Abstract:
The method of estimating and controlling driveline torque in a continuously variable hydro-mechanical transmission uses pressure data and other metrics of a hydrostatic power unit of the transmission in lieu of actual driveline torque data. A mechanical efficiency of the transmission is determined as a function of whether the power unit is operating in a power generation or regeneration mode, and the torque output of the power unit is estimated from that and other hydrostatic parameters. This is used to estimate a torque output of a planetary power unit of the transmission, and the torque on an output member of the driveline is then estimated using that value, and appropriate corrective action taken.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to estimating and controlling driveline torque in a continuously variable hydro-mechanical transmission, and more particularly, to a method that does so using pressure data and other metrics of the hydrostatic unit, in lieu of actual driveline torque data. 
       BACKGROUND ART 
       [0002]    Continuously variable hydro-mechanical transmissions are used in a variety of work machines, including for construction, earth moving, forestry, and agriculture. Reference in this regard, Weeramantry, U.S. Pat. No. 7,063,638 B2, issued Jun. 20, 2006, which discloses a representative continuously variable hydro-mechanical transmission. Typically, a continuously variable hydro-mechanical transmission will have a hydrostatic unit as one power input to a planetary gear set, and a mechanical connection to the engine of the machine as a second power input, with the output of the planetary connected via a clutch to one or more final gear reductions in connection with a load, e.g., the wheels, tracks or other drivers of the machine. 
         [0003]    An advantage of continuously variable hydro-mechanical transmissions is that they can provide a large speed range seamlessly. As another advantage, continuously variable hydro-mechanical transmissions are typically capable of lower gear ratios than transmissions with fixed gear ratios. As result, the engine and transmission combination can produce higher torques to the wheels, tracks, or other drivers, which is beneficial as it enables the work machine to pull harder. However, the higher torque can damage mechanical aspects of the transmission, particularly, the final gear reduction or output member of the driveline of the transmission. Typically, it is been found that damage to the final gear reduction or output member will occur if the torque is too high for a prolonged period of time. 
         [0004]    In agricultural applications, such as wherein a work vehicle such as when a tractor is pulling a large implement, or a deep subsurface tillage tool, a heavy wagon or cart, or the like, potentially damaging continuous high torque loads can be placed on the transmission driveline. Damage from intermittent or incidental high loads can also result from ground conditions, e.g., inclines, ruts, deep furrows, wet spots, transitions onto roads, and the like, when driving, and from contact with denser soil, buried objects such as stones or rock formations, large roots, and the like when doing subsurface tillage. 
         [0005]    To avoid such damage, one alternative is to limit engine torque output. However, often the engine supplies power to other systems of the work machine, e.g., auxiliary hydraulics, power take offs, and the like, and it can be problematic to reduce toque output to those systems also. The torque loads of these other systems typically vary and may be unknown, making accurately adjusting engine torque difficult. As another alternative, the transmission torque can be determined using an estimate of the engine torque and subtracting the torque loads of the other systems, or using maximum torque values for those systems, but this is often more complex, more costly and less accurate than desired. 
         [0006]    Thus, what is sought is a manner of determining driveline torque of a continuously variable hydro-mechanical transmission of a work machine, particularly in the vulnerable final gear reduction of the driveline, and limiting the torque for preventing damage to the transmission, without the shortcomings set forth above. 
       SUMMARY OF THE INVENTION 
       [0007]    What is disclosed is a method of estimating driveline torque of a continuously variable hydro-mechanical transmission of a work machine, particularly in the final gear reduction or output member, and limiting the torque for preventing damage to the transmission, without the shortcomings set forth above. 
         [0008]    During operation of a continuously variable hydro-mechanical transmission, operating parameters of the hydrostatic power unit, mainly a swash plate angle of a variable displacement pump, and/or the ratio of the final gear reduction of the driveline, will be automatically varied, continuously if required, by the transmission controller, to achieve and hold an inputted command, usually a speed command. The pump of the hydrostatic power unit is drivingly connected to the engine of the work machine, and the fluid motor is drivingly connected to the planetary power unit. In a power generation mode, the pump operates to pump pressurized fluid through the motor at a rate determined by the engine speed, a ratio of connecting gears, and the swash plate angle, to rotate the motor, which, in turn, drives an element of the planetary power unit, usually the ring gear. The direction of rotation is also determined by the swash plate angle. In a regeneration mode, the direction of power through the hydrostatic power unit is reversed, and the ring gear of the planetary power unit drives the fluid motor, operating it as a pump, and the pump as a motor. 
         [0009]    It has been observed that the fluid pressure condition in the hydrostatic power unit will be high when the transmission driveline is subjected to high torque loads, which is of concern for the purposes of the present invention, typically when the machine is moving slowly, or is stationary, under heavy load. An operational example would be a tractor pushing or pulling a heavy load, or towing an implement such as a deep subsurface tillage tool. As noted above, if prolonged, damage to the driveline will likely result, so it is desired to avoid this. 
         [0010]    It has also been observed that the hydrostatic power unit will have a mechanical efficiency which is a function of the pressure in that unit, swash plate angle, and pump speed. The efficiency will have a value of less than 1 when in the generation mode, and greater than 1 when in the regeneration mode. The motor of the hydrostatic power unit will have a mechanical efficiency which is a function of the pressure in that unit. Again, this pressure will be important for the purposes of the present invention only when high, approaching relief pressure, when potentially damaging driveline torque conditions are likely to be present. The driveline torque can reach a potentially damaging high level when the hydrostatic power unit is in the generation mode wherein the pump of that unit is functioning as a pump, and also when in the regeneration mode when the pump is being driven by the motor. The efficiency of the motor will preferably be determined by testing at least one high pressure for each operating mode, and recorded for later use. The efficiency value will be greater than 1 for the regeneration mode, and less than 1 for the generation mode. 
         [0011]    According to one aspect of the invention, it has been found that the pressure in the hydrostatic power unit will provide an indication of the torque on that unit, and if the operating mode, e.g., generation, regeneration, and the mechanical efficiency of the fluid motor and direction of operation thereof are known at the pressure, a relatively accurate estimation of the torque on the fluid motor can be made. In turn, the torque on the output of the planetary power unit can be estimated as a function of the torque on the fluid motor and ratios of gears of the planetary unit and those connecting it with the motor. The accuracy of this torque can be increased by knowing the efficiency of the planetary unit. The torque load on the driveline, particularly on the output member thereof, can then be estimated as a function of the estimated torque on the planetary unit output, and the ratio of gears connecting the planetary unit to the driveline output member. 
