Patent Publication Number: US-7720583-B2

Title: Vehicle control apparatus and method of suppressing vibration of vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Japanese Patent Application No. 205-173484 filed on Jun. 14, 2005, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a vehicle control apparatus that mainly controls a powertrain device of a vehicle, and a method of suppressing vibration of a vehicle, which reduces vibration on a spring of a vehicle. 
     2. Description of the Related Art 
     Japanese Patent Application Publication No. JP-A-2004-168148 describes a vehicle control apparatus for suppressing vibration of a vehicle. The vehicle control apparatus uses, as an instruction, a physical quantity corresponding to at least one of accelerator pedal operation, steering operation, and brake operation performed by a driver, and controls at least one of an engine and a brake according to the instruction. To suppress vehicle vibration, the vehicle control apparatus uses a movement model relating to vibration to correct the instruction input by the driver. That is, at least one of vertical vibration and/or torsional vibration caused by road surface reaction, which is applied to tires, vibration under a spring of a suspension, and vibration on the spring of the suspension, which is applied to a vehicle body itself. 
     However, in a conventional vehicle control apparatus, only a single movement model is used to correct the instruction input by the driver. Therefore, in a vehicle where the running characteristic can be changed according to the intention of the driver or the like, a conventional vehicle control apparatus may not be able to adequately suppress the vehicle vibration. Also, in the case where the instruction input by the driver is corrected using the single movement model as described above, the vehicle vibration may not be effectively suppressed if the environment around the vehicle or the running condition of the vehicle changes. 
     SUMMARY OF THE INVENTION 
     In view of the above, the invention provides a vehicle control apparatus and a method of suppressing vibration of a vehicle, which effectively suppresses vehicle vibration even if the driver or the like changes the running characteristic of the vehicle, or even if the driving environment around the vehicle or the running condition of the vehicle changes. 
     An aspect of the invention relates to a vehicle control apparatus that sets a target control amount related to running movement of a vehicle, and controls at least a powertrain device of the vehicle based on the target control amount. The vehicle control apparatus includes a controller. The controller sets the target control amount based on at least one of an instruction from a driver and an instruction from a predetermined control device provided in the vehicle. The controller determines at least one of the running characteristic of the vehicle set by the driver, the environment around the vehicle, and the running condition of the vehicle. The controller corrects the target control amount to suppress vibration on a spring of the vehicle. A correction amount, by which the target control amount is corrected, is adjusted according to the at least one of the running characteristic, the environment around the vehicle, and the determined running condition. 
     The controller may include a running characteristic determination device that determines the running characteristic of the vehicle set by the driver. The correction amount may be adjusted according to the determined running characteristic. 
     The controller includes the running characteristic determination device. The controller sets the target control amount based on the instruction provided by the driver using operation device such as an accelerator pedal, a brake pedal, and a steering wheel, and/or the instruction from the control device such as a cruise-controller. The running characteristic determination device determines the running characteristic of the vehicle that is set by the driver or the like, such as the mode where priority is given to acceleration, and the mode where priority is given to comfort. Further, the controller corrects the target control amount to suppress the vibration on the spring of the vehicle. When the controller corrects the target control amount, the correction amount, by which the target control amount is corrected, is adjusted according to the running characteristic. Thus, in the vehicle control apparatus, the target control amount is corrected to suppress the vibration on the spring of the vehicle taking into account the running characteristic set by the driver. Therefore, even if the driver changes the running characteristic of the vehicle, the vehicle vibration can be effectively suppressed. 
     In this case, the controller may have a damping characteristic for damping the vibration on the spring of the vehicle, and the damping characteristic may be adjusted according to the running characteristic determined by the running characteristic determination device. 
     The target control amount may be a target driving force of the vehicle. Also, the controller may include a quadratic notch filter, and a parameter for determining the damping characteristic may be adjusted according to the running characteristic. 
     Generally, a transfer function where the target driving force of the vehicle is used as an input and the suspension stroke of the vehicle (for example, the stroke of the rear suspension) is used as an output is generally expressed in the form of a quadratic/quartic transfer function. The quadratic/quartic transfer function includes two quadratic transfer functions. One of the quadratic transfer functions does not induce vibration, while the other induces vibration. Accordingly, the target driving force that is the target control amount is corrected using the quadratic notch filter that cancels the pole of the quadratic transfer function included the quadratic/quartic transfer function, which induces vibration. As a result, the vehicle vibration can be suppressed. When the target control amount is corrected using the notch filter, the parameter of the notch filter, that is, the parameter of the correction equation in the form of the quadratic/quadratic function, such as a frequency and a damping ratio, is changed according to the running characteristic set by the driver. As a result, even if the running characteristic of the vehicle is changed, the vehicle vibration can be effectively suppressed. 
     The controller may include an environment determination device that determines the environment around the vehicle, and the correction amount may be adjusted according to the determined environment. 
     The controller includes the environment determination device. The controller sets the target control amount based on the instruction that is provided by the driver using operation device such as the accelerator pedal, the brake pedal, and the steering wheel, and/or the instruction from the control device such as the cruise-controller. The environment determination device determines the environment around the vehicle such as the condition of the road on which the vehicle is running. Further, the controller corrects the target control amount to suppress the vibration on the spring of the vehicle. When the controller corrects the target control amount, the correction amount, by which the target control amount is corrected, is adjusted according to the environment around the vehicle determined by the environment determination device. Thus, in the vehicle control apparatus, the target control amount is corrected to suppress the vibration on the spring of the vehicle, taking into account the environment around the vehicle. Therefore, even if the environment around the vehicle changes, the vehicle vibration can be effectively suppressed. 
     In this case, the controller may have a damping characteristic for damping the vibration on the spring of the vehicle, and the damping characteristic may be adjusted according to the environment around the vehicle determined by the environment determination device. 
     The target control amount may be a target driving force of the vehicle. Also, the controller may include a quadratic notch filter, and a parameter for determining the damping characteristic may be adjusted according to the environment around the vehicle determined by the environment determination device. 
