ACTIVELY CONTROLLED SUSPENSION SYSTEM WITH CONTROL CHARACTERISTICS VARIABLE DEPENDING UPON VEHICULAR SPEED

Abstract: An actively controlled suspension system which successfully prevent a vehicular height from abruptly changing even when failure of a vertical acceleration sensor occurs. The actively controlled suspension system introduces a technology in performing suspension control, such as bouncing control, to shape a vehicular attitude indicative sensor signal, such as vertical acceleration indicative signal by slicing the sensor signal level at a predetermined slice level. In view of suppression of abrupt change of vehicular height at any vehicular driving condition, the slice level introduced in the present invention is variable depending upon a vehicle speed.

Description: The present invention relates generally to an actively controlled suspension system with variable suspension characteristics depending on the driving conditions of the vehicle for suppressing vehicular attitude changes. Such systems perform various controls, such as anti-rolling or anti-pitching or bouncing suppressive control. An actively controlled suspension system for bouncing suppressive control is described in DE-A-35 42 350. It comprises the following features: a plurality of cylinders each disposed between the vehicle body and a suspension member and defining therein a working chamber filled with a pressure medium with pressure being variable between a predetermined maximum pressure and a predetermined minimum pressure; a pressure medium source circuit connected to said working chambers, for supplying said pressure medium to said working chambers of said cylinders; a plurality of pressure control valves disposed within said pressure medium circuit and switchable between a first made for increasing pressure or said pressure medium within said working chamber, a second mode for decreasing said pressure of said pressure medium within said working chamber, and a third mode for maintaining said pressure of said pressure medium constant; sensor means for monitoring vehicle body attitude change for producing a vehicle body attitude change indicative signal; a controller for generating a control signal for controlling said pressure control valves so as to adjust the pressure of said pressure medium within said working chambers; and a vehicle speed sensor for monitoring a vehicle driving speed and producing a vehicle speed indicative signal representative of the monitored vehicle speed. Additionally, the system according to DE-A-35 42 350 comprises a steering wheel angle sensor. Depending on the values of the signals of the vehicle speed sensor and the steering angle sensor, the cylinders disposed between the vehicle body and the suspension members are controlled to act against rolling of the vehicle. In prior art suspension systems, failure of a sensor directly results in malfunctioning of the suspension control. For instance, when a vertical acceleration indicative sensor signal increases in stepwise fashion due to failure of a vertical acceleration sensor, the erroneously increased level of the vertical acceleration sensor signal is amplified through the control system to cause an abrupt change of the vehicular height. This clearly degrades riding comfort and drive feeling. Therefore, it is an object of the present invention to provide an actively controlled suspension system which successfully prevents abrupt changes of the vehicular heigth when a failure in a sensor signal, which is used for the control, occurs. The system of the present invention comprises the above listed features and is characterized in that: a slicer circuit is provided for slicing said vehicle body attitude change indicative signal out of predetermined upper and lower slice levels and outputting a sliced vehicle body attitude change indicative signal to said controller, in such a manner as to limit variation range of said control signal depending upon said sliced vehicle body attitude change indicative signal; and said controller is responsive to said vehicle speed indicative signal for varying said upper and lower slice levels in said slicer circuit. Preferably, the sensor means for monitoring a vehicle body attitude change is a vertical acceleration sensor. In this case, the system acts as a bouncing suppression system. The controller means may vary the slice level indicative value to be inversely proportional to the vehicle speed. Alternatively the controller means may vary the slice level indicative value in stepwise fashion according to a variation in the vehicle speed. The invention will now be explained by a preferred embodiment, accompanied by drawings. In the drawings: Fig. 1 is an illustration of the overall construction of the preferred embodiment of an actively controlled suspension system according to the present invention, in which a process of fail-detection and fail-safe operation is implemented; Fig. 2 is a sectional view of a pressure control valve employed in the preferred embodiment of the actively controlled suspension system of Fig. 1; Fig. 3 is another embodiment of the pressure control valve to be employed in the preferred embodiment of the actively controlled suspension system of Fig. 1 Fig. 4 is a circuit diagram of a hydraulic circuit to be employed in the preferred embodiment of the actively controlled suspension system according to the present invention; Fig. 5 is a chart showing relationship between an electric current value of a control signal to be output by a control unit and a hydraulic pressure in a pressure chamber of a hydraulic cylinder; Fig. 6 is a chart showing variation of a vertical acceleration indicative sensor signal in relation to a magnitude of vertical acceleration exerted on a vehicular body; Fig. 7 is a schematic block diagram showing an electric circuit including a control