GB2255390A

GB2255390A - Vehicle shock absorber adjustment

Info

Publication number: GB2255390A
Application number: GB9207917A
Authority: GB
Inventors: Stefan Gorny; Rainer Kallenbach; Andreas Klug; Udo Neumann; Stefan Otterbein
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1991-04-12
Filing date: 1992-04-10
Publication date: 1992-11-04
Anticipated expiration: 2012-04-10
Also published as: FR2675084B1; FR2675084A1; GB9207917D0; GB2255390B; JPH05104929A; DE4112007A1

Description

225 1 2) 5 S 7- PROCESSING OF SIGNALS DEPENDENT ON VEHICLE WHEEL DAMPING

The present invention relates to a system for formation of a signal in dependence on wheel damping in a vehicle, in particular in conjunction with damper regulation.

An efficient suspension system between the wheels and the chassis of a vehicle is essential for the wheel system of the vehicle. Such a suspension system, when of semi-active character, usually consists of a spring arrangement which has a fixed spring characteristic and is connected in parallel with a shock-absorber with an adjustable damping characteristic. A shock absorber with adjustable characteristic can be achieved by, for example, providing a piston of the shock absorber with a throttle valve having an alternative throughflow cross-section.

An efficient method for control or regulation of a adjustable suspension system is also of importance for the wheel system. By means of such a method, drive signals for adjustment of the suspension system are supplied on the basis of data from sensor signals which furnish information on the state of travel of the vehicle.

An efficient regulation of control procedure should ideally regulate or control the adjustable suspension in such a manner as to take into account travel safety on the one hand and to enable the highest po sible travel comfort for the vehicle occupants and/or a shock- sensitive load of the vehicle on the other hand. These are conflicting aims from the viewpoint of springing and shock absorbing.

- 2 A level of travel comfort is achieved by the softest possible suspension setting, whereas the hardest possible setting is usually desirable for a high level of travel safety.

A method of adjustable wheel damping in passenger and commercial vehicles is disclosed in DE-OS 39 18 735. In this case the drive signals for the control or regulation of the adjustable suspension are produced essentially by processing sensor signals in filters. These filters are so designed that the sensor signals, which provide information on the state of travel, are influenced with respect to their amplitude and/or course. Through this influencing, drive signals are produced for the adjustable suspension and are adapted to the respective state of travel in such a manner that a damper setting appropriate for travel safety is undertaken in critical travel situations and a setting appropriate to travel comfort is undertaken in non-critical travel situations.

A comfortable setting can be achieved by, for example, imparting to the suspension the softest possible setting, i.e. the adjustable shock absorbers provide low damping. A far more efficient control or regulation, for example with consideration of the movements of the vehicle chassis that determine travel comfort, can be achieved by a so-called frequencydependent "skyhook" regulation.

In skyhook regulation, the chassis movements are reduced and an improvement in the travelling comfort is effected, but without directly increasing travel safety. This regulation concept is generally known in the field of chassis behaviour regulation and is based on the model ideal of a shock absorber and/or springing system

3 - which engages at the vehicle chassis mass and is connected with an inertial fixed point (skyhook). Since an inertial shock absorber and/or springing system cannot in practice be readily realised, the suspension between the vehicle chassis and the wheel units is, as a substitute measure, appropriately regulated.

4 Aboul Nour, A.M.A.2 A number of publications (Crolla, D.A., Proceedings of the Institute of Mechanical Engineers, International Conference on Advanced Suspension, 22-25 October 1988, London; Margolis, D.L., Semi-Active Heave and Pitch Control for Ground Vehicles, Vehicle System Dynamics, 11 (1982), pp.31-42), describe a switching strategy known as "semi-active discrete skyhook damping" for suspension systems incorporating shock absorbers with shockabsorption characteristics adjustable in two stages (hard/soft), the characteristic being adjusted in dependence on chassic movement.

This switching strategy is shown in the following table:

shock absorber in shock absorber in tension phase compression phase Va > Vagr hard soft Va < -Vagr soft hard In this table, the chassis velocity in vertical direction at the points of engagement of the suspension system is represented by Va.