     As described above, the target driving force, which is the target control amount, is corrected using the quadratic notch filter that cancels the pole of one quadratic transfer function included in the quadratic/quartic transfer function where the target driving force is used as the input, and the suspension stroke of the vehicle is used as the output. As a result, the vehicle vibration can be suppressed. When the target control amount is corrected using the notch filter, the parameter of the notch filter, that is, the parameter of the correction equation in the form of the quadratic/quadratic function, such as the frequency and the damping ratio, is changed according to the environment around the vehicle. As a result, even if the environment around the vehicle changes, the vehicle vibration can be effectively suppressed. 
     The controller may include a running condition determination device that determines the running condition of the vehicle, and the correction amount may be adjusted according to the determined running condition. 
     The controller includes the running condition determination device. The controller sets the target control amount based on the instruction that is provided by the driver using operation device such as the accelerator pedal, the brake pedal, and the steering wheel, and/or the instruction from the control device such as the cruise-controller. The running condition determination device determines the running condition of the vehicle. For example, the running condition determination device determines whether the vehicle is turning. Further, the controller corrects the set target control amount to suppress the vibration on the spring of the vehicle. When the controller corrects the target control amount, the correction amount, by which the target control amount is corrected, is adjusted according to the determined running condition. Thus, in the vehicle control apparatus, the target control amount is corrected to suppress the vibration on the spring of the vehicle, taking into account the running condition of the vehicle. Therefore, even if the running condition changes, the vehicle vibration can be effectively suppressed. 
     In this case, the controller may have a damping characteristic for damping the vibration on the spring of the vehicle, and the damping characteristic may be adjusted according to the determined running condition. 
     The target control amount may be a target driving force of the vehicle. Also, the correction device may be a quadratic notch filter, and a parameter for determining the damping characteristic may be adjusted according to the running condition determined by the running condition determination device. 
     As described above, the target driving force (i.e., the target control amount) is corrected using the quadratic notch filter that cancels the pole of one quadratic transfer function included in the quadratic/quartic transfer function where the target driving force is used as the input, and the suspension stroke of the vehicle is used as the output. As a result, the vehicle vibration can be suppressed. When the target control amount is corrected using the notch filter, the parameter of the notch filter, that is, the parameter of the correction equation in the form of the quadratic/quadratic function, such as the frequency and the damping ratio, is changed according to the running condition of the vehicle. As a result, even if the running condition of the vehicle changes, the vehicle vibration can be effectively suppressed. 
     The controller may include running characteristic determination device an environment determination device and a running condition determination device. The running characteristic determination device determines the running characteristic of the vehicle set by the driver. The environment determination device determines the environment around the vehicle. The running condition determination device determines the running condition of the vehicle. The correction amount may be adjusted according to at least one of the determined running characteristic, the determined environment around the vehicle, and the determined running condition. 
     In this case, the controller may have a damping characteristic for damping the vibration on the spring of the vehicle, and the damping characteristic may be adjusted according to at least one of the determined running characteristic, the determined environment around the vehicle, and the determined running condition. 
     The target control amount may be a target driving force of the vehicle. Also, the controller may include a quadratic notch filter, and a parameter for determining the damping characteristic may be adjusted according to at least one of the determined running characteristic, the determined environment around the vehicle, and the determined running condition. 
     Another aspect of the invention relates to a method of suppressing vibration of a vehicle, which reduces vibration on a spring of a vehicle. The method includes the steps of setting a target control amount used to control at least a powertrain device of the vehicle based on at least one of an instruction from a driver and an instruction from a predetermined control device included in the vehicle; determining at least one of the running characteristic of the vehicle set by the driver, the environment around the vehicle, and the running condition of the vehicle; setting up a correction equation used to suppress the vibration on the spring of the vehicle based on at least one of the running characteristic, the environment around the vehicle, and the running condition; and correcting the target control amount using the correction equation. 
     In the method of suppressing vibration of a vehicle, the target control amount may be corrected by changing a damping characteristic for damping the vibration on the spring of the vehicle. The damping characteristic may be adjusted according to at least one of the running characteristic, the environment around the vehicle, and the running condition. 
     The target control amount may be a target driving force of the vehicle, and a parameter for determining the damping characteristic may be adjusted according to at least one of the running characteristic, the environment around the vehicle, and the running condition. 
     With the vehicle control apparatus and the method of suppressing vibration of a vehicle according to the invention, the vehicle vibration can be effectively suppressed even if the running characteristic of the vehicle is changed. In addition, the control apparatus and the method of suppressing vibration of the invention effectively suppress vehicle vibration even if the environment around the vehicle or the running condition of the vehicle changes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, advantages thereof, and technical and industrial significance of this invention will be better understood by reading the following detailed description of the example embodiments of the invention, when considered in connection with the accompanying drawings, in which: 
         FIG. 1  is a block diagram showing the configuration of a vehicle to which a vehicle control apparatus according to the invention is applied; 
         FIG. 2  is a control block diagram describing basic steps of control of an internal combustion engine and a transmission, which is executed by the vehicle control apparatus according to the invention; 
         FIG. 3  is a flowchart describing steps of correcting a target driving force of the vehicle while changing the parameter of a filter according to the running characteristic of the vehicle, the environment around the vehicle, and the running condition of the vehicle; 
         FIG. 4  is a flowchart describing a reference parameter setting process that is executed when the target driving force is corrected; 
         FIG. 5  is a flowchart describing a plant parameter setting process that is executed when the target driving force is corrected; 
         FIG. 6  is a flowchart describing the process of setting reference parameter coefficients according to the environment around the vehicle; 
         FIG. 7  is a flowchart describing the process of setting reference parameter coefficients according to the running condition of the vehicle; and 
         FIG. 8  is a flowchart describing the process of correcting the target driving force. 
     
