under controlled speed to the synchronization point Syn1 follows. In case of a hydraulic actuator by means of a flow resistance and a pressure control a predetermined volume flow is established that relates to the desired speed. If the speed of the input shaft changes fast in direction of the synchronization rotational speed nSyn1, the synchronization point Syn1 has been reached. Thereafter, the shifting rod is moved further under controlled pressure, wherein the control value is the deviation between the rotational speed nE of the input shaft and the synchronization rotational speed nSyn1.
When the synchronization rotational speed nSyn1 has been reached, the shifting rod is subjected to a tenth actuating force F10 that is provided for reaching of the end position of the gear G1.
After a waiting period the clutch is closed for transmitting torque. Thereafter, the shifting rod is subjected to an eleventh actuating force F11 acting in opposite direction. If the gear G1 has been finally engaged, an undercut in the teeth of the loose wheel prevents disengaging of the gear G1. If, however, the end position of G1 was not reached, the actuating force F11 of the synchronization process is turned off and a deviation of the rotational speed nE of the input shaft and the synchronization rotational speed nSyn1 results as this is depicted by the dotted line in the bottom diagram of FIG. 9. By means of the dotted line in the gearshift fork path over time it can be gathered that the gearshift fork is slid into the neutral position by the latching mechanism.

## Claims

1. In a motor vehicle, a method for failsafe disengaging of a gear and reaching a neutral position located between a first end position and a second end position of a gearshift fork in a shift transmission from a starting position of the gearshift fork where a first gear is engaged, said transmission being adapted to move the gearshift fork between the first end position where a first gear is engaged and a second end position where a second gear is engaged in a first direction and in a second direction; and synchronize a rotational speed of an input shaft of the shift transmission with a rotational speed of an output shaft of the shift transmission at a respective first synchronization point that is related to the first gear and a second synchronization point that is related to the second gear; said method comprising the steps of:
applying a latching force to the gearshift fork that pushes the gearshift fork in a direction of the neutral position when located in a first position, and away from the neutral position when the gearshift fork is located in a second position that is more remote from the neutral position than the first position, wherein the latching force preferably reaches a maximum in the neutral position;
determining a first time period Δt1 that is high enough so that the gearshift fork is pressed from the engaged gear into a section where the latching force starts urging the gearshift fork into the neutral position;
applying in a step S1 from a starting position of the gearshift fork where the first gear is engaged over the first time period Δt1 a first actuating force to the gearshift fork that is higher than the maximum latching force, said first actuating force acting in one of the first and second directions and pushing the gear shift fork from the starting position where the first gear is engaged within the first time period Δt1 into a section where the latching force starts urging the gearshift fork into the neutral position; and
terminating in a step S2 the first actuating force after the first time period Δt1 has expired to allow the latching force to urge the gearshift fork into the neutral position.

2. The method according to claim 1, further comprising prior to conducting step S1 performing the following step:
S0: opening a clutch of the motor vehicle separating the transmission or a partial transmission that relates to the engaged gear from power of the motor vehicle and determining that the actuating force acts in the direction of the neutral position.

3. The method according to claim 1, further comprising:
determining a change of the ratio of the rotational speed of the input shaft in comparison to the rotational speed of the output shaft due to a beginning synchronization action; and
terminating the first actuating force prior to expiration of the first time period Δt1 when a change of the ratio of the rotational speed of the input shaft in comparison to the rotational speed of the output shaft due to a beginning synchronization action is determined.

4. The method according to claim 1, further comprising applying after step S2 a fourth actuating force to the gearshift fork over a fourth time period, wherein the fourth actuating force acts in the opposite direction than the first actuating force and is smaller than the maximum latching force.

5. The method according to claim 4, further comprising applying after step S2 a fifth actuating force to the gearshift fork over a fifth time period, wherein the fifth actuating force acts in the opposite direction than the first actuating force and is lower than the first actuating force and higher than the maximum latching force, wherein subjecting to the fifth force ends after the fifth time period that is shorter than the time period that is required for a synchronization of the rotational speeds of the input and output shafts.

6. The method according to claim 5, further comprising determining the fifth time period, rotational speed changes, as well the fifth actuating force depending on the temperature and the presumably engaged gear.

7. The method according to claim 4, further comprising determining the fourth time period, rotational speed changes, as well as the fourth actuating force depending on the temperature and the presumably engaged gear.

8. The method according to claim 1, further comprising limiting a moving speed of the gearshift fork.

9. The method according to claim 1, further comprising determining rotational speed changes as well as the first actuating force depending on the temperature and the presumably engaged gear.