Abstract:
During state transitions, a clutch actuator position command includes an oscillating component called a dither. This dithering helps avoid jumps in the actuator position due to friction when the commanded position is changed gradually. Also, dither during a transition from fully released to slipping causes the natural frequency of the system to change gradually rather than abruptly. This permits use of another slipping clutch for active damping based on a measured oscillation.

Description:
TECHNICAL FIELD 
     The present disclosure relates to transmission control. More particularly, disclosure relates to use of dithering to reduce the occurrence of powertrain vibrations during clutch state transitions. 
     BACKGROUND 
     Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns. 
       FIG. 1  schematically illustrates a Dual Clutch Transmission (DCT). Input  20  is adapted for coupling to an engine crankshaft, potentially via a damper assembly that reduces the transmission of engine pulsations. Ring gear  22  is fixedly coupled to a differential to distribute power between two drive wheels. First output pinion  24  is fixedly coupled to first layshaft  26  and meshes with ring gear  22 . Second output pinion  28  is fixedly coupled to second layshaft  30  and also meshes with ring gear  22 . First friction clutch  32  selectively couples input  20  to solid shaft  34 , while second friction clutch  36  selectively couples input  20  to hollow shaft  38  which is concentric with solid shaft  34 . 
     Gears  40  and  42  are supported for rotation about first layshaft  26  and mesh with gears  44  and  46  respectively which are fixedly coupled to solid shaft  34 . Coupler  48  selectively couples gear  40  or  42  to first layshaft  26 . Gear  50  is supported for rotation about second layshaft  30  and meshes with gear  52  which is fixedly coupled to solid shaft  34 . Coupler  58  selectively couples gear  50  to second layshaft  30 . When couplers  48  or  58  have coupled one of gears  40 ,  42 , or  50  to the respective layshaft, a power flow path is established between solid shaft  34  and ring gear  22 . Each of these different power flow paths is associated with a different speed ratio. When clutch  32  is also engaged, a power flow path is established between input  20  and ring gear  22 . 
     Gears  60  and  62  are supported for rotation about second layshaft  30  and mesh with gears  64  and  66  respectively which are fixedly coupled to hollow shaft  38 . Coupler  68  selectively couples gear  60  or  62  to second layshaft  30 . Gears  70  and  72  are supported for rotation about first layshaft  26  and mesh with gear  66  and  60  respectively. Coupler  74  selectively couples gear  70  or  72  to first layshaft  26 . When couplers  68  or  74  have coupled one of gears  60 ,  62 ,  70 , or  72  to the respective layshaft, a power flow path is established between hollow shaft  38  and ring gear  22 . When clutch  36  is also engaged, a power flow path is established between input  20  and ring gear  22 . The speed ratios associated with clutch  36  are interleaved with the speed ratios associated with clutch  32  such that clutch  32  is used to establish odd numbered gear ratios and clutch  36  is used to establish even numbered gear ratios and reverse. 
     When a driver selects Drive with the vehicle stationary, coupler  48  is commanded to couple gear  42  to shaft  26  while clutch  36  is commanded to disengage. To launch the vehicle, clutch  32  is commanded to gradually engage. Similarly, when Reverse is selected with the vehicle stationary, coupler  74  is commanded couple gear  72  to shaft  26 . Then, clutch  36  is commanded to gradually engage to launch the vehicle. When cruising in an odd numbered gear, clutch  32  is engaged. To shift to an even numbered gear, clutch  36  is disengaged (if it was not already disengaged), and either coupler  68  or  74  pre-selects the destination power flow path. After the destination gear is pre-selected, clutch  32  is released and clutch  36  is engaged in a coordinated fashion to transfer power between the corresponding power flow paths and adjust the overall speed ratio. 