unit, of the preferred embodiment of the actively controlled suspension system of Fig. 1; Fig. 8 is a circuit diagram of a slice level setting circuit employed in the preferred embodiment of the suspension control circuit of Fig. 7; Fig. 9 is a graph shopwing relationship between a slice level setting signal value and slice level set by the slice level setting circuit of Fig. 8; Fig. 10 is a graph showing variation of the slice level setting signal in relation to a vehicle speed; Fig. 11 is a flowchart of a routine for deriving the slice level setting signal; Fig. 12 is a flowchart of a suspension control program for deriving a suspension control signal; Fig. 13 is a chart showing manner of slicing of a sensor signal; and Fig. 14 is a timing chart showing operation of a low-pass filter (LPF) employed in the preferred embodiment of the suspension control circuit of Fig. 7. Referring now to the drawings, particularly to Fig. 1, the preferred embodiment of an actively controlled suspension system, according to the present invention, is designed to generally perform suspension control for suppressing relative displacement between a vehicle body 10 and suspension members 24FL, 24FR, 24RL and 24RR provided in front-left, front-right, rear-left and rear-right suspension mechanisms 14FL, 14FR, 14RL and 14RR and rotatably supporting front-left, front-right, rear-left and rear-right wheels 11FL, 11FR, 11RL and 11RR, which suspension member will be represented by the reference numeral "24" as generally referred to, and suspension mechanisms will be represented by the reference numeral "14" as generally referred to, and whereby suppressing vehicle body attitude change. Respective front-left, front-right, rear-left and rear-right suspension mechanisms 14FL, 14FR, 14RL and 14RR include hydraulic cylinders 26FL, 26FR, 26RL and 26RR, which hydraulic cylinder will be hereafter represented by the reference numeral "26" as generally referred to. Each of the hydraulic cylinder 26 is disposed between the vehicle body 10 and the suspension member 24 to produce a damping force for suppressing relative displacement between the vehicle body and the suspension member. The hydraulic cylinder 26 has a cylinder body 26a. The cylinder body 26a defines a working fluid chamber therein. A piston 26c is disposed within the working fluid chamber to divide the working fluid chamber into a lower working chamber 26d and an upper reference pressure chamber 26e. As will be seen from Fig. 1, the working chamber 26d and the reference pressure chamber 26e are communicated each other via an orifice 26f defined through the piston 26c. The piston 26c is connected to the vehicle body 10 via a piston rod 26b. A suspension coil spring 36 is wound around the piston road 26b. The working chamber 26d of each hydraulic cylinder 26 is connected to a hydraulic pressure source unit 16 via a pressure supply line 52, a pressure drain line 54, a pressure control valve unit 28FL, 28FR, 28RL and 28RR which will be hereafter represented by the reference numeral "28" as generally referred to, and a pressure line 38. As seen from Figs. 1 through 3, the pressure line 38 is connected to a fluid path 26g defined through the piston rod 26b and the piston 26c. The working chamber 26d of the hydraulic cylinder 26 is further communicated with a pressure accumulator 34 via an orifice 32. Another pressure accumulators 18 are provided in the pressure supply line 52 for accumulating the excessive pressure generated by the pressure source unit 16. The pressure control valve unit 28 comprises a proportioning valve and is designed to be controlled by an electric control signal for varying valve position according to variation of current value of the control signal. Generally, the pressure control valve unit 28 controls magnitude of introduction and draining of the pressurized working fluid into and from the working chamber 26d for adjusting the pressure in the working chamber for setting the damping mode of the hydraulic cylinder 26. To control the valve position of the pressure control valve unit 28, a control unit 22 which comprises a microprocessor, is provided. The control unit 22 is connected to vertical acceleration sensors 21FR, 21RL and 21RR which monitors vertical accelerations exerted to the vehicle body at the portion of the front-right, rear-left and rear-right suspensions 14FR, 14RL and 14RR to produce vertical acceleration indicative signals GZFR, GZRL and GZRR. The vertical acceleration sensor will be hereafter represented by the reference numeral "21" as generally referred to. Similarly, the vertical acceleration indicative signal as generally referred to will be hereafter labeled as "Gz". In the shown embodiment, the vertical acceleration to be exerted on the vehicle body 10 at the portion of the front-left suspension 14FL is arithmetically derived on the basis of the vertical acceleration indicative signals GZFR, GZRL and GZRR. The control unit 22 is also connected to a vehicle speed sensor 20. Though the shown embodiment employs three vertical acceleration sensors 21FR, 21RL and 21RR for monitoring vertical accelerations exerted to the vehicle body at the portion of the front-right, rear-left and rear-right suspensions 14FR, 14RL and 14RR, it is, of course possible to provide additional vertical acceleration sensor for monitoring the vertical acceleration to be exerted on the vehicle body at the portion of the front-left suspension 14FL. Fig. 2 shows the detailed construction of the pressure control valve unit 28 to be employed in the shown embodiment of the actively controlled suspension system set forth above. The pressure control valve 28 comprises a valve housing 42 which housing a proportioning solenoid 43. The proportioning solenoid 43 is electrically connected to the control unit 22. The valve housing 42 defines a valve bore which