SUSPENSION DEVICE

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a suspension control system, and a method of controlling a suspension device.

2. Description of Related Art

[0002] As a known suspension control system installed on a vehicle, a hydraulic damper is described in, for example, Japanese Patent Application Publication No. 10-141415 (JP 10-141415 A). In the hydraulic damper, a piston that is in sliding contact with an inner circumferential surface of a cylinder is mounted on one end of a piston rod, and the other end of the piston rod is guided to extend to the outside, via a rod guide and an oil seal provided on the cylinder side. Also, in the hydraulic damper, a bushing is provided in a space between the rod guide and the oil seal, such that a certain friction arises between the bushing and the piston rod when the piston velocity is a very low velocity.

[0003] The hydraulic damper of the vehicle as described in JP 10-141415 A is constructed as described above in order to suppress minute vibration that occurs after large vibration occurs during running of the vehicle on a bad road; nonetheless, there is still room for further improvement in terms of suppression of vibration, for example.

SUMMARY OF THE INVENTION

[0004] The invention provides a suspension control system that is able to suppress vibration, and a method of controlling a suspension device. [0005] A suspension control system according to a first aspect of the invention includes a suspension device that connects a sprung member of a vehicle with an unsprung member of the vehicle, an actuator operable to adjust frictional force along a stroke direction of the suspension device, and a control device configured to control the actuator, based on a velocity direction of the sprung member parallel to the stroke direction of the suspension device, and a stroke velocity direction of the suspension device, so as to adjust the frictional force along the stroke direction of the suspension device.

[0006] In the suspension control system as described above, the control device may be configured to control the actuator, so that the frictional force along the stroke direction of the suspension device in the case where the velocity direction of the sprung member is the same as the stroke velocity direction of the suspension device becomes larger than the frictional force along the stroke direction of the suspension device in the case where the velocity direction of the sprung member is different from the stroke velocity direction of the suspension device.

[0007] In the suspension control system as described above, the control device may be configured to control the actuator based on the velocity direction of the sprung member and the stroke velocity direction of the suspension device, so as to adjust the frictional force along the stroke direction of the suspension device, when the stroke velocity of the suspension device is within a very low velocity region in which the stroke velocity is equal to or lower than a preset velocity, and a steering angle of the vehicle is equal to or smaller than a preset angle.

[0008] In the suspension control system as described above, the velocity direction of the sprung member may be a direction of a velocity vector of the sprung member, and the stroke velocity direction of the suspension device may be a direction of a stroke velocity vector of the suspension device.

[0009] A method of controlling a suspension device that connects a sprung member of a vehicle with an unsprung member of the vehicle, according to a second aspect of the, invention, includes adjusting frictional force along a stroke direction of the suspension device, based on a velocity direction of the sprung member parallel to the stroke direction of the suspension device, and a stroke velocity direction of the suspension device.

[0010] With the configurations as described above, vibration can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a schematic view showing the schematic configuration of a suspension control system according to one embodiment of the invention;

FIG. 2 is a graph useful for explaining the force generated in a damping mechanism (shock absorber) of the suspension control system according to the embodiment;

FIG. 3 is a diagram useful for explaining the tendency of the ride performance due to change of the frictional force in the suspension control system according to the embodiment;

FIG. 4 is a diagram useful for explaining the control logic of frictional force control in the suspension control system according to the embodiment;

FIG. 5 is a flowchart illustrating one example of frictional force control performed by the suspension control system according to the embodiment; and

FIG. 6 is a graph showing one example of sprung mass vibration when frictional force control is performed by the suspension control system according to the embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

[0012] One embodiment of the invention will be described in detail with reference to the drawings. It is, however, to be understood that this invention is not limited to this embodiment. Also, constituent elements of the following embodiment include those that can be easily replaced with the constituent elements by a person with

i

ordinary skill in the art, or those that are substantially identical with the constituent elements.

[0013] FIG. 1 is a schematic view showing the schematic configuration of a suspension control system according to one embodiment of the invention, and FIG. 2 is a graph useful for explaining the force generated in a damping mechanism (shock absorber) of the suspension control system according to the embodiment, while FIG. 3 is a diagram useful for explaining the tendency of the ride performance due to change of frictional force in the suspension control system according to the embodiment. FIG. 4 is a diagram useful for explaining the control logic of frictional force control in the suspension control system according to the embodiment, and FIG. 5 is a flowchart illustrating one example of frictional force control performed by the suspension control system according to the embodiment, while FIG. 6 is a graph showing one example of sprung mass vibration when frictional force control is performed by the suspension control system according to the embodiment.