         [0012]    Thus, according to a preferred aspect of the invention, a method of the invention includes a step of monitoring operation of the hydrostatic power unit of the transmission to determine whether that unit is operating in a generation mode or a regeneration mode. The method will then determine a mechanical efficiency of the hydrostatic power unit, in particular the motor thereof, as a function of at least the operating mode, a pressure therein and an operating speed thereof, e.g., motor speed. This can be, for instance, a selected constant value for a particular high pressure value, or it can be a stored value previously determined from testing as noted above. The torque output of the hydrostatic power unit will then be estimated as a function of at least the pressure therein and the mechanical efficiency of the motor. 
         [0013]    As a next step according to the invention, the torque output of the planetary power unit will be estimated, as a function of the estimated torque output of the hydrostatic power unit, a ratio of gears drivingly connecting the hydrostatic power unit to the planetary power unit, and ratios of the gears of the planetary unit. The torque on the output member of the driveline of the transmission will then be estimated as a function of the estimated torque output of the planetary power unit and ratios of gears drivingly connecting the planetary unit to the output member. 
         [0014]    According to another aspect of the invention, if the estimated torque on the output member is greater than a predetermined value, for instance, a threshold value above which damage to the driveline is likely to occur, then an operating parameter of the transmission will be changed to reduce the torque. As one example, if the machine is moving, the speed of movement can be lowered, but preferably without reducing engine speed, such that other systems run by the engine are not affected. As another example, the swash plate angle can be changed, to lower the pressure in the hydrostatic power unit and thus the torque output thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a side view of a representative work machine including a continuously variable hydro-mechanical transmission controlled according to the method of the invention; 
           [0016]      FIG. 2  is a simplified schematic representation of the work machine of  FIG. 1 , showing one of the embodiments of the transmission; 
           [0017]      FIG. 3  is a simplified schematic representation of the work machine, showing another embodiment of the transmission; 
           [0018]      FIG. 4  is a simplified schematic representation of another embodiment of the transmission; 
           [0019]      FIG. 5  a partial sectional view of an exemplary embodiment of a planetary power unit for the transmission of  FIG. 2 ; 
           [0020]      FIG. 6  is a high level flow diagram showing steps of the method of the invention; and 
           [0021]      FIG. 7  is a partial view of an exemplary embodiment of a planetary power unit for the transmission of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Referring now to the drawings, in  FIG. 1  a work machine  1  is shown, which is a tractor representative of those that can be used for a variety of uses, including, but not limited to, agriculture, construction, earth moving and forestry. Work machine  1  includes a power source  4  which will be, for instance, an internal combustion engine, and is mechanically coupled to a continuously variable hydro-mechanical transmission, three representative variants or embodiments of which are represented by numbers  10 A,  10 B and  10 C, like parts of which being identified by like numbers. Each of transmissions  10 A,  10 B and  10 C is controllably operable according to the method of the invention, for estimating and limiting driveline torque of the transmission, and the transmissions shown are intended to be exemplary of a wide range of possible hydro-mechanical architectures wherein the power is split between paths and different ranges are used, with which the present invention can be used. 
         [0023]    Referring also to  FIGS. 2 ,  3  and  4 , each of transmissions  10 A,  10 B and  10 C includes a hydrostatic power unit  12  and a planetary power unit  30  which are coupled to a driveline including a range gear set  58  mounted within a transmission housing  11  and coupled to a load L which here is the drive wheels of machine  1  as shown in  FIG. 1 . It should be understood that machine  1  can alternatively include a load L that comprises a track drive, or an operating system of the machine such as but not limited to, a power take off (PTO). 
         [0024]    Referring in particular to  FIG. 2 , hydrostatic power unit  12  of transmission  10 A includes a fluid pump  16  coupled by fluid conduits  17  in a closed loop to a fluid motor  18 . Power unit  12  includes a first input shaft  14  drivingly connected to pump  16  and a first output shaft  20  drivingly connected to motor  18 . Power unit  12  is coupled to a synchronous lockup clutch  24  by first output shaft  20 . Depending upon the desired speed of work machine  1  or the desired rpm of the load L, inputted to a processor based controller  100  by an input device  102  located preferably in operator cab  104  of machine  1 , clutch  24  will be automatically actuated by controller  100  to couple drive gear  26  to input shaft  36 , or drive gear  28  to input shaft  40 , to select an appropriate hydrostatic input gear range. At the same time, controller  100  also adjusts the angle of a swash plate of pump  16 . As an exemplary embodiment, pump  16  can be an electronically controlled variable displacement hydraulic pump. A hydrostatic power unit driving gear  7  coupled to the input shaft  6  from the power source  4  with the hydrostatic power unit driving gear  7  engaging a hydrostatic power unit driven gear  8  that is coupled to the first input shaft  14  drives the hydrostatic power unit  12 . 
         [0025]    Planetary power unit  30  is coupled to the power source  4  with a second input shaft  32  and the input shaft  6 . The planetary power unit  30  also includes a third input shaft  36 , a fourth input shaft  40  and a second output shaft  44 . The second input shaft  32 , the third input shaft  36 , the fourth input shaft  40  and the second output shaft  44  are all coaxial with the second input shaft  32  inside the hollow third input shaft  36  which in turn is within the fourth input shaft  40  as shown in  FIG. 5 . The planetary power unit  30  is selectively coupled to the load L; selectively coupled to the hydrostatic power unit  12 ; and coupled to the power source  4 , automatically by controller  100  utilizing various clutches as will be described below. The hydro-mechanical transmission  10 A also includes a load shaft  60  which is coupled to the load L and mounted for rotation in the housing  11 . An intermediate shaft  56  rotatably mounted in the housing  11  supports a range gear set  58  mounted for rotation in the housing  11  and selectively coupled to the planetary power unit  30  and the load shaft  60 . 