     Clutches  32  and  36  may be either dry or wet friction type clutches. One or more friction plates are fixedly coupled one of the elements while a housing with a pressure plate and a reaction plate is fixedly coupled to the other element. The friction plates are between the pressure plate and the reaction plate. If there is more than one friction plate, they are separated by separator plates that are also fixedly coupled to the housing. When the clutch is fully disengaged, the reaction plate and the pressure plate are spaced apart such that the friction plate can rotate relative to the housing with minimal drag torque. To engage the clutch, an actuator causes a normal force that squeezes the friction plate(s) between the pressure plate and the reaction plate. The torque capacity of the clutch is proportional to the normal force and also proportional to the coefficient of friction. The coefficient of friction may depend upon the relative speeds and on other factors such as the clutch temperature. Ideally, the coefficient of friction varies continuously with changes in relative speed, but some clutch materials depart from this ideal behavior and exhibit a sharp reduction in coefficient of friction between no slip and some slip. If the elements are rotating at different speeds, the clutch exerts torque on each element equal to the torque capacity in a direction tending to equalize the speeds. If the elements are at the same speed, then the clutch transfers as much torque as is applied up to the torque capacity. If the applied torque exceeds the torque capacity, then the clutch slips creating relative speed. 
     Some clutches use position controlled actuation in which a controller commands the actuator to move to a specified position. The actuator may be linked to the pressure plate and reaction plate through springs such that the clutch normal force may be adjusted by adjusting the actuator position as illustrated in  FIG. 2 . As the actuator moves through the disengaged region at  80 , the normal force is zero. After the actuator position passes a touchpoint  82 , the normal force increases in proportion to changes in the actuator position. When the controller commands a change in direction of actuator position, the normal force may remain constant for some distance before changing direction as shown at  84 , due to hysteresis. Some clutches may respond to other ways of adjusting the normal force, such as adjusting a hydraulic pressure instead of adjusting an actuator position. These other mechanisms may also be characterized by a touchpoint and hysteresis. 
     SUMMARY 
     A transmission includes an input, an output, first and second intermediate shafts, first and second clutches, first and second couplers, and a controller. The first and second clutches are configured to selectively couple the input to the first and second intermediate shafts, respectively. The first and second couplers are configured to selectively establish power flow paths between the first and second intermediate shafts respectively and the output. The transmission has a first natural frequency when the first coupler is engaged and the second coupler is disengaged. The controller is programmed to command an oscillating actuator position of the second clutch with a dithering frequency at least 2.5 times the natural frequency to dampen resonance. A dithering amplitude may exceed a hysteresis. For example, the commanded actuator position may follow a square waveform, a saw-tooth waveform, or other waveform. The actuator position of the second clutch may oscillate about a nominal commanded actuator position which gradually decreases as the second clutch transitions from a fully engaged state to a slipping state. The actuator position may oscillate about a nominal commanded actuator position which gradually increases as the second clutch transitions from a fully released state to a slipping state during a transition interval. During the transition interval, the clutch may have positive torque capacity during a portion of each cycle and be fully released during a remainder of each cycle. During this interval, the controller may control the first clutch to actively dampen a measured speed oscillation. 
     A method of controlling a transmission clutch includes issuing an actuator command according to a sum of a nominal component and an oscillating component and increasing the nominal component from a first value to a second value. The first value is less than a touchpoint by more than the amplitude of the oscillating component while the second value is greater than the touchpoint by more than the oscillating component. The actuator command may be a position. The transmission may be a dual clutch transmission, in which case the clutch may be released in response to the speed of a shaft reaching a target value. A second clutch may be controlled in a slipping condition to dampen a measured speed oscillation. 
     A method of controlling a transmission clutch includes issuing an actuator command according to a sum of a nominal component and an oscillating component with an amplitude and, while the amplitude exceeds a hysteresis, adjusting the nominal component to transition between clutch states. For example, the clutch may transition from a fully released state to a slipping state or may transition from a fully engaged state to a slipping state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of an exemplary dual clutch transmission. 
         FIG. 2  is a graph illustrating a relationship between actuator position and clutch normal force including the effect of hysteresis. 
         FIG. 3  is a graph illustrating a commanded actuator position during a Clutch Before Synch (CBS) event according to a first control method. 
         FIG. 4  is a graph illustrating a commanded actuator position during a Clutch Before Synch (CBS) event according to a second control method that utilizes dither. 
         FIG. 5  is a graph illustrating a commanded actuator position during a clutch release event according to a first control method. 
