Method and control system for downshifting an automatic transmission

Abstract: A control system of a downshift by an automatic transmission having an improved shift feeling by controlling pressure fed to a kickdown servo brake comprises a throttle valve sensor, an input shaft rpm sensor, an output shaft rpm sensor, a transmission control unit for outputting a control signal for a second-to-first speed shift after reducing hydraulic pressure fed to a predetermined state using every duty pattern when the vehicle is in the state of a second-to-first downshift, or a control signal for a second-to-first speed shift after the third-to-second speed shift is completed when the vehicle is in the state of a third-to-first speed shift, and first and second transmission control solenoid valves, and a hydraulic pressure control solenoid valve for changing the state of hydraulic pressure fed to every friction member.

Description: BACKGROUND OF THE INVENTION (a) Field of the Invention The present invention relates to a method and a control system for downshifting an automatic transmission gear, more particularity, to a method and a control system for downshifting an automatic transmission by controlling the hydraulic pressure fed to a kickdown servo brake so as to have an improved response to the downshift. (b) Description of the Prior Art Generally, when an automatic transmission gear is used in a vehicle, hydraulic pressure is controlled according to the transmission stage as a function of the running speed of a vehicle, and thereby, a corresponding transmission gear to a target transmission stage is automatically achieved. Namely, a torque converter operates from the output power of the engine, thereby controlling the fluid pressure. Signals applied from a transmission control system control the hydraulic pressure fed to corresponding valves. As a result, the corresponding transmission gear to the running state of the vehicle is achived. Therefore, the automatic transmission for a vehicle requires a minimum amount of effort on the part of the driver by removing the necessity for a driver to operate a clutch pedal. Also, the automatic transmission makes driving easy since there may be little possibility of an engine being stalled due to clutch engagement at an insufficient speed. The conventional method for controlling the automatic transmission is as follows. A transmission lever operated by a driver changes a plurality of ports feeding hydraulic pressure from an oil pump to the automatic transmission. The hydraulic pressure fed to the automatic transmission changes the operational state of a pressure valve controlling the operational state of friction members comprising clutches and a brake. In a transmission gear mechanism, one transmission stage can be selected by the operation of the friction members. Then a planetary gear is operated according to the selective operation of the friction members, and an adequate transmission gear ratio is fed to a drive gear. The power is fed to a drive gear and then to a differential gear of a final reduction device through a transfer driven gear engaged with the drive gear. As described above, in the conventional method, the plurality of electrical signals are changed by the transmission control system, thereby operating the transmission gear of a target transmission stage according to the running state of the vehicle. However, in a second-to-first speed downshift or a third-to-first speed downshift, there is a need to stop the operation of the kickdown servo brake since the kickdown servo brake operates only when the transmission is in a second speed or a fourth speed. Therefore, the transmission control system changes the operational state of a hydraulic pressure control solenoid valve thereby exhausting hydraulic pressure fed to a kickdown servo brake at the second speed via first and second speed shift solenoid valves. As a result, in the conventional transmission gear large shift shock is generated when hydraulic pressure fed to the kickdown servo brake is exhausted abruptly by the operation of the hydraulic pressure control solenoid valve. US 5,647,819 describes a speed change control method for an automatic transmission which is capable of reducing a shift shock and required shifting time period. In particular, a skip downshift from a fifth speed to a third speed is effected by way of a 4'-shift position which is not used in a normal shifting. Further, during a downshift from the fifth speed to the third speed, the electronic solenoid valves associated with supply/discharge of operation oil for the clutches and brakes are duty controlled to supply/discharge operation oil to or from brakes and clutches. US 4 742 732 is concerned with an automatic transmission system for a vehicle and describes a method of controlling a fourth-to-first downshift, a fourth-to-second downshift, a fourth-to-third downshift, a third-to-first downshift and a third-to-second downshift wherein each downshift is manually actuated by operating a selector lever of a manual valve and/or an auxiliary switch. After starting