         [0026]    The planetary power unit  30  comprises a primary sun gear  34  coupled to the second input shaft  32 , which is directly coupled to the power source via input shaft  6 . A secondary sun gear  38  is coupled to the third input shaft  36 , which is selectively coupled to the first output shaft  20  by synchronous lockup clutch  24  under control of controller  100 . A ring gear  42  is coupled to the fourth input shaft  40 , which is selectively coupled to the first output shaft  20  also with the synchronous lockup clutch  24  under control of the controller. A compound planetary gear cluster  46  mounted on a compound planetary gear carrier  48  engages with the primary sun gear  34 , the secondary sun gear  38  and the ring gear  42 . The compound planetary gear carrier  48  is coupled to the second output shaft  32  of the planetary power unit  30 . Compound planetary gear carrier  48  supports three compound planetary gears  47  which make up the compound planetary gear cluster  46 . 
         [0027]    The synchronous lockup clutch  24  is controlled by controller  100  to selectively engage driving gears  26  and  28  which engage third input shaft  36  and fourth input shaft  40 , respectively. When driving gear  26  is driven by the hydrostatic power unit  12 , it drives the secondary sun gear  38 . When driving gear  28  is driven by the hydrostatic power unit  12 , it drives the fourth input shaft  40 , which in turn drives the ring gear  42  within planetary power unit  30 . The above described power transmissions occur in the upstream side of unit  30  of the hydro-mechanical transmission  10 A. On the down stream side of unit  30  a single output shaft, designated as the second output shaft  44  is coupled within unit  30  with the compound planetary gear carrier  48 . The second output shaft  44  is coupled to the directional clutch  50 , which has a forward component  54  and a reverse component  52  which respectively drive gears  55  and  53  to control the forward or reverse directions of the work machine  1 , as selected by the operator through controller  100 . 
         [0028]    Intermediate shaft  56  is rotatably mounted in the housing  11  and supports a road range input gear  62 , which in turn engages a road range output gear  64  mounted on the load shaft  60 . A work range input gear  66  coupled to the intermediate shaft  56  engages a work range output gear  68  also mounted on the load shaft  60 . A reverse gear  70  is coupled to the intermediate shaft  56  and engages an idler reverse gear  72  mounted on the load shaft  60 . A range selector  74  is coupled to the load shaft and is controlled by the operator of machine  1  to select either the road range speeds or the work range speeds. In an exemplary embodiment of the hydro-mechanical transmission, the range selector  74  is a sliding collar or synchronizer  76 . 
         [0029]    Once the operator selects between the working range and road range speeds, controller  100  will automatically control the pump swash plate angle in the hydrostatic power unit  12  and the selection of one of the drive gears  26  or  28  coupled to the first output shaft  20  to achieve speed control. In low speeds, the hydrostatic drive is driven through ring gear  42 , which is coupled to the fourth input shaft  40  and is driven by driving gear  28 . The gear ratios in the planetary power unit  30  are designed so that a synchronous condition will occur at the most desirable speed within a given working range. With machine  1  starting from rest, the swash plate angle of the hydraulic motor  18  is automatically increased in order to increase machine or rpm speed until a synchronous speed is reached (i.e., the two sun gears,  34  and  38 , the ring gear  42  and the planet carrier  48 , supporting the compound planetary gear cluster  46  all rotate at the same speed). At that same speed, the synchronous lockup clutch  24  will be automatically actuated to disengage driving gear  28  and engage driving gear  26  to drive the secondary sun gear  38 . With such change occurring automatically at a synchronous speed it is “seamless” with little or no energy dissipation. With the hydrostatic drive power being delivered through the secondary sun gear  38 , the swash plate angle is reduced to increase speed of the compound planetary gear carrier  48  until a maximum speed of machine  1  is reached. It is also possible to engage both drive gears  26  and  28  with the synchronous lockup clutch  24  and with disconnect clutch  22  disconnecting output shaft  20  in which all gears of the planetary power unit  30  will be transmitting power and thereby providing a very high efficiency through the hydro-mechanical transmission  10 A. Under some operating conditions, controller  100  will completely disengage the hydrostatic power unit  12  from the planetary power unit  30  through the hydrostatic disconnect clutch  22 . In such instance, only direct mechanical power from the power source  4  is provided to the planetary power unit driving only the primary sun gear  34  which in turn drives the compound planetary gear cluster  46  and the second output shaft  44 . 
         [0030]    It is also possible for a full shuttle reverse in either the work range or road range by means of the directional clutch  50 . Since the directional change occurs downstream of the planetary power unit  30 , it is not necessary to change the swash plate position of the pump  16  in the hydrostatic power unit  12  if the same forward to reverse ratio is retained. 
         [0031]    The configuration of the hydro-mechanical transmission, described above provides that the synchronized ratio change gear speeds takes place on the input side (upstream side) of the planetary power unit  30  in the hydrostatic power unit  12  with only one output shaft  44  from planetary power unit  30 , under control of controller  100 . 
         [0032]    The control of the various clutches and the swash plate angle of the pump  16  in the hydrostatic power unit  12 , will be automatically controlled by controller  100 , using actuators  106  connected to controller via suitable conductive paths  108 , which can be wires of a wiring harness, a wired or wireless communications network or the like, and which also connect to input device  102 . Transmission  10 A also includes appropriate sensors, including pressure sensors  110  for sensing pressure conditions in conduits  17  connecting pump  16  and motor  18 , and speed sensors  112  for sensing a speed of first output shaft  20  and a speed of load shaft  60 , all connected to controller  100  via conductive paths  108 . Controller  100  is connected to power source  4 , also via conductive paths  108 , to receive data such as speed data, e.g., of input shaft  6 , therefrom. 
         [0033]    Referring in particular to  FIG. 3 , the second embodiment of a hydro-mechanical transmission  10 B eliminates the operator preselected work range or road range of speeds per se. However, seamless speed changes from zero to a maximum speed, such as 50 km per hour can be obtained through four gear ranges defined as range “A”, “B”, “C”, and “D” with synchronized shift points between each range to obtain the seamless speed changing. In this embodiment, the synchronized ratio changing is automatically controlled by a controller  100  and takes place on the output side (downstream side) of the compound planetary power unit  30  which has two coaxial output shafts  44  and  45 . As with transmission  10 A above, controller  100  is connected to the various actuators  106  of the clutches and pump  16 , pressure sensors  110 , and speed sensors  112 , and also to input device  102  and power source  4 , for receiving commands and data, via conductive paths  108 . 
         [0034]    Power source  4  of hydro-mechanical transmission  10 B selectively drives hydrostatic power unit  12  and planetary power unit  30 , which in turn drives a plurality of range gear sets  58  which are coupled to a load L, which, again, will typically be the wheels or tracks of machine  1 . Hydrostatic power unit  12  as shown in  FIG. 3  is contained within the hydro-mechanical transmission housing  11  but it may also be external to the housing  11  and accessed with appropriate couplings. The hydrostatic power unit  12  includes a pump  16  coupled to a motor  18  with the hydrostatic power unit  12  coupled to a first input shaft  14  and a first output shaft  20 . The power to the hydrostatic power unit  12  is provided by a driven gear  8  mounted on the first input shaft  14  and engaged with a hydrostatic power unit driving gear  7  mounted on the input shaft  6  of the power shaft  4 . The pump  16  is in fluid communication with the motor  18  by appropriate conduits  17 . The first output shaft  20  rotatably supports a gear for engaging a third input shaft of unit  30  as described below. 
