Method for reducing torsional shock of electric vehicle drive system

Abstract

A method for reducing torsional shock of a drive system of an electric vehicle, comprising: Applying, by an engine control unit, engine torque to an engine while increasing engine torque when a parking mechanism is engaged and a brake is released; Monitoring, by an engine speed sensor, an engine speed generated by the engine torque applied to the engine and transmitting the engine speed to the engine control unit; Calculating, by the engine control unit, an engine torque at a time when the engine speed fluctuates as a torsional shock of a part of the drive system; and Applying, by the engine control unit, a torque of the same magnitude as the calculated torsional torque to the engine to be transmitted to the part of the drive system when a request to release the parking mechanism is received, and performing a release of the parking mechanism.

Description

BACKGROUND(a) Technical FieldThe present disclosure relates to a method for reducing torsional shock of a drive system part of an electric vehicle, and more particularly to a method for reducing torsional shock of a drive system of an electric vehicle, in which torsion of a part of a drive system occurring during parking of the electric vehicle on a slope can be detected and shock caused by the torsion of the part of the drive system can be reduced.(b) Prior ArtIn general, as exemplarily shown in FIG. 1 of the related art, a drive system of an electric vehicle includes a motor 10 serving as a drive source, a reducer 20 connected to an output shaft of the motor 10, a drive shaft 30 coaxially connected to an output gear 21 of the reducer 20, etc. The drive system of the electric vehicle further includes a parking mechanism 22 that limits the output gear 21 of the reducer 20 by operation of a solenoid when a driver switches a shift lever to the parking (P) position.When the parking mechanism 22 is not engaged with the driven gear 21 of the reducer 20 during parking of the electric vehicle (specifically, on a slope), torsion energy is not generated at the drive shaft 30, but drive wheels are first rotated, the drive shaft 30 is rotated together with rotation of the drive wheels, and thus the vehicle is driven along a downhill travel path, thereby causing a safety accident.On the other hand, when the parking mechanism 22 is engaged with the output gear 21 of the reducer 20 during parking of the electric vehicle (specifically, on a slope), the drive shaft 30 coaxially connected to the output gear 21 of the reducer 20 is limited by the parking mechanism 22, and thus the vehicle is not driven but is pushed back by gravity. Specifically, the force of repulsive the vehicle by gravity is primarily transmitted to the ground contacting tires, and the force transmitted to the tires acts secondarily as a force to rotate the drive shaft 30. However, since the driven gear 21 of the reducer 20 is limited by the parking mechanism 22, the drive shaft 30 coaxially connected to the driven gear 21 is not rotated.Consequently, the vehicle is not moved because the parking mechanism 22 limits the output gear 21 of the reducer 20, but torsion energy equal to the force of repulsive the vehicle by gravity is generated at the input shaft 30. Thereafter, when parking of the vehicle is released (e.g., shifting from the P position to the driving (D) position), the drive shaft 30 on which torsion occurs is reset to an initial state before occurrence of torsion, and shock and vibration are generated from the vehicle during such a reset process.In other words, in the parked state of the electric vehicle on a slope (e.g., in the P position), torsional energy is accumulated on a part of the drive system such as the drive shaft 30 by inclination load of the vehicle, and thereafter, if the P position is released (e.g., shifting from the P position to the D position), if torsion accumulated on the part of the drive system such as the drive shaft 30 is released, a transient shock is generated, and since the weight of the vehicle and the degree of slope are increased, such torsion of the part of the drive system is increased and thus may cause larger shock and vibration.Therefore, in order to reduce such shock, an apparatus improving method for changing the structure of an output gear of a reducer or a parking mechanism to a structure in which backlash can be eliminated has been conventionally used, but such a method is not capable of reducing torsion energy of a part of a drive system, i.e., a fundamental cause of shock occurring when the P position is released, and is thus limited in shock reduction.As described above, torsional energy is accumulated on the part of the drive system such as the drive shaft 30 after the vehicle is parked on a slope, and thereafter, if the P position is released (e.g., shifting from the P position to the D position), an impact is generated in the vehicle by the torsional energy accumulated on the drive shaft 30, and specifically, the electric vehicle in which the engine 10 is connected to the drive shaft 30 through the reducer 20 does not include an impact reducing device such as a torque converter or a transmission gear, and thus such an impact is transmitted to a driver as it is.DE 10 2011 054 295 A1 discloses a device for binding torque for reducing vibration in a vehicle parking system. DE 10 2015 223 888 A1 discloses a system and a method for controlling an effect reduction of an electric vehicle. DE 10 2016 223 777 A1 discloses an anti-jerk control system and method for an environmentally friendly