the control routine and detecting the current speed ratio the selector lever position is detected. Thereafter, the target speed ratio is determined by judging the selector lever position. In detail, if the selector lever position is L (with the auxiliary switch set ON) it is decided that the target ratio is the first speed ratio. Similarly, it is decided that the target speed ratio is the second speed ratio when the selector lever position is position 2. With the auxiliary switch a fourth-to-third downshift can be manually actuated by turning the switch OFF. Furthermore, if the auxiliary switch is OFF a third-to-first and a third-to-second downshift can be actuated by the selector lever of the manual valve. When performing an n th -to-(n-1) th downshift the first step is to confirm that the current speed ratio is n th . If the current speed ratio is n th an n th -to-(n-1) th speed shift instruction is output. I.e. a signal is output to instruct a shift from the n th speed ratio to the (n-1) th speed shift ratio to the electromagnetic valves. Thereafter, the (n-1) th speed clutch engaging force is determined by map reading to output a signal to duty-control an electromagnetic valve. After completing the n th -to-(n-1) th speed shift an (n-1) th -to-(n-2) th downshift is performed in the same manner until the downshift to the target ratio is completed. The n th speed clutch engaging force is determined in dependence on the position of the selector lever and from the target ratio n th of the specific (n+1) th to-n th speed shift. As explained for a fourth-to-third downshift during fourth-to-second downshift controlling the third speed clutch engaging force to "medium" is to ensure that the front clutch is engaged earlier than normal so as to obtain a greater engine breaking effect. Summary of the invention It is an object of the present invention to provide a method and a control system for controlling downshifting an automatic transmission gear having an improved response to the running conditions of a vehicle and to simplify the hydrolic control system. This object is achieved by the method according to claim 1 and the control system according to claim 6. Additional improvements and refinements of the present invention are described in the corresponding subclaims. BRIEF DESCRIPTION OF THE DRAWINGS The above objective and feature of the present invention will be apparent from the following description of the embodiment with reference to the accompanying drawings. Fig. 1 is a block diagram for a control system of a downshift by an automatic transmission gear according to the embodiment of the present invention. Fig. 2 is a flow chart illustrating the control method of a downshift by an automatic transmission gear according to the embodiment of the present invention. Fig. 3A illustrates the duty pattern as a function of time when a second-to-first speed downshift is operated according to the embodiment of the present invention. Fig. 3B illustrates a graph showing the state of transmission stage as a function of when a second-to-first speed downshift according to the embodiment of the present invention is operated. Fig. 3C illustrates a graph showing the variable input as a function of time when a second-to-first speed downshift according to the embodiment of the present invention is operated. Fig. 3D illustrates a graph showing the variable rpm of a turbine shaft as a function of time when a second-to-first speed downshift according to the embodiment of the present invention is operated. Fig. 4A illustrates the duty pattern as a function of time when a third-to-first speed downshift according to the embodiment of the present invention is operated. Fig. 4B illustrates a graph showing the state of a transmission stage as a function of time when a third-to-first speed downshift according to the embodiment of the present invention is operated. DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to present the preferred embodiment of the present invention, an example of which is illustrated in the accompanying drawings. Referring to Fig. 1, the present invention employs, a throttle position sensor 11 for sensing the opening of a throttle valve, and outputting a corresponding electrical signal. The throttle opening is controlled by an accelerator pedal. A turbine shaft rpm sensor 12 is used for sensing the rpm of a turbine shaft and outputting a corresponding electrical signal. The turbine shaft is the input shaft to the automatic transmission. An output shaft rpm sensor 13 is used for sensing the rpm of the output shaft of the hydraulic pressure device of the automatic transmission, and outputting a corresponding electrical signal. The throttle position sensor 11, the turbine shaft rpm sensor 12 and the output shaft rpm sensor 13 are connected to a transmission control unit 2 which controls a duty ratio of hydraulic pressure fed to the kickdown servo brake, thereby outputting a control signal for a downshift when the vehicle is in the downshifting state. The transmission control unit 2 outputs control signals for first and second