         [0035]    Planetary power unit  30  of transmission  10 B includes a second input shaft  32 , a third input shaft  36 , a second output shaft  44  and a third output shaft  45  (see  FIG. 7  also). Unit  30  is selectively coupled to the load L, coupled to the hydrostatic power unit  12  and selectively coupled to the power source  4 . The unit  30  can be connected to a plurality of range gear sets  58  as will be described below. The second input shaft  32 , the third input shaft  36 , the second output shaft  44 , and the third output shaft  45  are coaxial with the third input shaft being hollow and the second input shaft  32  being supported within the third input shaft  36 . The second output shaft  44  is hollow and third output shaft  45  is supported within the hollow second output shaft  44 . The hydro-mechanical transmission  10 B also includes a load shaft  60  coupled to the load L and mounted for rotation in the housing. An intermediate shaft  56  supporting a plurality of range gear sets  58  is mounted for rotation in the housing and selectively coupled to unit  30  and the load shaft  60 . 
         [0036]    The planetary power unit  30  of the hydro-mechanical transmission  10 B comprises a primary sun gear  34 , which is coupled to the second input shaft  32 . A ring gear  42  is coupled to the third input shaft  36  and coupled to the first output shaft  20  with the hydrostatic power unit with the gear  26  engaging the third input shaft  36 . A compound planetary gear cluster  46  mounted on a compound planetary gear carrier  48  and engaged with the secondary sun gear  38  and the ring gear  42  is mounted within unit  30 . A compound planetary gear carrier  48  is coupled to the second output shaft  44 . The compound planetary gear cluster  46  includes three compound planetary gears  47 . 
         [0037]    In operation, the continuously variable hydro-mechanical transmission  10 B can be operated to have a combined hydrostatic and mechanical power flow by engaging the reverse clutch  52  or forward clutch  54  which respectively drive a reverse drive gear  53  and a forward drive gear  55  which in turn drives the first input shaft  20  and the second input shaft  32 . It is also possible to operate the hydrostatic mechanical transmission  10 B for a pure hydrostatic power flow by disengaging both clutches  52  and  54  in which case the second input shaft  32  is not directly driven by the power source  4 . In the pure hydrostatic mode, one range gear is coupled to carrier  48  and another range gear  58  is connected to the secondary sun gear  38  simultaneously. 
         [0038]    The plurality of arranged gear sets  58  comprise an A-range output gear  80  coupled to the intermediate shaft  56  and engaged with an A-range input gear  82  mounted on the second output shaft  44 . A B-range output gear  84  is coupled to the intermediate shaft  56  and engaged with a B-range input gear  86  mounted on the third output shaft  45 . A C-range output gear  88  coupled to the intermediate shaft  56  and engaged with a C-range input gear  90  is mounted on the second output shaft  44 . A D-range output gear  92  is coupled to the intermediate shaft  56  and engaged with D-range input gear  94  mounted on the third output shaft  45 . A plurality of range selectors  74  are coupled to the intermediate shaft to provide the selection of range gear sets, under control of controller  100 . A typical range selector  74  in this exemplary embodiment is a clutch  77  associated with the respective range gear sets. A main input drive gear  96  is coupled to the intermediate shaft  56  and engaged with a main output drive gear  98 , which is mounted on the load shaft  60 . 
         [0039]    As stated above in this embodiment, there is no selection for a work range or road range per se. However, the four ranges (A-D) provide a seamless transition between ranges similar to the work/road configuration previously described. Speed change from zero to maximum speed is achieved in a smooth and continuous manner by changing the swash plate angle of the pump  16  under control of controller  100 . For high efficiency, the first stall point of the motor  18  in the hydrostatic power unit  12  (i.e., ring gear  42  is a relative zero speed point) is selected in the 7 to 9 km per hour optimum speed range in order to transmit 100% of the power from the power source  4 . A full shuttle reverse is also available through the clutches  52  and  54  since the directional change occurs on the input side (upstream side) of the planetary power unit  30 . Since directional changes occur on the input side of unit  30 , it may be necessary to adjust the position of the swash plate in motor  18  depending upon the desired forward to reverse speed change ratio, and this is done automatically by controller  100 . In the low speed pure hydrostatic power flow regenerative heat is kept under control during prolonged creep operation of the work machine  1 . Also, in the pure hydrostatic power flow mode, different creep speed ranges can be achieved by engaging different combinations of the range clutches. For example, range gear set A,  80 ,  82  and B range set  84 ,  86  can be simultaneously engaged through their respective range selectors  74 . Similarly, range set  80  can be combined with C or D to obtain a different creep speed range as selected by the operator of the work machine  1 . With this embodiment, it is also possible to shuttle between forward and reverse in either the combined hydro-mechanical mode or the pure hydrostatic mode. Further, in this embodiment, the machine speed can be controlled independent of engine speed enabling constant output speed from the PTO during implement operation. 
         [0040]    Referring in particular to  FIG. 4 , the third embodiment of a hydro-mechanical transmission  10 C, like embodiment  10 B just discussed, eliminates the operator preselected work range or road range of speeds per se. Again, seamless speed changes from zero to a maximum speed, such as 50 km per hour can be obtained through four gear ranges defined as range “ 1 ”, “ 2 ”, “ 3 ”, and “ 4 ” with synchronized shift points between each range to obtain the seamless speed changing. The synchronized ratio changing is automatically controlled by the controller and again takes place on the output side (downstream side) of the planetary power unit  30  which is constructed in the above described manner and has two outputs: a secondary sun gear NS 2 , and planetary gear carrier N 13 . As with transmissions  10 A and  10 B above, the controller is connected to the various actuators  106  of the clutches and pump  16 , pressure sensors  110 , and speed sensors  112 , and also to an input device and power source  4  which is an engine, via conductive paths  108 . 
         [0041]    Power source  4  of hydro-mechanical transmission  10 B selectively drives hydrostatic power unit  12  and planetary power unit  30 , which in turn via secondary sun gear NS 2  and planetary gear carrier N 13 , will drive selected ones of a plurality of range gear sets  58  which are coupled to a load L, which, again, will typically be the wheels or tracks of machine  1 . Gear sets  58  are variously engageable by range selectors R 1 , R 2 , R 3  and R 4  under control of the controller. The hydrostatic power unit  12  includes a pump  16  in a fluid loop with a motor  18  with the hydrostatic power unit  12  coupled to power source  4  via an input gear N 6  and having an output gear N 10 . The power to the hydrostatic power unit  12  is provided by a driven gear N 4  mounted on the forward shaft and engaged with gear N 6 . Output gear N 10  is connected to ring gear NR of planetary power unit  30  via gears N 11  and N 12 . 
