Abstract:
The described system and method allow a controller to calibrate a transmission variator of a continuously variable transmission for torque control by obtaining static and dynamic qualities and parameters of the variator through an automated calibration procedure. The system and method employ a pair of transmission mode configurations and operational configurations in combination to obtain system-specific information. In this way, the system is able to calibrate out the system variations to provide effective feed forward torque control of the continuously variable transmission. In an embodiment, a first calibration operation is performed while the transmission is neutralized and a second calibration operation is performed while the transmission is engaged in a mode providing a fixed variator output speed.

Description:
TECHNICAL FIELD 
       [0001]    This patent disclosure relates generally to transmission systems for propulsion and, more particularly to a method and system for calibrating a torque provided by such a transmission. 
       BACKGROUND 
       [0002]    A system that provides a rotating shaft output can be classified by speed, power, and torque. Although these measures are related in some ways, the concept of torque may be more closely aligned with the experience of the user is operating such a machine. However, in certain environments, it is traditionally difficult to control torque accurately due to lack of adequate calibration. For example, effectively controlling torque in a continuously variable transmission is difficult without accurate system identification of the many hydrostatic variator static and dynamic qualities and parameters. More specifically, variability in hydrostatic variator system components such as valves and hydraulic pump and motor components can prevent the ability to control torque in the transmission. While open-loop torque control with closed-loop feedback may be used with some success, it does not entirely eliminate the need for efficient and accurate torque calibration. 
         [0003]    It will be appreciated that this background description has been created by the inventors to aid the reader, and is not to be taken as a reference to prior art nor as an indication that any of the indicated problems were themselves appreciated in the art. While the described principles can, in some regards and embodiments, alleviate the problems inherent in other systems, it will be appreciated that the scope of the protected innovation is defined by the attached claims, and not by the ability of the claimed invention to solve any specific problem noted herein. 
       SUMMARY 
       [0004]    The described principles allow a controller to obtain the necessary static and dynamic qualities and parameters to allow accurate torque control of a continuously variable transmission. In an embodiment, the system uses a series of pairs of transmission mode configurations and operational configurations in combination to obtain system-specific identification and variations. For example, unique spring strengths, component tolerances, and so on often endow a given variator with properties that differ from the analogous properties in a counterpart system of the same make and model. In certain embodiments, the present system is able to calibrate out the variations in order to allow effective feed forward torque control of a continuously variable transmission. 
         [0005]    The described method for calibrating hydrostatic transmission motor torque entails configuring and operating the transmission in specific mode/operation pairs to aid in system identification and calibration. In an embodiment, a method performs particular operations within particular transmission modes to allow system identification of the hydrostatic variator system. In an example implementation, the first such operation is performed while the transmission is neutralized. In this mode, the system commands a hydraulic pressure to the variator actuator component to identify the relationship between commanded hydraulic pressure and variator displacement. This relationship may be measured as a motor speed ratio (motor speed over pump speed) or may be measured by way of a swash plate angle sensor. Thus, although this description will generally refer to motor speed ratio to describe variator displacement, it will be appreciated that at every such instance, another measure such as swash plate angle may instead be used. 
         [0006]    In a next mode, the transmission in engaged such that the variator system is able to build circuit pressure with the hydraulic motor output shaft locked at fixed speed or zero speed. In this mode, the system commands hydraulic pressure to the variator actuator component to identify the relationship between commanded hydraulic pressure and variator circuit pressure. 
         [0007]    The relationships acquired from imposing these two mode/operation pairs provide calibration information to effectively control the transmission using feed forward torque control methods. 
         [0008]    Further and alternative aspects and features of the disclosed principles will be appreciated from the following detailed description and the accompanying drawings, of which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic system diagram of a variator for providing a variable output torque based on an applied control pressure differential in accordance with the disclosed principles; 
