Abstract:
A method for controlling the speed of internal combustion engines in heavy duty trucks and the like compensates for the overshoot, i.e., the difference between a targeted or commanded engine speed and a transient overspeed or underspeed. The method comprehends executing a program or subroutine where a throttle or engine speed change command is received by a controller, the engine speed change is monitored, a value of overshoot (on both an engine speed increase or decrease) is detected and the detected overshoot is subsequently utilized to temporarily reduce the speed change command, thereby effectively eliminating the overshoot and more positively and quickly arriving at the targeted engine speed.

Description:
TECHNICAL FIELD 
   The invention relates generally to control methods for internal combustion engines and more specifically to a control method which determines engine speed overshoot and compensates for such overshoot by subsequently, temporarily adjusting a speed change command by the determined overshoot value. 
   BACKGROUND 
   Particularly in two state or on/off control systems but also in proportional and more sophisticated control systems, overshoot is a common but unwanted operational reality. Overshoot may generally be defined as an undesirable and excess response to a control signal resulting in the controlled variable temporarily exceeding or overshooting the new, desired or target controlled value. The analysis of control overshoot and undershoot will not be addressed here beyond the acknowledgement that while overshoot or undershoot are generally undesirable and are to be minimized, such minimization carries with it compromises such as reduced speed of response and steady state errors, to name but two. 
   Control errors such as overshoot reside in many control systems, especially those associated with massive, mechanical devices. The manufacturer of motor vehicles and particularly heavy duty automated truck transmissions are often faced with control and overshoot challenges. Clearly, rapid, smooth and positive gear shifts are a most desired goal. However, each engine (and its electronic controller) with which a truck transmission may be mated will have slightly different speed, power and torque versus time characteristics. For example, in response to a throttle position change, one engine may accelerate and decelerate differently from another engine and may exhibit these differences in a distinct manner across various regions of the speed, power and torque curves. 
   For example, a command to one type or brand of engine to increase its speed from 1500 to 2000 rpm may achieve a first grouping of values of acceleration, elapsed time, overshoot and time to final, steady state speed, while another equally suitable type or brand of engine will exhibit another quite distinct grouping of values. 
   One of the significant areas of performance difference which implicates both the engine and its electronic control is overshoot, i.e., the tendency, upon receipt of a speed change command, to briefly exceed or overshoot either in a positive or negative direction, the new or target speed value. Such overshoot, if unaddressed, may result in an apparently poorly executed shift. For example, if a transmission/clutch controller determines during a downshift that the master clutch will be engaged when the engine speed 2000 rpm, the transmission/clutch controller will track the increasing engine speed and determine that at a specific future time, the engine speed will be 2000 rpm. Since at that specific future time, the engine speed will match the transmission input shaft speed in the newly selected gear, the master clutch should be engaged. Unfortunately, due to overshoot, the engine speed may briefly rise to 2050 rpm or 2075 rpm and then decay to 2000. If clutch engagement occurs above the 2000 rpm target speed and especially if it engages at or near peak rpm of 2075 rpm, a perceptible lurch will be experienced by the vehicle operator. Beyond momentary operator and passenger discomfort, such a lurch is indicative of a driveline torque surge and results in stress on the driveline components, especially the master clutch, which is highly undesirable. The present invention addresses the problem of engine/controller overshoot and detects the actual overshoot of an engine/controller combination and compensates for such overshoot. 
   SUMMARY 
   A method for controlling the speed of internal combustion engines in heavy duty trucks and the like compensates for the overshoot, i.e., the difference between a targeted or commanded engine speed and a transient overspeed or underspeed. The method comprehends executing a program or subroutine where a throttle or engine speed change command is received by a controller, the engine speed change is monitored, a value of overshoot (on both an engine speed increase or decrease) is detected and the detected overshoot is subsequently utilized to temporarily reduce the speed change command, thereby effectively eliminating the overshoot and more positively and quickly arriving at the targeted engine speed. 
   Thus it is an object of the present invention to provide a method for compensating for internal combustion engine overshoot in engine/controller systems. 
   It is a further object of the present invention to provide a method for detecting engine overshoot and utilizing such detected overshoot to compensate for such engine overshoot in subsequent operating cycles. 
   It is a still further object of the present invention to provide a method for detecting engine overshoot of a particular internal combustion engine and compensating for such overshoot in a particular engine/controller system. 
   Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiment and appended drawings wherein like reference numbers refer to the same component, element or feature. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic, plan view of truck tractor incorporating the present invention; 
       FIG. 2  is a graph illustrating both a learning cycle and an operating cycle of a method for reducing engine speed overshoot according to the present invention; and 
       FIGS. 3A and 3B  are flow charts of computer programs or software illustrating in diagrammatic form the steps of the method for reducing engine speed overshoot according to the present invention. 
   

   DESCRIPTION 
   Referring now to  FIG. 1 , a diagrammatic, plan view of a typical truck tractor incorporating the present invention is illustrated and generally designated by the reference number  10 . The truck tractor  10  includes a prime mover  12  which may be an internal combustion gas or Diesel engine having an output provided directly to a master friction clutch  14 . The master friction clutch  14  selectively and positively engages the output of the prime mover  12  to an input of a multiple speed gear change transmission  16 . The transmission  16  is preferably of the type currently designated an automated mechanical transmission (AMT) wherein gear or speed ratio changes of a main transmission, a splitter and a planetary gear assembly, for example, are all achieved by an automated, i.e., electric, hydraulic or pneumatic, shift and actuator assembly  18  under the control of a master microprocessor or controller  20 . The master microprocessor or controller  20  also includes a data and control link to an engine controller  22  which will typically include an engine speed sensor and a fuel control or metering device capable of adjusting and controlling the speed of the prime mover  12 . The master controller  20  also preferably provides control signals to a master friction clutch operator assembly  24  which controls the engagement and disengagement of the master friction clutch  14 . A throttle position sensor  26  senses the position of a vehicle throttle or accelerator pedal  28  and provides real time data regarding the position of the throttle pedal  28  to the master controller  20 . 
   The output of the transmission  16  is provided to a rear driveline assembly  30  which includes a rear propshaft  32  which drives a conventional rear differential  34 . The rear differential  34  provides drive torque to a pair of axles  36  which are in turn coupled to left and right tire and wheel assemblies  38  which may be either a dual configuration illustrated or a single left and right tire and wheel assembly. Suitable universal joints  42  may be utilized as necessary with the rear propshaft  32  to accommodate static and dynamic offsets and misalignments thereof. A stationary front axle  44  pivotally supports a pair of front tire and wheel assemblies  46  which are controllably pivoted by a steering linkage  48  which is coupled to and positioned by a steering wheel  52 . 
   As described, the present invention relates to learning the overshoot characteristics of an internal combustion engine in both the accelerating and decelerating modes, storing such positive and negative overshoot values and subsequently utilizing such overshoot values to compensate for such overshoot by temporarily reducing the target speed in an engine accelerating mode and temporarily increasing the target speed in an engine decelerating mode and, once the engine has achieved the adjusted target speed, allowing the engine or prime mover to seek and quickly achieve the actual target speed. 
   Turning now to  FIG. 2 , a graph  60  of rpm of the engine or prime mover  12  versus time presents two operating cycles of prime mover acceleration and deceleration: the first cycle being an overshoot detection and learning cycle and the second cycle representing subsequent cycles wherein the overshoot characteristics of the engine or prime mover  12  learned in the first cycle are utilized to compensate for and minimize overshoot. In the graph  60 , a dashed line  62  at all times represents the commanded speed of the prime mover  12  as signaled by the master controller  20  and a solid line  64  represents the actual rpm or rotational speed of the engine or prime mover  12 . By way of example, the prime mover  12  is initially rotating at approximately 1375 rpm. At a certain time, the master controller  20  provides and the prime mover  12  receives a command indicated by the dashed line  62 A to increase its speed from the current value of 1375 rpm to approximately 2000 rpm. The master controller  20  then provides a steady state output signal represented by the horizontal dashed line  62 B to maintain the speed of the engine or prime mover  12  at 2000 rpm. The speed of the prime mover  12 , of course, lags the command as illustrated by the sloping line  64 A. Furthermore, because of the inertia of the prime mover  12  and other factors, its speed overshoots to, for example, approximately 2075 rpm, as illustrated by the curve  64 B, and then settles back or decays to the commanded 2000 rpm as illustrated by the horizontal solid line  64 C. This (positive) overshoot value of approximately 75 rpm is stored in the master controller  20 . 