If this velocity exceeds a certain positive threshold Vagr (setting parameter), thus when a substantial upward movement of the chassis takes place, the respective shock absorber in the tension phase is switched over to a hard characteristic and in the compression phase to a soft characteristic. Conversely, in the case of a strong downward movement of the chassis, switching-over in the tension phase is to the soft characteristic and in the compression phase to the 5 hard characteristic. If there is no excessive chassis movement (1ValVagr), then the shock absorber operates in its soft setting in both compression and tension phase.

Shock absorbers with adjustable shock-absorption characteristics are described in, for example, DE-OS 33 04 815 and DE-OS 36 44 447.

As already indicated, considerations in respect of travel safety are also relevant as criteria for the setting of the shock absorption characteristic. A system which is intended to minimise dynamic fluctuations in wheel loading is described in DE application P 40 11 808. 8. Such systems supply control signals for adjustment of the shock absorption characteristic in dependence on the state of travel of the vehicle.

In US 4 936 425 there is disclosed a suspension regulation system in which resetting a semi-active shock absorber between a hard and a soft shock absorption phase is carried out when the relative speed of the two shock absorber coupling points is lower than a fixed predetermined threshold or the tyre deformation is smaller than a fixed predetermined threshold, according to which one of these two conditions is fulfilled first. However, such a drive in dependence on shock bsorber piston velocity is not optimal. Even if the deformation of the tyres is taken into consideration as criterion for alteration of the shock absorption characteristic, an optimum driving mode is not achieved. Moreover, taking the tyre deformation into consideration entails appreciable complication in respect of sensor systems.

There is thus a need to optimise the drive mode of an adjustable 5 shock absorber.

According to a first aspect of the present invention there is provided a system for formation of a signal in dependence on wheel damping in a vehicle with a shock absorber operatively coupled between a wheel unit of the vehicle and the vehicle chassis, the system comprising means to provide an actual value first signal having a value indicative of the relative movement of the differently coupled sides of the shock absorber at a time instant, means to provide an estimated value second signal having a value dependent on that of the first signal, and means to logically interlink the values of the first and second signals and in dependence on the interlinking to provide a signal for use in controlling a working magnitude of the vehicle.

According to a second aspect of the invention there is provided a system for regulation of suspension damping in a vehicle equipped with a shock absorber which is operatively coupled between a wheel unit of the vehicle and the vehicle chassis and the shock absorption characteristic of which is alterable in dependence on- operating parameters of the vehicle, the system being operable to alter the characteri5tic only in operating phases of the shock absorber in which low shock-absorbing forces are present.

In a preferred embodiment, the magnitude is that of 'a drive signal for alteration of the shock-absorption characteristic of the shock absorber and the alteration is effected in operating phases of small damping force by the shock absorber. These operating phases are generally in the region of the reversals of travel of a piston of the shock absorber. Alteration of the characteristic in these operating phases is of advantage in respect of minimisation of noise, since shock absorber "switching" noises mostlY occur when the shock absorber is adjusted at higher internal differential pressure. Moreover, adjustment in the region of the reversal points of the piston travel is of advantage with respect to the parameters of 10 discrete semi-active skyhook regulation.

In practice, however, it may not be sufficient to simply measure the shock absorber piston velocity and to adjust the shock absorber when a change in the sign of the velocity occurs. This is because:

1 1 Up to one scanning cycle elapses from the instant of the physical zero transition of the shock absorber piston velocity until the recognition thereof.

2. Approximately one regulation cycle elapses in the usual scanning regulator from the instant of recognition of a change in sign of the piston velocity until the completion of the setting of the new shockabsorption characteristic.

3. The shock absorber valve responsible for the change ib the shock absorption characteristic always suffers from a dead time and dynamic range. This means that a finite time period is present between the drive of the valve and the change in damping force.

Real signals are always noisy. Therefore, only relatively imprecise information is obtainable on the actual velocity of the shock absorber piston.