[0014] FIG. 1 represents a single-wheel model of a damping control system according to the embodiment of the invention. The single-wheel model shown in FIG. 1 is a kinetic model of a vehicle including a suspension device. In FIG. 1 , "c _s " represents the damping coefficient of a damping mechanism 8 which will be described later. "F _c " represents the suspension frictional force generated in a sliding portion 9 which will be described later. "k _s " represents the modulus of elasticity of a spring mechanism 7 which will be described later. "k _t " represents the stiffness (spring constant) of a. wheel 50. "n t," represents the mass (which may be called "sprung mass") of a sprung member 5 which will be described later. "m _w " represents the mass (which may be called "unsprung mass") of an unsprung member 6 which will be described later, "xt," represents the displacement (which may be called "sprung mass displacement") of the sprung member 5. "xw" represents the displacement (which may be called "unsprung mass displacement") of the unsprung member 6. "x _r " represents the displacement (which may be called "road surface displacement") of a road surface R. Here, "displacement" is a displacement of an object relative to its reference position as measured in the up-and-down direction of the vehicle, for example, a displacement measured in the vertical direction, for example. The amount of movement of the suspension device 2 in the axial direction as described later may also be referred to as "displacement".

[0015] In the suspension control system 1 according to this embodiment as shown in FIG. 1, the suspension device 2, which corresponds to each of four wheels 50 of the vehicle, is provided for supporting the wheel 50 on the vehicle body of the vehicle. The suspension control system 1 includes the suspension device 2, actuator 3, and an. ECU 4 as a control device. In the suspension control system 1 , one suspension device 2 and one actuator 3 are provided for each of the four wheels 50, and one ECU 4 is provided for the four wheels 50 (i.e., the four wheels 50 share a common ECU 4). In the following, the suspension device 2 and the actuator 3 for one of the four wheels 50 will be explained.

[0016] The suspension device 2 is provided between the sprung member 5 and unsprung member 6 of the vehicle, for connecting the sprung member 5 with the unsprung member 6. The sprung member 5 is a member supported by the suspension device 2, and includes the vehicle body. The unsprung member 6 is a member located closer to the wheel 50 than the suspension device 2, and includes a knuckle coupled to the wheel 50, a lower arm coupled to the knuckle, and so forth.

[0017] The suspension device 2 has a spring mechanism 7, and a damping mechanism 8. The spring mechanism 7 and the damping mechanism 8 are arranged in parallel with each other.

[0018] The spring mechanism 7 connects the sprung member 5 with the unsprung member 6, and produces spring force commensurate with a relative displacement between the sprung member 5 and the unsprung member 6, so as to apply the spring force to the sprung member 5 and the unsprung member 6. The spring mechanism 7 produces the spring force, by means of a coil spring 7a mounted on a piston rod 8c of the damping mechanism 8 which will be described later, or an air suspension mechanism (not shown), for example. The relative displacement between the sprung member 5 and the unsprung member 6 is a relative displacement in a direction in which the sprung member 5 and the unsprung member 6 come closer to each other or get away from each other in the stroke direction of the suspension device 2 (which may be called "suspension stroke, direction"). While the suspension stroke direction is illustrated herein as a direction parallel to. the vertical direction, the suspension stroke direction may be inclined by a given angle from the vertical direction. The spring mechanism 7 may be constructed, such that its elastic modulus k _s , or spring force, can be variably controlled.