is separated into a valve chamber 42L and a control chamber 42U by means of a partitioning member 42A. The partitioning member 42A is formed with a communication hole 42a. Above the communication hole 42a is defined the control chamber 42U. The valve chamber 42L and the control chamber 42U are aligned to each other across the communication hole 42a. Beneath the communication hole 42a and adjacent the top of the valve chamber 42L, a stationary throttling orifice defining member 44 is provided. The throttling orifice defining member 44 is formed with a fixed throttling rate of orifice. The throttling orifice defining member 44 defines with the partitioning member 42A a pilot chamber P. A valve spool 48 is thrustingly or slidingly disposed within the valve chamber 42L. The valve spool 48 defines an upper feedback chamber FU between the top end thereof and the throttling orifice defining member 44. The valve spool 48 also defines a lower feedback chamber FL between the lower end thereof and the bottom of the valve chamber 42L. Offset springs 50A and 50B are disposed within the upper and lower feedback chambers FU and FL, which offset springs exert spring forces to the valve spool 48 for resiliently restricting movement of the latter. The valve chamber 42L is communicated with an inlet port 42i, a drain port 42o and the communication port 42n which are defined through the valve housing 42. The inlet port 42i is connected to the pressure unit 16 via a supply line 52. On the other hand, the drain port 42o is connected to the pressure unit 16 via the drain line 54. The valve spool 48 is formed with an upper land 48b and a lower land 48a. The upper and lower lands 48b and 48a defines therebetween an annular pressure chamber 48c. The valve spool 48 is formed with a pilot path 48d communicating the pressure chamber 48c with the lower feedback chamber FL. A poppet valve member 56 is disposed within the control chamber 42U for thrusting or sliding movement therein. The poppet valve member 56 has a valve head opposing to the communication hole 42a. The poppet valve member 56 is operably associated with the proportioning solenoid 43 which comprises a plunger 58 which has a plunger rod 58A. The lower end of the plunger rod 58A of the plunger 58 opposes to the top end of the poppet valve 56. The poppet valve member 56 is driven by the plunger 58 to control the path area in the communication hole 42a according to the position of the plunger rod 58A. Therefore, the poppet valve member 56 adjusts the path area of the communication hole 42a to control fluid pressure to be introduced in the pilot chamber P. Poppet valve member 56 separates the control chamber 42U into upper and lower control chambers. In order to control the position of the poppet valve 56 for adjusting the pilot pressure in the pilot chamber P, a solenoid coil 60 is provided for energizing the deenergizing to cause axial shift of the plunger rod 58A. By adjusting the fluid pressure in the pilot chamber P, the pressure in the upper feedback chamber FU is adjusted to exert an axially driving force to the valve spool 48 to cause axial shift. By this, selective fluid communication between the inlet port 42i, the drain port 42o and the communication port 42n can be established to adjust the fluid pressure at the communication port 42n. Since the pressure at the communication port 42n is equal to the fluid pressure in the working chamber 26d of the pressure cylinder 26, the damping force created by the pressure cylinder can be adjusted. The inlet port 42i is also connected to the pilot chamber P via a fluid path 42s. On the other hand, the drain port 42o is connected to control chamber 42U via a fluid path 42t. In order to control the pressure at the communication port 42n, a control current I is applied to the solenoid coil 60. Fig. 3 shows alternative embodiment of the pressure control valve unit 28 to be employed in the preferred embodiment of the actively controlled suspension system, according to the invention. Fig. 3 shows the detailed construction of the hydraulic cylinder 115A and the pressure control valve 28. As will be seen from Fig. 3, the hollow cylinder housing 115a is formed with a port 115f communicating the upper fluid chamber 115d to an outlet port 118d of the pressure control valve 28 via a communication line 127. Though Fig. 3 does not show clear construction, the lower fluid chamber 115e is defined as an enclosed space and is filled with the viscous working fluid. The pressure of the working fluid in the lower fluid chamber 115e at an initial position of the piston 115c serves as a reference pressure and per se serves as resistance for downward movement of the piston. The pressure control valve 28 has a valve housing 118A having the aforementioned outlet port 118d, an inlet port 118b and a drain port 118c. Respective inlet port 118b, the drain port 118c and the outlet port 118d are connected to a valve bore 118a defined within the valve housing 118A. A valve spool 119 is disposed within the valve bore 118a for thrusting movement therein. The valve spool 119 has first, second and third lands 119a, 119b and 119c. As will be seen from Fig. 3, the third land 119c has smaller diameter than that of the first and second lands 119a and 119b. The third land 119c defines a fifth pressure control chamber 118h which is connected to the drain port 118c via a drain path 118f. An actuator piston 122c is also disposed within the valve bore 118a. The actuator piston 122c opposes the second land 119b in spaced apart relationship to define a second pressure control chamber 118i which is connected to the drain port 118c via a drain path 118e. An annular pressure chamber 118j is defined between the first and second lands 119a and 119b. The pressure chamber 118j is constantly communicated with the outlet port 118d and whereby communicated