5. In the case of low shock absorber piston velocities, such as result from vehicle travel over almost smooth roads, the velocity changes its sign at high frequency in random manner. In view of the noise problem indicated in 4 above, damage to the setting equipment as a result of overly frequent operation is a possibility.

A further advantage of a system embodying the invention is that it may avoid a too late change in the shock-absorption characteristic, thus when the piston velocity has for some time passed through the zero point (points 1 to 3 above), and may suppress random switching back and forth at low piston velocities (points 4 and 5 above).

For this purpose, first signals which represent the relative movements of both sides of the shock absorber at a time instant are provided and processed in order to obtain a second signal value dependent on the first signal value. If the first signal value represents for example, the detected actual shock absorber piston velocity at the time instant, then the second signal value represents the predicted velocity at the time instant plus a delay period. The signal values are logically interlinked and compared with at least one threshold, preferably two thresholds. As a result of the logical interlinking and thresold comparison, a vehicle magnitude is now produced, such as a drive signal for hard or soft setting of a shock absorber.

A prompt and unambiguous recognition of operating phases or changes in movement, such as the zero transitions of the shock absorber piston velocity, which are relevant for the regulation is made possibly by the extrapolation or prediction (estimation) of the signal course of the first signal value with the use of suitable filters, which can be of digital and/or analog type, as well as the use of a logic drive system.

Embodiments of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which:

Fig. 1 is a block diagram of a signal forming and shock absorber adjusting system embodying the invention; Fig. 2 is a flow chart illustrating formation of drive signals in the system of Fig. 1; and Fig. 3 is a di gram illustrating signal comparison with thresholds in the system of Fig. 1.

Referring now to the drawings there is shown in the upper part of Fig. 1 two signal paths 14 and 15 respectively for signals Dh/Dw and Zh/Zw from a signal generating block 18. The signal paths 14 and 15 are selectively couplable to a signal path 16, which conducts a signal h/w and leads to a shock absorber 20, by means of a controllable switch 11. In the lower part of the Figure 1, an output signal Xar' of a signal generating block 19 is applied to a signal processing block 12. A signal Xarp', which is fed to a-logic block 13, is present at the output of the block 12. The signal Xar' is also fed t'o the block 13. A drive signal for drive of the switch 11 is provided at the output of the block 13. Magnitudes VSW, VDEA, tmd, taz and tdr are fed by a block 17 to the block 13.

A total delay time tau for production of a change in the shockabsorption characteristic in the shock absorber, taking place at the piston velocity zero point (Vd = 0), essentially consists of three components:

1. A value tmd, which is known only approximately and is caused by the time-discrete measurement data acquisition.

A large constant value trz, which depends on the regulation cycle time.

A value tdr, which is dependent on the shock absorber type and caused by, for example, the dynamic range of the shock absorber. Other magnitudes, such as the shock absorber piston velocity itself, can influence the overall delay time tau, which can be expressed as 3.

tau = tmd + trz + tdr (1) In the following description, the time derivatives of the magnitudes are characterised by an indice symbol. Thus, for example, Vd'(t) represents the first time derivative and Vd''(t) the second time derivative of the magnitude Vd(t), which itself signifies piston velocity at a time instant t.

If the measured piston velocity Vd at each instant is developed into a Taylor series Vd(t+tau) = [Vd(t)+[Vd'(t)tau]+[1/2Vd''(t)tau 2] (2) and if this is discontinued after the second member (because the higher derivatives require more computing time and their signal quality rapidly reduces), there is obtained a linearly extrapolated estimated value for the piston velocity in tau seconds. If this value is zero or it changes its sign between two regulation cycles, a zero transition of the actual velocity is to be expected in tau seconds.

In the embodiment illustrated in Fig. 1, a discrete semi-active skyhook regulation is initiated in the block 18. Resetting of the shock absorption characteristic takes place in two stages, which means that the shock absorber is adjustable between a hard and a soft setting. However, the system is not restricted to a drive mode with only a two stage adjustment, but is equally applicable to shock absorbers adjustable in more stages, particularly, with a view to achieving noise minimisation during adjustment operations. Moreover, there is no restriction to just one particular regulation strategy: rather, any kind of adjustment requirement of a shock absorber can be influenced by an appropriately structured system.