           [0010]      FIG. 2  is a detailed schematic drawing of a hydraulic actuator for controlling the position of a variable-angle swash plate in a variator in accordance with the disclosed principles; 
           [0011]      FIG. 3  is a simplified logical schematic of control components and data flow to calibrate and operate the variator in accordance with the disclosed principles; 
           [0012]      FIG. 4  is a flow chart illustrating a process for calibrating a hydrostatic transmission in accordance with the disclosed principles; and 
           [0013]      FIG. 5  is a schematic architectural and flow diagram illustrating the use of calibration correlations to control torque in accordance with the disclosed principles. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    This disclosure relates to machines requiring a transmission to link a power source to the final ground-engaging mechanism, e.g., wheels, tracks, etc., and/or to another powered function or implement. Examples of such machines include machines used for mining, construction, farming, transportation, or any other industry known in the art. For example, the machine may be an earth-moving machine, such as a wheel loader, excavator, dump truck, backhoe, motor grader, material handler or the like. Moreover, one or more implements may be connected to the machine for a variety of tasks, including, for example, loading, compacting, lifting, brushing, and include, for example, buckets, compactors, forked lifting devices, brushes, grapples, cutters, shears, blades, breakers/hammers, augers, and others. In an example embodiment, the system is applied to a continuously variable transmission (CVT), such as may be used in wheel loader or motor grader machine applications. 
         [0015]    In overview, a hydrostatic transmission with a variable displacement pump and fixed or variable displacement motor is used in combination with mechanical gearing in a transmission powered by an engine that is operated at a substantially constant speed. The hydrostatic transmission has a system pressure (circuit pressure) and a gear (or mode) that determine the transmission output torque. Thus, the driveline torque is controlled by controlling the pressure in the hydrostatic transmission. In an embodiment, the control is a combination of open loop and closed loop control as opposed to pure closed loop control, which, in the inventor&#39;s experience, does not provide adequate response time. 
         [0016]    Within the hydrostatic transmission, torque control is accomplished by controlling the swash plate angle on a variable displacement pump. In turn, the swash plate angle is controlled by a swash plate actuator. In order to provide open loop control in an accurate and effective manner within this system, the spring pressure in the actuator and the correlation between the actuator position and system pressure are determined. This is necessitated by the fact that, due to manufacturing tolerances with respect to various components, there may be as much as a 30% difference in the operational and mechanical parameters of any particular hydrostatic transmission relative to another hydrostatic transmission of the same make and model. 
         [0017]      FIG. 1  is a detailed schematic drawing of a variator  100  for providing a variable output torque based on an applied control pressure differential. The variator  100  comprises a pump  101  and a motor  102 . The pump  101  comprises a variable angle swash plate  103  set by a swash plate actuator  104 . A number of pistons  105  in respective chambers ride on the swash plate  103  via sliding contacts, such that the range of movement of the pistons  105  is set by the angle of the swash plate  103 . The chambers for the pistons  105  are formed in a pump carrier  108  that is rotated via the pump input shaft  109 . 
         [0018]    The motor  102  comprises a similar arrangement including a number of pistons  106  in respective chambers. The pistons  106  of the motor  102  are slidably engaged upon a fixed swash plate  107 . The chambers of the pistons  105  of the pump  101  are in fluid communication with the chambers of the pistons  106  of the motor  102  via hydraulic fluid that fills the chambers and intervening conduits (not shown). The chambers for the pistons  106  are formed in a motor carrier  110  that rotates the motor output shaft  111 . As the angle of the swash plate  103  is varied, the amount of fluid displaced by the pistons  105  of the pump  101  (and thus the fluid volume received or taken from the chambers of the pistons  106 ) varies. 
         [0019]    Because of these interrelationships, the torque and/or output speed of the motor  102  varies with the angle of swash plate  103 . In overview, the swash plate actuator  104 , which in this example operates on differential hydraulic pressure, is driven via solenoid valves (not shown in  FIG. 1 ), e.g., one for each of two pressure values, controlled electronically by appropriate input signals from a transmission controller or the like. In this way, a controller can control the output speed of the variator  100  via the application of electrical signals to solenoid valves associated with the swash plate actuator  104 . 
         [0020]      FIG. 2  is a more detailed schematic drawing of the hydraulic actuator  104  for controlling the position of a variable-angle swash plate (not shown in  FIG. 2 ) in a variator  100  such as that shown in  FIG. 1 . The actuator  104  includes a number of interrelated elements including primarily two opposed pistons  200 ,  201  (or opposed chambers of a single piston) within respective cylinders  202 ,  203 . The pistons  200 ,  201  cooperate with the bores of their respective cylinders  202 ,  203  to form respective pressure chambers  204 ,  205  for containing pressurized hydraulic fluid. 