         [0042]    Planetary power unit  30  is constructed essentially as shown in  FIG. 7  but is numbered differently, including a primary sun gear NS 1  on a planetary input shaft connectable with power source  4  via a forward clutch  54  or a reverse clutch  52 . Power unit  30  is selectively coupled to the load L, coupled to the hydrostatic power unit  12  and selectively coupled to the power source  4 , under automatic control of the controller. For connection to the load L, the hydro-mechanical transmission  10 C includes an output shaft  60  coupled to the load L which carries an input gear N 18  engaged with an output gear N 17  on a range  1 / 2  shaft of range gear set  58 , and a gear N 22  engaged with a gear N 19  on a range  3 / 4  shaft. The range  1 / 2  shaft can be coupled to planetary power unit  30  via automatic operation of range selectors R 1  and R 3  for power flow through gears N 13  and N 14 , or N 15  and N 16 , respectively. The range  3 / 4  shaft can be coupled to unit  30  via range selectors R 3  and R 4  for power flow via gears N 13  and N 20 , or N 15  and N 21 . Range  1 / 2  shaft and range  3 / 4  shaft can also be simultaneously coupled to power unit  30 , to provide dual power flow. 
         [0043]    In operation, the continuously variable hydro-mechanical transmission  10 C can be operated to have a combined hydrostatic and mechanical power flow by engaging the reverse clutch  52  to power planetary power unit  30  via gears N 1 , N 3 , N 5  and N 7 , or engaging forward clutch  54  to power it via gears N 1 , N 8 , and N 2 . It is also possible to operate the hydrostatic mechanical transmission  10 C for a pure hydrostatic power flow by disengaging both clutches  52  and  54 . 
         [0044]    As stated above in this embodiment, there is no selection for a work range or road range per se. However, the ranges provide a seamless transition between ranges similar to the work/road configuration previously described. Speed change from zero to maximum speed is achieved in a smooth and continuous manner by changing the swash plate angle of the pump  16  under control of controller  100 . A full shuttle reverse is also available through the clutches  52  and  54  since the directional change occurs on the input side (upstream side) of the planetary power unit  30 . Since directional changes occur on the input side of compound planetary unit gear  30 , it may be necessary to adjust the position of the swash plate in motor  18  depending upon the desired forward to reverse speed change ratio, and this is done automatically by controller  100 . In the low speed pure hydrostatic power flow regenerative heat is kept under control during prolonged creep operation of the work machine  1 . Also, in the pure hydrostatic power flow mode, different creep speed ranges can be achieved by engaging different combinations of the range selectors R 1 -R 4 . 
         [0045]    As noted above, it has been observed that under some operating conditions, torque loads on components of the continuously variable hydro-mechanical transmissions  10 A,  10 B or  10 C can be sufficient, particularly if sustained, to damage the transmission. Such damage has been observed to be more prevalent in the driveline or output portions of the transmissions, that is, in the final gear reduction in connection with load shaft  60  and related elements. 
         [0046]    According to the invention, a method of estimating driveline torque of a of a work machine, particularly in the final gear reduction or output member, and limiting the torque for preventing damage to the transmission, and thus eliminating need for torque sensors, is provided. According to the invention, it has been observed that the fluid pressure condition in the hydrostatic power unit  12  will be high when the transmission driveline is subjected to high torque loads, typically when the machine is moving slowly, e.g., creep conditions, or is stationary, under heavy load. It has also been observed that the hydrostatic power unit  12  will have a mechanical efficiency which is a function of the pressure in that unit, swash plate angle, and speed of pump  16 . The efficiency will have a value of less than 1 when in the generation mode, and greater than 1 when in the regeneration mode. The motor  18  of the hydrostatic power unit  12  will have a mechanical efficiency which is a function of the pressure in that unit. This pressure will be important for the purposes of the present invention only when high, approaching relief pressure, when potentially damaging driveline torque conditions are likely to be present. The efficiency of the motor  18  can be determined by testing at relevant high pressure, e.g., near relief, and recorded for later use. The efficiency value will be greater than 1 for the regeneration mode, and less than 1 for the generation mode. 
         [0047]    It has been further found that the pressure in the hydrostatic power unit  12  will provide an indication of the torque on that unit, and if the operating mode, e.g., generation, regeneration, and the mechanical efficiency of the fluid motor  18  and direction of operation thereof are known or accurately estimated at the pressure, a relatively accurate estimation of the torque on the fluid motor  18  can be made. In turn, the torque on the output of the planetary power unit  30  can be estimated as a function of the torque on the fluid motor  18  and ratios of gears of the planetary unit  30  and those connecting it with the motor  18 . The accuracy of this torque estimation can be increased by knowing or estimating the efficiency of the planetary unit  30 . The torque load on the driveline, particularly on the output member, e.g. load shaft  60 , thereof, can then be estimated as a function of the estimated torque on the planetary unit output, and the ratio of gears connecting the planetary unit  30  to the driveline output member. 
         [0048]    Referring also to  FIG. 6 , a high level flow diagram  114  showing steps of a preferred method of the invention for estimating and limiting driveline torque is shown. The steps of diagram  114  will be performed automatically by the transmission controller, e.g., controller  100 , based on command values, e.g., swash plate angle commands and clutch actuation commands outputted to or feedback received from actuators  106 , input commands; sensor data received from pressure sensors  110  and speed sensors  112 ; and also data received from the machine power unit  4 . At block  116 , operation of the hydrostatic power unit of the transmission is monitored or read, to determine whether that unit is operating in a generation mode or a regeneration mode. This will be determined from a differential between pressure outputs of sensors  110 , and also which sensor has a higher or lower pressure, which will be indicative of whether the pump is acting as a pump (generation) or as a motor (regeneration), which will be determined at decision block  118 . The invention will then determine a mechanical efficiency of the hydrostatic power unit, in particular, of the motor thereof, as a function of at least the operating mode, e.g., generation or regeneration; pressure; and motor speed, as denoted by respective blocks  120  and  122 . The efficiency can be determined by looking up a stored value, previously obtained from testing. 
         [0049]    The torque output of the hydrostatic power unit will then be estimated as a function of at least the pressure therein and the mechanical efficiency of the motor, as denoted at block  124 . The torque in the motor T motor  in N*m in the hydro-mechanical operating mode can be estimated using the following equation. 
         [0000]    
       