with the upper fluid chamber 115d. On the other hand, the pressure chamber 118j shifts according to shifting of the valve spool 119 to selectively communicate the outlet port 118d with either the inlet port 118b or the drain port 118c. On the other hand, an pressure control chamber 118k is defined between the first and third lands 119a and 119c. The pressure control chamber 118k is in communication with the outlet port 118d via a pilot path 118g. A bias spring 122d is interposed between the actuator piston 122c end the valve spool 119. The actuator piston 122c contacts with an actuator rod 122a of an electrically operable actuator 122 which comprises an electromagnetic solenoid. The solenoid 122 comprises a proportioning solenoid. In order to increase the supply pressure of the working fluid, the spool valve 119 is shifted to the lower position to increase path area at a throttle constituted at the inner end of the inlet port 118b by means of the land 119a of the spool valve 119. On the other hand, in order to decrease the supply pressure of the working fluid, the spool valve is shifted to the upper position to decrease the path area at the throttle of the inner end of the inlet port 118b and opens the drain port 118c which is normally blocked by means of the land 119b of the spool valve. As seen from Fig. 3, the proportioning solenoid 122 comprises the actuator rod 122a and a solenoid coil 122b. The solenoid coil 122b is energized by suspension control signal from the control unit. In the shown embodiment of the pressure control valve, the working fluid pressure P at the outlet port 118d is variable according to the predetermined variation characteristics. Namely, when the control value represented by the suspension control signal is zero, the pressure at the outlet port 118d becomes an initial pressure determined according to a predetermined offset pressure. When the suspension control signal value in positive value increases, the fluid pressure at the outlet port 118d increases with a predetermined proportioning rate. Namely, by increasing of the suspension control value, the actuator rod 122a is driven downwardly in Fig. 3 at a magnitude toward to position to achieve increasing of the fluid pressure with the predetermined proportioning rate. The fluid pressure at the outlet port 118d saturate at the output pressure of the pressure unit. On the other hand, when the suspension control signal value decreases, the pressure decreases to zero to by shifting of the actuator rod 122a. The actuator rod 122a of the proportioning solenoid 122 is associated with the actuator piston 122c. Contact between the actuation rod 122a and the actuator piston 122c can be maintained by the resilient force of the bias spring 122d which normally biases the actuator piston toward the actuation rod. On the other hand, the spring force of the bias spring 122d is also exerted on the valve spool 119 to constantly bias the valve spool downwardly in Fig. 3. The valve spool 119 also receives upward hydraulic force from the pressure control chamber 118k. Therefore, the valve spool 119 is oriented at the position in the valve bore at the position where the downward bias of the bias spring 122d balances with the upward hydraulic force of the pressure control chamber 118k. Fig. 4 is a circuit diagram of a hydraulic circuit including the pressure source unit 16 and the working fluid chamber 26d of the hydraulic cylinder 26. Fig. 4 shows detailed circuit construction of the preferred embodiment of the hydraulic system to be employed in the suspension control system according to the invention. As set forth, the pressure source unit includes the pressure unit 16 which comprises a fluid pump, and is connected to the reservoir 16a via a suction pipe 201 which is driven by means of an automotive engine 200. The outlet of the pressure unit 16, through which the pressurized working fluid is fed, is connected to the inlet port 42i of the pressure control valve 18 via the supply line 52. A pressure regulating orifice 202 is disposed in the supply line 52 for suppressing pulsatile flow of the working fluid and whereby regulate the output pressure of the pressure unit 16 to be delivered to the pressure control valve 28. A feedback line 53 is connected to the upstream of the pressure regulating orifice 202 at one end. The other end of the feedback line 53 is connected to the upstream of the inlet of the pressure unit 16. Therefore, excessive fluid between the pressure unit 16 and the orifice 202 is fed back to the inlet side of the pressure unit. A pressure accumulator 203 is also connected to the supply line 52 to receive therefrom the pressurized fluid for accumulating the pressure. A one-way check valve 204 is disposed in the supply line 52 at the position upstream of the junction between the pressure accumulator 203 and the supply line 52. A pressure relief line 205 is also connected to the supply line 52 at the position intermediate between the pressure regulating orifice 202 and the one-way check valve 204, at one end. The other end of the pressure relief line 205 is connected to the drain line 54. A pressure relief valve 206 is disposed in the pressure relief line 205. The pressure relief valve 206 is responsive to the fluid pressure in the supply line 52 higher than a give value to drain part of the working fluid to the drain line for maintaining the pressure in the supply line 52 below the given pressure value. On the other hand, a shut-off valve 207 is disposed in the drain line 54. The shut-off valve 207 is also connected to the supply line 52 at upstream of the one-way check valve 204 to receive therefrom the pressure in the supply line as a pilot pressure, via pilot line 208. The shut-off valve 207 is designed to be maintained at open position as long as the pilot pressure to be introduced through the pilot