   At some subsequent time, as illustrated by the dashed line  62 D, the master controller  20  commands deceleration of the prime mover  12 , again for purposes of example, to 1375 rpm, and the master controller  20  then provides a steady state output represented by the horizontal dashed line  62 E. The speed of the prime mover  12  decays along the line  64 D. However, once again because of the inertia of the prime mover  12  and other factors, its speed overshoots, that is, goes lower than the desired 1375 rpm, as illustrated by the curve  64 E to approximately 1325 rpm, and then settles back to the commanded speed of 1375 rpm as illustrate by the horizontal line  64 F. This overshoot value, in the negative direction, of approximately 50 rpm, in the example, is also stored in the master controller  20 . 
   On all subsequent operating cycles, a command to change the speed of the engine or prime mover  12  is transmitted to the prime mover  12  but is corrected or adjusted by the previously detected quantitative overshoot values or functions thereof and stored in the master controller  20 . Thus, if the target speed of the engine or prime mover  12  is 2000 rpm, and the overshoot sensed in the previous cycle is 75 rpm, an adjusted target of 1925 rpm or a target value which is a function of the 75 rpm overshoot value will be provided to the prime mover  12  as indicated by the dashed line  62 G and the horizontal dashed line  62 H. The speed of the engine or prime mover  12  increases along the line designated  64 G. When the speed of the prime mover  12  approximately equals the adjusted or reduced target speed of 1925 rpm, the target speed is then readjusted to the full target speed as illustrated by the dashed line  62 I and the speed of the engine or prime mover  12  settles quickly at the desired target speed of 2000 rpm, as indicated by the horizontal line  64 l. Later, a reduction in the speed of the engine or prime mover  12  will be commanded as illustrated by the dashed line  62 J and the speed of the prime mover  12  will thus decay along the line  64 J. As the speed drops, the target speed will not be the actual final target speed, for example, 1375 rpm, but will be a slightly higher target speed, i.e. the target or commanded speed adjusted by the previously sensed deceleration overshoot, for example, 50 rpm or a value which is a function of this value. Thus the target speed at the end of the deceleration line  62 J will be 1425 rpm as indicated by the horizontal dashed line  62 K. When the prime mover  12  has decelerated to approximately this speed, the final target speed of 1375 rpm will be provided to the prime mover  12  as indicated by the line  62 L and its speed will quickly settle at the target speed of 1375 as indicated by the horizontal line  64 L. 
   Referring now to  FIG. 3A , a first computer program or software according to the present invention is illustrated and designated by the reference number  70 . This first computer program or software  70  corresponds to the learning activity on the left half of the graph  60  illustrated in  FIG. 2 . The first computer program or software  70  commences with a start or initialization step  72  which clears registers and which may include a process step  74  which sets an up or positive overshoot value (UOS) to zero and a negative or down overshoot value (DOS) also to zero. Alternatively, a median or average overshoot value which may be experimentally or empirically determined such as 50 for the UOS value and 30 for the DOS value may be set or stored as initial or default values. Additionally, stored UOS and DOS values may be averaged with new determined values to adjust, over time, these values to acknowledge and accommodate, for example, different operators&#39; habits or slowly shifting component performance. The program  70  then moves to a process step  76  which senses or determines activity and commands to the engine or prime mover  12 . Such commands and activity may include a final engine speed increase or up command (FESU), a final engine speed reduction or down command (FESD) and the change in engine speed (ΔES), either positive or negative, represented by the command which is the difference between the current speed of the engine or prime mover  12  and the final commanded speed. Alternatively, the sensed change in engine speed per unit time (dES/dt) may be utilized to determine whether the speed of the engine or prime mover  12  is increasing or decreasing. 
   Next, the program  70  moves to a decision point  78  which inquires whether the commanded change of speed of the engine or prime mover  12  is positive or negative, i.e., an increase (acceleration) or a decrease (deceleration) according to whether ΔES is greater than zero or less than zero, respectively. If ΔES is greater than zero, the speed of the engine or prime mover  12  is or will be increasing and the decision point is exited at YES. If ΔES is less than zero, the speed of the engine or prime mover  12  is or will be decreasing and the decision point is exited at NO. Alternatively, the decision point  78  may inquire whether the derivative of engine speed, i.e., change of engine speed per unit time (dES/dt) is greater than zero, i.e., is positive. If it is, the speed of the engine or prime mover  12  is increasing. If the derivative value dES/dt is less than zero, i.e., is negative, the speed of the engine or prime mover  12  is decreasing. 