vehicle. U.S. Pat. No. 9,211,810 B2 discloses a vehicle control device having a measurement unit which measures the torsional stress of the front drive shafts in a parking state.SUMMARYThe present invention provides a method for reducing torsional shock of a drive system of an electric vehicle, in which a torsional torque of a part of the drive system is calculated by applying a motor torque to a motor under the condition that a parking mechanism is engaged, and monitoring a motor speed, wherein a reverse torsional torque of the same magnitude as the calculated torsional torque is applied to the part of the drive system when a release of the parking mechanism is requested, and then the parking mechanism is released to reduce a shock caused by torsion of the part of the drive system when the parking mechanism is released.It is another object of the present invention to provide a method for reducing torsional shock of a drive system of an electric vehicle, in which a torsional torque of a part of the drive system is calculated using an engine rotational speed fluctuation amount or a vehicle acceleration fluctuation amount when a brake is released after the parking mechanism is engaged, a reverse torsional torque of the same magnitude as the calculated torsional torque is applied to the part of the drive system when release of the parking mechanism is requested, and then the parking mechanism is released to reduce shock caused by torsion of the part of the drive system when the parking mechanism is released.In one aspect, the present invention provides a method for reducing torsional shock ("torsional shock") of a drive system of an electric vehicle, comprising applying, by an engine controller, an engine torque to an engine while regularly increasing the engine torque when a parking gear ("parking gear") is engaged and a brake is released, monitoring, by an engine speed sensor, an engine speed generated by the engine torque applied to the engine, and transmitting the engine speed to the engine controller, calculating, by the engine controller, an engine torque at a time when the engine speed fluctuates as a torsional shock of a part of the drive system, and applying, by the engine controller, a torque having the same magnitude as the calculated torsional torque to the engine, to be transmitted to the part of the drive system when a request to release the parking mechanism is received and then performing a release of the parking mechanism.In another aspect, the present invention provides a method for reducing torsional shock of a propulsion system of an electric vehicle, which may include applying, by an engine controller, an engine torque to an engine while regularly increasing the engine torque when a request to release a parking mechanism is received under the condition that the parking mechanism is engaged, monitoring, by an engine speed sensor, an engine speed generated by the engine torque applied to the engine, and transmitting the engine speed to the engine controller, and maintaining, by the engine controller, an engine torque at a time when the engine speed fluctuates for a certain time (range), and then performing release of the parking mechanism.In another aspect, the present invention provides a method for reducing torsional shock of a drive system of an electric vehicle, which may include monitoring, by an engine speed sensor, an engine speed, and transmitting the engine speed to an engine controller when a parking mechanism is engaged and a brake is released, calculating, by the engine controller, an engine speed fluctuation amount and calculating a torsional torque of a part of the drive system based on the calculated engine speed fluctuation amount, and applying, by the engine controller, a torque having the same magnitude as the calculated torsional torque to an engine to be transmitted to the part of the drive system when a request to release the parking mechanism is received, and then performing release of the parking mechanism.In yet another aspect, the present invention provides a method for reducing torsional shock of a propulsion system of an electric vehicle, which may include monitoring, by a vehicle acceleration sensor, acceleration of the electric vehicle, and transmitting the acceleration to an engine controller when a parking mechanism is engaged and a brake is released, calculating, by the engine controller, an amount of acceleration variation and calculating a torsional torque of a part of the propulsion system based on the calculated amount of acceleration variation, and applying, by the engine controller, a torque having the same magnitude as the calculated torsional torque to a motor to be transmitted to the part of the propulsion system when a request to release the parking mechanism is received, and then performing release of the parking mechanism.BRIEF DESCRIPTION OF THE DRAWINGSThe above and other features of the present invention will now be described in detail with reference to exemplary embodiments thereof illustrated in the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein: FIG. 1 is a view schematically illustrating a drive system of an electric vehicle according to the related art; FIG. 2 is a control block diagram illustrating a reduction in torsional shock of a drive system of an electric vehicle according to an exemplary embodiment of the present invention; FIG. 3 is a flowchart illustrating a method for reducing torsional shock of a drive system of an electric vehicle according to an exemplary embodiment of the present invention; FIG. 4 is a flowchart illustrating a method for reducing