         [0021]    The pistons  200 ,  201  are joined by a bar  206  which has a central pivot pin  207  mounted thereon. The central pivot pin  207  interferes within a slot  208  in a swash plate arm  209 , such that the lateral position of the bar  206  establishes the position of the swash plate arm  209  and hence the angle of the swash plate itself (not shown). The bar  206  is biased to a central position by opposing springs  212 . As the bar  206  is displaced from this central position, there is a restoring force exerted by springs  212  that is proportional to the displacement. 
         [0022]    The lateral position of the bar  206  is determined by the positions of the pistons  200 ,  201  within the cylinders  202 ,  203 . The positions of the pistons  200 ,  201  are determined by the difference in hydraulic pressure between the piston chambers  204 ,  205 . Respective pressure valves  210 ,  211  independently control the pressure within chambers  204 ,  205 . In an example, the pressure valves  210 ,  211  are solenoid valves that supply hydraulic fluid at a pressure that is set by an applied current within limits set by a supply pressure. Thus, in the illustrated example, each valve  210 ,  211  has at least a current input (illustrated as inputs A and C) and a fluid input (illustrated as inputs B and D). Typically, solenoid valves can supply fluid at any pressure between zero and the fluid pressure at the fluid input B, D. The pressure response of a solenoid valve such as solenoid valves  210  and  211  to a current input is a function of various components and their tolerances. 
         [0023]    Because the distance between the pistons  200 ,  201  is fixed by the length of the bar  206 , it is the pressure differential between chambers  204 ,  205  rather than the absolute pressure within each chamber  204 ,  205  that establishes the position of the bar  206 . In particular, when the bar  206  is in such a position that the net displacement force differential between the pistons  200 ,  201  is equal to the net restoring force exerted by springs  212 , the system is in equilibrium. 
         [0024]    Considering  FIG. 2  in conjunction with  FIG. 1 , it will be appreciated that the torque supplied at output  111  is related to the pressure differential applied by valves  210 ,  211 . In particular, the fluid pressure within the hydraulic circuit between pistons  105  and  106  is related to the angle of swash plate  103 , and the angle of swash plate  103  is related to the pressure differential applied by valves  210 ,  211 . Thus, in torque-controlled applications, it is desirable to accurately correlate combinations of solenoid currents for valves  210  and  211  (or applied pressure differential in actuator  104 ) with expected associated output torques at output  111 . 
         [0025]    Before discussing the calibration process in further detail, the control infrastructure and informational flow within the system will be discussed.  FIG. 3  is a simplified logical schematic  300  of the control components and data flow associated with the mechanical components of  FIGS. 1 and 2  to calibrate and operate the variator  100  effectively. In particular, a variator controller  301  is provided for controlling the operation of the variator  100  via solenoid valves  210  and  211 . The variator controller  301  may be a dedicated variator controller, but more typically will also control a larger system, such as a transmission, associated with the variator  100 . The controller  301  may be of any suitable construction, however in one example it comprises a digital processor system including a microprocessor circuit having data inputs and control outputs, operating in accordance with computer-readable instructions stored on a computer-readable medium. Typically, the processor will have associated therewith long-term (non-volatile) memory for storing the program instructions, as well as short-term (volatile) memory for storing operands and results during (or resulting from) processing. 
         [0026]    In operation, the controller  301  receives a number of data inputs from the variator system  100  and provides a number of control outputs to the system  100 . In particular, the controller  301  has a first data input connected to circuit pressure sensors  302  or other torque sensing devices or sensors. Although it is possible to use a single pressure sensor, it is desirable to use multiple sensors to obtain more accurate pressure readings. The circuit pressure sensors  302  are positioned and adapted to sense the hydraulic pressure within the internal hydraulic circuit of the variator  100  (i.e., between pistons  105  and  106 ) and to provide signals related to the sensed pressures. A second data input to the controller  301  is linked to a pump speed sensor  303 . The pump speed sensor  303  is positioned and adapted to detect the rotational speed of the variator input shaft  108  and to provide a signal related to the sensed rotational input speed. A motor speed sensor  304  is linked to third data input of the controller  301 . The motor speed sensor  304  is positioned and adapted to detect the rotational speed of the variator output shaft  110  and to provide a signal related to the sensed rotational output speed. It will be appreciated that the pump displacement (e.g., derived from the stroke of actuator  103 ) or the angle of the swash plate  103  (e.g., derived from an angle sensor) can be used as an input in place of the motor speed ratio. 
