GB2327990A

GB2327990A - Method for determining the fill time of a transmission clutch

Info

Publication number: GB2327990A
Application number: GB9824449A
Authority: GB
Inventors: Randall M Mitchell; Todd D Creger; James R Talbott; Gregory L Williamson; Richard B League
Original assignee: Caterpillar Inc
Current assignee: Caterpillar Inc
Priority date: 1995-04-13
Filing date: 1996-02-26
Publication date: 1999-02-10
Anticipated expiration: 2016-02-26
Also published as: GB9824449D0; GB2327990B

Abstract

In an automatic transmission which includes a torque converter and a planetary transmission with both direction and speed friction devices (clutches/brakes), the time at which a fluid-operated direction friction device substantially fills with fluid during a shift in which one direction friction device is off-going and another is on-coming is determined by monitoring the input and output speeds of the torque converter. The speed ratio of the torque converter is calculated and compared a reference value. It is determined-that the on-coming direction friction device has filled when the torque converter speed ratio increases relative to the reference value. Also described is a method of determining the filling of a speed clutch by comparing the transmission output speed with an intermediate transmission speed.

Description

METHOD FOR DETERMINING THE FILL TIME OF A TRANSMISSION CLUTCH This invention relates generally to a method of transmission control and, more particularly, to a method of determining the fill time of an on-coming clutch.

Generally, a powershift transmission includes a number of gear elements coupling the input and output shafts, and a related number of clutches which are selectively engageable to activate gear elements for establishing a desired speed ratio between the input and output shafts.

The clutch may be of the band or disk type.

For example, the input shaft may be connected to the engine through a fluid coupling, such as a torque converter, and the output shaft is connected directly to the vehicle drive. Shifting from one gear ratio to another involves releasing or disengaging the off-going clutches associated with the current gear ratio and applying or engaging the on-coming clutches associated with the desired gear ratio.

To improve the shifting of the transmission, several transmission manufactures have utilized electronic control technology. Electronic control technology employs a direct interface between an electronic control unit and the clutches via a plurality of solenoid valves. The solenoid valves are modulated to control both the on-coming and offgoing clutch pressures in response to command signals from the electronic control unit.

To provide for precise timing of the shifting, it is desirous to determine the fill time of an on-coming clutch.

The fill time is the time required to fill an on-coming clutch cavity with fluid. During the fill period, the clutch piston strokes and the clutch plates compress.

However, the clutch cannot transmit significant torque until the compression is complete. Thus, for reliable clutch control during shifting, it is desirous to accurately determine the length of the fill period so as to know when the clutch is ready to transmit significant torque.

In an aspect of the present invention, a method is disclosed for determining when an-coming direction clutch substantially fills with fluid. The method includes the steps of: monitoring the torque convertor input speed and output speed; determining a torque converter speed ratio in response to the torque converter input speed and output speed; comparing the torque converter speed ratio to a reference value; and determining when an on-coming direction clutch substantially fills in response to the torque converter speed ration increasing relative to the reference value.

For a better understanding of the present invention, reference may be made to the accompanying drawings in which: Fig. 1 is a block diagram of an electronic control system of a vehicle drive train including a transmission; Figs. 2A,B are block diagrams illustrating respective embodiments of a transmission configuration; Figs. 3A,B,C are timing charts illustrating clutch commands for various types of transmission shifts; Fig. 4 is timing diagram that relates the clutch fill parameters of a clutch command to the clutch fill time; Fig. 5 is a top level flowchart of the program control for an executive program; Fig. 6 is a low level flowchart of the program control of an end of fill detecting algorithm for an on-coming direction clutch; Figs. 7A,B are timing diagrams that show a torque converter ratio and clutch slip for an on-coming direction clutch; Fig. 8 is a low level flowchart of the program control of an end of fill detecting algorithm for an on-coming speed clutch; and Fig. 9 is a timing diagram that shows the result of a difference between an actual carrier speed and a reference carrier speed.

Referring now to the drawings, Fig. 1 illustrates a electronic control system of a power train 100 that includes an internal combustion engine 105, a fluidic torque converter 110, a multi-speed fluid operated power transmission 115, and a vehicle drive train 120. The engine 105 is connected to the torque converter 110 via shaft 125, the torque converter 110 is connected to the transmission 115 via shaft 130, and transmission 115 is connected to the vehicle drive 120 via shaft 135.