torsional shock of a drive system of an electric vehicle according to an exemplary embodiment of the present invention; FIG. 5 is a flowchart illustrating a method for reducing torsional shock of a drive system of an electric vehicle according to an exemplary embodiment of the present invention; FIG. 6 is a flowchart illustrating a method for reducing torsional shock of a drive system of an electric vehicle according to an exemplary embodiment of the present invention; FIGS. 7A and 7B are waveform diagrams showing variations of vehicle acceleration and engine speed when torsion of a drive shaft corresponding to a part of a drive system occurs according to an exemplary embodiment of the present invention; FIGS. 8A and 8B are schematic views illustrating release of torsion of the drive shaft by a method for reducing torsion shock of a drive system of an electric vehicle according to an exemplary embodiment of the present invention; and FIG. 9 is a view illustrating a force applied to an electric vehicle parked on a slope according to an exemplary embodiment of the present invention.It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. In the figures, reference numerals designate the same or equivalent parts of the present invention throughout the several figures of the drawing.DETAILED DESCRIPTIONIt will be understood that the term "vehicle" or "vehicular" or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, combustion, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from sources other than petroleum).Although an example embodiment is described as using a plurality of units to perform the example process, it should be understood that the example processes may also be performed by one or a plurality of modules. It should also be understood that the term controller / controller refers to a hardware device that includes a memory and a processor. The memory is configured to store the modules, and the processor is specifically configured to execute the modules to perform one or more processes described further below.The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular numbers "a", "an" and "the / s" are intended to include the plural numbers as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "with," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. As used herein, the term "and / or" includes all and any combinations of one or more of the associated listed items.Reference will now be made in detail to various exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in conjunction with exemplary embodiments, it is to be understood that the present description is not intended to limit the invention to the exemplary embodiments. On the contrary, it is intended that the invention cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments within the spirit and scope of the invention as defined by the appended claims.FIG. 2 is a control block diagram illustrating a reduction in torsional shock of a drive system of an electric vehicle according to the present invention. As exemplarily shown in FIG. 2, a drive system of an electric vehicle may include an engine 10 serving as a drive source, a reducer 20 connected to an output shaft of the engine 10, and an input shaft 30 coaxially connected to an output gear 21 of the reducer 20. In addition, the drive system of the electric vehicle may further include a parking mechanism 22 that limits the output gear 21 of the reducer 20 by operation of a solenoid when a driver switches a shift lever to the parking (P) position.Further, a plurality of controllers configured to reduce a torsion shock of the drive system according to the present invention may include an engine controller 40 configured to operate the engine 10, an engine speed sensor 41 and an acceleration sensor 42 connected to an input side of the engine controller 40 to transmit electric signals to the engine controller 40, and a shift controller 50 configured to operate the parking mechanism 22 based on a gear shift signal of a gear shift unit 51.Hereinafter, methods for reducing torsional shock of the drive system having the above-described configuration according to respective exemplary embodiments of the present invention will be described.First EmbodimentFIG. 3 is a flowchart illustrating a method for reducing torsional shock of a drive system of an electric vehicle according to an exemplary embodiment of the present invention. First, it may be confirmed whether the parking mechanism 22 is engaged and whether a brake is released (operation S 101).When a driver shifts the shift lever to the P position through the gear shifting unit 51, a P position signal is transmitted to the shift controller 50, the shift controller 50 may be configured to transmit the P position signal to the engine controller 40, and the engine controller 40 may be configured to determine that the parking mechanism 22 is in an engaged state.Further, when the driver releases a brake pedal, a brake release signal is transmitted to the engine controller 40, and the engine controller 40 may be configured to determine that the brake is in a released state.Thereafter, the engine controller 40 may be configured to apply an engine torque to the engine 10 while regularly increasing the engine torque and at the same time monitoring an engine speed (operation S 102). Specifically, the engine speed sensor 41 may be configured to monitor the engine speed generated by the engine torque applied to the engine 10 and transmit the engine speed to the engine controller 40.Thereafter, it may be confirmed whether a variation in engine speed occurs (operation S 