[0019] The damping mechanism 8 connects the sprung member 5 with ^' the unsprung member 6, and produces damping force for damping or attenuating relative movement between the sprung member 5 and the unsprung member 6. The relative movement between the sprung member 5 and the unsprung member 6 takes place in a direction in which the sprung member 5 and the unsprung member 6 come closer to each other or get away from each other in the suspension stroke direction. The damping mechanism 8 produces damping force commensurate with the relative velocity between the sprung member 5 and the unsprung member 6 during the relative movement, so as to damp or attenuate the relative movement. For example, a shock absorber is used as the damping mechanism 8. The shock absorber may include, for example, a cylinder 8a that is connected to one of the sprung member 5 and the unsprung member 6 and contains a working fluid, and a piston rod 8c connected to the other of the sprung member 5 and the unsprung member 6. The piston rod 8c ^' has a piston portion 8b that reciprocates within the cylinder 8a. In the suspension device 2, the cylinder 8a and the piston rod 8c move relative to each other to make strokes, for relative displacement between the sprung member 5 and the unsprung member 6. In other words, the suspension stroke direction is a direction in which the cylinder 8a and the piston rod 8c move relative to each other, typically, a direction in which the sprung member 5 and the unsprung member 6 move relative to each other. The damping mechanism 8 may be constructed such that its damping coefficient c _s , or damping force, can be variably controlled. In this case, a shock absorber whose damping coefficient c _s can be variably controlled may be used as the damping mechanism 8. As a mechanism that variably controls the damping coefficient c _s , a device operable to rotate a rotary valve of the piston portion so as to vary the flow passage area of an oil passage that communicates an upper chamber of the piston with a lower chamber thereof may be employed. It is, however, to be understood that the damping mechanism 8 is not limited to this arrangement, but a damping mechanism constructed otherwise may be used, or the damping mechanism 8 may be constructed such that its damping coefficient c _s cannot be variably controlled.

[0020] The actuator 3 is able to adjust frictional force (which may be called "suspension frictional force") generated along the stroke direction of the suspension device 2.

[0021] Here, the suspension frictional force F _c is frictional force that acts on the sliding portion 9 of the suspension device 2. The sliding portion 9 of the suspension device 2 is a portion of the suspension device 2 which slides in accordance with strokes thereof, and may include, for example, a sliding region between the piston rod 8c and cylinder 8a of the damping mechanism 8 (shock absorber), a sliding region of a seal member provided between the piston rod 8c and the cylinder 8a, and so forth. Namely, the suspension frictional force F _c is frictional force generated along the stroke direction in the sliding portion 9 in accordance with strokes of the suspension device 2.

[0022] The actuator 3 is arranged to be able to variably control the suspension frictional force F _c generated in the sliding portion 9. For example, various devices, such as a device that can vary the fastening force of the seal member that provides the sliding portion 9, by means of a piezoelectric element, or the like, and a device that can vary the force with which one of the piston rod 8c and the cylinder 8a is pressed against the other, may be used. It is, however, to be understood that the actuator 3 is not limited to the above arrangements, but may be constructed otherwise so as to be able to adjust the suspension frictional force F _c .

[0023] The ECU 4 controls the actuator 3, so as to adjust the suspension frictional force F _c . Here, the ECU 4 is configured to control each portion of the vehicle on which the suspension control system 1 is installed. The ECU 4 is an electronic control unit that consists principally of a known microcomputer including CPU, ROM, RAM, and an interface. For example, various sensors, such as a sprung mass acceleration sensor 10 as a sprung mass acceleration detector, an unsprung mass acceleration sensor 11 as an unsprung mass acceleration detector, and a steering angle sensor 12 as a steering angle detector, and each portion of the vehicle on which the suspension control system 1 is installed, are electrically connected to the ECU 4. The sprung mass acceleration sensor 10 is placed on the sprung member 5. The sprung mass acceleration sensor 10 is operable to detect the acceleration (which may be called "sprung mass acceleration") of the sprung member 5 in the suspension stroke direction, typically, in the vertical direction of the sprung member 5. The unsprung mass acceleration sensor 11 is placed on the unsprung member 6. The unsprung mass acceleration sensor 11 is operable to detect the acceleration (which may be called "unsprung mass acceleration") of the unsprung member 6 in the suspension stroke direction, typically, in the vertical direction of the unsprung member 6. The steering angle sensor 12 detects the steering angle of the vehicle on which the suspension control system 1 is installed, more specifically, the steering angle of the steering wheel as the amount of operation of the steering wheel. The ECU 4 receives electric signals (detection signals) corresponding to the detection results from various sensors, and outputs drive signals to respective portions of the vehicle on which the suspension control system 1 is installed, according to the received detection results, so as to control driving thereof.

[0024] FIG. 2 is a graph useful for explaining the force generated by the damping mechanism (shock absorber) 8. In FIG. 2, the horizontal axis indicates the velocity (mm/s) of the piston portion 8b, and the displacement (mm) of the piston portion 8b, and the vertical axis indicates the axial force (N). FIG. 2 shows an example of very low velocity region in which the stroke velocity of the suspension device 2 is very low, such as when the vehicle runs on a good road where the road surface R has a relatively small number of irregularities. Here, the stroke velocity of the suspension device 2 corresponds to the velocity of expansion and contraction of the suspension device 2 (the velocity of relative displacement between the cylinder 8a and the piston rod 8c). The very low velocity region is a velocity region in which an absolute value of the stroke velocity is larger than Om/s and smaller than 0.002m/s. FIG. 2 schematically represents the force (total axial force) generated by the damping mechanism 8 when vibration having a very small amplitude of about 0.2mm is applied at a frequency of about 1.5Hz (a very low velocity of 0.002m/s), as an example.