Referring now to Figs. 2A,2B, a block diagram of two transmission gear sets are shown. For example, Fig. 2A represents a track-type tractor transmission gear set 205, and Fig. 2B represents a wheeled vehicle transmission gear set 210. The transmission of Fig. 2A includes five clutches, where C1 is a reverse direction clutch, C2 is a forward direction clutch and clutches C3-C5 are speed clutches. The transmission of Fig. 2B includes six clutches, where C1 is a reverse direction clutch, C2 is a forward direction clutch and clutches C3-C6 are speed clutches. Gear shifts are accomplished by selectively engaging and disengaging combinations of the clutches. The clutches are actuated by hydraulic pressure and upon engagement, require fill time before torque is transmitted between a driving and a driven friction element. That is, the fill time is the elapsed time that the clutch piton moves from the released to the engaged position. The clutches are selectively engaged and disengaged by solenoid operated proportional pressure control valves 215. The hydraulic circuit of the transmission includes a positive displacement pump 220 that supplies pressurized hydraulic fluid from the sump or reservoir 225 to the clutches through the control valves 215. Moreover, a relief valve 230 may be added to regulate the valve supply pressure.

Referring back to Fig. 1, the control portion of the power train will now be discussed. An operator initiates a desired shift via an operator shift handle 140, which produces a gear selection signal. An electronic control module 147 receives the gear selection signal and responsively controls the operation of the solenoid control valves 215. The electronic control module 147 may also receive various other input signals representative of vehicle system parameters. Such other inputs signals may include a neutralizing signal from a neutralizer pedal 145, an engine speed signal from an engine speed sensor 150, an transmission input speed signal, TX, from a transmission input speed sensor 155, a transmission intermediate speed signal, TN, from a transmission intermediate speed sensor 160, a transmission output speed signal, T,, from a transmission output speed sensor 165, and a transmission oil temperature signal from a transmission oil temperature sensor 170. The sensors are conventional electrical transducers such as potentiometer, thermistors and/or magnetic speed pickups.

Note that, although the intermediate speed sensor 160 is shown adjacent clutch C4, the intermediate speed sensor 160 may be adapted to sense the rotational speed of any one of the clutches or ring gears in the illustrated transmissions. The transmission speed signals are helpful to determine the individual rotational speeds of any one clutch or ring gear. For example, the clutch slip of any one of the speed clutches may be determined in response to the transmission output and intermediate speed signals.

Additionally, the clutch slip of any one of the direction clutches may be determined in response to the transmission input and intermediate speed signals. Such determinations are based on the specific gear ratios of the transmission in a manner well known in the art.

Internally, the electronic control module 147 comprises a number of conventional devices including a microprocessor with an internal clock and memory, an input/output device, and an array of proportional solenoid current drivers. One solenoid driver are dedicated to each solenoid control valve 215. The microprocessor delivers a command signal proportional to the desired solenoid current, and the current driver uses a Pulse Width Modulated voltage to produce the desired current. The solenoid control valves 215 are configured to maintain a flow of oil to the clutch sufficient to maintain a clutch pressure proportional to the solenoid current. Thus, the microprocessor can control the clutch pressure proportional to the command signal delivered to the solenoid driver circuit. Proportional solenoid current drivers are well known in the art, and need not be further discussed.

The microprocessor utilizes arithmetic units to control the transmission shifting in accordance with software programs. Typically, the programs are stored in read-only memory, random-access memory or the like. The programs are discussed in relation to various flowcharts.

The clutch commands that control the various types of shifts are shown with respect to Figs. 3A,B,C. Note that the direction clutch is used to absorb the majority of the torque transmitted by the shift. Thus, the direction clutch is the last clutch to be fully engaged in a shift.

For example, in the case of a speed shift, only after the on-coming speed clutch is fully engaged, will the direction clutch be re-engaged.

A speed shift is shown in relation to Fig 3A. The clutch command, which controls the clutch pressures is shown with respect to time. The on-coming speed clutch enters a pulse, ramp and hold phase. As shown, the oncoming clutch command is pulsed at a high level for a predetermined amount of time to quickly open the solenoid control valve in order to begin filling the clutch. Then the clutch command is ramped to accurately control the fluid flow to the clutch. Finally the clutch command is maintained at a hold level to completely fill the clutch.

The value of the hold level is high enough to insure completion of clutch fill, yet low enough to prevent excessive torque transmission when the clutch plates "touch-up." After the on-coming clutch is filled, the oncoming clutch enters a modulation phase. The corresponding clutch command is controlled either closed-loop or openloop to gradually increase the clutch pressure in order to cause a desired decrease in clutch slip. Once the oncoming clutch locks-up or engages, the clutch command is increased to a maximum pressure level.