transmission control solenoid valves 31 and 32 to change their operational state, and thereby control transmission stage. The transmission control unit 2 also outputs a control signal to a hydraulic pressure control solenoid valve 33 to change the operational state of pressure fed to the friction members. Referring to Fig. 2, the preferred embodiment of the present invention comprises the following steps. The operation of the transmission control unit 2 and every corresponding device starts if necessary when power is applied S10. The transmission control unit 2 reads the signal output from the above-mentioned sensors 11, 12 and 13 S20. The transmission control unit 2 detects the rpm of the output shaft of the transmission (No) and the opening of the throttle valve (TH) S30. The transmission control unit 2 determines whether or not the detected opening of the throttle valve (TH) and the rpm of the output shaft of the transmission (No) satisfies the following predetermined conditions S40, (1) 300rpm < No < 900rpm (2) TH > 1.25V (3) Ne > 2000rpm; wherein Ne is the engine rpm. If the above-mentioned conditions are met, the transmission control unit 2 determines whether or not the vehicle is in the downshift state S50. If the vehicle is in the downshift state, the transmission control unit 2 determines whether or not the vehicle is in the second-to-first speed shift state S60. If the vehicle is in the second-to-first speed shift state, the transmission control unit 2 reads the signal input from the throttle position sensor 11. The signal input is a voltage corresponding to the opening of a throttle valve which is controlled by the accelerator pedal S70. The transmission control unit 2 detects a voltage corresponding to the opening of the throttle valve S80. The transmission control unit 2 computes every variable value of the duty pattern to be set according to the voltage value S90. Referring to Fig. 3A, the corresponding variable values to the voltage, determined by the opening of the throttle valve (TH), are as follows: TH Dr a 1 a 2 dD 1.25V<TH<3.5V 60% 30%/s Upgrade 10%/s Upgrade 50% 3.5V<TH<5V 53% 40% Upgrade 15% Upgrade 7% The transmission control unit 2 controls the operation of the hydraulic pressure solenoid valve 33 according to the predetermined duty ratio S100. The transmission control unit 2 determines whether or not the vehicle mechanically completes the transmission S110. The transmission control unit 2 reads the signals from the above-mentioned sensors at regular intervals. If the value obtained by multiplying the rpm of the output shaft detected by the signal from the output shaft rpm sensor 13 by the transmission ratio of the first speed is the same as the rpm of the input shaft detected by the signal from the turbine shaft rpm sensor 12, it is determined that a first shift to a first speed is completed. If the vehicle completes the first shift to the first speed, the transmission control unit 2 measures time S120 and after a predetermined time period has elapsed S130 changes the operational state of the first and second transmission control solenoid valves 31 and 32 to enable the second-to-first speed downshift S140. Then, the transmission control unit 2 maintains the operation of the hydraulic pressure solenoid valve in the first speed according to the duty ratio set at the first speed S150 (as shown in Fig. 3b). The transmission control unit 2 controls the duty ratio of the hydraulic pressure solenoid valve before performing step S150. Fig. 3c illustrates a change rate of line pressure when the operation of the hydraulic pressure solenoid valve is controlled by the predetermined duty pattern. Fig. 3d illustrates variable rpm of the engine. Since the pressure value fed to every variable friction member is reduced as illustrated in Fig 3c as time passes, the shift shock generated when the operational states of the friction members are changed is reduced. Referring to step S60, if the vehicle is not in the second-to-first speed downshift state, the transmission control unit 2 determines whether or not the vehicle is in a third-to-first speed shift state S160. If the transmission is not in the third-to-first speed shift state, the transmission control unit 2 returns to step S20 and reads the signals from the sensors to determine the running condition of the vehicle. If the vehicle is in the third-to-first speed shift state, the transmission control unit 2 operates the third-to-second speed shift. If the vehicle is in third speed, the operational state of the first and second transmission control solenoid valves 31 and 32 is OFF. When the vehicle is shifted to second speed, the operational state of the second transmission control solenoid valve 32 is changed to ON. The control signal for changing the operational state of the second transmission control solenoid valve 32 from OFF to On is output S170. Then, the transmission control unit 2 measures time S180 and after a predetermined time period passes 190, the transmission control unit 2 controls the operation of the hydraulic pressure solenoid valve according to