[0025] As the suspension device 2 vibrates in the vertical direction, friction occurs to the sliding portion 9. As shown in FIG. 2, when the stroke velocity is within a very low velocity region, the force generated by the damping mechanism 8 of the suspension device 2 constructed as described above consists mainly of coulomb friction force (a) that provides the suspension frictional force F _c , and the elastic (viscoelastic) friction force (b), and is hardly influenced by the very-low-velocity damping force (c) of the damping mechanism 8. Here, the coulomb friction force (a) is typically static friction force that appears in the sliding portion 9, and the elastic (viscoelastic) friction force (b) is typically dynamic friction force that appears in the seal member, or the like, of the sliding portion 9 at the start of strokes.

[0026] In the suspension control system 1 , it was found that, in the very low velocity region of the stroke velocity as described above, there is incompatibility as indicated in FIG. 3, regarding the ride performance or ride quality depending on the relationship between the frequency of the road surface input (i.e., vibration) applied to the suspension device 2 and the suspension frictional force F _c . Namely, if the suspension control system 1 controls the suspension frictional force F _c that, appears in the sliding portion 9 to be relatively small frictional force, in an attempt to improve the ride performance against the road surface input of a middle frequency region (which may be called "middle-frequency vibration") applied to the suspension device 2, the ride performance against the road surface input of a low frequency region (which may be called "low-frequency vibration") deteriorates, and tends to give a "floaty" feeling to a passenger. On the other hand, if the suspension control system 1 controls the suspension frictional force F _c that appears in the sliding portion 9 to be relatively large frictional force, in an attempt to improve the ride performance against the low-frequency vibration, for example, the ride performance against the middle-frequency vibration deteriorate, and tends to give a "rough" feeling to a passenger. Therefore, it may not be possible to make the ride performance against the low-frequency vibration compatible with the ride performance against the middle-frequency vibration, simply by making the suspension frictional force F _c relatively large, or making the suspension frictional force F _c relatively small. The above-indicated low-frequency region is, for example, a region that is higher than OHz and lower than 1.5Hz. On the other hand, the middle-frequency region is, for example, a region that is equal to or higher than 1.5Hz and equal to or lower than 8Hz.

[0027] Thus, in the suspension control system 1 of this embodiment, the ECU 4 performs frictional force control by controlling the actuator 3 according to conditions of the suspension device 2, and adjusting the suspension frictional force F _c , so as to make the ride performance against the low-frequency vibration compatible with the ride performance against the middle-frequency vibration, and properly suppress sprung mass vibrations.

[0028] More specifically, the ECU 4 performs frictional force control by controlling the actuator 3 and adjusting the suspension frictional force F _c , based on the velocity direction of the sprung member 5 (which may be called "sprung mass velocity direction") parallel to the suspension stroke direction, and the stroke velocity direction of the suspension device 2. In this manner, the ECU 4 changes friction characteristics of the damping mechanism (shock absorber) 8 according to the conditions of the suspension device 2.

[0029] Here, the sprung mass velocity direction is the direction of a velocity vector of the sprung member 5, which is, typically, the upward vertical direction (+ (positive)), or downward vertical direction (- (negative)). The stroke velocity direction is the direction of a stroke velocity (velocity of expansion and contraction) vector of the suspension device 2, which is, typically, the upward vertical direction (expanding direction, + (positive)), or downward vertical direction (contracting direction, - (negative)).

[0030] The ECU 4 of this embodiment performs frictional force control when the vehicle travels straight on a good road where the stroke velocity is within a very low velocity region. In this embodiment, the ECU 4 performs frictional force control when the stroke velocity of the suspension device 2 is within a very low velocity region that is equal to or lower than a preset speed, and the steering angle of the vehicle is equal to or smaller than a preset angle. With the frictional force control thus performed, the ECU 4 improves the vehicle performance when the stroke velocity is within the very low velocity region.