line 208 is held at a pressure level higher than or equal to a given pressure level. At the open position, the shut-off valve maintains fluid communication between the inlet side and outlet side thereof so that the working fluid in the drain line 54 may flow therethrough to the reservoir tank 16a. On the other hand, the shut-off valve 207 is responsive to the pilot pressure drops below the given pressure level to be switched into shut-off position. At the shut-off position, the shut-off valve blocks fluid communication between the drain port 42o and the reservoir tank 16a. In parallel relationship to the shut-off valve, a pressure relief valve 209 is provided. The pressure relief valve 209 is disposed in a by-pass line 210 connecting the upstream side and downstream side of the shut-off valve 207. The pressure relief valve 209 is normally held at closed position to block fluid communication therethrough. On the other hand, the pressure relief valve 209 is responsive to a fluid pressure in the drain line 54 upstream thereof, higher than a set pressure, e.g. 30 kgf/cm², in order to establish fluid communication between the upstream side and downstream side of the shut-off valve to allow the excessive pressure at the upstream side drain line 54 to be drained therethrough. Therefore, the pressure relief valve 209 limits the maximum pressure at the set pressure. The set pressure of the pressure relief valve 209 corresponds to a predetermined offset pressure. An oil cooler 211 is disposed in the drain line 54 for cooling the working fluid returning to the reservoir tank 16a. Pressurized fluid supply operation to be taken place by the pressure source unit as set forth above will be discussed herebelow. While the automotive engine 200 is running, the fluid pump as the pressure unit 16 is driven. Therefore, the working fluid in the reservoir tank 16a is sucked via the suction pipe 201 and pressurized through the pressure unit 16. The pressurized working fluid is discharged from the outlet of the pressure unit 16 and fed to the pressure control valve 28 via the supply line 54 including the pressure regulating orifice 202 and the one-way check valve 204. When the pressure control valve 28 is in the lower position of Fig. 2, the pressurized working fluid passes the pressure control valve and introduced into the working chamber 26d of the hydraulic cylinder 26. On the other hand, when the pressure control valve 28 is shifted to block communication between the supply line 52 and the working chamber 26d, the line pressure in the supply line increases. When the line pressure in the supply line 52 becomes higher than a set pressure of the pressure relief valve 206 in the pressure relief line 205, the excessive pressure higher than the set pressure is fed to the drain line 54 via the pressure relief valve 206 and thus returned to the reservoir tank 16a. The fluid pressure in the supply line 52 is also fed to the shut-off valve 207 via the pilot line 208. As set forth, the shut-off valve 207 is placed at open position as long as the pilot pressure introduced through the pilot line 208 is held higher than or equal to the set pressure thereof. Therefore, fluid communication between the pressure control valve 28 and the reservoir tank 16a is maintained. At this position, the working fluid is thus returned to the reservoir tank 16a via the drain line 54 via the shut-off valve 207 and the oil cooler 211. Since the shut-off valve 207, even at the open position, serves as a resistance to the fluid flow. Therefore, the fluid pressure in the drain line 54 upstream of the shut-off valve 207 becomes excessively higher, i.e. higher than the off-set pressure P₀. Then, the pressure relief valve 209 becomes active to open for allowing the excessive pressure of the working fluid to flow through the by-pass line 210. When the engine 200 stops, the pressure unit 16 cease operation. By stopping of the pressure unit 16, the working fluid pressure in the supply line 52 drops. According to drop of the pressure in the supply line 52, the pilot pressure to be exerted to the shut-off valve 207 via the pilot line 208 drops. When the pilot line 208 drops below or equal to the set pressure, the shut-off valve 207 is switched into shut-off position to block fluid communication therethrough. As a result, the fluid pressure in the drain line 54 upstream of the shut-off valve 207 becomes equal to the pressure in the working chamber 26d. Therefore, even when the working fluid leaks through a gap between the spool valve 48 and the inner periphery of the valve bore, it will not affect the fluid pressure in the working chamber 26d. This is advantageous to maintain the suspension characteristics of the suspension systems irrespective of the engine driving condition. As seen from Fig. 5, the working fluid pressure in the working chamber 26d of the hydraulic cylinder 26 varies according to variation of the current value of the control signal applied to the pressure control valve unit 28. As seen from Fig. 5, hydraulic pressure in the working chamber 26d varies between a maximum pressure P max which is saturation pressure of the pressure source unit 16 and a minimum pressure P min which is set at a magnitude in view of a noise component to be contained in the control signal. As seen from Fig. 5, the maximum hydraulic pressure P max corresponds to the maximum current value I max of the control signal and the minimum hydraulic pressure P min corresponds to the minimum current value I min of the control signal. Furthermore, the hydraulic pressure level as labeled P n represents neutral pressure at the neutral current I n . As seen, the neutral current value I n is set at an intermediate value between the maximum current value I max and the minimum current value I min . On the other hand, the vertical acceleration indicative signals GZFR, GZRL