As described in the introduction, shock absorbers adjustable in two stages are controlled in the discrete semi-active skyhook regulation strategy in such a manner that a compression phase or tension phase requirement for adjustment of the shock- absorption characteristic is provided according to the movement of the vehicle chassis at the point of coupling of the shock absorber. If the shock absorber is disposed in its tension phase (Xar' positive) and the vertical chassis movement is upwardly, i.e. away from the road, then the shock absorber is reset to hard in order to damp the chassis movement. If the shock absorber is in the compression phase during such upward chassis movements, then it is reset to soft for minimisation of the chassis movement. The tension phase requirement in this case of upward chassis movement is thus hard (signal Zh) and the compression phase requ irement is soft (signal Dw). Analogous considerations apply to the opposite chassis movements (see also the table in the introduction). These compression and tension phase requirements are represented in Fig. 1 by the signal paths 14 and 15.

The path 14, which carries the compression phase requirement as information from the actual skyhook regulation block 18, has either the signal Dh (compression phase, hard) or the signal Dw (compression phase, soft). The path 15, which carries the tension phase requirement as information from the block 18, has either the signal M (tension phase, hard) or the signal Zw tension phase, soft). The drive signal "hard" or "soft" is then passed to the shock absorber 20 by means of the signal path 16 under the control of the switch 11.

The signal Xar' representing the stroke velocity or the shock absorber piston velocity, is formed in the block 19. This can be effected, for example, in that signals from suitable sensors, which detect the stroke movements such as the stroke travel and/or the stroke velocity of the suspension and/or the pressure in the shock absorber, are suitably processed. If, for example, the shock absorber i's connected at one side with the vehicle chassis and operatively connected directly at its other side with the wheel unit, the stroke velocity or the differentiated stroke travel represents the shock absorber piston velocity. If the shock absorber is not connected directly with the wheel unit or the chassis, sensors which reproduce the pressure differences in the shock absorber, for example, can provide a measure for the piston velocity.

The switching conditions for the controllable switch 11 are determined by the blocks 12 and 13. The associated signal Xarp' is determined from the signal Xar' in the block 12. The signal Xar' represents, as already stated, the actual shock absorber piston velocity Vd at the time t and the signal value Xarp' represents the predicted piston velocity Vd at the time t+tau (equation (2)). The block 12 is characterised by a transmission behaviour which is as follows:

The transmission behaviour is selected to be such that the output magnitude Xar(t+tau) of the block 12 corresponds to the predicted value of the magnitude Xar'(t) according to the equation (2). A linear or quadratic extrapolation appears appropriate for practical application, because of the limited signal quality. The block 12 can be an electronic digital device, which, for example, processes a differential equation representing the transmission properties in computer units, or an electronic analog device, which for example, replicates a differential equation representing the transmission properties by electronic components. Moreover, a computer-controlled design is possible. In particular, the block 12 can be a 'continuous or discretely realised finite pulse response filter, i.e. non-recursive filter or transversal filter, or a digital infinite pulse response filter, i.e. recursive filter. The design of 1 such filters is known in the art; cf, for example, the manual U.Tietze, Ch.Schenk, Halbleiterschaltungstechnik, 9th edition, Springer-Verlag, Berlin, Heidelberg, New York, London, Paris, Tokyo, 1989.

In the case of a linear extrapolation (finite pulse response filter of second order), the transmission behaviour of the entire predictive filter (block 12) results as G(Xarp'/Xar') = s2_+ sr(2Dw)+(tauw2)l +.w2 s2 + (2Dws) + w2 (3) In the case of a quadratic extrapolation (finite response filter of second order), the transmission behaviour of the entire predictive filter (block 12) results as G(Xarp'/Xar') = s2rl+(1/2tauw2)l + s[(2Dw)+(tauw2)l + W2 S2 + (2Dws) + w2.. (4) The magnitudes used in the equations (3) and (4) have the following significations:

s the Laplace operator, D the Lehr damping amount (setting parameter) and - w the limit frequency (setting parameter).