the duty pattern illustrated in Fig. 4a so that a downshift to a second speed can be achieved S200. The control duty pattern of the hydraulic pressure solenoid valve 33 for downshifting to second speed is the same as that for the third-to-second downshift. The transmission control unit 2 determines whether or not the vehicle completes a third-to-second speed shift S210. If a value obtained by multiplying the rpm valve of the output shaft detected by the signal from the output shaft rpm sensor 13 by the transmission ratio set at the second speed is the same as the rpm of the input shaft detected by the signal from the turbine shaft rpm sensor 12, it is determined that the vehicle completes the shift to the second speed. If it is determined that the vehicle completes the first third-to-second speed shift, the transmission control unit 2 measures time S220, and after a predetermined time period passes S230, the transmission control unit 2 returns to step S70 and performs the steps to S150 for the second-to-first speed shift. Since the second-to-first speed shift is gradually operated after the predetermined period from the completion of the first second-to-first speed shift, the operational state of the hydraulic pressure solenoid valve is stabilized thereby improving the response to a downshift. Namely, the shift shock generated during the third-to-first speed shift can be reduced by operating the third-to-second speed shift, then second-to-first speed shift gradually after the predetermined period. Accordingly, the control system of a downshift by an automatic transmission according to the preferred embodiment of the present invention can provide reduced shift shock by stopping reaction force members of a second speed so as to complete the first shift to the first speed, then changing the solenoid valve to be in the second-to-first speed shift. Also, the control system for downshifting the an automatic transmission gear according to the preferred embodiment of the present invention can provide reduced shift shock by gradually operating the downshift to a first speed after the completion of the downshift to a second speed when the third-to-first speed shift is operated. As a result, the hydraulic pressure device can be protected due to reduced shift shock and the load fed to the corresponding friction members.

Claims: A method for controlling a down shift by an automatic transmission gear of a vehicle, comprising the steps of: determining (S40) if a running state of the vehicle satisfies predetermined conditions by using output signals of a throttle valve sensor (11) for sensing an opening of a throttle valve, and an output shaft rpm sensor (13) for sensing rpm of an output shaft; determining (S50) if the vehicle is in a state for down shift when the running state of the vehicle satisfies predetermined conditions; determining (S60) if the vehicle is in a state of a second-to-first speed shift if the vehicle is in a state for down shift; and outputting a control signal for a second-to-first speed shift to a first and a second transmission control solenoid valve (31, 32) after reducing hydraulic pressure to be fed to friction members to a predetermined state by means of a hydraulic pressure control solenoid valve (33) that is duty controlled in accordance with a duty pattern determined as a function of said throttle valve sensor output signal when the vehicle is in a state of a second-to-first speed down shift; or outputting a control signal for a third-to-second speed shift to said first and second transmission control solenoid valves (31, 32) when the vehicle is in a state of a third-to-first speed down shift, and performing said second-to-first down shift after the third-to-second down shift is completed. The method according to claim 1, characterized in that the running state of the vehicle satisfies said predetermined conditions when said output signal of said throttle valve sensor (11) is indicating a throttle valve opening greater than 1.25 Volts; and said rpm of said output shaft is greater than 300 rpm and less than 900 rpm. The method according to claim 1 or 2, characterized by detecting variable values according to said throttle valve opening thereby setting a duty pattern if the vehicle is in the state of a second-to-first speed down shift, outputting a control signal for controlling the operation of said hydraulic pressure control solenoid valve (33) according to said duty pattern: and outputting a control signal for said first and second transmission control solenoid valves (31, 32) thereby completing a shift to a first speed if a predetermined state of the transmission is completed by the operation of said hydraulic pressure control solenoid valve (33) in accordance with to said duty pattern. The method according to claim 1 or 2, characterized by outputting a control signal for a third-to-second speed shift to said first and second transmission control solenoid valves (31, 32); outputting a control signal for controlling the operation of said hydraulic pressure control solenoid valve (33) according to said duty