         FIG. 6  is a graph illustrating a commanded actuator position during a clutch release event according to a second control method that utilizes dither. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     Referring to the transmission schematic of  FIG. 1 , the transmission is prepared for launch in a forward direction by sliding coupler  48  to couple gear  42  to shaft  26 , thus pre-selecting 1st gear. Then, to start the vehicle moving, the torque capacity of clutch  32  is gradually increased. Couplers  58 ,  68 , and  74  and clutch  36  may be disengaged during this process. Shaft  38  and gears  60 ,  62 ,  64 ,  66 ,  70 , and  72  all tend to remain stationary during this process because they have inertia and no torque acts upon them. In order to prepare for a shift into 2nd gear, gear  60  must be coupled to shaft  30  by sliding coupler  68 . Coupler  68  may have limited capability to quickly and smoothly synchronize the speeds of gear  60  and shaft  30  before engagement. Therefore, clutch  36  may be used to bring the speed of gear  60  close to the speed of shaft  30  before using coupler  68  to complete the synchronization and coupling. Using of one of the friction clutches  32  and  36  in this way is called a Clutch Before Synchronization (CBS) event. CBS events may be performed in many different gear ratios as part of the process of pre-selecting a new gear ratio. 
     During a CBS event, the torque capacity of the clutch must be accurately controlled at a level far below the clutch&#39;s maximum design torque capacity. In order to accomplish this, the actuator must be positioned very close to the touchpoint. Small errors in actuator position may result in either zero torque capacity or a torque capacity several times as large as desired. If the torque capacity is zero, synchronization does not happen. If the torque capacity is substantially more than desired, then the speed of the shaft rapidly increases past the target speed until the clutch becomes fully engaged. The clutch torque changes almost instantaneously as the clutch becomes fully engaged, which results in almost instantaneous changes in torque in many other components. These very rapid changes in torque can set off vibrations at a natural frequency of the transmission and driveline which vehicle occupants find unpleasant. 
       FIG. 3  illustrates one potential way to command a clutch actuator during a CBS event. Dotted line  86  represents the controller&#39;s best estimate of the actuator position corresponding to the touchpoint. Dotted line  88  represents the controller&#39;s best estimate of the actuator position corresponding to the desired torque capacity during the CBS event. At  90 , the controller commands the actuator position to a value slightly less than the estimated touchpoint. Then, the controller gradually increases the actuator position at a predetermined ramp rate at  92 , while monitoring one or more speed sensors to determine the progress of the event. When the controller determines at  94  that the clutch capacity is near the desired value, the controller commands a constant actuator position at  96 . At  98 , the controller determines that the CBS event is complete and commands the actuator to the disengaged position. In practice, this method has not proven sufficiently robust. Due to friction in the actuator mechanism, the actuator does not necessarily respond linearly to small changes in commanded position. Instead, it may stick and then jump by more than desired. Sometimes, the clutch torque capacity may suddenly change from less than the desired level to substantially more than the desired level. When this happens, the clutch may become fully engaged and set off a transmission and driveline vibration. 
       FIG. 4  illustrates an improved method of commanding a clutch actuator during a CBS event. Between  90  and  98 , the actuator command is a sum of a nominal component as indicated by  FIG. 3  and an oscillating component, as shown at  92 ′ and  96 ′. The oscillating component is called a dither. Preferably the dithering amplitude is greater than the hysteresis. The dither reduces the tendency of the actuator to stick in a position and then jump by more than desired. At the same time, dither removes the sharp change in slope of normal force as a function of actuator position near the touchpoint and replaces it with a more gradual change in slope. The result is that actuator hysteresis and touchpoint estimation errors are both mitigated by dither. The frequency of the dither is preferably at least 2.5 times the natural frequency to avoid exciting a vibration. Although a square waveform is illustrated in  FIG. 4 , other waveforms may also be employed such as a saw-tooth waveform or a sinusoidal waveform. The inventors have determined experimentally that the method of  FIG. 4  is substantially less likely to produce annoying vibrations than the method of  FIG. 3 . 
     In some cases, one of the friction clutches  32  or  36  will be in a slipping state while the other clutch performs a CBS event. For example, the clutch  36  CBS event to engage 2nd gear may occur during vehicle launch prior to clutch  32  being fully engaged. When a clutch is in a slipping condition, it is sometimes feasible to actively control the slipping clutch to reduce the magnitude of a vibration at a powertrain natural frequency. The controller determines the frequency and phase of the vibration using speed sensors or other sensors. Using this information, the controller varies the torque exerted by the slipping clutch at the same frequency as the vibration with a phase angle calculated to reduce the vibration. 