and GZRR output by the vertical acceleration sensors 21FR, 21RL and 21RR vary linearly according to variation of the vertical accelerations respectively exerted on the vehicle body 10 at the front-right, rear-left and rear-right suspensions 14FR, 14RL and 14RR, as shown in Fig. 6. As will be seen from Fig. 6, the value of the vertical acceleration indicative sensor signal to be produced by the vertical acceleration sensor 21 varies across a predetermined neutral acceleration indicative value GZ n which represents the vertical acceleration exerted on the vehicle body is zero. In the shown embodiment, the vertical acceleration indicative signal GZ is maintained at a value greater than the neutral acceleration indicative value GZ n while the upward acceleration is exerted on the vehicle body to cause rebounding motion of the vehicle body and at a value smaller than the neutral acceleration indicative value GZ n while the downward acceleration is exerted on the vehicle body for causing bounding direction displacement of the vehicle body relative to the suspension member. As shown in Fig. 7, the control unit 22 comprises the microprocessor 68 including an input/output interface 80, an arithmetic circuit 82 and a memory 84. The vertical acceleration sensors 21FR , 21RL and 21RR are connected to low-pass filter circuits (LPF) 70a, 70b and 70c via slicer circuits 69a, 69b and 69c to feed the vertical acceleration indicative signals GZFR, GZRL and GZRR. Analog-to-digital (A/D) converters 71a, 71b and 71c are designed to convert analog form vertical acceleration indicative signal into a digital data representative of the vertical acceleration magnitude. The data output from the A/D converters 71a, 71b and 71c will be hereafter referred to as "vertical acceleration data". Each of the slicer circuits 69a, 69b and 69c slices the vertical acceleration indicative signal GZFR, GZRL and GZRR at preset slice levels +E and -E to shape the vertical acceleration indicative signal from the associated one of the vertical acceleration sensors 21FR, 21RL and 21RR. Each of the slicer circuits 69a, 69b and 69c comprises a pair of diodes 76a and 76b arranged at mutually opposite polarity. The diodes 76a and 76b are connected in series to variable direct current sources 78a and 78b. The variable direct current sources 78a and 78b output voltages +E and -E which serve as positive and negative slice levels. The variable direct current sources 78a and 78b are adjustable of the output voltage level E by a slice level setting signal Z input from the micropprocessor 68. The output voltage level E of each variable direct current source 78a and 78b varies in relation to the slice level setting signal Z as shown in Fig. 9. On the other hand, the microprocessor 68 varies the value of the slice level setting signal Z based on the vehicle speed data S of the vehicle speed sensor 20 according to the characteristics shown in Fig. 10. The low-pass filter circu ts 70a, 70b and 70c are designed for receiving the shaped vertical acceleration indicative signals GZFR, GZRL and GZRR from associated vertical acceleration sensors 21FR , 21RL and 21RR and integrating the received acceleration indicative signals to derive vertical stroke speed indicative signals vzFR, vzRL and vzRR. The vertical stroke speed indicative signals vzFR, vzRL and vzRR of the low-pass filters 70a, 70b and 70c are input to the input/output interface 80 of the microprocessor 68.The analog-to-digital converters 71a, 71b and 71c receive the stroke speed indicative signal from respective associated low-pass filters 70a, 70b and 70c in analog form and output a digital signal representative of the vertical stroke speed. This output data of the A/D converters 71a, 71b and 71c will be hereafter referred to as "stroke speed data". The arithmetic circuit 78 processes the input vertical acceleration data and the stroke speed data to derive the values of the control signal SC to be fed to driver circuits 74a, 74b, 74c and 74d which are respectively connected to the actuators of the pressure control valve units 28FL, 28FR, 28RL and 28RR. The control signals SC are fed to the driver circuits 74a, 74b, 74c and 74d via digital-to-analog (D/A) converters 73a, 73b, 73c and 73d. The driver circuits 74a, 74b, 74c and 74d generate electric current I corresponding to the control signal values for controlling the valve position of the pressure control valve units 28 and adjust the hydraulic pressure in the pressure chambers 26d of the hydraulic cylinder 26 according to the characteristics shown in Fig. 5 The process of processing the vertical acceleration data and the stroke speed data to be performed in the microprocessor will be discussed herebelow with reference to Figs. 11 and 12. The routines shown in Figs. 11 and 12 are designed to be executed as interrupt routines to be executed interrupting a main program which governs various suspension control programs, such as anti-rolling control routine, anti-pitching control routine and so forth, and serves as a background job. In the practical embodiment, the routines shown in 11 and 12 are executed every 20 ms for example. Fig. 11 shows a routine for deriving the slice level setting signal value Z on the basis of the vehicle speed indicative signal value S. Immediately after starting execution, the vehicle speed indicative signal s input from the vehicle speed sensor 20 is read at a step S102. Practically, the vehicle speed indicative signal s is a pulse signal having a frequency proportional to the vehicle speed. Based on the read vehicle speed indicative signal, a vehicle speed data S is derived at a step S104. In practice, the vehicle speed data S is derived by counting number of pulses of the vehicle speed indicative signal s. Based on the vehicle speed data S, the slice level setting signal value Z is derived according