The off-going speed and direction clutch commands are decreased to a low pressure level for a predetermined time period before the on-coming speed clutch is modulated. The predetermined time period is set to allow for end of fill detection and to reduce torque interruption.

Once the on-coming speed clutch is engaged, the direction clutch then enters a modulation phase. Once the direction clutch is engaged, then the clutch command is increased to a maximum pressure level to complete the shift.

Note that, clutch lock-up may be indicated by monitoring clutch slip.

A direction shift is shown in relation to Fig. 3B.

Here, the speed clutch command remains unchanged while the direction clutch command is modified. The on-coming direction clutch command is pulsed, then ramped down to a hold level to fill the clutch. The off-going direction clutch pressure is decreased to a low pressure level a predetermined time before the on-coming direction clutch enters the modulation phase. The modulation phase uses a closed-loop pressure control to result in a desired decrease in clutch slip. Once the on-coming clutch locksup, then the clutch command is increased to a maximum pressure level to complete the shift.

A combination or direction and speed clutch shift is shown in relation to Fig. 3C. The on-coming speed clutch command is pulsed and ramped down to a hold level in order to fill the clutch. After the on-coming speed clutch is filled, the clutch command is modulated until the clutch locks-up, then increased to a maximum command level to fully engage the clutch. However, a predetermined time period prior to the on-coming speed clutch entering the modulation phase, the off-going speed and direction clutch commands are decreased to release the clutches. Moreover, a predetermined time period after the on-coming speed clutch begins filling, the on-coming direction clutch command is pulsed then ramped down to a hold level in order to fill the clutch. The on-coming direction clutch command is then modulated after the on-coming speed clutch is engaged. Once the on-coming direction clutch locks-up, the clutch command is increased to a maximum level to fully engage the clutch to complete the shift.

The present invention determines the magnitude of the clutch fill period of an on-coming clutch. The clutch fill period represents the time period in which the on-coming clutch substantially fills, and is referred to as the actual fill time as shown in Fig. 4. Thus, in order to determine the actual fill time, the end of fill (EOF) for the on-coming clutch must be detected.

Figs. 5, 6, and 8 are flowcharts representative of computer program instructions executed by the computerbased control unit of Fig. 1 in carrying out the shift control technique of the present invention. In the description of the flowcharts, the functional explanation marked with numerals in angle brackets, < nnn > , refers to blocks bearing that number.

Reference is now made to Fig. 5, which represents an executive or main loop program of an adapting control, which directs the sequential execution of various subroutines. The program first determines whether a new gear has been selected < 505 > and whether the new shift satisfies certain conditions < 510 > . For example, whether the vehicle travel speed is above a predetermined value and whether the shift represents a predetermined type of shift that is amenable to end of fill detection. If the shift qualifies, then the program control determines the values of several variables used in the various subroutines, including: COUNTER, TARGET VALUE, and the initial direction associated with a clutch slip value of a predetermined oncoming direction clutch. These variables will be discussed in more detail below. Moreover, several parameters associated with the clutch command will be initialized.

While a direction shift in is progress, the program control detects the end of fill (EOF) of a predetermined on-coming direction clutch and determines the actual fill time < 520 > . Once the actual fill time has been determined, the program control adapts the fill parameters of the predetermined direction clutch based on the magnitude of the actual fill time < 525 > . Next, while a speed shift in is progress, the program control detects the EOF of a predetermined on-coming speed clutch and determines the corresponding or actual fill time < 530 > . Once the actual fill time has been determined, the program control adapts the fill parameters of the predetermined speed clutch based on the magnitude of the actual fill time < 535 > .

The preferred end-of-fill (EOF) detection methods will now be discussed. The present invention utilizes two different methods to detect the EOF, depending upon whether the clutch being engaged is a speed clutch or a direction clutch. For example, the illustrated transmissions are designed such that the direction clutches are the energy absorbing clutches. Thus, during all shifts, the direction clutches are the last to be engaged. (This also includes speed shifts where the direction clutch is the same for both the of going gear and the oncoming gear.) Consequently, during a shift, the direction clutch fills, begins engaging, and begins transmitting torque between the transmission and the torque converter. Advantageously, the present invention monitors the output torque of the torque converter and when the output torque suddenly increases (indicating an upshift) or decreases (indicating a downshift), EOF of the direction clutch is said to occur.

The present invention monitors the torque converter output torque by determining a speed ratio of the torque converter and comparing the torque converter speed ratio to a reference value.