[0031] The ECU 4 may calculate the stroke velocity of the suspension device 2 in the following manner, for example. In the following explanation, x' represents first order differential of x, and x" represents second order differential of x. The ECU 4 calculates the sprung mass velocity Xb' and the unsprung mass velocity x _w ' by integrating the sprung mass acceleration Xb" and the unsprung mass acceleration x _w ", based on the sprung mass acceleration X " and unsprung mass acceleration x _w " detected by the sprung mass acceleration sensor 10 and the unsprung mass acceleration sensor 11 , respectively. Then, the ECU 4 calculates a difference between the sprung mass velocity Xb' and the unsprung mass velocity x _w ', namely, the relative velocity (Xb'-x _w ') between the sprung member 5 and the unsprung member 6, based on the calculated sprung mass velocity Xb' and unsprung mass velocity x _w ', and sets the relative velocity (Xb'-x _w ') as the stroke velocity (expansion/contraction velocity) of the suspension device 2. The ECU 4 may calculate the stroke velocity in another manner, for example, based on the detection result of a sensor that detects the amount of expansion or contraction (stroke displacement) of the suspension device 2. Also, the ECU 4 may obtain the steering angle of the vehicle, based on the steering angle of the steering wheel detected by the steering angle sensor 12. Further, the above-indicated preset velocity is set in advance, according to stroke characteristics, etc. of the suspension device 2, based on on-vehicle evaluation, for example. The preset velocity is set to a velocity at which the stroke velocity can be determined as being within a very low velocity region, typically, set to a velocity, e.g., 0.015m/s, which is higher than Om/s and lower than 0.002m/s. Also, the preset angle is set in advance, according to straight-ahead running performance of the vehicle, steering characteristics, etc., based on on-vehicle evaluation, for example. The preset angle is set to an angle at which it can be determined that the vehicle is running straight ahead, and is set to, for example, about ±5 degrees.

[0032] It was found that when the suspension device 2 as described above is subjected to low-frequency vibration (first-frequency vibration), the sprung mass velocity direction tends to be the same as the stroke velocity direction, since the vibration is applied with a relatively long period. On the other hand, when the suspension device 2 is subjected to middle-frequency vibration (second-frequency vibration) having a higher frequency than the low-frequency vibration, the sprung mass velocity direction tends to be different from the stroke velocity direction, since the vibration is applied with a relatively short period.

[0033] In the view of the above finding, the ECU 4 controls actuator 3 for control of frictional force, so as to make the suspension frictional force F _c in the case where the sprung mass velocity direction is the same as the stroke velocity direction larger than the suspension frictional force F _c in the case where the sprung mass velocity direction is different from (opposite to) the stroke velocity direction. Namely, the ECU 4 makes the suspension frictional force F _c relatively large when the sprung mass velocity direction is the same as the stroke velocity direction, in other words, when low-frequency vibration is presumed to be applied to the suspension device 2. On the other hand, the ECU 4 makes the suspension frictional force F _c relatively small when the spmng mass velocity direction is different from the stroke velocity direction, in other words, when middle-frequency vibration is presumed to be applied to the suspension device 2.

[0034] In this embodiment, an on-off control logic as shown in FIG. 4 is designed for properly suppressing sprung mass vibration, and the ECU 4 performs frictional force control based on the on-off control logic. FIG. 4 schematically illustrates the suspension device 2, without illustrating the unsprung member 6, etc.

[0035] The ECU 4 makes the suspension frictional force F _c relatively large when the relationship between the velocity directions is that in a condition (B) or a condition (D) where the ride performance is required to be improved against low-frequency vibration, namely, when the sprung mass velocity direction is the same as the stroke velocity direction. In this case, in the condition (B) where the sprung mass velocity direction is the upward direction (+), and the stroke velocity direction is the expanding direction (+), the ECU 4 controls the actuator 3, so as to apply relatively large suspension frictional force F _c downward as indicated by a black arrow in FIG. 4. In the condition (D) where the sprung mass velocity direction is the downward direction (-), and the stroke velocity direction is the contracting direction (-), the ECU 4 controls the actuator 3, so as to apply relatively large suspension frictional force F _c upward as indicated by a black arrow in FIG. 4.