    
    
     DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS 
     In the following description and the accompanying drawings, the present invention will be described in more detail with reference to example embodiments.  FIG. 1  is a block diagram showing a vehicle to which a vehicle control apparatus according to the invention is applied. In  FIG. 1 , a vehicle  1  includes a driving power source, such as an internal combustion engine (not shown), for example, a gasoline engine or a diesel engine. The internal combustion engine may include devices such as a fuel injection device  2 , an ignition device  3 , and an electronically controlled throttle valve  4  (hereinafter, simply referred to as “throttle valve  4 ”). The vehicle  1  includes a transmission  5  that transmits power generated by the internal combustion engine to driving wheels. For example, the transmission  5  may be, for example, an automatic transmission or a continuously variable transmission. The vehicle  1  further includes an electronically controlled brake system, a steering device, and an electronically controlled suspension. The electronically controlled brake system includes a brake actuator  6  that is electronically controlled according to the operation amount of a brake pedal. The steering device includes an actuator  7  for steering that is electronically controlled according to the operation amount of a steering wheel, such as a variable gear mechanism and an electric assist unit. The electronically controlled suspension includes a plurality of shock absorbers  8  that are electronically controlled to change damping ratio. 
     The internal combustion engine and the transmission constitute a powertrain device of the vehicle  1 . The powertrain device is controlled by an electronic control unit for controlling the powertrain device (hereinafter, referred to as “powertrain control ECU”)  10 . The powertrain control ECU  10  includes a first processor  11  and a second processor  12 . Each of the first processor  11  and the second processor  12  includes a CPU, ROM, RAM, an input/output interface, and a storage device (none of them are shown). Control programs are stored in the ROM. The RAM is used as a work area where data is stored and programs are executed. The powertrain control ECU  10  is connected to an accelerator pedal sensor  14 , a brake sensor  15 , and a steering angle sensor  16  via a bus (not shown) and the like. 
     The accelerator sensor  14  detects the operation amount of an accelerator pedal operated by a driver, and transmits a signal indicating the detected value to the powertrain control ECU  10 . The brake sensor  15  detects the operation amount of a brake pedal operated by the driver, and transmits a signal indicating the detected value to the powertrain control ECU  10 . Further, the steering sensor  16  detects the steering angle that indicates the steering amount of a steering wheel operated by the driver, and transmits a signal indicating the detected value to the powertrain control ECU  10 . The powertrain control ECU  10  controls the aforementioned fuel injection device  2 , the ignition device  3 , the throttle valve  4 , and the transmission  5  to meet the driver&#39;s need, according to the driver&#39;s instruction indicated by the signals from the sensors  14  to  16 , and values detected by other sensors (not shown). In this embodiment, the fuel injection device  2  and the ignition device  3  of the internal combustion engine is controlled mainly by the first processor  11  of the powertrain control ECU  10 . The throttle valve  4  is controlled mainly by the second processor  12  of the powertrain control ECU  10 . In addition, the second processor  12  controls the transmission  5 , and functions as a so-called cruise-controller that assists the driver in driving the vehicle  1 . The second processor  12  may also drive the vehicle instead of the driver. The powertrain control ECU  10  does not necessarily control both of the internal combustion engine and the transmission  5 . That is, the powertrain control ECU  10  may be configured to control one or both of the internal combustion engine and the transmission  5 . 
     In addition, the powertrain control ECU  10  is connected to an environmental information-obtaining device  17  and a mode-switch  18  via a bus (not shown). In the embodiment, the environmental information-obtaining device  17  includes, for example, a navigation system, a vehicle information communication system (VICS), and an image-capturing unit or an inter-vehicle sensor that obtains the inter-vehicle distance. The environmental information-obtaining device  17  obtains information on the condition of the road on which the vehicle  1  is running, the inter-vehicle distance, and the like. The environmental information-obtaining device  17  supplies the obtained information to the powertrain control ECU  10 . The environmental information-obtaining device  17  may include a radar unit that can obtain various kinds of information. 
     The mode-switch  18  is used to change the damping ratio of each of the plurality of shock absorbers  8  in the electronically controlled suspension system. By operating the mode-switch  18 , the running characteristic of the vehicle  1 , that is, the running mode can be changed. In this embodiment, when the driver turns off the mode-switch  18 , the damping ratio of each shock absorber  8  is set to a standard value. As a result, the running mode of the vehicle  1  is set to a normal mode. When the mode-switch  18  is turned on, and a “first mode” is selected, the damping ratio of each shock absorber  8  is set to a value lower than the standard value. As a result, the running mode of the vehicle  1  is set to a power mode. In the power mode, priority is given to acceleration performance of the vehicle  1  over suppression of vibration of the vehicle  1 . In a “second mode” that may be selected using the mode-switch  18 , the damping ratio of each shock absorber  8  is set to a value higher than the standard value. As a result, the running mode is set to a comfort mode. In the comfort mode, priority is given to suppression of vehicle vibration  1  over acceleration performance of the vehicle  1 . 