   If the decision point  78  is exited at YES, the program  70  moves to a decision point  82  which inquires whether a commanded change in engine speed is greater than a predetermined value (PV). This predetermined value (PV) is an experimentally or empirically determined value which ensures that the learning activity of the program  70  is associated with a sufficiently large change in speed of the engine or prime mover  12  that a substantial and sensible overshoot of the speed of the engine or prime mover  12  will be experienced. In other words, if only a small change (ΔES) of the speed of the prime mover  12  is commanded, overshoot will typically be negligible or small. Thus, a predetermined value (PV) of 200 or 300 rpm or more will typically be suitable. A smaller predetermined value will allow the program  70  to learn with a smaller change in speed of the engine or prime mover  12  but such smaller change in speed may not result in detection of an optimum or suitable overshoot value. 
   Correspondingly, if the decision point  78  is exited at NO, the program  70  moves to a decision point  84  which determines whether the absolute value of engine speed difference (ΔES) is greater than a predetermined value (PV). This predetermined value may be the same value as utilized in the process step  82  but will more typically be a smaller value since the negative overshoot of the decelerating engine or prime mover  12  will typically be smaller than the positive overshoot of the accelerating engine or prime mover  12 . Thus, the predetermined value (PV) for the decision point  84  may be 100 rpm or more or less. 
   With regard to both decision points  82  and  84 , if the commanded engine speed change (ΔES) is below the predetermined value, both the decision points  82  and  84  are exited at NO and the first program  70  returns to the beginning of the process step  76  which once again senses activity of the engine or prime mover  12  to detect a commanded increase or decrease of the speed of the engine or prime mover  12 . 
   Returning then to the decision point  82 , if the commanded speed change of the engine or prime mover  12  is greater than the predetermined value (PV), the decision point  82  is exited at YES and the first program  70  moves to a process step  86  which monitors and determines the resulting maximum speed of the engine or prime mover  12  in response to the command of the master controller  20  to increase the speed of the engine or prime mover  12 . Next, the first program  70  moves to a process step  88  which sets or resets the value of up or positive overspeed, (UOS) to the difference between the maximum sensed speed of the engine or prime mover  12  and the commanded final engine speed. This difference is the positive overshoot which is evidenced by the curve  64 B in  FIG. 2 . At this point, the first program  70  has learned the positive or accelerating overshoot value (UOS) of the prime mover  12  and the first program  70  is exited at the process step  90 . 
   Returning to the decision point  84 , if the absolute value of the change of speed of the engine or prime mover  12  is greater than the predetermined value (PV), the decision point  84  is exited at YES and the first program  70  moves to a process step  92  which senses the minimum speed of the engine or prime mover  12 . Once the minimum speed has been sensed, the program  70  moves to a process step  94  which sets the negative or down overshoot value (DOS) to the difference between the commanded final decelerated speed of the engine or prime mover  12  and the actual sensed minimum speed. This represents the curve  64 E in  FIG. 2 . The program  70  then exits at the process step  90 . 
   Turning now to  FIG. 3B , the positive or up overshoot value (UOS) and the negative or down overshoot value (DOS) learned in the first program or software  70  is now utilized in a second and similar computer program or software  100 . This second computer program or software  100  corresponds to the activity on the right half of the graph  60  illustrated in  FIG. 2 . The second program  100  which may follow directly from the first program  70  begins with an initialization step  102  and moves to a process step  104  which senses the activity of the engine or prime mover  12  much as the process step  76  functions in the first program  70 . That is, data regarding a final increased engine speed command (FESU), a final decreased engine speed command (FESD), a change in the engine speed (ΔES) or alternatively, a change in engine speed per unit time, which both indicate whether the speed of the engine or prime mover  12  is increasing or decreasing are provided to the master controller  20 . 
   The second program  100  then moves to a decision point  106  which determines whether the commanded change in engine speed (ΔES) is greater than zero or less than zero and thus whether the engine is accelerating or decelerating, respectively. If the commanded change in engine speed (ΔES) is greater than zero, i.e., positive, the engine or prime mover  12  is accelerating and the decision point  106  is exited at YES. If the commanded change in engine speed (ΔES) is less than zero, i.e., negative, the engine or prime mover  12  is decelerating and the decision point  106  is exited at NO. Alternatively, the decision point  106  can inquire whether the commanded or sensed change in the speed of the engine or prime mover  12  per unit of time (dES/dt) is greater than zero, i.e., positive, and thus that the engine or prime mover  12  is accelerating or is less than zero, i.e., negative, and thus that the engine or prime mover  12  is decelerating. 