[0036] On the other hand, the ECU 4 makes the suspension frictional force F _c relatively small when the relationship between the velocity directions is that in a condition (A) or a condition (C) where the ride performance is required to be improved against middle-frequency vibration, namely, when the sprung mass velocity direction is different from (opposite to) the stroke velocity direction. In this case, in the condition (A) where the sprung mass velocity direction is the upward direction (+), and the stroke velocity direction is the contracting direction (-), the ECU 4 controls the actuator 3, so as to apply relatively small suspension frictional force F _c upward as indicated by a black arrow in FIG. 4. In the condition (C) where the sprung -mass velocity direction is the downward direction (-), and the stroke velocity direction is the expanding direction (+), the ECU 4 controls the actuator 3, so as to apply relatively small suspension frictional force F _c downward as indicated by a black arrow in FIG. 4.

[0037] The suspension frictional force F _c may be expressed by Eq. (1 ) as indicated below, for example. In Eq. (1), "f _c " represents an absolute value of the suspension frictional force F _c . Namely, the direction of the suspension frictional force F _c is determined depending on the relationship in magnitude between the sprung mass velocity x _b ' and the unsprung mass velocity x _w '.

[0038] In this embodiment, the ECU 4 calculates the product of the sprung mass velocity Xb' and the stroke velocity (x _b '-x _w '), for example, and sets the product as a frictional force selection determination value (which may be called "determination value"). Then, the ECU 4 selects one of high frictional force and low frictional force, based on whether the determination value is equal to or larger than a threshold value, or "0" in this embodiment. The ECU 4 sets a frictional force control value F _ccon , based on the determination value X '( _b '- _w ') as the product of the sprung mass velocity x _b ' and the stroke velocity (x _b '-x _w '), using Eq. (2) as indicated below, for example. The frictional force control value F _CCO n is a control value used when the actuator 3 is controlled so as to control the suspension friction force F _c . .

Fccon- F _cmax Xb (Xb ^— Xw )— 0

Fccon- Fcmin Xb' (Xb'-Xw')<0 (2)

[0039] Namely, when the determination value X _b '(x _b '-Xw') is a positive value (x _b '(xb'-Xw')≥0), which means that the sprung mass velocity direction is the same as the stroke velocity direction, the ECU 4 sets the frictional force control valve F _CCO n to a relatively large value. In this case, the ECU 4 sets the frictional force control value F _ccon to, for example, the maximum value F _cma x of the frictional force control value, which can be realized by the actuator 3. On the other hand, when the determination value X '(xb'-Xw') is a negative value (xb'(Xb'-Xw') ^< 0) _? which means that the sprung mass velocity direction is different from the stroke velocity direction, the ECU 4 sets the frictional force control value F _ccon to a relatively small value. In this case, the ECU 4 sets the frictional force control value F _CCO n to, for example, the minimum value F _cm i _n of the frictional force control value, which can be realized by the actuator 3.

[0040] Then, the ECU 4 controls the actuator 3, based on the frictional force control value F _ccon set as described above, so as to actually adjust the suspension frictional force F _c . Namely, when the sprung mass velocity direction is the same as the stroke velocity direction, and the ride performance against low-frequency vibration is required to be improved, the ECU 4 sets the frictional force control value F _CCO n to the maximum value Fcmax of the frictional force control value, and controls the actuator 3 based on the maximum value F _cmax , so as to make the suspension frictional force F _c that actually acts on the sliding portion 9 relatively large. In this case, the suspension frictional force F _c becomes the maximum value that can be realized by the actuator 3. On the other hand, when the sprung mass velocity direction is different from the stroke velocity direction, and the ride performance against middle-frequency vibration is required to be improved, the ECU 4 sets the frictional force control value F _CC on to the minimum value F _cm j _n of the frictional force control value, and control the actuator 3 based on the minimum value F _cm i _n , so as to make the suspension frictional force F _c that actually acts on the sliding portion 9 relatively small. In this case, the suspension frictional force F _c becomes the minimum value that can be realized by the actuator 3.

[0041] Next, one example of frictional force control performed by the ECU 4 will be explained with reference to the flowchart of FIG. 5. The control routine illustrated in FIG. 5 is repeatedly executed in control cycles of several milliseconds to several tens of milliseconds.

[0042] Initially, the ECU 4 determines whether the absolute value of the steering angle of the vehicle (i.e., the steering angle of the steering wheel) is equal to or smaller than a preset angle (e.g., 5°), based on the detection result obtained by the steering angle sensor 12 (step ST1). If the ECU 4 determines that the absolute value of the steering angle of the vehicle (i.e., the steering angle of the steering wheel) is larger than the preset angle (step ST 1 : NO), the ECU 4 finishes the current control cycle, and proceeds to the next control cycle.