     The powertrain control ECU  10  is connected to an ECBECU (ECU for the electronically-controlled brake)  20 , a steering ECU  30 , and a suspension ECU  40  via bus (not shown) and the like, or by wireless communication. The ECBECU  20  controls the aforementioned electronically controlled brake system. That is, the ECBECU  20  controls the brake actuator  6  and the like based on values detected by the sensors including the brake sensor  15 . The ECBECU  20  in the embodiment can execute vehicle stability control to ensure stability of the vehicle  1  while the vehicle  1  turns. The steering ECU  30  controls the steering device of the vehicle  1 . That is, the steering ECU  30  controls the actuator  7  for steering, and the like based on values detected by the sensors including the steering sensor  16 . The suspension ECU controls the aforementioned electronically controlled suspension. That is, the suspension ECU executes the control to change the damping ratio of each shock absorber  8  according to the operation of the mode-switch  18  operated by the driver. A throttle valve opening amount sensor, a vehicle speed sensor, a longitudinal acceleration sensor, a yaw rate sensor, and the like (none of them are shown) supply information necessary for the control to the powertrain control ECU  10 , the ECBECU  20 , the steering ECU  30 , and the suspension ECU  40 . 
       FIG. 2  is a control block diagram describing basic steps of control executed by the powertrain control ECU  10 . Basically, the control related to  FIG. 2  is executed by the first processor  11  of the powertrain control ECU  10 . As shown in  FIG. 2 , the first processor  11  includes a target acceleration determination portion  111 , a target driving-force obtaining portion  112 , a driving force adjustment portion  114 , a filter  115 , and a control-amount setting portion  116 . The target acceleration determining portion  111  obtains the target acceleration of the vehicle  1  based on the operation amount of the accelerator pedal indicated by the signal from the accelerator pedal sensor  14 , using, for example, a map that defines the relation between the operation amount of the accelerator pedal and the target acceleration of the vehicle  1 . Then, the target acceleration determining portion  111  transmits the signal indicating the obtained value to the driving power adjustment portion  114 . 
     The driving force adjustment portion  114  sets a target driving force Pt of the internal combustion engine based on the signal from the target driving-force obtaining portion  112  and at least one of an instruction from the driver and an instruction from the second processor  12  that is the control device included in the vehicle  1 . In the embodiment, the second processor  12  of the powertrain control ECU  10  functions as the so-called cruise-controller that assists the driver in driving the vehicle  1 . The second processor  12  may also drive the vehicle  1  instead of the driver. Therefore, when the driver provides an instruction to execute the cruise control, the second processor  12  transmits an instruction for the driving force necessary for the cruise control to the driving force adjustment portion  114  of the first processor  11 . In this case, basically, the driving force adjustment portion  114  sets the target driving force Pt of the internal combustion engine by adding the driving force requested by the second processor  12  to the target driving force obtained by the target driving-force obtaining portion  112 . When the sum of the target driving force obtained by the target driving-force obtaining portion  112  and the driving force requested by the second processor  12  is not in a predetermined guard range, the driving force adjustment portion  114  execute guard processing so that the target driving force Pt becomes equal to the upper limit value or the lower limit value of the guard range. When the target driving force Pt is not in the guard range, this guard processing may invalidate the instruction from the second processor  12 . 
     The filter  115  corrects the final driving force Pt set by the driving force adjustment portion  114  to suppress vibration on a spring of the vehicle  1 . In the embodiment, as the filter  115 , a quadratic notch filter is used. Output from the filter  115 , that is, a corrected target driving force Ptc is supplied to the control-amount setting portion  116 . The control-amount setting portion  116  sets the control amount of each of the fuel injection device  2 , the ignition device  3 , the throttle valve  4 , and the transmission  5  based on the corrected target driving force Ptc. The first processor  11  or the second processor  12  generates and transmits a control signal to each of the fuel injection device  2 , the ignition device  3 , the throttle valve  4 , and the transmission  5  based on the control amount set by the control-amount setting portion  116 . Thus, the internal combustion engine and the transmission  5  of the vehicle  1  are controlled to meet the driver&#39;s need. 
     In this embodiment, a quadratic notch filter is provided as the filter  115  for the first processor  11  of the powertrain control ECU  10  for the following reason. For example, in the case where the vehicle  1  is a rear-wheel-drive vehicle, a transfer function where the target driving force of the vehicle is used as an input and the rear suspension stroke of the vehicle is used as an output is generally expressed in the form of a quadratic/quartic transfer function represented by the following equation (1). 
     
