Abstract:
A method of controlling a vehicle transmission during a gear change thereof, the method comprising operating a plurality of power shift clutches to disengage the power shift input gearing and the power shift output gearing, disengaging the driving connection between the range shift input shaft and the range shift output shaft by means of the range selection gearing, engaging at least two of the power shift clutches to vary the rotational speed of the power shift output shaft and hence the range shift input shaft, and operating the range selection gearing to establish a driving connection between the range shift input shaft and the range shift output shaft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 USC 119(a)-(d) to United Kingdom Patent Application No. GB 0619501.0 filed on Oct. 3, 2006, which is incorporated by reference in its entirety herein. 
   BACKGROUND OF THE INVENTION 
   This invention relates a method of controlling a vehicle transmission. ‘Vehicle transmission’ here refers to a transmission to be interposed between a prime mover of the vehicle and ground engaging means of the vehicle which provides a plurality of gear ratios by which the ground-engaging means can be driven by the prime mover. The transmission has been devised primarily, but not exclusively, for a material handling vehicle, such as, for example, a tractor, loader type or similar vehicle for industrial and/or agricultural use, in which the ground engaging means typically comprises wheels but could also comprise crawler tracks. 
   Vehicles such as tractors or loaders have conventionally used transmission which provide a range of forward gear ratios. With a range of such transitions that the vehicle needs to develop useful tractive effort over a wide range of speeds. For example, in bottom gear the vehicle may be designed to have a maximum speed of 1.5 kph and in top gear have a maximum speed of 65 kph, to permit the vehicle to operate at low speed whilst under load, but enable the vehicle to travel at an adequate speed when travelling along a road. 
   To provide a wide range of close ratios, it is known to provide a transmission having effectively a pair of gear boxes, a first gear box drivingly connected to the engine and providing a first range of ratios and a reverse gear, and a second gear box connected to the output of the first gear box and providing a plurality of selectable gear ranges. The output of the second gear box is then connected to the ground engaging means. Consequently, where the first gear box provides six gears and the second gear box provides four gear ranges, it will be apparent that there are 24 torque paths through the transmission. It would be apparent that there may be a number of different torque paths which provide the same or overlapping speed ranges. Ideally, the selected gear should use the lowest possible range gear provided by the second gearbox. Advantageously, gear selection is performed by an electronic control unit which controls a solenoid to supply fluid to power shift clutches or operates a solenoid to move the synchromesh clutches in response to a gear shift demand from the operator. However, a problem with the second gear box, when it uses synchromesh clutches, is that it is desirable to effect the range shift as quickly as possible. For example, when travelling up an incline, it is desirable that an up-shift is effected as quickly as possible, because a slow gear change can result in the vehicle slowing excessively or even rolling backward. This is because, unlike power shift gear boxes, synchromesh gear boxes do not permit an unbroken supply of torque during upshift as it is necessary for the intermeshing parts of the gears to synchronise. A fast gearshift will also improve operator ‘feel’, by presenting a more responsive transmission. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the invention, we provide a method and control unit for controlling a vehicle transmission during a gear change thereof, the vehicle transmission having a power shift gearbox and a range shift gearbox, the power shift gearbox having a power shift input shaft, a power shift output shaft, power shift input gearing and power shift output gearing, and a plurality of power shift clutches to drivingly connect the power shift input gearing and the power shift output gearing, the range shift gearbox having a range shift input shaft drivingly connected to the power shift output shaft, a range shift output shaft and range selection gearing to drivingly connect the range shift input shaft and range shift output shaft, the method comprising the steps of operating the plurality of power shift clutches to disengage the power shift input gearing and the power shift output gearing, disengaging the driving connection between the range shift input shaft and the range shift output shaft by means of the range selection gearing, engaging at least two of the power shift clutches to retard the rotational speed of the power shift output shaft and hence the range shift input shaft, and operating the range selection gearing to establish a driving connection between the range shift input shaft and the range shift output shaft. 
   The method may include the subsequent step of controlling the plurality of power shift clutches to establish a driving connection between the power shift input shaft and power shift output shaft. 
