GB2271535A

GB2271535A - A vehicle suspension system

Info

Publication number: GB2271535A
Application number: GB9221562A
Authority: GB
Inventors: David Burke
Original assignee: Lotus Cars Ltd
Current assignee: Lotus Cars Ltd
Priority date: 1992-10-14
Filing date: 1992-10-14
Publication date: 1994-04-20
Anticipated expiration: 2012-10-14
Also published as: GB9221562D0; GB2271535B

Abstract

A vehicle suspension system comprises actuator means (26, 27, 28, 29) for connection between a vehicle body and a wheel and hub assembly (eg. 10, 11, 12, 13), force sensor means (45, 46, 47, 48) for measuring the force transmitted to the vehicle body by the actuator means (26, 27, 28, 29) and for generating a signal indicative thereof, position sensor means (53, 54, 55, 56) for measuring the length or displacement of the actuator means (26, 27, 28, 29) and for generating a signal indicative thereof, and processor means (100) which processes the signals generated by the force sensor means (45, 46, 47, 48) and the position sensor means (53, 54, 55, 56) and generates a control signal for controlling the length and/or the rate of extension/contraction of the actuator means, characterised in that the processor means (100) generates the control signal by using a mathematical model of a passive suspension system with a spring and damper connected between the vehicle body and the wheel and hub assembly (eg. 10, 11, 12, 13). <IMAGE>

Description

A VEHICLE SUSPENSION 8Y8TEM AND METHOD8 OF USING A VEHICLE 8U8PEN8ION SYSTEM The present invention relates to a vehicle suspension system and methods of using a vehicle suspension system.

Traditional suspension systems for vehicles use springs and dampers acting between the vehicle body and the wheel and hub assemblies of the vehicles. The springs and dampers react to forces arising during vehicle motion from uneveness in road surfaces and inertial forces generated during vehicle maneouvres (e.g. braking, accelerating and cornering) by allowing motion of the wheel and hub assemblies relative to the vehicle body and applying resistive and damping forces on such motion.

Active suspension systems have been developed to overcome the shortcomings of the traditional passive suspension systems. Such active suspension systems are described in patent specifications such as EP 0114757.

The control strategy of such active systems approaches the problem of vehicle ride and handling in a different manner to that of passive systems. The active systems separate the control of forces due to braking, acceleration and cornering from the control of forces due to ride inputs deriving from road uneveness. The control systems make use of extensive measurements of vehicle parameters (eg yaw rate, lateral acceleration, etc) to achieve such separation of inputs.

The present invention provides a vehicle suspension system comprising: actuator means for connection between a vehicle body and a wheel and hub assembly of the vehicle, force sensor means for measuring the force transmitted to the vehicle body by the actuator means and for generating a signal indicative thereof, position sensor means for measuring the length or displacement of the actuator means and for generating a signal indicative thereof, and processor means which processes the signals generated by the force sensor means and the position sensor means and generates a control signal for controlling the length and/or the rate of extension/contraction of the actuator means, characterised in that the processor means generates the control signal by using a mathematical model of a passive suspension system with a spring and damper connected between the vehicle body and the wheel and hub assembly.

Unlike the active suspension systems of the prior art the vehicle suspension system of the present invention synthesizes a passive suspension system. The active suspension system of the prior art deliberately do not synthesize passive systems since they attempt to improve on the performance of such. The applicants have realised that an active suspension system can be used as a powerful development tool in the development of passive suspensions for a vehicle.

Preferably, the spring rate and the damping rate of the model of the passive system are variable, whereby the user can use the vehicle suspension system to synthesize a plurality of different passive suspension systems.

Without having to have components made and fitted to a vehicle, development engineers can test the suitability many different passive systems for a vehicle. The vehicle suspension system has ride and handling characteristics which correspond to those of a passive system and which can be tuned without the need to change mechanical components. Tuning is even possible during motion.