       
         
           
             
               
                 
                   
                     
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     The quadratic/quartic transfer function includes two quadratic transfer functions G 1  (s) and G 2  (s). When the equation (1) is set up, a damping ratio ζ 1  of the quadratic transfer function G 1  (s) induces vibration. A damping ratio ζ 2  of the quadratic transfer function G 2  (s) does not induce vibration. Therefore, the quadratic transfer function G 2  (s) of the equation (1) does not induce vibration, but the quadratic transfer function G 1  (s) induces vibration. Accordingly, the target driving force Pt that is the target control amount is corrected using the filter  115 . The filter  115  is the quadratic notch filter that cancels the pole of the quadratic transfer function G 1  (s) included in the quadratic/quartic transfer function represented by the equation (1), which induces vibration. As a result, the vehicle vibration  1  can be suppressed. 
     The quadratic notch filter that cancels the pole of the quadratic transfer function G 1  (s) in the equation (1) is expressed in the form of a quadratic/quadratic transfer function represented by the following equation (2). 
     
       
         
           
             
               
                 
                   
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     In the equation (2), ω m  is a reference frequency, ζ m  is a reference damping ratio, ω p  is a plant frequency of a driving system of the vehicle  1  that functions as a plant, and ζ p  is a plant damping ratio. The first processor  11  of the powertrain control ECU  10  is provided with the filter  115  that corrects the target driving force Pt based on the correction equation (2). 
     In this case, the parameters of the equation (2), that are, the reference frequency ω m , the reference damping ratio ζm, the plant frequency ω p , and the plant damping ratio ζ p  are changed according to the changes in the running characteristic (i.e., the running mode) of the vehicle  1 , the environment around the vehicle  1 , the running condition of the vehicle  1 , and the like. That is, when the target driving force Pt is corrected using the filter  115 , the reference frequency ω m , the reference damping ratio ζ m , the plant frequency ω p , and the plant damping ratio ζ p  that are used to set the damping characteristic of the filter  115  (correction equation) are changed according to the running mode of the vehicle  1  set by the driver, the environment around the vehicle  1 , and the running condition of the vehicle  1 . As a result, vehicle vibration  1  can be constantly and effectively suppressed. 
     In the vehicle  1  according to the embodiment, a routine shown in  FIG. 3  is executed to correct the target driving force Pt while changing the parameters used to set the damping characteristic of the filter  115  according to the running characteristic of the vehicle  1 , the environment around the vehicle  1 , and the running condition of the vehicle  1 . The routine in  FIG. 3  is repeatedly executed by the first processor  11  of the powertrain control ECU  10  at predetermined time intervals. When the routine is executed, the first processor  11  obtains the values of the parameters indicating the state of the vehicle  1 , such as the vehicle speed of the vehicle  1 , and the operation amount of the accelerator pedal or the brake pedal (S 1 ). Then, the first processor  11  executes a reference parameter setting process to set each reference parameter of the filter  115  according to the running mode of the vehicle  1  (S 2 ). 
     After the process in step S 2  is executed, the first processor  11  executes a plant parameter setting process to set each plant parameter of the filter  115  according to the running mode of the vehicle  1  (S 4 ). Further, the first processor  11  determines whether the environment around the vehicle  1  needs to be taken into account when the filter  115  executes a correction process (S 5 ). If the first processor  11  determines that the environment around the vehicle  1  needs to be taken into account when the filter  115  executes the correction process based on a predetermined criterion (YES in step S 5 ), the first processor  11  executes the process of setting a reference parameter coefficient by which each reference parameter of the filter  115  is multiplied according to the environmental information obtained by the environmental information-obtaining device  17  (S 6 ). If it is determined that the environment around the vehicle  1  does not need to be taken into account when the filter  115  executes the correction process (NO in step S 5 ), the process of setting the reference parameter coefficients (S 6 ) is skipped. Subsequently, the first processor  11  executes the process of setting the reference parameter coefficient by which each reference parameter of the filter  115  is multiplied, according to the running condition of the vehicle  1  (S 8 ). Then, the first processor  11  executes the process of correcting the target driving force Pt using the filter  115  (S 10 ). 
       FIG. 4  is a flowchart describing the reference parameter setting process in step S 2 . As shown in  FIG. 4 , the first processor  11  of the powertrain control ECU  10  determines whether the mode-switch  18  is ON (S 20 ). If the first processor  11  determines that the mode-switch  18  is off and the driver sets the running mode of the vehicle  1  to the normal mode (NO in step S 20 ), the first processor  11  reads a reference frequency ω m0  and a reference damping ratio ζ m0  for the normal mode from a predetermined storage device. Then, the first processor  11  stores the values in a predetermined storage area as the reference parameters used as basic parameters (S 22 ). 
     Meanwhile, if the first processor  11  determines that the mode-switch  18  is on (YES in step S 20 ), the first processor  11  determines whether the mode-switch  18  is set to the first mode where priority is given to the acceleration performance of the vehicle  1  (S 24 ). If the first processor  11  determines that the mode-switch  18  is set to the first mode, and the driver sets the running mode of the vehicle  1  to the power mode (YES in step S 24 ), the first processor  11  reads the reference frequency ω m1  and the reference damping ratio ζ m1  for the power mode from the predetermined storage device, and stores the values in a predetermined storage area as the reference parameters used as basic parameters (S 26 ). 
     If the first processor  11  determines that the mode-switch  18  is on in step S 20 , and determines that the mode-switch  18  is not set to the first mode in step S 24 , it is determined that the driver sets the running mode of the vehicle  1  to the second mode, that is, the comfort mode where priority is given to suppression of the vehicle vibration  1  (NO in step S 24 ). Accordingly, if a negative determination is made in step S 24 , the first processor  11  reads a reference frequency ω m2  and a reference damping ratio ζ m2  for the comfort mode from the predetermined storage device, and stores the values in a predetermined storage area as the reference parameters used as basic parameters (S 28 ). 
     The reference parameters ω m0  and ζ m0  for the normal mode, ω m1  and ζ m1  for the power mode, ω m2  and ζ m2  for the comfort mode are set in advance through experiment and analysis, and are stored in the storage device of the powertrain control ECU  10 . Hereinafter, the reference parameters ω m0 , ω m1 , and ω m2  will be sometimes collectively referred to as “ω mi ”, and the reference parameters ζ m0 , ζ m1 , and ζ m2  will be sometimes collectively referred to as “ζ mi ”. The suffix “i” signifies the running mode of the vehicle  1  set by the mode-switch  18 . The suffix “0” signifies the normal mode, the suffix “1” signifies the power mode, and the suffix “2” signifies the comfort mode. 