         
           
             
               T 
               motor 
             
             = 
             
               
                 
                   
                     P 
                     HSU 
                   
                   · 
                   
                     V 
                     motor 
                   
                 
                 
                   2 
                   · 
                   π 
                 
               
               · 
               
                 η 
                 Motor 
               
             
           
         
       
     
         [0000]    where P HSU  is the pressure in the hydrostatic power unit in Pa (N/m 2 ); V motor  is volume of the motor in m 3 ; and n motor  is the mechanical efficiency of the motor. The pressure is positive for non-regenerative mode and negative for re-generation mode. The motor mechanical efficiency is a function of hydrostatic power unit pressure, but as noted above, is only of concern when output torque is high. When torque is high, the hydrostatic power unit pressure will also be high, such that the mechanical efficiency will need only be determined near the relief pressure. The motor efficiency is also slightly a function of the swash plate angle and pump speed, but it has been found that this can be neglected. The best practice has been found to be to measure the efficiency using torque sensors or gauges, as part of a temporary or removable test apparatus, prior to installation of the motor or hydrostatic power unit, and then to save the test values and look up them up later during the transmission operation. 
         [0050]    As a next step, the torque output of the planetary power unit will be estimated, as a function of the estimated torque output of the hydrostatic power unit, a ratio of gears drivingly connecting the hydrostatic power unit to the planetary power unit, and ratios of the gears of the planetary power unit, as denoted at block  126 . 
         [0051]    The torque at the planetary output can be estimated as follows. 
         [0000]    For Range  1  and  3  of transmission  10 C of  FIG. 4 , the planetary output torque is denoted T P13 . 
         [0000]    
       
         
           
             
               T 
               
                 P 
                  
                 
                     
                 
                  
                 13 
               
             
             = 
             
               
                 
                   
                     T 
                     m 
                   
                   · 
                   
                     ( 
                     
                       
                         N 
                         12 
                       
                       
                         N 
                         10 
                       
                     
                     ) 
                   
                 
                 
                   K 
                   1 
                 
               
               · 
               
                 ( 
                 
                   
                     K 
                     1 
                   
                   - 
                   1 
                 
                 ) 
               
             
           
         
       
     
         [0000]    where N 12  is the number of teeth of gear N 12 ; N 10  is the number of teeth of gear N 10 ; and K 1  is determined according to the formula below. 
         [0000]    
       
         
           
             
               K 
               1 
             
             = 
             
               
                 - 
                 
                   
                     N 
                     R 
                   
                   
                     N 
                     
                       P 
                        
                       
                           
                       
                        
                       2 
                     
                   
                 
               
               · 
               
                 
                   N 
                   
                     P 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   N 
                   
                     S 
                      
                     
                         
                     
                      
                     1 
                   
                 
               
             
           
         
       
     
         [0000]    where N R  is the number of teeth of the ring gear of the planetary; N P1  is the number of teeth of each planet gear NP 1 ; N P2  is the number of teeth of each planet gear NP 2 ; and N S1  is the number of teeth of the primary sun gear NS 1 .
 