Referring now to Fig. 6, the EOF detection strategy for the direction clutch is discussed. The program control first determines the actual torque converter ratio by dividing the torque converter output speed by the torque converter input speed < 605 > ; then, determines the difference between the actual torque converter ratio and a reference value referred to as the null torque convertor ratio < 610 > . The null torque converter ratio is defined as the torque converter ratio at which the torque converter output is zero. For example, the null torque converter ratio is based on well known torque converter output characteristic curves. The on-coming direction clutch slip is then determined < 615 > by using well known methods.

Two tests are made to determine if the EOF has occurred. The first consists of two parts (1) determining if the absolute value of the direction clutch slip is approaching zero slip < 620 > and (2) determining if the torque convertor ratio is diverging from the null torque ratio < 625 > . If both parts are true, then EOF is said to have occurred because the above conditions indicate that the on-coming direction clutch is transmitting significant torque.

This is better illustrated with reference to Figs. 7A and 7B, where Fig. 7A shows the torque convertor ratio-and Fig. 7B shows the clutch slip relative to time. At point 1, the torque converter speed ratio is shown to be diverging or moving away from the null toque converter ratio, while the clutch slip is shown to be decreasing.

The second test also consists of two parts (1) determining if the direction clutch slip is decreasing < 620 > (as referenced to the initial direction clutch slip) and (2) if the magnitude difference between the actual torque converter ratio and the null torque convertor ratio is greater than a predetermined value. If both parts are true, EOF has already occurred. For example, this is shown by point 2 on Figs. 7A,7B.

It is noted that the waveforms illustrated on Figs.

7A,7B are in simplified form. The actual waveforms will have varying degrees of fluctuations. Consequently, it may be desirable to perform the subroutine shown in Fig. 6 several times to ensure that EOF has occurred. For example, in the executive routine, the program control sets COUNTER equal to five. If decision blocks 625 and 630 indicate EOF, then COUNTER is decremented by one.

Consequently, once COUNTER is decremented to zero, then EOF is flagged < 645 > . However, if decision blocks 620 and 630 indicate that EOF has not occurred, then COUNTER is incremented by one.

The EOF detection for an on-coming speed clutch will now be discussed. The EOF detection strategy uses the occurrence of one event to help determine the occurrence of another event. Here, the release of an off-going speed clutch signifies the EOF of an on-coming speed clutch. For example, once an on-coming speed clutch fills, the clutch pressure rises engaging the clutch, which in turn, causes the clutch to begin transmitting torque. This transmitted torque causes the off-going speed clutch to release (due to the low clutch pressure).

One method to determine whether an off-going speed clutch has released is to monitor the slip of that clutch.

For example, an engaged clutch will not normally slip; but, when the clutch pressure is lowered, the clutch will begin to slip. Consequently, once the engaged clutch begins to slip, that clutch is said to be released. Thus, once the slip of an off-going speed clutch becomes greater than a reference slip value, then EOF of an on-coming speed clutch is said to occur.

Another method to determine whether an off-going speed clutch has released is to monitor the speed of the carrier that connects the direction clutch stage to the speed clutch stage. For example, referring to Figs. 2A and 2B, the illustrated transmission consists of essentially two stages where the &num;2 carrier connects the direction clutch stage to the speed clutch stage. During a speed shift, the # 2 carrier initially rotates at a speed associated with the off-going speed clutch, then eventually rotates at a speed associated with the on-coming speed clutch.

Consequently, once the on-coming speed clutch substantially fills, the clutch begins to transmit torque; and the #2 carrier speed changes from the speed associated with the off-going speed clutch to the speed associated with the oncoming speed clutch - at a rate proportional to the torque that is produced by the on-coming clutch. It is at this initial change in speed that indicates EOF of the on-coming speed clutch.

In view of this, the present invention compares the actual #2 carrier speed with a reference carrier speed.

The reference carrier speed represents the carrier speed as if the on-coming speed clutch were completely engaged.

Once the difference between the two speeds is less than a predetermined target value, then EOF is said to occur.

Based on the planetary gear relationship of the #3 clutch, the actual #2 carrier speed is determined in accordance with the following equation: carrier speed = transmission output speed + (Nn/Ns) * &num;3 clutch slip)/(l+(NR/Ns)) Eq.1 1 where: NR = the number of teeth of the clutch #3 ring gear; Ns = the number of teeth of the clutch #3 sun gear; and the &num;3 clutch slip is the ratio of the input clutch speed to the output clutch speed.