     After the reference frequency ω mi  and the reference damping ratio ζ mi  are set in step S 22 , S 26  or S 28 , the plant parameter setting process is executed in step S 4 .  FIG. 5  is a flowchart describing the plant parameter setting process in step S 4 . In this case, the first processor  11  of the powertrain control ECU  10  determines whether the mode-switch  18  is on (S 40 ). If the first processor  11  determines that the mode-switch  18  is off, and the running mode of the vehicle  1  is set to the normal mode (NO in step S 40 ), a plant frequency ω p0  and a plant damping ratio ζ p0  for the normal mode are read from the predetermined storage device, and the values are stored in a predetermined storage area as the plant parameters used as basic parameters (S 42 ). 
     If the first processor  11  determines that the mode-switch  18  is on (YES in step S 40 ), the first processor  11  determines whether the mode-switch  18  is set to “the first mode” (S 44 ). If the first processor  11  determines that the mode-switch  18  is set to “the first mode”, and the driver sets the running mode of the vehicle  1  to the power mode (YES in step S 44 ), the first processor  11  reads a plant frequency ω p1  and a plant damping ratio ζ p1  for the power mode from the predetermined storage device, and stores the values in a predetermined storage area as the plant parameters used as basic parameters (S 46 ). If the first processor  11  determines that the mode-switch  18  is on and the mode-switch  18  is not set to the first mode (NO in step S 44 ), the first processor  11  reads a plant frequency ω p2  and a plant damping ratio ζ p2  for the comfort mode from the predetermined storage device, and stores the values in a predetermined storage area as the plant parameters used as basic parameters (S 48 ). 
     The plant parameters ω p0  and ζ p0  for the normal mode, ω p1  and ζ p1  for the power mode, ω p2  and ζ p2  for the comfort mode are set in advance through experiment and analysis, and are stored in the storage device of the powertrain control ECU  10 . Hereinafter, the plant parameters ω p0 , ω p1 , and ω p2  will be sometimes collectively referred to as “ω pi ”, and the plant parameters ζ p0 , ζ p1 , and ζ p2  will be sometimes collectively referred to as “ζ pi ”. The electronically controlled suspension of the vehicle  1  has the function of adjusting the vehicle height. For example, in the case where a plurality of vehicle heights can be set in each running mode, a plurality of plant parameters corresponding to the plurality of vehicle heights may be provided for each of the normal mode, the power mode, and the comfort mode, and the plant parameters corresponding to the selected vehicle height may be set. 
     In step S 42 , and step S 46  or step S 48 , the plant frequency ω pi  and the plant damping ratio ζ pi  are set. If an affirmative determination is made in step S 5 , the process of setting reference parameter coefficients is executed in step S 6 .  FIG. 6  is a flowchart describing the process of setting the reference parameter coefficients according to the environment around the vehicle  1 . As shown in  FIG. 6 , when the reference parameter coefficients are set according to the environment around the vehicle, the first processor  11  of the powertrain control ECU  10  obtains the information on the environment around the vehicle  1  from the navigation system and the like of the environmental information-obtaining device  17  (S 60 ). The first processor  11  determines whether the vehicle  1  is running on a highway such as an express way (S 62 ). If the first processor  11  determines that the vehicle  1  is running on the highway (YES in step S 62 ), the first processor  11  reads, from the predetermined storage device, a reference parameter coefficient e 1  and a reference parameter coefficient E 1 , and stores the values in a predetermined storage area (S 63 ). When the vehicle  1  is running on the highway, the reference frequency ω mi  that is set by the reference parameter setting process in step S 2  is multiplied by the reference parameter coefficient e 1 , and the reference damping ratio ζ mi  that is also set by the reference parameter setting process in step S 2  is multiplied by the reference parameter coefficient E 1 . 
     If the first processor  11  determines that the vehicle  1  is not running on the highway (NO in step S 62 ), the first processor  11  determines whether the vehicle  1  is running on a winding road based on the information on the environment around the vehicle (S 64 ). If the first processor  11  determines that the vehicle  1  is running on the winding road (YES in step S 64 ), the first processor  11  reads, from the predetermined storage device, a reference parameter coefficient e 2  and a reference parameter coefficient E 2 , and stores the values in a predetermined storage area (S 65 ). When the vehicle  1  is running on the winding road, the reference frequency ω mi  that is set by the reference parameter setting process in step S 2  is multiplied by the reference parameter coefficient e 2 , and the reference damping ratio ζ mi  that is also set by the reference parameter setting process in step S 2  is multiplied by the reference parameter coefficient E 2 . 
     If the first processor  11  determines that the vehicle  1  is not running on the winding road (NO in step S 64 ), the first processor  11  determines whether the vehicle  1  is running on a low-friction coefficient road (hereinafter, referred to as “low-μ road”) based on the information on the environment around the vehicle (S 66 ). If the first processor  11  determines that the vehicle  1  is running on the low-μ road (YES in step S 66 ), the first processor  11  reads, from the predetermined storage device, a reference parameter coefficient e 3  and a reference parameter coefficient E 3 , and stores the values in a predetermined storage area (S 67 ). When the vehicle  1  is running on the low-μ road, the reference frequency ω mi  that is set by the reference parameter setting process in step S 2  is multiplied by the reference parameter coefficient e 3 , and the reference damping ratio ζ mi  that is also set by the reference parameter setting process in step S 2  is multiplied by the reference parameter coefficient E 3 . 
     If the first processor  11  determines that the vehicle  1  is not running on the low-μ road (NO in step S 66 ), the first processor  11  obtains the distance between the vehicle  1  and a preceding vehicle from the image-capturing unit and the like (S 68 ). The image-capturing unit included in the environmental information-obtaining device  17  obtains the inter-vehicle distance. The first processor  11  determines whether the distance between the vehicle  1  and the preceding vehicle is less than a threshold value (S 70 ). If the first processor  11  determines that the distance between the vehicle  1  and the preceding vehicle is less than the threshold value (YES in step S 70 ), the first processor  11  reads, from the predetermined storage device, a reference parameter coefficient e 4  and a reference parameter coefficient E 4 , and stores the values in a predetermined storage area (S 71 ). When the distance between the vehicle  1  and the preceding vehicle is less than the threshold value, the reference frequency ω mi  that is set by the reference parameter setting process in step S 2  is multiplied by the reference parameter coefficient e 4 , and the reference damping ratio ζ mi  that is also set by the reference parameter setting process in step S 2  is multiplied by the reference parameter coefficient E 4 . 