In a first preferred embodiment the processor means controls the rate of extension/contraction of the actuator means calculating the velocity demand signal as a function of the signals generated by the force sensor means and the position sensor means.

In the prior art active suspension systems the processor generally considers signals from transducers such as yaw rate gyrometers, velocity sensors, etc., when generating a velocity demand signal. It is the incorporation of such variables in the suspension control system that allows the active suspension systems to function more effectively than passive systems. However, only force and displacement need be considered to model a passive system.

In the first preferred embodiment, the processor means calculates the velocity demand as a non-linear function of the measured force and the measured length or displacement, the non-linear function being variable with the measured force and/or the demanded velocity and/or the measured length or displacement.

Normal passive springs and dampers are non-linear (with displacement) in their performance. Therefore it is very important that the system can model non-linearity of response.

The system can be used to more accurately model a passive system if non-linear functions are used; for instance damping and "bump-stop" characteristics can be modelled effectively.

In a second preferred embodiment the processor means preferably calculates the velocity demand as a linear function of the measured force and the measured length or displacement of the actuator means. In this way the system can model a normal linear passive spring and damper arrangement.

Preferably, the processor of the first or second preferred embodiments calculates a demanded velocity for the actuator as the product of a damping co-efficient and the difference between the measured force for the actuator and the product of a spring rate co-efficient with the measured displacement.

The system calculates a velocity demand for the actuator means. This is particuarly convenient since the spool valves for a hydraulic actuator control rate of fluid flow and therefore actuator velocity.

Preferably, the damping co-efficient and the spring rate co-efficient used by the processor means to calculate the demanded velocity are variable by the user of the suspension system, whereby the system can synthesize a plurality of passive spring and damper arrangements. The development engineer can simply change the spring rate and damping coefficient of the synthesized passive system, quickly and easily. Testing of passive system become a far easier task.

In a third preferred embodiment of a vehicle suspension system according to the invention the actuator means comprises a plurality of actuators connected to a plurality of wheels and the processor means uses a mathematical model of a passive suspension which includes one or more passive roll bars. The system can be used by a development engineer to test passive systems with rolls bars in use on a vehicle without the need for manufacture or fitting of the roll bars.

In the third preferred embodiment the processor means models a passive roll bar connected between two wheels of the vehicle by calculating the difference between the forces measured by the force sensor means for the two wheels and by modifying the control signals for the actuators connected to the two wheels as a function of the difference between the measured forces.

The system works by comparing forces transmitted to the body from wheels on the same axle and thus is able to model a passive roll bar connected between the wheels.

Preferably, the functional relationship between the control signals for the wheels and the difference between the measured forces is variable in the third preferred embodiment by the user of the system, whereby the user can use the system to synthesize a plurality of different passive suspension systems with one or more roll bars.

The user can test how several different roll bar arrangements perform in use with a vehicle without the need to manufacture or fit them.

In the third preferred embodiment the processor means preferably modifies the control signals for the two wheels by adding to the measured force signals of the actuators for a first wheel a force correction signal and subtracting from the measured force signal for the other wheel the same force correction signal. A roll moment arises due to an imbalance between forces on wheels on the same axle and the system calculates the imbalance to model the response of a roll bar connected between the wheels.

Preferably, the force correction signal is calculated as the product of a chosen constant and the difference between the measured forces for the two wheels.

A roll bar with linear response can thus be modelled.

A fourth preferred embodiment comprises additionally a lateral accelerometer which generates a signal indicative of the lateral acceleration of the vehicle, wherein the processor means modifies the control signal for the actuator means with variations in the measured lateration acceleration.

A fifth preferred embodiment comprises additionally a longitudinal accelerometer which generates a signal indicative of the longitudinal acceleration of the vehicle, wherein the processor means modifies the control signal for the actuator means with variations in the measured longitudinal acceleration.