to the characteristics shown in Fig. 10, at a step S106. Practically, the slice level setting signal value Z in relation to the vehicle speed S is set in a form of a table in the memory 84. Therefore, derivation of the slice level setting signal value Z is performed by looking up the table in terms of the vehicle speed data S. The slice level setting signal having the value Z derived at the step S106 is then output to the slicer circuits 69a, 69b and 69c via a D/A converter 72 at a step S108. By the slice level setting signal from the microprocessor 68, the voltage levels +E and -E defining the slice levels in the variable direct current source 78a and 78b of the slicer circuit 69a, 69b and 69c are set. Therefore, the vertical acceleration signals of the vertical acceleration sensor 21FR, 21RL and 21RR are shaped by the slicer circuit 69a, 69b and 69c. Namely, the vertical acceleration signal having the signal level higher than the upper slice level +E and having the signal level lower than the lower slice level -E is removed and replaced with the slice levels as shown in Fig. 13. Since response characteristics of the low-pass filters 70a, 70b and 70d are variable depending upon the input signal level, the pressure variation speed of the working fluid in the working chamber 26d is variable depending upon the input level to the low-pass filters. Therefore, when the vertical acceleration indicative signals GZFR, GZRL and GZRR vary to the level g₁ as shown by phantom line in Fig. 13, the pressure in the working chamber 26d may vary to reach the maximum pressure in a period t₁ as shown by phantom line in Fig. 14 unless the vertical acceleration indicative signal level is not sliced. Whereas, as shown by the solid lines in Figs. 13 and 14, the period to reach the maximum pressure can be expended to t₂ in Fig. 14 by slicing the vertical acceleration indicative signal level at the slice level E and set at the level g₂ corresponding to the slice level. In the routine of Fig. 12, the vertical stroke data vzFR, vzRL and vzRR are read out immediately after starting execution at a step S202. The read vertical stroke data vzFR, vzRL and vzRR are temporarily stored in a temporary register of the input/output interface 80. At a step S204, vertical stroke VZFR, VZRL and VZRR are derived on the basis of the read vertical stroke date vzFR, vzRL and vzRR. Thereafter, the vertical stroke data VZFL is derived on the basis of the read vertical stroke VZFR, VZRL and VZRR at a step S206. practically, when a vehicle rolls, one of the vertical stroke VZRL and VZRR becomes a positive value and the other becomes a negative value. In such case, the positive value and the negative value of the vertical stroke VZRL and VZRR represent approximately the same magnitude of vertical stroke in the opposite directions. Therefore, by comparing the vertical stroke VZRL and VZRR, the vehicular rolling condition can be detected. If the vehicular rolling condition is detected, the vertical stroke VZFL of the front-left suspension 14FL can be derived by obtaining the value representative of the vertical stroke magnitude equal to that represented by the vertical stroke VZFR in opposite direction to that monitored in the front-right suspension. Similarly, when the vehicle pitches, one of the vertical stroke VZFR and VZRR becomes a positive value and the other becomes a negative value. In such case, the positive value and the negative value of the vertical stroke VZFR and VZRR represent approximately the same magnitude of vertical stroke in the opposite directions. Therefore, by comparing the vertical stroke VZFR and VZRR, the vehicular pitching condition can be detected. If the vehicular pitching condition is detected, the vertical stroke VZFL of the front-left suspension 14FL can be derived by obtaining the value representative of the vertical stroke magnitude equal to that represented by the vertical stroke VZRL in opposite direction to that monitored in the rear-left suspension. At a step S208, a control current value I for the pressure control valve units 28FL, 28FR, 28RL and 28RR of respective front-left, front-right, rear-left and rear-right suspensions is calculated. The control current value I is calculated by the following equation: I = VZ x Kz where VZ is VZFL, VZFR, VZRL and VZRR; and Kz is a predetermined constant. After deriving the control current value I at the step S208, the control signal value SC is derived at a value corresponding to the control current value I at a step S210. After outputting the control signal SC at the step S210 process goes END. As will be appreciated, according to the shown embodiment, abrupt change of the pressure in the working chamber which causes abrupt change of the vehicular height, is successfully prevented. Furthermore, according to the present invention, since the slice level is variable depending upon the vehicular speed so that the slice level is lowered according to increasing of the vehicle speed, vehicular height change at high vehicle speed condition becomes satisfactory modelate to assure prevention of discomfortable height change. While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding of the invention, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention set out in the appended claims. For example, though the shown embodiment adjusts the slice level setting signal level to vary linearly according to increasing of the vehicle speed, it may possible to adjust the slice level setting signal level in stepwise fashion. Furthermore, though the shown embodiment is diuscussed for bouncing control, it may be possible to apply the similar technology in slicing the sensor signal for preventing abrupt change of the pressure in the working chamber, to any of other suspension control technologies, such as anti-rolling control, anti-pitching control and so forth.