     If the first processor  11  determines that the distance between the vehicle  1  and the preceding vehicle is greater than or equal to the threshold value (NO in step S 70 ), the first processor  11  may determine that the vehicle  1  is running on an uncrowded ordinary road, and the first processor  11  reads, from the predetermined storage device, a reference parameter coefficient e 0  and a reference parameter coefficient E 0 , and stores the values in a predetermined storage area (S 73 ). When the vehicle  1  is normally running on the ordinary road, the reference frequency ω mi  that is set by the reference parameter setting process in step S 2  is multiplied by the reference parameter coefficient e 0 , and the reference damping ratio ζ mi  that is also set by the reference parameter setting process in step S 2  is multiplied by the reference parameter coefficient E 0 . The reference parameter coefficients e 0 , E 0 , e 1 , E 1 , e 2 , E 2 , e 3 , E 3 , e 4 , and E 4  are determined empirically, and stored in the storage device of the powertrain control ECU  10 . Hereinafter, the reference parameter coefficients e 0 , e 1 , e 2 , e 3 , and e 4  will be sometimes collectively referred to as “ej”, and the reference parameter coefficients E 0  E 1 , E 2 , E 3 , and E 4  will be sometimes collectively referred to as “Ej”. The suffix “j” signifies the environment around the vehicle. The suffix “0” signifies that the vehicle  1  is normally running on the ordinary road, the suffix “1” signifies that the vehicle  1  is running on the highway. The suffix “2” signifies that the vehicle  1  is running on the winding road. The suffix “3” signifies that the vehicle  1  is running on the low-μ road. The suffix “4” signifies that the distance between the vehicle  1  and the preceding vehicle is short. 
     After the process in step S 4  is executed, or after the reference parameter coefficients ej and Ej are set in step S 63 , S 65 , S 67 , S 71 , or S 73 , the process of setting reference parameter coefficients is executed in step S 8 .  FIG. 7  is a flowchart describing the process of setting the reference parameter coefficients according to the running condition of the vehicle  1 . As shown in  FIG. 7 , when the reference parameter coefficients are set according to the running condition of the vehicle  1 , the first processor  11  of the powertrain control ECU  10  obtains the steering angle that indicates the steering amount of the steering wheel operated by the driver, based on the signal from the steering angle sensor  16  (S 80 ). Then, the first processor  11  determines whether the vehicle  1  is running straight based on the obtained steering angle (S 82 ). 
     In this embodiment, the aforementioned reference parameters ω mi  and ζ mi  are set on the assumption that the vehicle  1  is running straight. Therefore, if the first processor  11  determines that the vehicle  1  is running straight (YES in step S 82 ), the first processor  11  sets a reference parameter coefficient “c” to “1”, and sets a reference parameter coefficient “C” to “1” (S 83 ). The reference parameter coefficients “c” and “C” are set according to the running condition of the vehicle  1 . The reference frequency ω mi  that is set by the reference parameter setting process in step S 2  is multiplied by the reference parameter coefficient “c”. The reference damping ratio ζ mi  that is also set by the reference parameter setting process in step S 2  is multiplied by the reference parameter coefficient “C”. 
     If the first processor  11  determines that the vehicle  1  is not running straight (NO in step S 82 ), the first processor  11  obtains a yaw rate of the vehicle  1  from a yaw rate sensor (S 84 ), and determines whether the vehicle  1  is turning (S 86 ). If the first processor  11  determines that the vehicle  1  is turning (YES in step S 86 ), the first processor  11  sets the reference parameter coefficient “c” to “cs”, and sets the reference parameter coefficient “C” to “Cs” (S 87 ). The values “cs” and “Cs” are set in advance through experiment and analysis, and are stored in the storage device of the powertrain control ECU  10 . 
     Further, if the first processor  11  determines that the vehicle  1  is not turning in step S 86  (NO in step S 86 ), the first processor  11  determines whether the ECBECU  20  is executing the vehicle stability control (S 88 ). If the first processor  11  determines that the ECBECU  20  is not executing the vehicle stability control (NO in step S 88 ), the first processor  11  determines that the vehicle  1  is running straight, and sets the reference parameters “c” and “C” to “1” (S 83 ). If the first processor  11  determines that the ECBECU  20  is executing the vehicle stability control (YES in step S 88 ), the first processor  11  turns a predetermined correction flag off to prohibit the processes in step S 2  and subsequent steps of the routine in  FIG. 3  (S 89 ). When the ECBECU  20  is executing the vehicle stability control, priority needs to be given to the driving stability of the vehicle  1  over suppression of vehicle vibration  1 . Accordingly, in this embodiment, when the ECBECU  20  is executing the vehicle stability control, the correction flag is turned off to prohibit the process of correcting the target driving force Pt, which may affect the accuracy of the vehicle stability control. 
     After the reference parameters ω mi  and ζ mi , the plant parameters ω pi  and ζ pi , and the reference parameter coefficients ej and Ej, “c” and “C” are set in steps S 2  to S 8 , the process of correcting the target driving force Pt is executed in step S 10 .  FIG. 8  is a flowchart describing the process of correcting the target driving force. As shown in  FIG. 8 , the first processor  11  determines whether the correction flag is on (S 100 ). If the first processor determines that the correction flag is off (NO in step S 100 ), the first processor determines that the ECBECU  20  is executing the vehicle stability control. Therefore, in this case, the processes in step S 102  and subsequent steps are skipped, and the process of correcting the target driving force Pt, which may affect the accuracy of the vehicle stability control, is not executed. 
     If the first processor  11  determines that the correction flag is on (YES in step S 100 ), the first processor  11  reads, from the predetermined storage area, the plant frequency ω p  and the plant damping ratio ζ p  which are set by the plant parameter setting process in step S 4  (S 102 ). Further, the first processor  11  reads, from the predetermined storage area, the reference frequency ω mi  and the reference damping ratio ζ mi  which are set by the reference parameter setting process in step S 2 . Also, the first processor  11  reads, from the predetermined storage area, the reference parameter coefficients ej and Ej that are set based on the environment around the vehicle in step S 6  and the reference parameter coefficients “c” and “C” that are set based on the running condition in step S 8  (S 104 ). 
     After the process in step S 104  is executed, the first processor  11  substitutes the values of the plant frequency ω p  and the plant damping ratio ζ p  read in step S 102  into the aforementioned equation (2). The first processor  11  also substitutes the following equations into the aforementioned equation (2).
 