[0043] If the ECU 4 determines that the absolute value of the steering angle of the vehicle (i.e., the steering angle of the steering wheel) is equal to or smaller than the preset angle (step ST1 : YES), the ECU 4 integrates the sprung mass acceleration xt>", based on the detection result obtained by the sprung mass acceleration sensor 10 (step ST2).

[0044] Then, the ECU 4 calculates the sprung mass velocity Xb', based on the result of integration of the sprung mass acceleration Xb" obtained in step ST2 (step ST3).

[0045] Then, the ECU 4 calculates the unsprung mass velocity x _w ' by integrating the unsprung mass-acceleration x _w ", based on the detection result obtained by the unsprung mass acceleration sensor 11, and calculates he stroke velocity (Xb'-x _w ') _> based on the sprung mass velocity b' calculated in step ST3 and the unsprung mass velocity x _w '. Then, the ECU 4 determines whether the absolute value of the stroke velocity (xb'-x _w ') is equal to or smaller than a preset velocity (e.g., 0.015m/s) (step ST4). If the ECU 4 determines that the absolute value of the stroke velocity (x _b '-x _w ') is larger than the preset velocity (step ST4: NO), the ECU 4 finishes the current control cycle, and proceeds to the next control cycle.

[0046] If the ECU 4 determines that the absolute value of the stroke velocity (xb'- w') is equal to or smaller than the preset velocity (step ST4: YES), the ECU 4 calculates the product of the sprung mass velocity x _b ' calculated in step ST3 and the stroke velocity (x _b '-x _w ') calculated in step ST4, and sets the result of calculation as a determination value Xb'(xb'-x _w ') (step ST5).

[0047] Then, the ECU 4 determines whether the determination value Xb'(x _b '-Xw') calculated in step ST5 is equal to or larger than 0 (threshold value) (step ST6).

[0048] If the ECU 4 determines that the determination value Xb'(x _b '-Xw') is equal to or larger than 0 (step ST6: YES), the ECU 4 requires high frictional force, and sets the frictional force control value F _ccon to a relatively large value, for example, the maximum value Fomax of the frictional force control value _, (step ST7) .

[0049] Then, the ECU 4 controls the actuator 3, based on the frictional force control value F _ccon (in this example, the maximum value F _cma x of the frictional force control value) set in step ST7, so as to make the suspension frictional force F _c actually applied to the sliding portion 9 relatively large (step ST8). Then, the ECU 4 finishes the current control cycle, and proceeds to the next control cycle.

[0050] . On the other hand, if the ECU 4 determines that the determination value Xb'(xb'-Xw') calculated in step ST5 is smaller than 0 (step ST6: NO), the ECU 4 requires low frictional force, and sets the frictional force control value F _CCO n to a relatively small value, for example, the minimum value F _cm in of the frictional force control value (step ST9).

[0051] Then, the ECU 4 controls the actuator 3, based on the frictional force control value F _CCO n (in this example), the minimum value F _cm j _n of the frictional force control value) set in step ST9, so as to make the suspension frictional force F _c actually applied to the sliding portion 9 relatively small (step ST8). Then, the ECU 4 finishes the current control cycle, and proceeds to the next control cycle. [0052] FIG. 6 is a graph showing one example of simulation of sprung mass vibration in the vehicle on which the suspension control system 1 constructed as described above is installed. In FIG. 6, the horizontal axis indicates the frequency of the vibration applied, and the vertical axis indicates the power spectral density of the sprung mass vibration. FIG. 6 shows one example of simulation results in the case where the stroke velocity (Xb'- w') is very low, e.g., is about 0.015m/s. In FIG. 6, solid line LI indicates a simulation result in the case where the frictional force control as described above is performed in the suspension control system 1 of this embodiment, and dashed line L2 indicates a simulation result in the case where the suspension frictional force F _c is fixed to a relatively small value (e.g., F _c =0N) in a suspension control system according to Comparative Example 1 , while dashed line L3 indicates a simulation result in the case where the suspension frictional force F _c is fixed to a relatively large value (e.g., F _C =20N) in a suspension control system according to Comparative Example 2.

[0053] As is apparent from the solid line LI of FIG. 6, the suspension control system 1 of this embodiment performs frictional force control, more specifically, controls the actuator 3 according to the conditions of the suspension device 2 for adjustment of the suspension frictional force F _c , so that sprung mass vibration can be suppressed when low-frequency vibration is applied to the suspension device 2, and sprung mass vibration can also be suppressed when middle-frequency vibration is applied to the suspension device 2, as compared with the dashed lines L2, L3 representing the suspension control systems according to Comparative Examples 1 , 2. Consequently, the suspension control system 1 can suppress both the "floaty" feeling caused by the low-frequency vibration, and the "rough" feeling caused by the middle-frequency vibration, so as to make the ride performance against the low-frequency vibration compatible with the ride performance against the middle-frequency vibration.