Preferably a lateral accelerometer for generating a signal indicative of the lateral acceleration of the vehicle is provided in addition to the longitudinal accelerometer, the processor means adding to or subtracting from the measured force signal for the actuator a force correction term calculated as a function of the measured longitudinal and lateral accelerations.

By measuring the lateral and longitudinal accelerations of the vehicle, the vehicle suspension system of the invention can more accurately model a passive suspension system since it can synthesise the jacking, anti-drive and anti-squat forces generated by a passive suspension system.

The present invention also provides a method of operating an active vehicle suspension system characterised in that the active vehicle suspension system is used to synthesize a passive suspension system for the vehicle.

Whilst in the past active suspension systems have been used to provide suspensions with performance improved over passive systems, the applicant has appreciated that they can be used to develop and test passive systems.

The present invention further provides a method of testing a plurality of different passive suspension systems for a vehicle including the steps of providing the vehicle with an active suspension system and using the active suspension system to synthesize each of the different passive suspension systems.

Different passive suspension systems can be tested in use with a vehicle without the need for time consuming and costly manufacturing and fitting processes.

Preferably in the method of testing the vehicle is provided with an active suspension system as previously described and preferably the force sensor means and the position sensor means of the active suspension system are used to provide information on the handling of the vehicle.

The system of the invention not only enables a passive suspension system to be synthesized, it also provides useful data on vehicle handling.

The present invention also provides a method of manufacturing a passive suspension system which includes the step of testing the passive suspension system using the above-described method of testing.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a first embodiment of a vehicle suspension system according to the invention; Figure 2 is a schematic representation of a second embodiment of a vehicle suspension system according to the invention.

With reference to Figure 1, the vehicle suspension system of the invention can be seen to comprise four wheel and hub assemblies 10, 11, 12 and 13. The wheel and hub assemblies are respectively connected to the vehicle body by suspension arms 14, 15, 16 and 17 which are each pivotally connected to the vehicle body.

Acting in parallel between the suspension arms 14, 15, 16 and 17 and the vehicle body are four hydraulic actuators 26, 27, 28 and 29. The hydraulic actuators respectively comprise pistons 22, 23, 24 and 25 movable within cylinders. The pistons 22, 23, 24 and 25 are respectively connected by links 18, 19, 20 and 21 to the suspension arms 14, 15, 16 and 17, the links being pivotally connected to the suspension arms. Each piston 22, 23, 24, 25 divides its respective cylinder into two chambers, an upper chamber A and a lower chamber B.

Four servo-valves 39, 40, 41 and 42 are provided for controlling the flow of hydraulic fluid to and from the hydraulic actuators 26, 27, 28 and 29. Each servo-valve is a spool valve which can connect the upper chamber A of its respective actuator to a source of pressurised fluid 34, e.g. a mechanical or electrically powered pump, or to an exhaust for pressurised fluid 43, 44, e.g. return lines to the pump of the source 34. The bottom chambers B of the actuators 26, 27, 28 and 29 are permanently connected to the source of pressurised fluid 34.The surfaces of the upper faces of the pistons 22, 32, 24 and 25 acting in the upper chambers A of the actuators 26, 27, 28 and 29 are of greater area than the lower sides of the pistons acting in chamber B due to the presence of the links 18, 19, 20 and 21 in the chambers B (typically, the links are chosen such that the area of each piston acting in an upper chamber A is twice as large as the area of the same piston acting in the corresponding lower chamber B).

Therefore, when the chamber A of an actuator is connected to the source of pressurised fluid 34 there is a net resultant downward force on the piston and when the upper chamber A of an actuator is connected to an exhaust for fluid 43, 44 then there is a net resultant upward force on the piston. The spool valves are each controllable to meter the flow of fluid to and from the upper chambers A and thereby control the velocity of motion of the pistons 22, 23, 24 and 25 by regulating the rate of flow of fluids into or out of the chambers A.

