Methods for shifting a vehicle transmission

A continuously variable transmission mechanically coupled with an automated mechanical transmission is adjusted during a shifting process. At the completion of the shifting process, an engine mechanically coupled with the automated mechanical transmission is provided a fueling pulse. Additionally, the speed of an output shaft of the automated mechanical transmission is filtered by a second order variable time constant low pass filter.

BACKGROUND

1. Field of the Invention

The invention relates to methods for shifting a vehicle transmission.

2. Background Art

A powertrain system for a vehicle generally includes an engine and a multi-ratio transmission. An output shaft of the engine is mechanically coupled to an input shaft of the transmission. Before shifting from a first gear to a second gear, the input shaft may need to achieve a target speed. The target speed is based on the gear ratio associated with the second gear and the speed of an output shaft of the transmission. Fuel supplied to the engine is temporarily reduced or eliminated to reduce the input shaft speed to the target speed. The shifting process is completed when the input shaft speed approaches the target speed. The time for the input shaft speed to reach the target speed depends on the dynamics associated with the operation of the engine.

In one example, the process of shifting the transmission involves disengaging a dog clutch from a first gear and subsequently engaging the dog clutch with a second gear. Engaging the second gear may be accompanied by a lurching movement caused by slack in gearing of the transmission. Shifting with a dog clutch may cause large fluctuations in the speed of the output shaft of the transmission.

SUMMARY

An engine power transformer may include a continuously variable transmission. The engine power transformer may be mechanically coupled with an input shaft of an automated mechanical transmission and an engine. The continuously variable transmission may be adjusted during a shifting process to synchronize the speeds of the input shaft and an output shaft of the automated mechanical transmission. At the completion of the shifting process, the engine may be provided a fueling pulse to eliminate slack in gearing of the automated mechanical transmission. A target ratio for sheaves of the continuously variable transmission may be based on a filtered speed of the output shaft of the automated mechanical transmission and a target speed of the engine. A second order variable time constant low pass filter may be used to filter the output shaft speed.

DETAILED DESCRIPTION

FIG. 1is a schematic diagram of a vehicle powertrain system10. In the embodiment ofFIG. 1, system10includes an engine12, an engine power transformer (EPT)14, and an automated mechanical transmission (AMT)16. An input shaft18of the EPT14may be mechanically coupled with an output shaft20of the engine12via a clutch22. A coupling shaft24mechanically couples the EPT14and AMT16. An output shaft26of the AMT16may be mechanically coupled with a drive axle of a vehicle (not shown).

FIG. 2is a schematic diagram of an embodiment of the EPT14. EPT14includes a step up gear set28, a continuously variable transmission (CVT)30, a step-down gear set32, and a planetary gear set34. The step up gear set28in the illustrated embodiment includes gears36,38,40,42,44,46. Gear36rotates with input shaft18and has a speed of ω0. Gears38,40rotate with a shaft48and have a speed of ω1. Gears42,44rotate with a shaft50and have a speed of ω2. Gear46is mechanically coupled with the CVT30by the shaft52and has a speed of ω3.

CVT30includes a first variator sheave54and a second variator sheave56that are mechanically coupled via belt58. Variator sheaves54,56have speeds of ω3, ω4respectively. Variator sheaves54,56may be adjusted axially to alter their effective diameters thereby altering the CVT ratio ω3/ω4.

The Step-down gear set32includes a pair of gears60,62. Gear60is mechanically coupled with CVT30via shaft64. Gear62is mechanically coupled to the coupling shaft24. Gears60,62rotate at speeds ω4, ω5respectively. The coupling shaft24is mechanically coupled with the step down gear set32by the planetary gear set34. Coupling shaft24rotates at a speed of ω6.

The EPT14transfers torque from the input shaft18to the coupling shaft24and has an EPT ratio of ω0/ω6.

FIG. 3is a plot of the EPT ratio versus the CVT ratio for EPT14. The variator sheaves54,56are adjusted as described with reference toFIG. 2above to change the EPT ratio of EPT14. In the embodiment ofFIG. 3, as the CVT ratio increases, the EPT ratio increases. In other embodiments, as the CVT ratio increases, the EPT ratio may decrease.