         [0027]    In order to detect a desired torque, the controller  301  also receives a data input from the operator interface  307 , e.g., an accelerator setting. The operator may be human or automated, and the operator interface  307  may vary accordingly. As noted above, the variator  100  operates in discrete modes, which may be automatically set and/or set based on user input. 
         [0028]    The torque calibration values  308  derived by the variator controller  301  are stored during calibration and are retrieved by the variator controller  301  during actual operation. Based on the various available inputs as discussed above, the controller  301  calculates and provides appropriate control signals such that the variator  100  provides an output torque closely corresponding to the desired output torque. In particular, the controller  301  provides two solenoid control signals  305 ,  306  to control the operation of the actuator  104  and thus the operation of the variator  100 . The solenoid control signals  305 ,  306  include a first solenoid control signal  305  to control a first one  210  of the actuator pressure valves and a second solenoid control signal  306  to control a second one  211  of the actuator pressure valves. 
         [0029]    As noted above, however, there may be a significant variation in the qualities and characteristics of a given hydrostatic transmission from what is expected. This can be largely attributed to unavoidable differences in the characteristics of the solenoid valves, e.g., solenoid valves  210 ,  211 . For example, variations in the solenoid valve springs, windings, spools, cages, and so on can have a significant effect on the valve&#39;s pressure/current relationship. Other variations within the transmission, e.g., in fluid composition, piston tolerances, etc., may additionally play a secondary role in causing variations between ostensibly identical transmission components. 
         [0030]    Another potential source of variability lies in the port plate timing system. In particular, the port plate timing, i.e., the timing with which pressurized fluid is admitted to the motor side of the variator from the pump side of the variator, is often adjustable. While this timing may be calibrated and set, there may be either miscalibration or drift resulting in a discrepancy in the variator response. Any or all of these sources of discrepancy can result in erroneous operation of the variator, wherein the application of parameters expected to yield a certain output torque does not in fact yield the expected torque. Thus, calibration of such a transmission is important to obtain the torque calibration values  308  to enable accurate open loop torque control. 
         [0031]    In an embodiment, the hydrostatic transmission is calibrated by the process  400  described in the flow chart of  FIG. 4 . At stage  401  of the process  400 , the transmission is placed in neutral. This may be executed by the variator controller  301  or by a separate transmission controller. With the transmission in neutral, the variator controller  301  commands hydraulic pressure to the variator actuator  104  at stage  403  to identify a displacement correlation  410  between commanded hydraulic pressure to the actuator  104  and variator displacement. The latter may be measured as a motor speed ratio or as a swash plate angle, which is associated with the motor speed ratio after accounting for losses. The displacement correlation is partly reflective of the spring pressure of the bias springs  212  within the actuator  104 , as well as the rotational inertia effect. 
         [0032]    It will be appreciated that pump valve variations occurring as a function of pressure will be partly encompassed during this step, but that other pump valve variations outside of the range exercised in this step will not be included until later calibration steps. To avoid including the pump valve variation effects twice, the contribution of such variations gleaned in the first calibration stage may later be subtracted out, or may be otherwise accounted for. 
         [0033]    The remainder of the calibration process is performed with the transmission in a configuration such that the variator  100  is able to build circuit pressure, e.g., with the hydraulic motor output shaft locked at fixed speed or at zero speed. Thus, at stage  405 , the transmission is placed into gear, i.e., it is no longer in neutral. The variator controller  104  then commands hydraulic pressure to the variator actuator  104  at stage  407  to identify a circuit pressure correlation  411  between commanded hydraulic pressure and variator circuit pressure. 
         [0034]    In one contemplated embodiment, this state is achieved by intentionally creating a clutch tie-up at a transmission synchronous point. In other words, in a two-clutch system, when the synchronous point is reached, instead of deactivating an off-going clutch and activating an oncoming clutch, both clutches are activated. It will be appreciated that such synchronous points occur at zero output speed as well as certain other non-zero output speeds. Whichever synchronous point is selected, the clutch tie-up will prevent the output speed from changing from the output speed associated with that synchronous point. At that point then, the circuit pressure may be increased and decreased without resulting in a change in output speed, and without a change in swash plate angle, thus allowing circuit pressure to be built. 