103). To this end, the engine control device 40 may be configured to confirm whether or not a variation in engine speed occurs based on a signal transmitted from the engine speed sensor 41. Upon confirming that a variation in engine speed occurs, the engine controller 40 may be configured to calculate an engine torque at a time when the engine speed varies, i.e., engine torque at a time when the engine speed starts to vary, as a torsion torque of a drive system part (e.g., the drive shaft 30) (operation S 104).As exemplarily shown in FIG. 8A, a torsional torque is generated at the input shaft 30 when the parking mechanism 22 is engaged with the output gear 21, and as exemplarily shown in FIG. 8B, an engine torque at the time when the engine speed starts to fluctuate is transmitted to the input shaft 30, and thus a torsional torque of the input shaft 30 can be released. Thus, the motor controller 40 may be configured to calculate a motor torque at the time when the motor speed starts to fluctuate, as the torsion torque of the drive system part (e.g., the drive shaft 30), and store the torsion torque of the drive system part.In other words, the magnitude of the motor torque at the time when the motor speed starts to fluctuate is the same as the magnitude of the torsional torque generated at the drive shaft 30, and thus motor torque at the time when the motor speed starts to fluctuate can be calculated as the torsional torque of the drive shaft 30, which is a part of the drive system, and can be stored. Thereafter, it may be confirmed whether a request to release the parking mechanism 22 is received (operation S 105).When the driver shifts the shift lever from the P position to the driving (D) position through the gear shifting unit 51, a D position signal is transmitted to the shift controller 50, wherein the shift controller 50 may be configured to transmit the D position signal to the engine controller 40, and the engine controller 40 may be configured to determine that the request to release the parking mechanism 22 is received. Thereafter, the motor controller 40 may be configured to apply motor torque to the motor 10 to eliminate torsion of the driveshaft 30 before an actual release of the parking mechanism 22, and at this time, the motor torque having the same magnitude as the torsion torque calculated in operation S 104 may be applied to the motor 10 (operation S 106).When the motor torque having the same magnitude as the torsion torque calculated in operation S 104 is applied to the motor 10, the applied motor torque can be sequentially transmitted to the output gear 21 of the reducer 20 connected to the output shaft of the motor 10 and the input shaft 30 coaxially connected to the output gear 21 of the reducer 20, and thus torsion of the input shaft 30 can be eliminated.As exemplarily shown in FIG. 8B, a torque (i.e., the motor torque at the time when the motor speed starts to fluctuate) of the same magnitude as the torsion torque calculated in operation S 104 may be transmitted to the driveshaft 30 before an actual release of the parking mechanism 22, and thus a torsion torque of the driveshaft 30 may be released. Thereafter, the motor controller 40 may be configured to transmit a parking mechanism release permission signal to the shift controller 50, the shift controller 50 may be configured to apply an operation control signal to the solenoid to operate the parking mechanism 22, the parking mechanism 22 may be disengaged from the output gear 21 of the reducer 20 by operation of the solenoid, and thereby actual release of the parking mechanism 22 may be performed.As described above, since after parking the vehicle on a slope, a torsional torque generated on the driveshaft 30 is calculated, and when release of the parking mechanism 22 is requested, actual release of the parking mechanism 22 can be performed on the condition that the torsional torque of the driveshaft 30 is removed, an impact conventionally applied to a vehicle due to torsional energy accumulated on a driveshaft when a parking mechanism is released can be reduced.Second EmbodimentFIG. 4 is a flowchart illustrating a method for reducing torsional shock of a drive system of an electric vehicle according to an exemplary embodiment of the present invention. Unlike the method according to the above-described embodiment, the method according to this exemplary embodiment of the present invention is characterized in that a torsion torque of the drive shaft 30 corresponding to a part of the drive system is not calculated in advance, and when a request to release the parking mechanism 22 is received, the torsion torque of the drive shaft 30 is released, and then an actual release of the parking mechanism 22 can be performed.First, it may be determined whether a request to release the parking mechanism 22 is received on condition that the parking mechanism 22 is in the engaged state (operation S 201). When a driver shifts the shift lever from the P position to the D position through the gear shifting unit 51, a D position signal is transmitted to the shift controller 50, the shift controller 50 may be configured to transmit the D position signal to the engine controller 40, and the engine controller 40 may be configured to determine that the request to release the parking mechanism 22 is received.Thereafter, the engine controller 40 may be configured to apply an engine torque to the engine 10 while regularly increasing the engine torque and at the same time monitoring an engine speed (operation S 202). Specifically, the engine speed sensor 41 may be configured to monitor