Abstract:
The described system and method provide improved transmission performance and response with closed loop torque feedback by implementing situational gain scheduling and nonlinear control techniques for continuously variable transmissions. The system uses contextual information regarding the operation of the machine to determine a gain to be applied in associated PID control logic. In an embodiment, the determined gain is applied in the integral portion of the closed loop controller.

Description:
TECHNICAL FIELD 
       [0001]    This patent disclosure relates generally to transmission systems for propulsion and, more particularly to a method and system for exercising closed loop control of the torque of the transmission output. 
       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 due to the slow frequency response and non-linear behavior of the transmission. 
         [0003]    While simple PID control schemes may be used, such systems will generally not be able accurately control the output torque of the transmission. This is primarily due to the control lag, and may result in oscillation, instability, and a generally poor user experience. Although the application of gains can speed response time, it may also result in instability and over-control. In particular, while inadequate gains will not be sufficient to alleviate the lag problem, excessive gains may cause control ringing or under-damped oscillation. 
         [0004]    It will be appreciated that this background description has been created by the inventors to aid the reader, and represents concepts known to the inventors. It is not a discussion of, nor reference to, prior art, nor is this section intended to imply that any of the indicated problems were themselves appreciated in the art. While the principles described herein can, in some regards and embodiments, avoid the problems described, 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 
       [0005]    The disclosed techniques and structures provide improved transmission performance and response with closed loop torque feedback by implementing situational gain scheduling and nonlinear control techniques for continuously variable transmissions. In an embodiment, the system determines and employs contextual information regarding the operation of the machine, such as machine operator inputs and transmission dynamics, to identify the machine operation state, machine dynamics, and transmission dynamics. 
         [0006]    The state information is employed, in conjunction with operator inputs and transmission dynamics information to gain schedule associated PID control logic. For very low system frequency response transmission systems especially, the use of this method for gain scheduling, e.g., with respect to the integral portion of the closed loop controller, has a significant impact in facilitating effective closed loop torque control. 
         [0007]    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 
         [0008]      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; 
           [0009]      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; 
           [0010]      FIG. 3  is a simplified logical schematic of control components and data flow to control the variator in accordance with the disclosed principles; 
           [0011]      FIG. 4  is a logical flow schematic illustrating the manner in which gain scheduling is executed in accordance with the disclosed principles; and 
           [0012]      FIG. 5  is a data flow and operational schematic illustrating the manner in which gain scheduling is executed in accordance with the disclosed principles. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    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 other powered functions or implements. Examples of such machines include those used for mining, construction, farming, transportation, or any other industry known in the art. 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) as may be used in wheel loader or motor grader machine applications. 
         [0014]    In overview, the described system and control method may be used in any suitable machine, including but not limited to medium sized machines. Moreover, the described configuration may alternatively be used with a generator/motor combination to replace a hydrostatic transmission. As noted above, simple speed control does not work in this type of transmission due to the slow response. The described system provides torque control for a hydrostatic transmission via a combination of open loop control, closed loop control, and a variable gain, selected as a function of work parameters. 
         [0015]    The hydrostatic transmission includes a variable displacement pump and motor, and is used in combination with mechanical gearing in a transmission powered by an engine, e.g., one that is operated at a substantially constant speed. A pressure and gear setting determine the transmission output torque. In other words, the drive line is torque-controlled by controlling the actuator/circuit pressure in the hydrostatic transmission. The combination of open loop control and closed loop control overcomes difficulties that would be experienced during the use of closed-loop-only controls. In physical terms, the control is accomplished by controlling the swashplate angle on the variable displacement pump. The swashplate angle is in turn controlled by the swashplate actuator. 
         [0016]    The closed loop portion of the system uses a PID controller. In certain situations, excessively high gain factors in the closed loop part of the system can cause the system to become unstable. On the other hand, small conservative gain factors sacrifice response time. The present control method has been developed to use different gain factors depending on machine operating parameters such as speed, torque, and direction. 
         [0017]    The method applies higher gain factors to provide a better response time in situations wherein empirical data shows that the system will tolerate, i.e., remain stable with, high gain factors. In other situations, when the system is at a risk of instability if higher gains are used, lower gain factors are applied. The applied gain scheduling data may be stored in look-up maps or may be implemented in a state machine. 
         [0018]    With this overview in mind, specific details of the described principles and system will now be discussed.  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 . 
         [0019]    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. 
         [0020]    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 . 
         [0021]      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. 
         [0022]    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. 
         [0023]    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. 
         [0024]    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. 
         [0025]    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 like the present system, it is desirable to 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 . 