         [0035]    It will be appreciated that the actuator pressures used in stage  407  will cover a range that includes but also exceeds the range of actuator pressures used in stage  403 . As noted above, the pressure effects gleaned during calibration step  403  may be subtracted out at stage  407  or may be subtracted out during later use of the calibration values, so as to avoid double counting any portion of the range. 
         [0036]    In this way, the calibration values obtained in stage  403  are applicable to transmission equations used during zero torque conditions, i.e., to adjust speed with open loop equations while torque is zero, such as during synching prior to engaging a clutch. The calibration obtained at stage  407  is applicable to feed forward control when torque is non-zero, e.g., for torque control with the transmission engaged. 
         [0037]    At stage  409 , the calibration is completed and the displacement correlation  410  and circuit pressure correlation  411  acquired from performing the two calibration operations are used during actual operation to effectively control the transmission using feed forward torque control methods. 
         [0038]    As can be seen, both calibrations are run at fixed pump speeds in the described embodiment. However, there is often an effect, be it small or large, of pump speed and circuit pressure as well. Thus, in systems where this relationship is strong, calibrations taken at one pump speed become less applicable as the actual pump speed during operation diverges from the calibration pump speed. Thus, in systems wherein the pressure response is known or expected to be more highly dependent upon pump speed, each calibration step may be executed at multiple pump speeds, and extrapolation may be used if needed to reach intermediate values. 
         [0039]      FIG. 5  is a schematic architectural and flow diagram illustrating in greater detail the use of the calibration correlations to control torque as in stage  409  of process  400 . The flow  500  of  FIG. 5  begins with a torque command  501 , e.g., from a user via a user interface, or from a controller, e.g., automatically changing torque to account for an increase in grade, etc. The torque command  501  is processed by the feed forward torque equations  503  in view of the calibration correlations  505  (i.e., the displacement correlation  410  and the circuit pressure correlation  411 ) to produce one or more valve commands  507 . 
         [0040]    The one or more valve commands  507  are provided to the transmission  509 . In particular, the one or more valve commands  507  act as input commands to the pump valves  511 , which respond by adjusting to the commanded level. The pump valves  511  control the actuator  513  (e.g., swash plate actuator  104 ). The position of the actuator  513  modifies the operation of the variator  515  (e.g., variator  100  of  FIG. 1 ), which provides a motor torque output  517  substantially matching the initial torque command  501 . In this manner, accurate and efficient feed forward control of torque is provided. 
         [0041]    Although the foregoing discussion pertains by way of example to certain calibration techniques that remove the effects of inertia, it will be appreciated that this simplification may be forgone if so desired. Thus, for example, in a system where inertial effects during acceleration are largely quantified, the zero acceleration calibration of stage  411  may instead be reconfigured to allow acceleration, with the effects of inertia then being accounted for and decoupled from the calibration results. 
       INDUSTRIAL APPLICABILITY 
       [0042]    The described principles are applicable to machines requiring a transmission to link a power source to the final ground-engaging mechanism, e.g., wheels, tracks, etc., and/or to another powered function or implement. Examples of such machines include machines used for mining, construction, farming, transportation, or any other industry known in the art. For example, the machine may be an earth-moving machine, such as a wheel loader, excavator, dump truck, backhoe, motor grader, material handler or the like. Exemplary implements include, without limitation, buckets, compactors, forked lifting devices, brushes, grapples, cutters, shears, blades, breakers/hammers, augers, and others. 
         [0043]    Within such applications, the described principles apply to the operation of hydrostatic and hydraulic continuously variable transmissions to allow accurate torque control using a feed forward control configuration. This allows the operation of the host machine to be controlled in a more effective manner than simple traditional speed control or other alternative control strategies. 
         [0044]    It will be appreciated that the foregoing description provides useful examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for the features of interest, but not to exclude such from the scope of the disclosure entirely unless otherwise specifically indicated. 
         [0045]    Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. For example, the illustrated calibration steps may optionally be executed in reverse order, and other alternative orders and steps may be practicable where logically appropriate without departing from the described principles. 
         [0000]    
       