The servo-valves 39, 40, 41 and 42 are controlled by electrical control signals generating an electronic processor 100. The processor 100 is in the preferred embodiment of the invention a digital processor. Digital to analogue converters are provided either to the processor 100 or in the servo-valves 26, 27, 28 and 29, so that the final analogue drive required to move the spools of the valves can be provided.

The processor 100 receives a number of signals from a number of sensors provided to sense various vehicle parameters. Four load cells 45, 46, 47 and 48 are provided between the actuators 26, 27, 28 and 29 and rubber isolators attached to the vehicle body. Each load cells measures the force transmitted to the vehicle body through an actuator and generates a signal indicative of the measured force.

Four Linear Variable Induction Transducers (L.V.I.T's) 53, 54, 55 and 56 are respectively provided to measure the displacement of the pistons 22, 23, 24 and 25. Each LVIT generates a signal indicative of the displacement of its respective piston.

The processor 100 uses a mathematical model of a passive spring and damper system for each corner of the vehicle (i.e. for each actuator). Considering actuator 26 in one corner, the equation governing the force/displacement relationship of a mass, spring and damper system is as follows: M*DDX1 - Cd*DX1 - k*X1 = 0 Where N = vehicle mass Cd = damper coefficient k = spring rate X1X = measured displacement DX1 = velocity DDX1 = acceleration In the above described preferred embodiment M*DDX1 = Fml, the measured force. Thus the equation can be re-arranged to provide a demanded velocity DX1 DX1 = 1 * (Fml - K*X1) Cd The X1 is measured by the L.V.I.T. 53 and the Fml by the force transducer 45.The damping coefficient Cd and the spring rate K are variables which can be selected by the user of the vehicle. Thus, a development engineer can test the handling of a vehicle with different suspension set-ups (i.e. springs of different ratios and dampers of different coefficients) without the need of have different springs and dampers made and attached to the car.

Typically various sensors would be provided on the car to evaluate its handling performance for different suspension set ups. For instance a yaw gyrometer could be provided to measure the yaw rate of the vehicle (the rate at which the vehicle turns about a vertical axis) and a steering angle sensor and a velocity sensor could be provided to enable the calculation of a desired yaw rate demanded by the driver. In this way the difference between demanded yaw rate and actual yaw rate for different suspension settings could be measured.

The load sensors and L.V.I.T's on the vehicle themselves provide valuable information on the handling of the vehicle, for instance pitching and rolling moments and displacements can be easily calculated by comparison of measurements.

The invention provides a way in which different passive suspension arrangements can be tested on a vehicle quickly and cheaply.

In the above embodiment a passive suspension system with linear characteristics is modelled. However, the demanded velocity calculated can be modified as a function of the measured force and displacement and the velocity calculated by the processor 100 so that a damper can be modelled which has non-linear characteristics. This is very important since the real spring and damper arrangements of vehicles are non-linear in response to displacement. The damping characteristics and the bump stop characteristics of a real vehicle passive suspension system (i.e. the characteristics of the system as it approaches its limits of motion) can only be closely modelled if non-linearity in response is accounted for.

In a similar manner the spring stiffness K can be calculated by the processor 100 as a function of force, displacement and velocity so that a model of a rising rate spring can be provided. In either case the operator would program into the processor 100 a chosen functional relationship.

The suspension system of the invention can also be used to model a passive suspension system having anti-roll bars. To do this the measured forces (measured by the force transducers) are modified as a function of the roll displacement of the vehicle.