FIG. 4Ais a schematic diagram, side view, of an embodiment of the AMT16. AMT16includes a plurality of gears64,66,68,70,72,74. Gears66,68,72rotate with a shaft76. A dog clutch78may engage either gear70or gear74to change the effective gear ratio of AMT16between coupling shaft24and the output shaft26. When the dog clutch78engages gear70, gear74free spins and torque from the coupling shaft24is transferred to the output shaft26via gears64,66,68,70. When the dog clutch78engages gear74, gear70free spins and torque from the coupling shaft24is transferred to the output shaft26via the gears64,66,72,74. In alternative embodiments, the AMT16may include any desired number of gears configured to transfer torque from the coupling shaft24to the output shaft26.

FIG. 4Bis a schematic diagram, front view, of the gear70. Gear70includes an opening80for receiving the dog clutch78(FIG. 4A). Similarly, gear74(FIG. 4A) has an opening (not shown) similar to that of gear70for receiving the dog clutch78.

FIG. 5is a flow chart of an embodiment of a shifting process for system10(FIG. 1). At82, the AMT16is in gear, e.g., the dog clutch78is engaged with gear70. At84, the fuel command to engine12is cut. At86, the AMT16is pulled to neutral, e.g., the dog clutch78is disengaged with gear70. At88, the input shaft18and output shaft26are synchronized. At90, the AMT16is pushed in gear, e.g., the dog clutch78is engaged with gear74. At92, the engine12is refueled.

FIG. 6is a flow chart of an embodiment of the synchronization portion of the shifting process ofFIG. 5. At94, the engine speed is commanded to a target speed. At96, the CVT30(FIG. 2) is slewed, that is, the effective diameters of sheaves54,56are adjusted in a continuous fashion.

Referring toFIGS. 1,2and4A, the speed of the input shaft18is related to the speed of the output shaft26by the following
ωout*EPT Ratio*AMT Ratio=ω0
where
ωout=speed of the output shaft26
ω0=speed of the input shaft18
AMT Ratio=effective gear ratio of the AMT16
The AMT16, in the embodiment ofFIG. 4A, has two effective gear ratios:
AMT Ratio1=effective gear ratio of the gears64,66,68,70AMT Ratio2=effective gear ratio of the gears64,66,72,74
If the dog clutch78is engaged with gear70, the AMT16has an effective gear ratio of AMT Ratio1. If the dog clutch78is engaged with gear74, the AMT16has an effective gear ratio of AMT Ratio2. Prior to shifting from gear70to gear74, the speed of the input shaft18is related to the speed of the output shaft26by the following
ω0=ωout*ω0/ω6*AMT Ratio1
As explained with reference toFIG. 5, the speeds of the input shaft18and output shaft24are synchronized when shifting from one gear to another gear. For example, the target speed for the input shaft18, when shifting to gear74, is given by the following
ω0Target=ωout*ω0/ω6*AMT Ratio2
As explained with reference toFIG. 6, in addition to cutting the fueling level command to the engine12, the CVT ratio may also be altered by adjusting the variator sheaves54,56.

FIG. 7Ais an example plot of the coupling shaft speed and target speed versus time for the automated mechanical transmission ofFIG. 4A. Shortly after the start of the shifting process, the coupling shaft speed begins to decrease as the fueling level command to the engine12has been cut. This rate of initial decrease depends on the dynamics of the engine12. When the coupling shaft speed is approximately 50% of the target speed, the diameter of variator sheave54is decreased and the diameter of variator sheave56is increased. The AMT16is put in gear once the coupling shaft speed is approximately equal to the target speed.

FIG. 7Bis a plot of the commanded CVT ratio and actual CVT ratio versus time and shows the increase in CVT ratio during the slewing of CVT30.

FIG. 8is an example plot of the commanded fuel level versus time associated with the shifting process ofFIG. 5. Slack in the gearing of the EPT14may become apparent during the shifting process described above. A pulsed fuel command98is given at the beginning of the refueling of the engine12(FIG. 1) to eliminate the effect of this slack by gently taking up the slack in a very controlled manner. In the embodiment ofFIG. 8, the pulsed fuel command98is at 20% of the maximum engine torque command and has a duration of 80 milliseconds. In other embodiments, the level and duration of the command may be dictated by the system dynamics.