The EOF detection strategy for the speed clutch is shown with reference to Fig. 8. The program control determines the actual carrier speed < 805 > , a reference carrier speed < 810 > , and determines the absolute value of the difference between the actual and reference speeds < 815 > . The actual carrier speed is determined with respect to Eq. 1 and the reference carrier speed is determined according to the following equation: reference carrier speed = reduction speed ratio [i] * transmission output speed Eq. 2 where: the reduction speed ratio (i] is the reduction speed ratio for the predetermined engaging speed clutch. It is noted that the reduction speed ratio is dependant upon the specific configuration of the transmission and is determined in a manner well known in the art.

The program control determines whether the absolute difference between the actual and reference speeds is greater than TARGET < 820 > . TARGET is determined at the start of the shift and its value is a predetermined percentage of the absolute difference between the actual and reference speeds determined at the start of shift. For example, TARGET may be 90% of the absolute difference between the actual and reference speeds determined at the start of shift. Thus, if the absolute difference between the actual and reference speeds is less than the target value, then EOF is said to have occurred. This is shown more particularly with reference to Fig. 9, where the absolute difference between the actual and reference speeds is shown with respect to time.

However, if the absolute difference between the actual and reference speeds is greater than the target value, then the program control determines if the clutch slip of the on-coming speed clutch is less than a reference slip value < 825 > . The reference slip value represents some value below which the on-coming speed clutch is assumed to be engaged. Consequently, if the on-coming speed clutch has engaged (the on-coming clutch slip is less than the reference slip value), then EOF has already occurred.

Industrial Applicability The present invention is advantageously applicable to controlling a clutch-to-clutch shift of a powershift transmission of a construction machine such as a wheel loader, bulldozer, or the like. One such shift may be a speed shift that changes from one speed ratio to another speed ratio. A speed ratio is defined as the transmission input speed or torque converter speed divided by the output speed. Thus, a low gear range has a high speed ratio and a higher gear range has a lower speed ratio. To perform an upshift, a shift is made from a high speed ratio to a low speed ratio. In the type of transmission involved in this invention, the upshift is accomplished by disengaging a clutch associated with the higher speed ratio and engaging a clutch associated with the lower speed ratio to thereby reconfigure the gear set to operate at the lower speed ratio. With respect to a speed shift, such as an upshift, the direction clutch is used to absorb the torque created by the shift. Thus, during an upshift, the direction clutch is disengaged, then re-engaged after the on-coming speed clutch is engaged. In this manner the direction clutch absorbs the torque created during the shift.

The present invention helps to control the timing of the clutch-to-clutch shifts by accurately determining the actual fill time of the on-coming clutch in order- to achieve high quality shifting. For example, at the start of each shift, the on-coming clutch command is pulsed at a high level for a predetermined period of time then the command is ramped down from a initial level to a hold level. Optimally, the initial ramp level is chosen to create a desired amount of fluid flow to quickly stroke the clutch piston and gain torque capacity of the on-coming clutch. However, if the end of fill occurs too early, the resulting clutch pressure causes the clutch to transmit torque to quickly, which results in a harsh shift.

Alternately, if the end of fill occurs too late, the clutch does not achieve sufficient pressure until the modulation phase, which causes a sudden (rather than gradual) increase in clutch torque, which also results in a harsh shift.

Advantageously, by monitoring the actual fill period of a predetermined clutch, the clutch timing can be accurately controlled to result in good quality shifts.

Claims

1. A method for determining when a clutch substantially fills with fluid, the clutch being one of a plurality of speed and direction clutches associated with an automatic transmission, the transmission being adapted to receive torque from an engine through a torque convertor, the transmission producing a shift from a first transmission ratio to a second transmission ratio through disengagement of an off-going clutch associated with the first transmission ratio and engagement of an on-coming clutch associated the second transmission ratio, the method comprising the steps of: monitoring the torque converter input speed and output speed; determining a torque converter speed ratio in response to the torque converter input speed and output speed; comparing the torque converter speed ratio to a reference value; and determining when an on-coming clutch substantially fills in response to the torque converter speed ratio increasing relative to the reference value.

2. A method, as set forth in claim 1, including the steps of: determining the difference between the torque converter speed ratio and the reference value; comparing the difference to a predetermined valuerand determining when an on-coming direction clutch substantially fills in response to the difference being greater than the predetermined value.

3. A method, as set forth in claim 2, including the steps of: monitoring the slip of an on-coming speed clutch; comparing the on-coming speed clutch slip to a reference slip value; and determining when the on-coming speed clutch substantially fills in response to the on-coming speed clutch slip being less than the reference slip value.

4. A method, as set forth in claim 2, including the steps of: determining when an off-going speed clutch releases; and determining when an on-coming speed clutch substantially fills based on when the off-going speed clutch releases.