Defining the four measured corner displacements of the vehicle as X1 to X4 (X1 and X2 being the displacements of the wheels at the front of the vehicle and X3 and X4 being the displacements of the rear wheels) then: Rxf = X - X Rxr = X - X When Rxf = front roll Rxr = rear roll Two parameters Krf and KRr are then taken as the synthesised front and rear roll siffnesses.The necessary force correction terms FCRl to FCR4 for the four wheels can then be calculated as follows: Front Roll Force Correction F CR1 - FCR2 = KRf * Rxf Since the synthetic roll bar is assumed to be connected between the two front wheels (as in a normal passive system) the applied forces in the wheels must be equal and opposite in sign: FCRl = (KRf * Rxf)*1/2 and FCR2 = (KRf * Rxf)*l/2 Rear Roll Force Correction In a similar fashion the rear axle force correction terms are calculated: FCR3 (KRr * Rxr)*l/2 FCR4 (KRr * * Rxr)*1/2 The velocity for each wheel (taking one wheel as an example) is therefore calculated as follows:: DX1 = 1 * (Fm - FCR1 - K*X1) Cd In this way a passive system incorporating roll bars is modelled and a development engineer can easily test out different damping characteristics for modelled roll bars in arrangements with different springs rates and different damping rates for modelled corner springs and dampers.

This greatly speeds the development process, which before required production of different components.

Jacking, anti-dive and anti-squat forces arise in a normal passive suspension system due to the transmission of lateral and longitudinal forces through the springs and dampers or the suspension linkeages. These forces are generated by lateral and longitudinal accelerations of the vehicle. In Figure 2 a modification of the suspension system of Figure 1 is shown which can model the effects of such forces.

The second embodiment of the invention shown in Figure 2 is the same as the first embodiment in most respects. However, it has two additional sensors; lateral accelerometer 200 and longitudinal accelerometer 300.

Additional force correction terms are calculated for the four corners: FCjl = Kjf * ny + Kaf * nx F ;2 = Kjf * ny - Kaf * nx F .3 = Kjr * ny + Kar * nx Fcj4 = Kjr * ny - Kar * nx where F .1 to F .4 = force correction terms Kjf = jacking force coefficient for front wheels Kjr = jacking force coefficient for rear wheels Kaf = anti-drive/anti-squat coefficient for front wheels Kar = anti-drive/anti-squat coefficient for rear wheels ny = measured lateral acceleration nx = measured longitudinal acceleration The negative signs in the equations for FCj2 and Fcj4 represent the fact that during cornering the lateral forces act in opposite directions for the inner and outer wheels.

The force correction terms for jacking and anti-drive/squat could be used on their own by the processor to generate a modified velocity demand, but in the preferred embodiment of the present invention are used in combination with the roll-bar simulation force correction terms. A variable Fcor is generated by the processor 100 as follows for each corner, for example: Fcorl = FCR1 + Fcjl The demanded velocity signal generated by the processor 100 is calculated for each corner using one corner as an example as: DX1 = 1/Cd*(Fml-Fcorl-K*X1) The active suspension thus simulates a passive suspension system with anti-roll bars and takes account of anti-jacking forces, anti-drive forces and anti-squat forces.

A more versatile embodiment of the invention could be provided with hub accelerometers mounted on the four wheel and hub assemblies of the vehicle. The signals generated could be used to generate further force correction terms to enable simulation of wheels of different masses (without the need to attach wheels of different masses to the test vehicles). Thus the system can be used to enable a vehicle to be tested with similar wheels of various masses, without the need for the actual manufacture and/or fitting of the wheels to the vehicle.

Claims

1. A vehicle suspension system comprising: actuator means for connection between a vehicle body and a wheel and hub assembly of the vehicle, force sensor means for measuring the force transmitted to the vehicle body by the actuator means and for generating a signal indicative thereof, position sensor means for measuring the length or displacement of the actuator means and for generating a signal indicative thereof, and processor means which processes the signals generated by the force sensor means and the position sensor means and generates a control signal for controlling the length and/or the rate of extension/contraction of the actuator means, characterised in that the processor means generates the control signal by using a mathematical model of a passive suspension system with a spring and damper connected between the vehicle body and the wheel and hub assembly.

2. A vehicle suspension system as claimed in claim 1 wherein the spring rate and the damping rate of the model of the passive system are variable, whereby the user can use the vehicle suspension system to synthesize a plurality of different passive suspension systems.