For determination of the value tau according to the equation (1), the magnitudes tmd, trz and tdr described more closely above are fed from the block 17 to the block 13. These magnitudes-can assume constant values or can, for example, be matched in dependence on the state of travel or as setting parameters to a vehicle or to the remaining components of the adjustable suspension.

The signal value Xarp', which represents the shock absorber piston velocity Vd at the time t+tau, is ascertained in the block 12 through processing of the signal Xar', which represents the piston velocity Vd at the time t. A signal preview in the case of signals Xar' of limited noise is made possible by a low-pass filter characteristic of the block 12.

The signals Xar' are compared in the block 13 with the associated signal values Xarp' and with threshold values. This is clarified by reference to the flow diagram shown in Fig. 2.

After the start in a step 201, the signals Xar' and Xarp' respectively ascertained in the block 19 and block 12 are read in together with the threshold values VSW and VDEA in a step 202. The threshold values VSW and VDEA are provided at the output of the block 17 (Fig. 1) and can be chosen to be constant as setting parameters with a view to minimising switching noise on change in the damping characteristic. However, the threshold values can be dependent on the dynamic damping range and/or on signal noise, in particular on the noise of the signal Xar, and/or magnitudes which represent and/or influence the state of travel.

In a step 203, the signal Xar' is compared in terms of amount with the first threshold value VSW. If the signal Xar', representing the shock absorber piston velocity at the time t, lies- within the switching band +/-VSW, i.e. the magnitude of Xar' is less than VSW, a If Xar' lies outside the switching band +/ the magnitude of Xarl is greater than or equal to VSW, no adjusting signal h/w is passed onto the shock absorber 20. This is step 204 then follow VSW i.e.

- indicated by the direct transition to an end step 213. In the step 204, the signal XarpI is compared in terms of amount with the second threshold value VDEA. If the signal Xarp', representing the shock absorber piston velocity at the time t+tau, lies outside the dead band +/4DEA, i.e. the magnitude of Xarp' is greater than VDEA, a step 205 then follows. If Xar' lies within the dead band +/4DEA, i.e. the magnitude of Xar' is equal to or less-than VDEA, a step 209 follows. In the step 205 or 209, the product of Xar' and Xarp' is examined in respect of its sign.

In the following description, Fig. 3 will be referred to for further clarification of the flow diagram of Fig. 2. In Fig. 3, the piston velocity M is entered as a function of time t. The piston velocity, represented by the signal Xar', is marked by a circle and the piston velocity at the time t+tau, represented by the signal Xarp', is marked by a cross.

Examples a, b, c and d:

If the signal Xar' lies within the switching band +/-VSW (interrogation in step 203) and the signal Xarp' lies outside the dead band +/4DEA (interrogation in step 204) and the signs of the two signals Xar' and Xarp' are different (Xar'.Xarp' is 1-ess than 0, i.e. output signal "Y" in step 205), then in the step 206 according to the stgn of the signal Xarp' either the compression phase adjustment signal Dh/Dw is passed to the shock absorber 20 in a step 208 (Xarp' greater than 0, example a) or the tension phase adjustment signal Zh/Zw (Xarp' greater than 0, example b) is passed to the shock absorber 20 in a step 207. If the signs of the two signals Xarl and Xarp' are equal (Xar'.Xarp' is greater than or equal to 0, i.e. output signal "N" in step 205), then no adjustment signal h/w is passed to the shock absorber 20 and the next step is the end step 213 (examples c and d).' Examples e, f and 9:

If the signal Xar' 1 ies within the switching band +/-VSW (interrogation in step 203) and the signal Xarp' lies within the dead band +/4DEA (interrogation in step 204) and the signs of the two signals Xar' and Xarp' are equal (Xar'.Xarp' is greater than 0, i.e. output signal "V in step 209), then in a step 210 according to the sign of the signal Xarp' either the tension phase adjustment signal Zh/N (Xarp' greater than 0, example e) is passed to the shock absorber 20 in a step 211 or the compression phase adjustment signal Dh/Dw (Xarp' less than 0, example f) is passed to the shock absorber 20 in a step 212. If the signs of the two signals Xar' and Xarp' are different (XarlAarp' is less than 0, i.e. output signal "N" in step 209), no adjustment signal h/w is passed to the shock absorber 20 and the next step is the end step 213 (example g).