FIG. 9is a block diagram of an embodiment of a second order variable time constant low pass filter99. A measured speed100of the output shaft26is fed into a pair of comparators102,104. A delay106delays the measured speed100for a predetermined duration, e.g., 1 sec. The output of the delay106is also fed into the comparator104. Comparator104takes the difference between the measured speed100and the output of delay106. This difference is then scaled by a scaling factor107and the absolute value taken at108resulting in an error value. A look-up table110outputs a time constant adjustment for a given error value.

FIG. 10is a plot of the time constant adjustment versus error value in the look-up table110. In the embodiment ofFIG. 10, the time constant adjustment is 1 for low values of error and 0.5 for high values of error. In alternative embodiments, the time constant adjustment versus error data may be different.

Referring toFIG. 9, a filtered speed112of the output shaft26is fed into the comparator102. Comparator102takes the difference between the measured speed100and filtered speed112. This difference is fed into a product block114along with a filter gain116and the time constant adjustment from the look-up table110. This product is fed into a comparator118along with the output from a first order filter block120. Comparator118takes the difference between the product and the output from the first order filter block120. This difference is fed into a product block122along with a filter gain124. The resulting product is fed into the first order filter block120. The output of the first order filter block120is fed into a first order filter block126.

In the example ofFIG. 9, K is the gain, Tsis the time constant, and z is the discrete transfer function variable of the first order filter blocks120,126.

In alternative embodiments, any n-order variable time constant low pass filter capable of accomplishing the methods described herein may be used. For example, a third order low pass filter may be used that adjusts its time constant according to the input signal amplitude fluctuation.

FIG. 11Ais an example plot of the measured unfiltered output shaft speed versus time of the AMT16(FIG. 1). The output shaft speed may be measured, for example, with a speed sensor. During a shifting process as described above, the speed of the output shaft26(FIG. 1) experiences non-smooth behavior, or large fluctuations, due to the dynamics associated with shifting gears. As described below, such behavior may result in a CVT ratio that also exhibits large fluctuations.

The filter99(FIG. 9) selectively filters the measured output shaft speed. Prior to the shifting process, the measured speed100(FIG. 9) of the output shaft26(FIG. 1) is relatively smooth. Thus, the output of the delay106(FIG. 9) is approximately equal to the measured speed100resulting in a relatively small error value. The time constant adjustment, fromFIG. 10, is approximately 1. A time constant adjustment of approximately 1 does not alter the time constant of the filter99. During the shifting process, the measured speed100of the output shaft26is not relatively smooth. The output of the delay106is different from the measured speed100resulting in a relatively large error value. The time constant adjustment, fromFIG. 10, takes on values less than 1. A time constant adjustment of less than 1 alters the time constant of filter99, resulting in lower cut-off frequencies and increased phase lag. After the shifting process, the measured speed100of the output shaft26is again relatively smooth. The output of the delay106is approximately equal to the measured speed100resulting in a relatively small error value. The time constant adjustment, fromFIG. 10, is again approximately 1 which returns the time constant of the filter99to its preshifting value.

FIG. 11Bis an example plot of the filtered output shaft speed versus time of the AMT16(FIG. 1). The filtered output shaft speed ofFIG. 11Bis smoother than the unfiltered output shaft speed ofFIG. 11A.

The filtered output shaft speed is used to determine the CVT ratio. Multiplying the filtered output shaft speed by the appropriate effective gear ratio of AMT16, i.e., AMT Ratio1or AMT Ratio2, yields the filtered coupling shaft speed, ω6.

During an up-shift process as described herein, the target input shaft speed, ω0, is determined by the target speed for the engine output shaft20after the shifting process is complete. The quotient of the target input shaft speed and the coupling shaft speed yields the target EPT ratio. Referring toFIG. 3, a target CVT ratio is determined based on the target EPT ratio. Sheaves54,56of the EPT30may be adjusted to achieve the target CVT ratio.