         
               
               
             
           
               
                   
               
               
                 ELEMENT NUMBER 
                 DESCRIPTION 
               
               
                   
               
             
             
               
                 VARIATOR 
                 100 
               
               
                 PUMP 
                 101 
               
               
                 MOTOR 
                 102 
               
               
                 VARIABLE ANGLE SWASH PLATE 
                 103 
               
               
                 SWASH PLATE ACTUATOR 
                 104 
               
               
                 PUMP PISTONS 
                 105 
               
               
                 MOTOR PISTONS 
                 106 
               
               
                 FIXED SWASH PLATE 
                 107 
               
               
                 PUMP CARRIER 
                 108 
               
               
                 PUMP INPUT SHAFT 
                 109 
               
               
                 MOTOR CARRIER 
                 110 
               
               
                 MOTOR OUTPUT SHAFT 
                 111 
               
               
                 OPPOSED PISTON 
                 200 
               
               
                 OPPOSED PISTON 
                 201 
               
               
                 CYLINDER 
                 202 
               
               
                 CYLINDER 
                 203 
               
               
                 PRESSURE CHAMBER 
                 204 
               
               
                 PRESSURE CHAMBER 
                 205 
               
               
                 ACTUATOR BAR 
                 206 
               
               
                 CENTRAL PIVOT PIN 
                 207 
               
               
                 BAR SLOT 
                 208 
               
               
                 SWASH PLATE ARM 
                 209 
               
               
                 PRESSURE VALVE 
                 210 
               
               
                 PRESSURE VALVE 
                 211 
               
               
                 OPPOSING SPRINGS 
                 212 
               
               
                 SIMPLIFIED LOGICAL SCHEMATIC 
                 300 
               
               
                 VARIATOR CONTROLLER 
                 301 
               
               
                 CIRCUIT PRESSURE SENSORS 
                 302 
               
               
                 PUMP SPEED SENSOR 
                 303 
               
               
                 MOTOR SPEED SENSOR 
                 304 
               
               
                 FIRST SOLENOID CONTROL SIGNAL 
                 305 
               
               
                 SECOND SOLENOID CONTROL SIGNAL 
                 306 
               
               
                 OPERATOR INTERFACE 
                 307 
               
               
                 TORQUE CALIBRATION VALUES 
                 308 
               
               
                 CALIBRATION PROCESS 
                 400 
               
               
                 FIRST STAGE OF CALIBRATION PROCESS 
                 401 
               
               
                 SECOND STAGE OF CALIBRATION PROCESS 
                 403 
               
               
                 THIRD STAGE OF CALIBRATION PROCESS 
                 405 
               
               
                 FOURTH STAGE OF CALIBRATION PROCESS 
                 407 
               
               
                 FIFTH STAGE OF CALIBRATION PROCESS 
                 409 
               
               
                 DISPLACEMENT CORRELATION 
                 410 
               
               
                 CIRCUIT PRESSURE CORRELATION 
                 411 
               
               
                 PROCESS FLOW 
                 500 
               
               
                 TORQUE COMMAND 
                 501 
               
               
                 FEED FORWARD TORQUE EQUATIONS 
                 503 
               
               
                 CALIBRATION CORRELATIONS 
                 505 
               
               
                 VALVE COMMANDS 
                 507 
               
               
                 TRANSMISSION 
                 509 
               
               
                 PUMP VALVES 
                 511 
               
               
                 ACTUATOR 
                 513 
               
               
                 VARIATOR 
                 515 
               
               
                 MOTOR TORQUE OUTPUT 
                 517