It can be recognised in this embodiment that shock absorber adjustmentis permitted within a switching band +/-VSW when a zero transition of the shock absorber piston velocity is to be expected within the time tau. If however, the predicted piston velocity lies within a dead band around the zero point of the velocity, then adjustment is permitted only if the values of the velocity within the time tau do not lie on both sides of the velocity zero point. Consequently, a random switching back and forth at low piston 5 velocities is avoided.

By means of a system embodying the invention, operating phases of low shock absorber piston velocities or low shock absorber forces can be recognised in a preview time tau and if appropriate utilised for adjustment of the shock-absorption characteristic. In particular, the adjustments are made at the zero transitions of the piston velocity. Random adjustments are avoided by the aforedescribed embodiment merely at low piston velocities (dead band +/VDEA.

The threshold values can be adapted to the shock absorber switching means for different adjustments of the shock-absorption characteristic and for different shock absorber piston velocities.

In addition, to the dependencies of the delay time tau described in equation (1), the value of tau can be chosen as a setting parameter in respect of minimising switching noise during a change in the shock-absorption characteristics. Damping force transients and thereby noise are minimised through adjustment at the correct time.

Claims

1. A system for formation of a signal in dependence on wheel damping in a vehicle with a shock absorber operatively coupled between a wheel unit of the vehicle and the vehicle chassis, the system comprising means to provide an actual value first signal having a value indicative of the relative movement of the differently coupled sides of the shock absorber at a time instant, means to provide an estimated value second signal having a value dependent on that of the first signal, and means to logically interlink the values of the first and second signals and in dependence on the interlinking to provide a signal for use in controlling a working magnitude of the vehicle.

2. A system as claimed in claim 1, wherein the magnitude is that of drive signal for alteration of the shock-absorption characteristic of the shock absorber.

3. A system as claimed in claim 1 or claim 2, wherein the first signal value represents the actual velocity of a piston of the shock absorber at the time instant.

4. A system as claimed in claim 3, wherein the second signal value represents'the estimated velocity of the piston after a delay period following the time instant.

1

5. A system as claimed in claim 1 or claim 2, wherein the first signal value is a processed signal from a sensor sensing at least one of a parameter of a stroke associated with said relative movement and a pressure difference within the shock absorber.

6. A system as claimed in claim 5, wherein the stroke parameter is at least one of stroke travel and stroke velocity of the wheel unit.

7. A system as claimed in any one of the preceding claims, the means to logically interlink being arranged to compare the second signal value with first threshold values.

8. A system as claimed in claim 7, the means to logically interlink being arranged to compare the second signal value with second threshold values.

9. A system as claimed in claim 8, wherein the threshold values are selected in dependence on at least one of the shock absorber dynamic range, signal noise, noise minimisation on alteration of the shockabsorption characteristic of the shock absorber and a magnitude indicative of or influencing vehicle travel.

10. A system as claimed in claim 4, wherein the delay period is selected ih dependence on at least one of the shock absorber dynamic range, signal noise, noise minimisation on alteration of the shockabsorption characteristic of the shock absorber and a magnitude indicative of or influencing vehicle travel.

11. A system for regulation of suspension damping in a vehicle equipped with a shock absorber which is operatively coupled between a wheel unit of the vehicle and the vehicle chassis and the shockabsorption characteristic of which is alterable in dependence on operating parameters of the vehicle, the system being operable to alter the characteristic only in operating- phases of the shock absorber in which low shock-absorbing forces are present.

12. A system as claimed in claim 11, wherein the system includes a signal formation system as claimed in any one of claims 1 to 10.

13. A system substantially as hereinbefore described with reference to the accompanying drawings.