the engine speed generated by the engine torque applied to the engine 10 and transmit the engine speed to the engine controller 40. Thereafter, it may be confirmed whether a variation in engine speed occurs (operation S 203). To this end, the engine control device 40 may be configured to confirm whether a variation in engine speed occurs based on a signal transmitted from the engine speed sensor 41.In response to confirming that a variation in engine speed occurs, the engine controller 40 may be configured to maintain an engine torque at a time when the engine speed varies, i.e., an engine torque at a time when the engine speed starts to vary, for a certain time (operation S 204). In other words, in response to confirming that a variation in engine speed occurs, the engine controller 40 no longer increases the engine torque, and may be configured to maintain the engine torque at the time when the engine speed starts to vary, as the torque applied to the engine 10 for a certain period of time.Specifically, the motor torque at the time when the motor rotation speed starts to fluctuate can be sequentially transmitted to the output gear 21 of the reducer 20 connected to the output shaft of the motor 10 and the input shaft 30 coaxially connected to the output gear 21 of the reducer 20, and thus torsion of the input shaft 30 can be eliminated.As exemplarily shown in FIG. 8B, the engine torque may be transmitted to the driveshaft 30 at the time when the engine speed starts to fluctuate before an actual release of the parking mechanism 22, and thus a torsional torque of the driveshaft 30 may be released.Thereafter, the motor controller 40 may be configured to transmit a parking mechanism release permission signal to the shift controller 50, the shift controller 50 may be configured to apply an operation control signal to the solenoid to operate the parking mechanism 22, the parking mechanism 22 may be disengaged from the output gear 21 of the reducer 20 by operation of the solenoid, and thereby actual release of the parking mechanism 22 may be performed.As described above, since when a request to release the parking mechanism 22 is received, actual release of the parking mechanism 22 can be performed on condition that the torsional torque of the driveshaft 30 is removed, shock conventionally applied to a vehicle due to torsional energy accumulated on a driveshaft when a parking mechanism is released can be reduced.Third EmbodimentFIG. 5 is a flowchart illustrating a method for reducing torsional shock of a drive system of an electric vehicle according to an exemplary embodiment of the present invention. First, it may be confirmed whether the parking mechanism 22 is engaged and whether a brake is released (operation S 301).When a driver shifts the shift lever to the P position through the gear shifting unit 51, a P position signal is transmitted to the shift controller 50, the shift controller 50 may be configured to transmit the P position signal to the engine controller 40, and the engine controller 40 may be configured to determine that the parking mechanism 22 is in the engaged state. Further, when the driver releases a brake pedal, a brake release signal is transmitted to the engine controller 40, and the engine controller 40 may be configured to determine that the brake is in the released state.Thereafter, an engine speed fluctuation amount may be monitored and calculated (operation S 302). Specifically, when the engine speed sensor 41 monitors the engine speed and transmits the engine speed to the engine controller 40, the engine controller 40 may be configured to calculate the engine speed fluctuation amount.As exemplarily shown in FIG. 7A, if the vehicle is parked on a flat surface, even if the brake is released after the parking mechanism 22 is engaged, torsion is not generated on the drive shaft 30, and thus an engine speed does not fluctuate in a state before starting the vehicle. In contrast, as exemplarily shown in FIG. 7B, if the vehicle is parked on an incline or inclined road surface, when the brake is released after the parking mechanism 22 is engaged, torsion is generated on the drive shaft 30, the vehicle is at the same time pushed slightly back, and an engine speed fluctuates.Thus, the engine control device 40 may be configured to calculate a difference between an upper peak and a lower peak, i.e., a peak-to-peak value when the engine speed fluctuates, as the engine speed fluctuation amount. Thereafter, the engine controller 40 may be configured to calculate a current torsion torque of the driveshaft 30 corresponding to a part of the drive system based on the calculated engine speed fluctuation amount, and store the calculated current torsion torque of the driveshaft 30 (operation S 303).For this purpose, when the current torsional torque of the driveshaft 30 is calculated based on the calculated engine speed fluctuation amount, engine speed fluctuation amounts according to road slopes measured in advance by tests and torsional torques of the driveshaft 30 according to the engine speed fluctuation amounts measured in advance by tests are constructed as map data, and the map data may be stored in the engine controller 40.Therefore, the engine control device 40 may be configured to store the engine rotation fluctuation amount calculated in operation S 302 in the map data, acquire the current torsional torque of the driveshaft 30 corresponding to the engine rotation fluctuation amount stored in the map data, and store the current torsional torque of the driveshaft 30. Thereafter, it may be confirmed whether a request to