     The natural frequency of a transmission and driveline system changes depending upon the state of engagement of clutches and couplers. The system may have one natural frequency when a particular clutch is fully released, a second natural frequency when the clutch is slipping, and a third natural frequency when the clutch is fully engaged. During a CBS event, the system natural frequency will transition from the first value to the second value. If the CBS event results in an unintentional full engagement of a clutch, the natural frequency may even be equal to the third value for a portion of the time. When a CBS event is controlled according to the method of  FIG. 3 , the change in natural frequency occurs abruptly at the time that the actuator position crosses the touchpoint  86 . Therefore, active damping control using the other clutch is ineffective during the CBS event and may even re-inforce the vibration. Therefore, active damping control using the slipping clutch may be suspended during a CBS event. 
     When a CBS event is controlled according to the method of  FIG. 4 , the natural frequency changes gradually. During a portion of phase  92 ′, the actuator position rapidly alternates between positions on opposite sides of the touchpoint. The clutch spends a portion of the time in the fully open condition and a portion of the time in the slipping condition. Since the dithering frequency is substantially higher than either relevant natural frequency, the effective natural frequency during this period of time is a weighted average of the two natural frequencies with weighting factors based on the proportion of time spend in each clutch state. As the average position about which the dithering is performed gradually increases, the proportion of time spent in a slipping state gradually increases. Thus, the effective natural frequency gradually changes from the frequency associated with a released clutch to the frequency associated with a slipping clutch. By continuously monitoring the frequency of any vibration during this time period, the controller may be able to effectively dampen such vibrations using active control of the slipping clutch. 
     Clutch release events, like CBS events, can set off powertrain vibrations.  FIG. 5  illustrates one potential way to command a clutch actuator during a clutch release event. Dotted line  102  represents the controller&#39;s best estimate of the actuator position at which the torque capacity is equal to the current clutch torque based on the static coefficient of friction. At  104 , the controller commands the actuator position to a value slightly higher than  102 . Then, the controller gradually decreases the actuator position at a predetermined ramp rate at  106 , while monitoring one or more speed sensors to determine when clutch slip actually begins. When the clutch begins to slip, the torque capacity may drop due to a change in friction coefficient. While the clutch is slipping, the transmitted torque is equal to the torque capacity. Due to friction in the actuator mechanism, the actuator does not necessarily respond linearly to the small changes in commanded position at  106 . Instead, it may stick and then jump by more than desired. If this sudden change in actuator position occurs as the clutch begins slipping, it increases the magnitude of the torque change and increases the likelihood of setting of a vibration and/or increases the likely magnitude of such a vibration. At  108 , the controller continuously adjusts the commanded actuator position in order to maintain a desired level of clutch slip using feedback control based on speed measurements. Due to friction and hysteresis in the actuator mechanism, the actual torque capacity does not always respond in direct proportion to these small changes in commanded actuator position. This limits the ability of the controller to maintain the desired slip within a narrow band. If the desired slip is small, the clutch may become unintentionally fully engaged. If that happens, the change in friction coefficient will likely result in further torque fluctuations and increased likelihood of setting off vibrations. When torque is no longer desired, the actuator is commanded to a fully released position at  110 . 
       FIG. 6  illustrates an improved method of commanding a clutch actuator during a clutch release event. At  106 ′ and  108 ′, the actuator command is a sum of a nominal component as indicated by  FIG. 5  and an oscillating component called dither. Preferably the dithering amplitude is greater than the hysteresis. The dither reduces the tendency of the actuator to stick in a position and then jump by more than desired. The frequency of the dither is preferably at least 2.5 times the natural frequency to avoid exciting a vibration. Although a saw-tooth waveform is illustrated in  FIG. 6 , other waveforms may also be employed. Although  FIGS. 5 and 6  illustrate releasing a clutch from an intentional fully locked state, dither is also useful when maintaining a small degree of slip for an extended period. While attempting to maintain a low degree of slip using closed loop control, the clutch may accidentally become fully engaged. Dither makes accidental full engagement less likely and helps to better control the subsequent release back into the slipping state. Similarly, dither is useful while controlling the rate at which slip decreases to zero during an intentional full engagement and for controlling the transition from slipping to a fully released state. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.