3. A vehicle suspension system as claimed in claim 1 or claim 2, wherein the processor means controls the rate of extension/contraction of the actuator means by generating a velocity demand signal, the processor means calculating the velocity demand signal as a function of only the signals generated by the force sensor means and the position sensor means.

4. A vehicle suspension system as claimed in claim 3, wherein the processor means calculates the velocity demand as a non-linear function of the measured force and the measured length or displacement, the non-linear function being variable with the measured force and/or the demanded velocity and/or the measured length or displacement.

5. A vehicle suspension system as claimed in claim 3, wherein the processor means calculates the velocity demand as a linear function of the measured force and the measured length or displacement of the actuator means.

6. A vehicle suspension system as claimed in claim 4 or claim 5, wherein the processor calculates a demanded velocity for the actuator as the product of a damping co-efficient and the difference between the measured force for the actuator and the product of a spring rate co-efficient and the measured displacement.

7. A vehicle suspension system as claimed in claim 5, wherein the damping co-efficient and the spring rate co-efficient used by the processor means to calculate the demanded velocity are variable by the user of the suspension system, whereby the system can be used to synthesize a plurality of passive spring and damper arrangements.

8. A vehicle suspension system as claimed in any one of the preceding claims, wherein the actuator means comprises a plurality of actuators connected to a plurality of wheels and the processor means generates a plurality of control signals for the actuators using a mathematical model of a passive suspension which includes one or more passive roll bars connected between wheels on transversely opposite sides of the vehicle.

9. A vehicle suspension system as claimed in claim 8, wherein the processor means models a passive roll bar connected between two wheels by calculating the difference between the forces measured by the force sensor means for the two wheels and modifying the control signals for the actuator connected to the two wheels as a function of the difference between the measured forces for the wheels.

10. A vehicle suspension system as claimed in claim 9, wherein the functional relationship between the control signals for the wheels and the difference between the measured forces is variable by the user of the system, whereby the user can use the system to synthesize a plurality of different passive suspension systems with one or more roll bars.

11. A vehicle suspension system as claimed in claim 9 or 10, wherein the processor means modifies the control signals for the two wheels by adding to the measured force signal of the actuator for a first wheel a force correction signal and subtracting from the measured force signal for the other wheel the same force correction signal.

12. A vehicle suspension system as claimed in claim 11, wherein the force correction signal is calculated as the product of a chosen constant and the difference between the measured forces for the two wheels.

13. A vehicle suspension system as claimed in any one of the preceding claims comprising additionally a lateral accelerometer which generates a signal indicative of the lateral acceleration of the vehicle, wherein the processor means modifies the control signal for the actuator means with variations in the measured lateral acceleration.

14. A vehicle suspension system as claimed in any one of the preceding claims comprising additionally a longitudinal accelerometer which generates a signal indicative of the longitudinal acceleration of the vehicle, wherein the processor means modifies the control signal for the actuator means with variations in the measured longitudinal acceleration.

15. A vehicle suspension system as claimed in claim 14, comprising additionally lateral accelerometer for generating a signal indicative of the lateral acceleration of the vehicle wherein the processor means adds to or substracts from the measured force signal for the actuator a force correction term calculated as a. function of the measured longitudinal and lateral accelerations.

16. A method of operating an active vehicle suspension system characterised in that the active vehicle suspension system is used to synthesize a passive suspension system for the vehicle.

17. A method of testing a plurality of different passive suspension systems for a vehicle including the steps of providing the vehicle with an active suspension system and using the active suspension system to synthesize each of the different passive suspension systems.

18. A method as claimed in claim 17, wherein the vehicle is provided with an active suspension system as claimed in any one of claims 1 to 15.

19. A method as claimed in claim 18, wherein the force sensor means and the position sensor means of the active suspension system are used to provide information on the handling of the vehicle.

20. A method of manufacturing a passive suspension system including the step of testing the passive suspension system using a method as claimed in claims 17, 18 or 19.

21. A vehicle suspension system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.