         [0026]    To better understand the torque control provided by the presently describe principles, the control infrastructure and informational flow within the system will be discussed.  FIG. 3  is a simplified logical schematic  300  of the data flow associated with the mechanical components of  FIGS. 1 and 2  to 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. 
         [0027]    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. 
         [0028]    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. 
         [0029]    The gain values  308  derived by the variator controller  301  are retrieved by the variator controller  301  during operation. Based on the various available data 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. 
         [0030]    As noted above, however, it is difficult to accurately control torque in this type of transmission due to the slow response time. Thus, the described system provides torque control via a combination of open loop control, closed loop control, and a variable gain that depends upon various work parameters. In other words, the present control method applies different gain factors depending on machine operating parameters such as acceleration, speed (output speed, pump speed, machine speed), torque, and direction, as well as operator input, e.g., from an accelerator peddle, gear selector/shift, which provides an indication of future machine operation. The system applies higher gain factors to provide a better response time in situations wherein the system will tolerate high gain factors, but when the system is at a risk of instability, lower gain factors are applied. The applied gain data  308  may be stored in one or more look-up maps or may be implemented via a state machine. 
         [0031]    In an embodiment, a reset is applied to prevent the gain from climbing improperly during major machine operations. Thus, for example, the system will reset the I term when shifting from one clutch to another in this embodiment. Instead of resetting the I term, the system may optionally lower the gain value during such operations. 
         [0032]      FIG. 4  is a logical flow schematic illustrating the manner in which gain scheduling is executed in accordance with the disclosed principles. In particular, the control logic  400 , which may be implemented within or as part of the variator controller  301 , includes a gain provision module  401  and a gain application module, i.e., PID module  403 . 
         [0033]    The PID module  403  implements a proportional (P)/integral (I)/derivative (D) algorithm to minimize a deviation from the desired torque by adjusting parameters such as solenoid signal  1  ( 305 ) and solenoid signal  2  ( 306 ). In brief, the proportional value contributes a reaction based on the magnitude of the deviation, while the other two terms relate to the occurrence over time and the rate of deviation. In particular, the integral value contributes a reaction based on a sum of recent deviations, and the derivative value contributes a reaction based on the rate at which the deviation is changing. In other words, a low and rapidly increasing deviation might have a similar correction applied as a higher but more slowly increasing deviation. 
         [0034]    The gain provision module  401  includes a situational state calculator  405  and a situational gain scheduler  407 . The situational state calculator  405  determines a state of the machine based on machine operational values such as speed, torque, direction, and any other value in a given implementation that may have a significant effect on the transmission control response characteristics. Each state is associated with a particular correlated gain or set of correlated gains. After the state of the machine system is identified within the situational state calculator  405 , the situational gain scheduler  407  selects a gain based on the system state. Several gain curves  409  are illustrated with the situational gain scheduler  407 , reflecting that different curves may apply for different machine states, and that the appropriate gain in a given state may be derived from the curve for that state. 
         [0035]    Once the appropriate gain has been identified, it is passed to the PID module  403  for calculation of appropriate hydraulic pressure valve control signals to implement the desired machine behavior, e.g., the desired torque. As noted above, the gain values may be applied within the integral portion of the PID operation in a particular embodiment of the described principles, although such is not required in every embodiment. 
         [0036]    Given the process and architecture for deriving corrective values based on the system state, the machine configuration for applying those corrective values is shown schematically in  FIG. 5 . In particular,  FIG. 5  is a data flow and operational schematic illustrating the manner in which gain scheduling is executed in accordance with the disclosed principles. 
         [0037]    The flow  500  of  FIG. 5  begins with a torque command  501 , e.g., from a user via a user interface. Machine situational information  503  is also provided, e.g., from appropriate sensors. The torque command  501  and machine situational information  503  are provided to a closed loop torque controller  505  and an open loop torque controller  507 . Other machine elements may also feed into the closed loop torque controller  505 . For example, acceleration and deceleration pedals can feed into the closed loop torque controller  505  and may be used to modify the torque command as appropriate. Similarly, brakes, shifters, etc. may also affect the operation of the closed loop torque controller  505 . 
         [0038]    The results of these controllers, i.e., closed loop torque controller  505  and open loop torque controller  507 , are provided to the transmission  509 . In particular, the results of the closed loop torque controller  505  and open loop torque controller  507  are used to control the pump valves  511  (e.g., pressure valves  210 ,  211 ), 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 gain-compensated control of torque is provided. 
       INDUSTRIAL APPLICABILITY 
       [0039]    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. 
         [0040]    Within such applications, the described principles apply to the operation of hydrostatic and hydraulic continuously variable transmissions to allow accurate torque control using a combined closed loop/open loop 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. 
         [0041]    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. 
         [0042]    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.