For Range  2  and  4 , the planetary output torque is denoted T P24 .
 
         [0000]    
       
         
           
             
               T 
               
                 P 
                  
                 
                     
                 
                  
                 24 
               
             
             = 
             
               
                 
                   
                     T 
                     m 
                   
                   · 
                   
                     ( 
                     
                       
                         N 
                         12 
                       
                       
                         N 
                         10 
                       
                     
                     ) 
                   
                 
                 
                   ( 
                   
                     
                       
                         ( 
                         
                           
                             K 
                             2 
                           
                           - 
                           1 
                         
                         ) 
                       
                        
                       
                         K 
                         1 
                       
                     
                     
                       ( 
                       
                         
                           K 
                           2 
                         
                         - 
                         
                           K 
                           1 
                         
                       
                       ) 
                     
                   
                   ) 
                 
               
               · 
               
                 [ 
                 
                   
                     
                       
                         ( 
                         
                           
                             K 
                             2 
                           
                           - 
                           1 
                         
                         ) 
                       
                       · 
                       
                         K 
                         1 
                       
                     
                     
                       ( 
                       
                         
                           K 
                           2 
                         
                         - 
                         
                           K 
                           1 
                         
                       
                       ) 
                     
                   
                   - 
                   1 
                 
                 ] 
               
             
           
         
       
       
         
           and 
         
       
       
         
           
             
               K 
               2 
             
             = 
             
               
                 
                   N 
                   
                     S 
                      
                     
                         
                     
                      
                     2 
                   
                 
                 
                   N 
                   
                     P 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
               · 
               
                 
                   N 
                   
                     P 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   N 
                   
                     S 
                      
                     
                         
                     
                      
                     1 
                   
                 
               
             
           
         
       
     
         [0000]    where N S2  is the number of teeth of the secondary sun gear NS 2 . 
         [0052]    The torque on the output member of the driveline of the transmission will then be estimated as a function of the estimated torque output of the planetary power unit and ratios of gears drivingly connecting the planetary power unit to the output member, as denoted at block  128 . 
         [0000]    The output torque can now be written in terms of the planetary torque for each range as follows.
 
For Range  1 , the output torque T 01  is estimated using the equation
 
         [0000]    
       
         
           
             
               T 
               
                 O 
                  
                 
                     
                 
                  
                 1 
               
             
             = 
             
               
                 
                   T 
                   P 
                 
                 
                   
                     ( 
                     
                       
                         N 
                         13 
                       
                       
                         N 
                         14 
                       
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         N 
                         17 
                       
                       
                         N 
                         18 
                       
                     
                     ) 
                   
                 
               
               · 
               
                 η 
                 plantery 
               
             
           
         
       
     
         [0000]    where N 13  is the number of teeth of the gear N 13 ; N 14  is the number of teeth of gear N 14 ; N 17  is the number of teeth of gear N 17 ; N 18  is the number of teeth of gear N 18 , and n is the efficiency of the planetary power unit, which can be known or determined through testing.
 
For Range  2 , the output torque T 02  is estimated using the equation
 
         [0000]    
       
         
           
             
               T 
               
                 O 
                  
                 
                     
                 
                  
                 2 
               
             
             = 
             
               
                 
                   T 
                   P 
                 
                 
                   
                     ( 
                     
                       
                         N 
                         15 
                       
                       
                         N 
                         16 
                       
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         N 
                         17 
                       
                       
                         N 
                         18 
                       
                     
                     ) 
                   
                 
               
               · 
               
                 η 
                 plantery 
               
             
           
         
       
     
         [0000]    where N 15  is the number of teeth of the gear N 15 ; and N 16  is the number of teeth of gear N 16 . 
         [0053]    For Range  3  the output torque T 03  is estimated using the equation 
         [0000]    
       
         
           
             
               T 
               
                 O 
                  
                 
                     
                 
                  
                 3 
               
             
             = 
             
               
                 
                   T 
                   P 
                 
                 
                   
                     ( 
                     
                       
                         N 
                         13 
                       
                       
                         N 
                         20 
                       
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         N 
                         20 
                       
                       
                         N 
                         22 
                       
                     
                     ) 
                   
                 
               
               · 
               
                 η 
                 plantery 
               
             
           
         
       
     
         [0000]    where N 19  is the number of teeth of the gear N 19 ; N 20  is the number of teeth of gear N 20 ; N 21  is the number of teeth of gear N 21 ; and N 22  is the number of teeth of gear N 22 . 
         [0054]    For Range  4 , the output torque T 04  is estimated using the equation 
         [0000]    
       
         
           
             
               T 
               
                 O 
                  
                 
                     
                 
                  
                 4 
               
             
             = 
             
               
                 
                   T 
                   P 
                 
                 
                   
                     ( 
                     
                       
                         N 
                         15 
                       
                       
                         N 
                         21 
                       
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         N 
                         19 
                       
                       
                         N 
                         22 
                       
                     
                     ) 
                   
                 
               
               · 
               
                 η 
                 plantery 
               
             
           