release the parking mechanism 22 is received (operation S 304).When the driver shifts the shift lever from the P position to the D position through the gear shifting unit 51, a D position signal is transmitted to the shift controller 50, the shift controller 50 may be configured to transmit the D position signal to the engine controller 40, and the engine controller 40 may be configured to determine that the request to release the parking mechanism 22 is received. Thereafter, the engine controller 40 may be configured to apply an engine torque to the engine 10 to eliminate torsion of the driveshaft 30 before an actual release of the parking mechanism 22, and at this time, the engine torque having the same magnitude as the torsion torque calculated based on the engine rotation fluctuation amount in operation S 303 may be applied to the engine 10 (operation S 305).When the motor torque having the same magnitude as the torsion torque calculated based on the motor rotation fluctuation amount in operation S 303 is applied to the motor 10, the applied motor torque can be sequentially transmitted to the output gear 21 of the reducer 20 connected to the output shaft of the motor 10 and the input shaft 30 coaxially connected to the output gear 21 of the reducer 20, and thus torsion of the input shaft 30 can be eliminated. Thereafter, the motor controller 40 may be configured to transmit a parking mechanism release permission signal to the shift controller 50, the shift controller 50 may be configured to apply an operation control signal to the solenoid to operate the parking mechanism 22, the parking mechanism 22 may be disengaged from the output gear 21 of the reducer 20 by operation of the solenoid, and thereby actual release of the parking mechanism 22 may be performed.As described above, after parking the vehicle on a slope or inclined road surface, a torsional torque generated on the driveshaft 30 can be calculated more accurately using the engine speed fluctuation amount. When a request to release the parking mechanism 22 is received, an actual release of the parking mechanism 22 may be performed on condition that the torsion torque of the driveshaft 30 is removed, and a shock conventionally applied to a vehicle due to torsion energy accumulated on a driveshaft when a parking mechanism is released may be reduced.Fourth EmbodimentFIG. 6 is a flowchart illustrating a method for reducing torsional shock of a drive system of an electric vehicle according to an embodiment of the present invention. First, it may be confirmed whether the parking mechanism 22 is engaged and whether a brake is released (operation S 401).When a driver shifts the shift lever to the P position through the gear shifting unit 51, a P position signal is transmitted to the shift controller 50, the shift controller 50 may be configured to transmit the P position signal to the engine controller 40, and the engine controller 40 may be configured to determine that the parking mechanism 22 is in the engaged state. Further, when the driver releases a brake pedal, a brake release signal is transmitted to the engine controller 40, and the engine controller 40 may be configured to determine that the brake is released. Thereafter, an acceleration fluctuation amount of the vehicle may be monitored and calculated (operation S 402). Specifically, when the acceleration sensor 42 monitors acceleration of the vehicle and transmits the acceleration to the engine controller 40, the engine controller 40 may be configured to calculate the amount of acceleration fluctuation.As exemplarily shown in FIG. 7A, if the vehicle is parked on a flat surface, even if the brake is released after the parking mechanism 22 is engaged, torsion is not generated on the driveshaft 30, and thus vehicle acceleration does not fluctuate in a state before starting the vehicle. In contrast, as exemplarily shown in FIG. 7B, if the vehicle is parked on an incline or inclined road surface, when the brake is released after the parking mechanism 22 is engaged, torsion is generated on the drive shaft 30, the vehicle is at the same time pushed slightly back, and vehicle acceleration fluctuates.Thus, the engine control device 40 may be configured to calculate a difference between an upper peak and a lower peak, i.e., a peak-to-peak value when the acceleration fluctuates, as the acceleration fluctuation amount. Thereafter, the motor controller 40 may be configured to calculate a current torsion torque of the drive shaft 30 corresponding to a part of the drive system based on the calculated acceleration fluctuation amount, and store the calculated current torsion torque of the drive shaft 30 (operation S 403). For this purpose, when the current torsional torque of the driveshaft 30 is calculated based on the calculated acceleration variation amount, engine rotational speed variation amounts according to road slopes measured in advance by tests and torsional torques of the driveshaft 30 according to the engine rotational speed variation amounts measured in advance by tests are constructed as map data, and the map data may be stored in the engine controller 40.Therefore, the motor controller 40 may be configured to store (e.g., add) the acceleration variation amount calculated in operation S 402 into the map data, obtain the current torsional torque of the driveshaft 30 corresponding to the acceleration variation amount stored in the map data, and store the current torsional torque of the driveshaft 30. Thereafter, it may be confirmed whether a request to release the parking mechanism 22 is received (operation S 404).When