Claims: An actively controlled suspension system including a plurality of suspension members movably fixed to a vehicle body, each rotatably supporting a road wheel, said system comprising: a plurality of cylinders (26), each disposed between the vehicle body and a suspension member and defining therein a working chamber filled with a pressure medium with pressure being variable between a predetermined maximum pressure and a predetermined minimum pressure; a pressure medium source circuit (16, 38, 52 and 54) connected to said working chambers, for supplying said pressure medium to said working chambers of said cylinders; a plurality of pressure control valves (28) disposed within said pressure medium circuit and switchable between a first mode for increasing pressure of said pressure medium within said working chamber, a second mode for decreasing said pressure of said pressure medium within said working chamber, and a third mode for maintaining said pressure of said pressure medium constant; sensor means (21) for monitoring vehicle body attitude change for producing a vehicle body attitude change indicative signal; a controller (22) for generating a control signal for controlling said pressure control valves so as to adjust the pressure of said pressure medium within said working chambers; and a vehicle speed sensor (20) for monitoring a vehicle driving speed and producing a vehicle speed indicative signal representative of the monitored vehicle speed; characterized in that a slicer circuit (69) is provided for slicing said vehicle body attitude change indicative signal out of predetermined upper and lower slice levels and outputting a sliced vehicle body attitude change indicative signal to said controller, in such a manner as to limit variation range of said control signal depending upon said sliced vehicle body attitude change indicative signal; and said controller (22) is responsive to said vehicle speed indicative signal for varying said upper and lower slice levels in said slicer circuit. The actively controlled suspension system as set forth in claim 1, characterized in that said controller (22) varies said upper and lower slice levels in said slicer circuit (69) such that the difference between said upper and lower slice levels is decreasing with an increasing value of said vehicle speed indicative signal. The actively controlled suspension system as set forth in claim 2, characterized in that said controller (22) varies said upper and lower slice levels in inverse proportion to said vehicle speed indicative signal value. The actively controlled suspension system as set forth in claim 2, characterized in that said controller (22) varies said upper and lower slice levels in stepwise fashion in accordance with the variation of said vehicle speed indicative signal value. The actively controlled suspension system as set forth in claim 2, characterized in that said controller (22) modifies pressure control step-response characteristics for controlling pressure of said pressure medium within said working chamber in such a manner as to vary the variation speed of said pressure depending upon the signal level of said vertical acceleration indicative signal. The actively controlled suspension system as set forth in one of the claims 1 to 5, characterized in that said sensor means (21) includes a vertical acceleration sensor for monitoring a vertical acceleration exerted on said vehicle body and causing attitude change of said vehicle body, for producing a vertical acceleration indicative signal as said vehicle body attitude change indicative signal. The actively controlled suspension system as set forth in one of the claims 1 to 5, characterized in that said controller (22) derives a stroke signal representative of bouncing and rebouncing motion of said vehicle body on the basis of said vertical acceleration indicative signal to derive said control signal depending upon said stroke signal, so as to suppress bouncing of said vehicle. The actively controlled suspension system as set forth in claim 7, characterized in that said controller (22) includes a low pass filter (70) for deriving said stroke signal on the basis of said vertical acceleration indicative signal. The actively controlled suspension system as set forth in claim 8, characterized in that said low pass filter (70) includes an integrator for integrating a value of said vertical acceleration indicative signal in such a manner as to derive said stroke signal therefrom.

Inventors: KAWABATA, KAZUNOBU
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1988
Main classification: B60G17
IPC codes: B60G17