   The plurality of power shift clutches may comprise a first plurality of power shift clutches operable to connect the power shift input gearing to an intermediate gearing and a second plurality of power shift clutches operable to connect the intermediate gearing to the power shift output gearing, and wherein the step of operating the plurality of power shift clutches to disengage the power shift input gearing and the power shift output gearing may comprise disengaging the first plurality of power shift clutches. 
   The step of operating the power shift clutches may comprise dithering the fluid pressure supplied to the clutches. 
   The method may be carried out during an up-shift gear change. 
   The step of engaging at least two of the power shift clutches may retard the rotational speed of the power shift output shaft. 
   The step of operating at least two of the power shift clutches may comprise operating at least two of the second plurality of power shift clutches. 
   The rotational speed of the range shift input shaft may be retarded until the ratio of the rotational speeds of the range shift input shaft and the range shift output shaft reaches a desired value. 
   The desired value may correspond to the rotational speeds of the meshing gears of the range shift gearing being substantially equal. 
   The method may be carried out during a down shift gear change. 
   The step of engaging at least two of the powershift clutches may comprise engaging one of the first plurality of clutches and one of the second plurality of clutches. 
   The powershift clutches are operated to select a higher ratio torque path through the powershift gear box. 
   The method may comprise the step of receiving a gear shift request from an operator control, and identifying that the shift requires a range shift change. 
   The rotational speed of the range shift input shaft may be monitored by a first sensing means. 
   The rotational speed of the range shaft output shaft may be monitored by a second sensing means. 
   Data from the first and second sensing means may be compared to determine whether further variation of the speed of the input gearing is required. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings, wherein: 
       FIG. 1  is a sectional view of a transmission embodying the present invention, 
       FIG. 2  is a diagrammatic illustration of the transmission of  FIG. 1 , 
       FIG. 3 , is a diagrammatic illustration of an electronic control unit for use with the transmission in  FIG. 1 , 
       FIG. 4  is an illustration of a first method of operating the transmission of  FIG. 1 , and 
       FIG. 5  is an illustration of a second method of operating the transmission of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   Referring now to  FIGS. 1 and 2 , a transmission embodying the present invention is generally shown at  10 . The transmission  10  comprises a first, power shift, gearbox generally shown at  11  and a second, range shift gear box generally shown at  12 . The power shift gear box  11  has an input connection shown at  13  to receive drive from an engine or other prime mover. The engine input  13  is connected via a main clutch  14  to an input shaft  15  which drives input gearing generally shown at  16 . The input gearing  16  comprises a main gear  17  supported on the input shaft  15 , which drives a first input gear  18  and a second input gear  19  through idler wheels at  18   a ,  19   a  respectively on  FIG. 2 . 
   The power shift gear box  11  further comprises an output shaft  20  which in this example is co-axial with the input shaft  15 . The power shift gearbox  11  has an output gearing as generally shown as  21  comprising a first output gear  22  and second output gear  23  mounted on the output shaft  20 . The first output gear  22  is in mesh with third output gear  24  and second output gear  23  is in mesh with fourth output gear  25 . 
   The power shift gear box  11  further comprises intermediate gearing shown at  26  comprising a first intermediate gear shown at  27 , a second intermediate gear shown at  28  and a third intermediate gear shown at  29 . The second intermediate gear  28  is in mesh with intermediate gears  27  and  29 . 
   To provide torque paths through the power shift gear box, six power shift clutches are provided labelled A, B, C, D, E and R. Power shift clutch A is operable to provide a driving connection between the first input gear  18  and third intermediate gear  29 ; power shift clutch B is operable to provide a driving connection between second input gear  19  and first intermediate gear  27 ; power shift clutch C is operable to provide a driving connection between first intermediate gear  27  and the third output gear  24 ; power shift clutch D is operable to provide a driving connection between the third intermediate gear  29  and the fourth output gear  25 ; and power shift clutch E is operable To provide a driving connection between the second intermediate gear  28  and the output shaft  20 . 
   Power shift clutch R is operable to provide a reverse gear, and is operable to connect the input shaft  15  to second intermediate gear  28 . 