the driver shifts the shift lever from the P position to the D position through the gear shifting unit 51, a D position signal is transmitted to the shift controller 50, the shift controller 50 may be configured to transmit the D position signal to the engine controller 40, and the engine controller 40 may be configured to determine that the request to release the parking mechanism 22 is received. Thereafter, the engine controller 40 may be configured to apply an engine torque to the engine 10 to eliminate torsion of the driveshaft 30 before an actual release of the parking mechanism 22, and at this time, the engine torque may be applied to the engine 10 with the same magnitude as the torsion torque calculated based on the engine speed in operation S 403 (operation S 405).When the motor torque having the same magnitude as the torsion torque calculated based on the acceleration fluctuation amount in operation S 403 is applied to the motor 10, the applied motor torque can be sequentially transmitted to the output gear 21 of the reducer 20 connected to the output shaft of the motor 10 and the input shaft 30 coaxially connected to the output gear 21 of the reducer 20, and thus torsion of the input shaft 30 can be eliminated. Thereafter, the motor controller 40 may be configured to transmit a parking mechanism release permission signal to the shift controller 50, the shift controller 50 may be configured to apply an operation control signal to the solenoid to operate the parking mechanism 22, the parking mechanism 22 may be disengaged from the output gear 21 of the reducer 20 by operation of the solenoid, and thereby actual release of the parking mechanism 22 may be performed.As described above, since, after parking the vehicle on a slope, a torsional torque generated on the driveshaft 30 can be calculated more accurately using the amount of acceleration fluctuation, and when a request to release the parking mechanism 22 is received, an actual release of the parking mechanism 22 can be performed on condition that the torsional torque of the driveshaft 30 is removed, an impact conventionally applied to a vehicle due to torsional energy accumulated on a driveshaft when a parking mechanism is released can be reduced.The torque having the same magnitude as the present torsional torque of the driveshaft 30 calculated based on the engine rotational fluctuation amount and applied to the engine 10 in the previous exemplary embodiment and the torque having the same magnitude as the present torsional torque of the driveshaft 30 calculated based on the acceleration fluctuation amount and applied to the engine 10 in this exemplary embodiment may be used as correction factors when calculating a torque to alleviate an impact generated on the vehicle due to torsional energy accumulated on the driveshaft 30.FIG. 9 is a view illustrating a force applied to an electric vehicle parked on an incline or inclined road surface.If an electric vehicle having a weight m is parked at an inclination at a certain angle θ, a force F 1 applied to the electric vehicle by gravity may be calculated as Equation 1.In Equation 1 above, g indicates acceleration due to gravity.The electric vehicle can be pushed by the force F 1, and at this time, since a parking mechanism is in the engaged state, the force F 1 causes torsion of a drive shaft.A magnitude F 2 of the torsion of the drive shaft can be calculated by Equation 2 below.In Equation 2 above, m indicates a weight of the electric vehicle, g indicates acceleration due to gravity, θ indicates an angle of inclination, and instead of a gear ratio, a reduction ratio of a reducer of the vehicle may be set in Equation 2. Above Equation 2 is also expressed as Equation 3 based on an engine torque F 3.When the parking mechanism is released (the P position is released), an impact caused by torsion of a drive shaft corresponding to a part of a drive system can be reduced by applying the torque F 3 to a motor. However, the torque F 3 is a torque if torsion of the drive shaft is ideally generated, and torsion of the drive shaft can actually be incorrectly calculated in the following cases: (1) if the vehicle is supported by a stopper or a brake wedge on the ground; (2) if a parking brake is engaged before parking; (3) if the weight of the vehicle is changed; and (4) if an imprecision in measurement of an acceleration sensor and a measurement error occur.Therefore, the torque having the same magnitude as the present torsional torque of the driveshaft 30 calculated based on the engine rotational fluctuation amount and applied to the engine 10 in the third embodiment and the torque having the same magnitude as the present torsional torque of the driveshaft 30 calculated based on the acceleration fluctuation amount and applied to the engine 10 in the fourth embodiment can be used as correction factors when calculating an engine torque F 3 to alleviate an impact generated on the vehicle due to torsional energy accumulated on the driveshaft 30.As is apparent from the above description, a method for reducing torsional shock of a drive system of an electric vehicle according to the present invention may have effects as follows. Since, after parking the vehicle on a slope, a torsional torque generated at a driveshaft corresponding to a part of the drive system can be calculated, and when release of a parking mechanism is requested, actual release of the parking mechanism can be performed on condition that the torsional torque of the driveshaft is removed, an impact conventionally applied to a vehicle due to torsional energy accumulated at a driveshaft when a parking mechanism is released can be reduced.The invention has been described in detail with reference to exemplary embodiments thereof. It will be appreciated, however, by those skilled in the art that changes may be made in these exemplary embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims

A method for reducing torsional shock of a propulsion system of an electric vehicle, comprising: applying, by an engine controller, an engine torque to an engine while increasing engine torque when a parking mechanism is engaged and a brake is released; monitoring, by an engine speed sensor, an engine speed generated by the engine torque applied to the engine and transmitting the engine speed to the engine controller; calculating, by the engine controller, an engine torque at a time when the engine speed varies as a torsional shock of a portion of the propulsion system; and applying, by the engine controller, a torque having the same magnitude as the calculated torsional torque to the engine to be transmitted to the portion of the propulsion system when a request to release the parking mechanism is received and performing a release of the parking mechanism.The method of claim 1, wherein the torque having the same magnitude as the calculated torsional torque is sequentially transmitted to an output gear of a reducer connected to an output shaft of the motor and an input shaft coaxially connected to the output gear of the reducer to eliminate torsion of the input shaft.A method for reducing torsional shock of a propulsion system of an electric vehicle, comprising: applying, by an engine controller, an engine torque to an engine while increasing the engine torque when a request to release a parking mechanism is received under the condition that the parking mechanism is engaged; monitoring, by an engine speed sensor, an engine speed generated by the engine torque applied to the engine and transmitting the engine speed to the engine controller; and maintaining, by the engine controller, an engine torque at a time when the engine speed fluctuates for a certain period of time and performing a release of the parking mechanism.The method according to claim 3, wherein the motor torque at the time when the motor speed fluctuates is sequentially transmitted to an output gear of a reducer connected to an output shaft of the motor and an input shaft coaxially connected to the output gear of the reducer to eliminate torsion of the input shaft.A method for reducing torsional shock of a propulsion system of an electric vehicle, comprising: monitoring, by an engine speed sensor, an engine speed, and transmitting the engine speed to an engine controller when a parking mechanism is engaged and a brake is released; calculating, by the engine controller, an engine speed variation amount and calculating a torsional torque of a portion of the propulsion system based on the calculated engine speed variation amount; and applying, by the engine controller, a torque having the same magnitude as the calculated torsional torque to an engine to be transmitted to the portion of the propulsion system when a request to release the parking mechanism is received and performing a release of the parking mechanism.The method according to claim 5, wherein, in calculating the torsional torque of the part of the drive system based on the calculated engine speed fluctuation amount, the torsional torque of the part of the drive system is calculated from map data including engine speed fluctuation amounts according to road slopes measured in advance and torsional torques of the part of the drive system according to the engine speed fluctuation amounts measured in advance.The method of claim 5, wherein the torque having the same magnitude as the calculated torsional torque is sequentially transmitted to an output gear of a reducer connected to an output shaft of the motor and an input shaft coaxially connected to the output gear of the reducer to reduce torsion of the input shaft.A method for reducing torsional shock of a propulsion system of an electric vehicle, comprising: monitoring, by a vehicle acceleration sensor, acceleration of the electric vehicle, and transmitting the acceleration to an engine controller when a parking mechanism is engaged and a brake is released; calculating, by the engine controller, an amount of acceleration variation and calculating a torsional torque of a portion of the propulsion system based on the calculated amount of acceleration variation; and applying, by the engine controller, a torque having the same magnitude as the calculated torsional torque to an engine to be transmitted to the portion of the propulsion system when a request to release the parking mechanism is received, and performing release of the parking mechanism.The method according to claim 8, wherein, in calculating the torsion torque of the part of the drive system based on the calculated acceleration variation amount, the torsion torque of the part of the drive system is calculated from map data including acceleration variation amounts according to road slopes measured in advance and torsion torques of the part of the drive system according to the acceleration variation amounts measured in advance.The method of claim 8, wherein the torque of the same magnitude as the calculated torsional torque is sequentially transmitted to an output gear of a reducer connected to an output shaft of the motor and an input shaft coaxially connected to the output gear of the reducer to eliminate torsion of the input shaft.

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