[0054] The suspension control system 1 according to the embodiment as described above includes the suspension device 2, actuator 3, and the ECU 4. The suspension device 2 connects the sprung member 5 and unsprung member 6 of the vehicle. The actuator 3 is able to adjust the frictional force along the stroke direction of the suspension device 2. The ECU 4 controls the actuator 3, based on the velocity direction of the sprung . member 5 parallel to the stroke direction of the suspension device 2, and the stroke velocity direction of the suspension device 2, so as to adjust the frictional force along the stroke direction of the suspension device 2.

[0055] Accordingly, the suspension control system 1 can make the frictional force along the stroke direction relatively large when the low-frequency performance is required to be improved, more specifically, when the sprung mass velocity direction is the same as the stroke velocity direction. Also, the suspension control system -1 can make the frictional force along the stroke direction relatively small when the middle-frequency performance is required to be improved, more specifically, when the sprung mass velocity direction is different from the stroke velocity direction. Consequently, the suspension control system 1 makes the ride performance against low-frequency vibration compatible with the ride performance against middle-frequency vibration, and the low-frequency performance and middle-frequency performance of the vehicle, which could be otherwise incompatible with each other, can be appropriately made compatible with each other, so that the sprung mass vibration can be appropriately suppressed.

[0056] Further, according to the suspension control system 1 of the embodiment as described above, the ECU 4 performs the frictional force control as described above, when the stroke velocity is within a very low velocity region that is equal to or lower than a given velocity, and when the steering angle of the vehicle is equal to or smaller than a preset angle. Accordingly, the suspension control system 1 can limit the timing of execution of the frictional force control to the time when the vehicle travels straight ahead on a good road, or when both the ride performance against low-frequency vibration and the ride performance against middle-frequency vibration are required to be improved. In other cases, the frictional force is fixed to a set value, so that the sprung mass vibration can be more efficiently suppressed.

[0057] The suspension control system according to the invention is not limited to the above-described embodiment, but various changes or modifications may be made within the ranges as described in the appended claims. [0058] While the ECU 4 also serves as the control device in the illustrated embodiment, the invention is not limited to this arrangement. For example, the control device may be configured, separately from the ECU 4, to supply and receive information, such as detection signals, drive signals, and control commands, to and from the ECU 4.

[0059] While the ECU 4 performs frictional force control while the vehicle is travelling straight on a good road with the stroke velocity being within a very low velocity region in the illustrated embodiment, the invention is not limited to this arrangement, but the frictional force control may be executed in cases other than the case of straight-ahead running on a good road. Namely, the ECU 4 may perform frictional force control in other cases than the case where the stroke velocity of the suspension device 2 is within a very low velocity region in which the stroke velocity is equal to or lower than a preset velocity, and the steering angle of the vehicle is equal to or smaller than a preset angle.

[0060] In the illustrated embodiment, when the ECU 4 sets the frictional force control valve F _ccon .to a relatively large value, the ECU 4 uses the maximum value F _cmax of the frictional force control value as the frictional force control value F _ccon , for example. When the ECU 4 sets the frictional force control value F _ccon to a relatively small value, the ECU 4 uses the minimum value F _cm i _n of the frictional force control value as the frictional force control value F _ccon , for example. However, the invention is not limited to the use of these values. The ECU 4 may set the frictional force control value F _ccon to a certain value, so that the suspension frictional force F _c in the case where the sprung mass velocity direction is the same as the stroke velocity direction becomes larger than the suspension frictional force F _c in the case where the sprung mass velocity direction is different from the stroke velocity direction. Namely, the suspension frictional force F _c in the case where the sprung mass velocity direction is the same as the stroke velocity direction may not be the maximum value that can be realized by the actuator 3, and the suspension frictional force F _c in the case where the sprung mass velocity direction is different from the stroke velocity direction may not be the minimum value that can be realized by the actuator 3. Rather, the suspension frictional force F _c may be controlled to certain values so as to achieve the ride performance required with respect to the low-frequency vibration, and the ride performance required with respect to the middle-frequency vibration, respectively.