   Power shift clutches A, B and R provide a first plurality of clutches which are referred to as being on the “input side” of the first gear box  11  as they are operable to connect the input gearing  16  to the intermediate gearing  26 . Power shift clutches C, D, and E provide a second plurality of the output clutches and are referred to as being on the “output side” of the first gear box  11  as they are operable to connect the output gearing  21  to the intermediate gearing  26 . A torque path is provided by engaging one of the first plurality of power shift clutches and one of the second plurality of power shift clutches. Consequently, it will be apparent that there are nine torque paths through the first gear box  11 , six forward gears and three reverse gears. 
   The output shaft  20  drives a power shift output gear  30   a  which supplies drive to the range shift gear box  12  through gear  30   b.    
   The range shift gear box  12  comprises a range shift input shaft  31  driven through gear  30   b  and a range-shift output shaft  32 . The range shift output shaft  32  is connected via forward output gearing generally shown at  33  to provide front wheel drive to the front wheel axle assembly generally shown at  34 , and is also connected through the rear output gearing generally shown at  35  to provide drive to a rear axle assembly generally shown at  36 . The range shift gear box  12  comprises range shift gearing  137  to drivingly connect the range shift input shaft  31  and the range shift output shaft  32 . 
   In this example, the range selection gearing  137  comprises four pairs of gears, one gear of each pair being mounted on the range shift input shaft  31  and the other on the range shift output shaft  32 . To provide a high ratio, the first range shift input gear  37  is mounted on the range shift input shaft  31  and is in mesh with first range shift output gear  38  provided on the range shift output shaft  32 . Similarly, to provide a medium gear ratio, second range shift input gear  39  is in mesh with second range shift output gear  40 ; third range shift input gear  41  is in mesh with third range shift output gear  42  to provide a low range and fourth range shift input gear  43  is in mesh with fourth range shift output gear  44  to provide an extra low gear range. A first synchromesh clutch is provided as shown at  45  to selectively clutch one of first range shift gear  37  and second range shift gear  39  to the range shift input shaft  31 . A second synchromesh clutch  46  is provided operable to clutch one of the third range shift output gear  42  and fourth range shift output gear  44  to the range shift output shaft  32 . The first synchromesh clutch  45  and second synchromesh clutch  46  are interlocked such that only one of the clutches  45 ,  46  can be engaged at a time. Consequently, it will be apparent that there are four possible torque paths through the range shift gear box  12 . Each torque path is selectable by operating one of the first synchromesh clutch  45  and the second synchromesh clutch  46 , while the other of the first synchromesh clutch  45  and the second synchromesh clutch  46  is in neutral, to provide the desired driving path. 
   When operated the synchromesh clutches  45 ,  46  and powershift clutches may be ‘dithered’, that is the fluid pressure supplied to the powershift clutches or the electrical power sent to the synchromesh clutches may be varied slightly about the control level to avoid ‘stickiton’. 
   To control the transmission  10 , an electronic control unit is provided as diagrammatically illustrated in  FIG. 3 . The control unit  50  receives input on line  51  from an operator control requesting a gear shift. The control unit  50  is also operable to receive signals on line  53  from sensors generally shown at  54 ,  55  in  FIGS. 1 and 2  to measure the speed rotation of the range shift input shaft  31  and range shift output shaft  32 . The controller is operable to control the synchromesh clutches  45 ,  46 , as shown by lines  54 , and to control the power shift clutches A, B, C, D, E, R as illustrated by lines  55 . In general. the controller  50  is operable to receive an input from the operator control  52  and control the power shift clutches and synchromesh clutches accordingly to provide the next desired gear or torque path through the transmission  10 . 
   When an up-shift gear change in the range shift gear box  12  is required, a first method of controlling the gear change is shown in the flow diagram of  FIG. 4 . The gear change may be requested by an operator through an operator control  52 , or by an automatic gear selection system. The demand for a gear change is sent to the controller  50  at step  60 . At step  61 , the controller  50  verifies that the gear change requires an up-shift in the range shift gear box  12 , and may at this stage notify the operator, for example by operating a display or sounding an audible alert. The input gearing  16  and output gearing  17  of the first gearbox  11  are disengaged, as shown in step  62 . At step  63 , the range shift selection gearing is disengaged, by operating the first synchromesh clutch  45  or second synchromesh clutch  46  so that both synchromesh clutches  45 ,  46  are in neutral. 