         
       
     
         [0000]    where 
       T O1 =M O1 ·M P13 ·T m    
     T O2 =M O2 ·M P24 ·T m    
     T O3 =M O3 ·M P13 ·T m    
     T O4 =M O4 ·M P24 ·T m    
       [0055]    where 
         [0000]    
       
         
           
             
               M 
               
                 P 
                  
                 
                     
                 
                  
                 13 
               
             
             = 
             
               
                 
                   ( 
                   
                     
                       N 
                       12 
                     
                     
                       N 
                       10 
                     
                   
                   ) 
                 
                 
                   K 
                   1 
                 
               
               · 
               
                 ( 
                 
                   
                     K 
                     1 
                   
                   - 
                   1 
                 
                 ) 
               
             
           
         
       
       
         
           
             
               M 
               
                 P 
                  
                 
                     
                 
                  
                 24 
               
             
             = 
             
               
                 
                   ( 
                   
                     
                       N 
                       12 
                     
                     
                       N 
                       10 
                     
                   
                   ) 
                 
                 
                   ( 
                   
                     
                       
                         ( 
                         
                           
                             K 
                             2 
                           
                           - 
                           1 
                         
                         ) 
                       
                       · 
                       
                         K 
                         1 
                       
                     
                     
                       ( 
                       
                         
                           K 
                           2 
                         
                         - 
                         
                           K 
                           1 
                         
                       
                       ) 
                     
                   
                   ) 
                 
               
               · 
               
                 [ 
                 
                   
                     
                       
                         ( 
                         
                           
                             K 
                             2 
                           
                           - 
                           1 
                         
                         ) 
                       
                       · 
                       
                         K 
                         1 
                       
                     
                     
                       ( 
                       
                         
                           K 
                           2 
                         
                         - 
                         
                           K 
                           1 
                         
                       
                       ) 
                     
                   
                   - 
                   1 
                 
                 ] 
               
             
           
         
       
       
         
           
             
               M 
               
                 O 
                  
                 
                     
                 
                  
                 1 
               
             
             = 
             
               
                 1 
                 
                   
                     ( 
                     
                       
                         N 
                         13 
                       
                       
                         N 
                         14 
                       
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         N 
                         17 
                       
                       
                         N 
                         18 
                       
                     
                     ) 
                   
                 
               
               · 
               
                 η 
                 plantery 
               
             
           
         
       
       
         
           
             
               M 
               
                 O 
                  
                 
                     
                 
                  
                 2 
               
             
             = 
             
               
                 1 
                 
                   
                     ( 
                     
                       
                         N 
                         15 
                       
                       
                         N 
                         16 
                       
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         N 
                         17 
                       
                       
                         N 
                         18 
                       
                     
                     ) 
                   
                 
               
               · 
               
                 η 
                 plantery 
               
             
           
         
       
       
         
           
             
               M 
               
                 O 
                  
                 
                     
                 
                  
                 3 
               
             
             = 
             
               
                 1 
                 
                   
                     ( 
                     
                       
                         N 
                         13 
                       
                       
                         N 
                         20 
                       
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         N 
                         19 
                       
                       
                         N 
                         22 
                       
                     
                     ) 
                   
                 
               
               · 
               
                 η 
                 plantery 
               
             
           
         
       
       
         
           
             
               M 
               
                 O 
                  
                 
                     
                 
                  
                 4 
               
             
             = 
             
               
                 1 
                 
                   
                     ( 
                     
                       
                         N 
                         15 
                       
                       
                         N 
                         21 
                       
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         N 
                         19 
                       
                       
                         N 
                         22 
                       
                     
                     ) 
                   
                 
               
               · 
               
                 η 
                 plantery 
               
             
           
         
       
     
         [0056]    If the estimated torque on the output member is greater than a predetermined value or limit, for instance, a threshold value above which damage to the driveline is likely or expected to occur, as determined at decision block  130 , then an operating parameter of the transmission will be changed to reduce the torque. In this regard, the speed of the machine or output member can be read from sensor  112 , and if the speed is greater than a predetermined value, as determined at decision block  132 , the speed of movement can be adjusted, e.g., lowered, as denoted by block  134 , but preferably without reducing power source speed, such that other systems run by the engine are not affected. This may entail adjusting the swash plate angle, to lower the pressure in the hydrostatic power unit and thus the torque output thereof. If, at block  136  the speed is not greater than the predetermined value, the hydrostatic power unit can also be adjusted, in the just described manner, to lower pressure therein and thus torque. 
         [0000]    The output torque is related to the drawbar force (or traction force), 
         [0000]        F   drawbar   =T   O ·Ratio Final     —     Drive /Tire_Radius
 
         [0000]    And, if torque loads on the range clutches is important, the torque at each clutch can be related to the output torque of the transmission as follows, 
         [0000]    
       
         
           
             
               T 
               
                 C 
                  
                 
                     
                 
                  
                 1 
                  
                 C 
                  
                 
                     
                 
                  
                 2 
               
             
             = 
             
               
                 T 
                 O 
               
               · 
               
                 ( 
                 
                   
                     N 
                     17 
                   
                   
                     N 
                     18 
                   
                 
                 ) 
               
             
           
         
       
       
         
           and 
         
       
       
         
           
             
               T 
               
                 C 
                  
                 
                     
                 
                  
                 3 
                  
                 C 
                  
                 
                     
                 
                  
                 4 
               
             
             = 
             
               
                 T 
                 O 
               
               · 
               
                 ( 
                 
                   
                     N 
                     19 
                   
                   
                     N 
                     22 
                   
                 
                 ) 
               
             
           
         
       
     
         [0057]    It will be understood that the foregoing descriptions are for preferred embodiments of this invention and that the invention is not limited to the specific forms shown. Other modifications may be made in the design and arrangement of other elements without departing from the scope of the invention as expressed in the appended claims.