pattern for a third-to-second speed down shift; detecting variable values according to said throttle valve opening thereby setting a duty pattern if the vehicle is in the state of a third-to-first speed down shift, outputting a control signal for controlling the operation of said hydraulic pressure control solenoid valve (33) according to said duty pattern; and outputting a control signal for said first and second transmission control solenoid valves (31, 32) thereby completing a shift to a first speed if a predetermined state of the transmission is completed by the operation of said hydraulic pressure control solenoid valve (33) in accordance with to said duty pattern. The method according to claim 3, characterized by determining that said predetermined state of the transmission is completed if a value obtained by multiplying the rpm value of an output shaft by a transmission ratio of a second speed is the same as the rpm value of an input shaft detected by an input shaft rpm sensor (12). A control system for controlling a down shift by an automatic transmission gear of a vehicle, comprising: a throttle valve sensor (11) for sensing an opening of a throttle valve wherein said variable opening of said throttle valve changes an electrical signal to be output; an input shaft rpm sensor (12) for sensing revolution per minute (rpm) of an input shaft wherein the rotational speed of said input shaft changes an electrical signal to be output; an output shaft rpm sensor (13) for sensing rpm of an output shaft wherein the rotational speed of said output shaft changes an electrical signal to be output; a transmission control unit (2) connected to said throttle valve sensor (11), said input shaft rpm sensor (12) and said output shaft rpm sensor (13), and for outputting control signals; a first and a second transmission control solenoid valve (31, 32) controlled by corresponding control signals output from said transmission control unit (2); and a controlled by corresponding control signals output from said transmission control unit (2); chraracterized in that said transmission control unit (2) determines if the vehicle is in a state of a downshift when the running state of the vehicle satisfies predetermined conditions, outputs a control signal to said hydraulic pressure control solenoid valve (33) for hydraulic pressure control according to a duty pattern set as a function of the throttle valve sensor output signal, and outputs a control signal for a second-to-first speed shift to a first and a second transmission control solenoid valve (31, 32) after reducing hydraulic pressure to be fed to friction members to a predetermined state by means of said hydraulic pressure control solenoid valve (33) that is operated in accordance with said duty pattern when the vehicle is in a state of a second-to-first speed down shift, or outputs a control signal for a third-to-second speed shift to said first and second transmission control solenoid valves (31, 32) when the vehicle is in a state of a third-to-first speed down shift, and performs said second-to-first down shift after the third-to-second down shift is completed. wherein said variable opening of said throttle valve changes an electrical signal to be output; The control system of claim 6, characterized in that said automatic transmission gear of a vehicle comprises an input shaft, a plurality of friction members, and an output shaft, the input shaft being driven by an engine, said engine having a throttle valve; and said transmission control unit (2) generates a downshift control signal as a function of outputs of said throttle valve sensor (11), input shaft sensor (12) and output shaft sensor (13) after the hydraulic pressure fed to said friction members by said hydraulic pressure control solenoid valve (33) is reduced. The control system of claim 7 characterized in that the duty pattern computed by said transmission control unit (2) is a function of the output signal of said throttle valve sensor (11), said downshift control signal being generated after a predetermined time after the automatic transmission reaches a predetermined state. The control system of claim 8 characterized in that said predetermined state of the automatic transmission occurs when the rotational speed of said output shaft multiplied by a first speed ratio of the automatic transmission is equal to the rotational speed of said input shaft. the control system of claim 7 characterized in that said hydraulic pressure control solenoid valve (33) for feeding hydraulic pressure to said friction members, being controlled by a third-to-second duty pattern, and after a predetermined time controlled by a second-to-first duty pattern, said duty patterns being a function of the output signal of said throttle valve sensor (11), said downshift control signal being generated after a predetermined time after said automatic transmission reaches a predetermined state.

Inventors: LEE HEE-YONG, LEE, HEE-YONG
Applicants: HYUNDAI MOTOR CO LTD, HYUNDAI MOTOR COMPANY
Application year: 1996
Main classification: F16H61
IPC codes: F16H59, F16H61