ω m =ω mi   ×e   j   ×c  
 
ζ m =ζ mi   ×E   j   ×C  
 
Then, the first processor  11  converts the equation (2) into which the reference parameters and the plant parameters are substituted, using Tustin model (i.e., the first processor  11  executes Tustin conversion), and obtains a correction equation H (z) that defines the damping characteristic of the filter  115  using the following equation (3). Thus, the filter  115  outputs the corrected target driving force Ptc represented by the equation, Ptc=Pt×H(z) (S 106 ).
 
                     H   ⁡     (   z   )       =             b   12     ×     z     -   2         +       b   11     ×     z     -   1         +     b   10             a   12     ×     z     -   2         +       a   11     ×     z     -   1         +   1       =       N   1       D   1                 (   3   )               
In the equation (3), z −1  is the value of Pt×H (z) or Ptc obtained during the routine executed last time, that is, the corrected target driving force that is obtained during the routine executed last time; and z −2  is the value of Pt×H (z) or Ptc obtained during the routine executed two times before, that is, the corrected target driving force obtained during the routine executed two times before.
 
     As described above, in the vehicle  1 , the first processor  11  of the powertrain control ECU  10  executes the aforementioned processes. When the filter  115  (i.e., the correction means) corrects the target driving force Pt, the damping characteristic of the filter  115  is changed according to the running mode (running characteristic) set by the driver using the mode-switch  18 , the information on the environment around the vehicle  1  obtained by the environmental information-obtaining device  17  that includes the navigation system, and information on the running condition of the vehicle  1  such as information on whether the vehicle is turning. Accordingly, in the vehicle  1  that includes the powertrain control ECU  10 , the target driving force Pt is appropriately corrected to suppress the vibration on the spring of the vehicle, taking into account the running characteristic set by the driver, the environment around the vehicle  1 , and the running condition of the vehicle  1 . Therefore, even if the driver changes the running characteristic of the vehicle, or even if the environment around the vehicle  1  or the running condition of the vehicle  1  changes, the vehicle vibration  1  can be effectively suppressed.