   At step  64 , a pair of the clutches of the second plurality of clutches (C, D, E) are engaged. In this example, this is clutch pair C and D as these are the lowest ratio gears but any appropriate pair of clutches may be engaged as desired. Whichever clutch pair is used, the inter-meshing pair of gears in the output gearing  21  will be attempting to drive meshing pairs of the intermediate gearing  26  at different speeds, with a consequent retardation of the shaft  20  and correspondingly if the range shift gear box input shaft  31 . The clutch pair can be pulsed appropriately to provide a desired retardation until the rotational speeds of the range shift input shaft  31  and range shift output shaft  32  are at a desired ratio. In particular the desired ratio may be selected such that the rotational speeds of the meshing part of the required input and output gear of the next selected gear of the range shift gear box  12  are substantially equal. In this example, the speed sensors  54 ,  55  are used to detect the rotational speed of the shafts  31 ,  32 , but it will be apparent that sensors may be located anywhere else as appropriate. 
   Alternatively, a predetermined dwell time may be allowed to elapse. This is a pre-programmed operational time in which the clutch pair is operated depending on which gear is being selected. This alternative may not require data from sensors  54 ,  55 . 
   At step  66 , the selected gear is engaged by operating the appropriate synchromesh clutch  45 ,  46  and selecting the appropriate gears from the input side clutches A, B, R and output side clutches C, D, E of the power shift gear box  11 . 
   When a down-shift gear change in the range shift gear box  12  is required, a second method of controlling the gear change is shown in the flow diagram of  FIG. 5 . A gear change  70  is requested, for example by an operator through an operator control  52  or through an automatic gear selection system. At step  71 , the controller  50  verifies that the gear change requires a down-shift In the range shift gear box  12 , and may at this stage notify the operator, for example, by operating a display or sounding an audible alert. The input gearing  16  and output gearing  17  of the first gear box  11  are disengaged as shown in step  72 . At step  73 , the range shift selection gear may be disengaged, by operating the first synchromesh clutch  45  or second synchromesh clutch  46  such that both synchromesh clutches  45 ,  46  are in neutral. At step  74 , a pair of power shift clutches are engaged comprising one of the first plurality of clutches and one of the second plurality of clutches. In this example, this is the clutch pair B and E, connecting the input gearing  16  and  17  output gearing to provide a torque path through the power shift gear box  11 . In this example, B and E correspond to the highest ratio torque path through the gear box  11 . As the output shaft  20  is in driving connection with the input shaft  15 , the range shift input shaft  31  is consequentially being driven through the power shift gear box  11 . Accordingly, the range shift input shaft  31  is sped up, bringing the input gear of the range shift gear box  12  closer to the synchroniser speed. As shown at step  75 , the range shift input shaft  31  can be driven as desired, for example by using a standard dwell time, or by using the speed sensors  54 ,  55  to detect the rotational speeds of the shafts  31 ,  32 , or otherwise as desired. At step  76 , none of the first synchromesh clutch  45  and second synchromesh clutch  46  is operated to select the appropriate gear ratio in the range shift gear box  12 . By bringing the speeds of the input and output shafts  31 ,  32  together, this reduces the amount of work that the synchronisers of the synchromesh clutches  45 ,  46  have to perform to engage the selected gear, which result in a faster gear shift, less wear on the synchronisers, and improved operator feel. 
   In either method, where the power shift clutches are released, it may be preferable only to partially release the power shift clutches. In the present example, the power shift clutches A, B, C, D, E, R typically have 4 mm of travel, but only 0.25 mm of that range of travel brings the clutches into engagement. Accordingly by only partially releasing the power shift clutches during steps  63  and  73 , the time taken to re engage the clutches can be reduced as it is not necessary for the clutches to move across the full range of travel. 
   Consequently, by operating the gear box in this manner, it is possible to speed up gear shifts involving a change in range selection, providing improved operation and operator comfort. 
   When used in this specification and claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. 
   The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.