   If the decision point  106  is exited at YES, the program  100  moves to a process step  108  which sets a temporary target speed (TESU) for the speed of the engine or prime mover  12  to a value which is the commanded final engine speed (FESU) minus the up overshoot value determined in the program  70  discussed directly above. Alternatively, the up overshoot value (UOS) may be a function of a sensed variable such as the speed of the engine or prime mover  12  before this speed increase event occurred or the change of position of the throttle pedal  28 , a throttle kickdown increasing the UOS value by a predetermined factor or value and a partial throttle change reducing the UOS value by a predetermined factor or value. For purposes of example and simplicity, it will be assumed that the sensed overshoot is 75 rpm and that the final target speed of the engine or prime mover  12  (FESU) is 2000 rpm. Thus, the process step  108  sets the target speed (TESU) at 1925 rpm. Then the second program  100  moves to a process step  112  which senses the actual speed of the engine or prime mover  12 . 
   Next, a decision point  114  is entered which inquires whether the previously set temporary target engine speed (TESU) minus the current speed (ES) of the engine or prime mover  12  is less than a small error or tolerance value (TOL). Typically, the error or tolerance value (TOL) is a small whole number less than 10 r.p.m. but which may be raised or lowered to suit particular component variables. If the adjusted or temporary target speed (TESU) set in the process step  108  minus the speed (ES) of the engine or prime mover  12  is not less than the error or tolerance value (TOL), the decision point  114  is exited at NO, a process timer  116  times out a short interval and the speed of the engine or prime mover  12  is again sensed in the process step  112 . This cycle repeats until the temporary target speed (TESU) set in the process step  108  minus the speed (ES) of the engine or prime mover  12  is less than the error or tolerance value (TOL). When it is, the decision point  114  is exited at YES and the second program  100  enters a process step  116  which then resets the commanded engine speed to be the actual, initially commanded engine speed (FESU) which, in the example given, is 2000 rpm. As noted above, the engine or prime mover  12  then quickly and without significant overshoot moves to the final targeted speed (FESU) of 2000 rpm and the second program  100  exits at a step  120  to be repeated as frequently as activity of the engine or prime mover  12  necessitates. 
   Returning to the NO output of the decision point  106 , the second program  100  enters a process step  122  which sets a temporary deceleration target speed (TESD) of the engine or prime mover  12  as the commanded or final target speed (FESD) plus the down (deceleration) overshoot (DOS) value. Alternatively, the down overshoot value (DOS) may be a function of a sensed variable such as the speed of the engine or prime mover  12  before this speed decrease event occurred so the change of position of the throttle pedal  28 ; a throttle lift off increasing the DOS value by a predetermined factor or value and a partial throttle reduction reducing the DOS value by a predetermined factor or value. The program  100  then moves to a process step  124  which senses the actual speed of the engine or prime mover  12 . Next, a decision point  126  is entered which determines whether the actual measured speed (ES) of the engine or prime mover  12  minus the temporary target deceleration speed (TESD) is less than a small error or tolerance value (TOL). If it is not, the decision point  126  is exited at NO and an interval timer  128  is allowed to run and elapse whereupon the speed of the engine or prime mover  12  is once again sensed in the process step  124 . The cycle is repeated until the speed (ES) of the engine or prime mover  12  minus the temporary target deceleration speed (TESD) is less than the error or tolerance value (TOL). When it is, the decision point  126  is exited at YES and a process step  132  is entered which sets the final engine speed as the initially commanded speed (FESD) which is then quickly arrived at without significant overshoot. The second program  100  then moves to the exit step  120  and, as noted above, is repeated as necessary. 
   It will be appreciated that although the foregoing invention has been described in relation to an internal compulsion engine, it is equally suitable for use with other controlled devices, especially mechanical devices, exhibiting overshoot as a control variable is adjusted. 
   The foregoing disclosure is the best mode devised by the inventors for practicing this invention. It is apparent, however, that methods incorporating modifications and variations will be obvious to one skilled in the art of control methods for internal combustion engines. Inasmuch as the foregoing disclosure is intended to enable one skilled in the pertinent art to practice the instant invention, it should not be construed to be limited thereby but should be construed to include such aforementioned obvious variations and be limited only by the spirit and scope of the following claims.