GB2230358A

GB2230358A - Height regulating system for a vehicle with air suspension

Info

Publication number: GB2230358A
Application number: GB8907967A
Authority: GB
Inventors: Engelbert Tillhon; Juergen Pischke
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1989-04-08
Filing date: 1989-04-08
Publication date: 1990-10-17
Anticipated expiration: 2009-04-08
Also published as: GB8907967D0; DE4003781A1; GB2230358B

Description

DESCRIPTION HEIGHT REGULATING SYSTEM FOR A VEHICLE WITH AIR SUSPENSION

State of the Art The present invention relates to a height regulating system of the kind described in the precharacterising clause of claim 1. Such a system has already been proposed wherein compressed air is admitted to the air spring when a distance sensor senses that the height of the vehicle body is below a desired value and air is released from the air spring when the distance sensor senses that the body height is above the desired value. To avoid the adjustment in response to transient fluctuations of the body height relative to the axle, particularly when the vehicle is moving along a rough road, the measured height (h) is filtered with a time constant (tl) before comparison with the desired value (ho). To obtain a good smoothing of the error signal, the time constant W) of the filter should be large. However, a large time constant leads to a phase delay between the actual value and the filtered value. This has the consequence that the height regulating operation commences late. The filter is thus of limited usefulness.

When the filtered actual value reaches the desired value (h.) the adjusting operation is immediately terminated. This can lead to a premature switching off if the desired value is reached in the event of a momentary change in the height (h) of the vehicle body above the axle, but further adjustment can only take place after the relatively long delay (td) which is 1htended to avoid regulating operations consequent upon temporary changes in the body height (h), e.g. when cornering. Accordingly, rapid and accurate regulation is not obtained during travelling in certain circumstances.

Therefore it was proposed to choos e an average time constant so that the phase displacement does not act on the height regulation in a negative sense and so that, by sufficient filtering, the effect of unevenness of the road surface is suppressed as much as-possible.

It is an object of the invention to obtain improved height control. Advantages of the Invention The above object is achieved by adopting the features set forth in the claim. This has the advantage that improved response to adjustment of the desired height can be obtained in the first mode while the longer delay time (td) effective only in the second mode prevents repeated regulating operations in the event of transient and relatively short term fluctuations in the body height (h) Drawings The invention is further described, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a diagram of an air suspension system for a vehicle fitted with a height regulating system; Fig. 2 is a block diagram of the height regulating system according to the invention; and Fig. 3 is a series of graphs illustrating operation of the height regulating system. Description of the Exemplary Embodiment

Fig. 1 shows diagrammatically a part of a vehicle body 10 attached to an axle 12 by an air suspension 14. The air suspension 14 comprises an air spring bellows 16 and a height measuring sensor 20, each arranged between the vehicle body 10 and the axle 12. The height measuring sensor 20 can be a simple potentiometer or variable resistor or an inductive transducer. The air spring bellows 16 is filled and emptied as required by a compressed air reservoir 22 and a solenoid valve arrangement 24. The reservoir 22 is charged from a compressor (not shown) via a line 26 containing a nonreturn valve 28. The valve arrangement 24 comprises a 3-port, 2-position valve 30 and a 2port, 2-position valve 32 in a line 34 between the reservoir 22 and the suspension bellows 16. The valves are spring-biassed to their positions shown and are operated by solenoids 36,38 with the assistance of air pressure from the reservoir 22. Actuation of both valves 30,32 connects the air spring bellows 16 to the reservoir 22 in order to feed compressed air to the air suspension 14 and thereby increase the height h of the vehicle body 10 relative to the axle 12. Actuation of the valve 32 alone connects the air spring.bellows 16 via the unactuated valve 30 to exhaust and thereby decrease the height h of the vehicle body 10.

Referring now to Fig. 2, the output h of the sensor 20 is processed in filters 40 and 42 to produce smoothed signals NIV, and NIV2 respectively. The time constants tl and t2 of the filters 40 and 42 are such that t2> t, ' t, being, for example 0.4 sec. and t2 2 to 3 secs. The smoothed signals NIV, and NIV2 are compared with an adjustable desired height (ho) of the vehicle body 10 in respective comparators 41 and 43. The output of the comparator 41, corresponding to the shorter time constant tI, is fed to a delay stage 54 which can be set so as to be by-passed and whose output is connected to a control device 44. The control device 44 controls energisation of the solenoids 36 and 38 of the valves 30 and 32 via amplifier end stages 46 and 48. However, height regulation by means of the"control device 44 can normally only commence when there is a-continuous output signal NIV1-t ho from the comparator 41 for a period longer than the delay period td of the delay stage 54, which may be 15 to 25 secs.

The adjustable desired value h. is derived by way of example from a potentiometer 50. The.desired value ho is differentiated in a component 52 and the signal, hl which results upon any adjustment of the desired height ho is applied to the delay stage 54 so that its delay time is set to zero and the signal from the comparator 41 is fed immediately to the control device without any delay, whereby height regulation by means of the control device 44 can commence immediately the desired height ho is adjusted. The output of the comparator 43 is also applied to the delay stage, but so that the delay by the delay stage 54 is re-introduced when the output of the comparator 43 returns to zero, whereby the solenoids 36, 38 are now energised in response to the correction signal derived by comparing NIVI with ho but only after the delay period td.

Fig. 3A and Fig. 3B show the values NIV, and NIV2 smoothed with smaller and larger time constants t, and t2, respectively, when the instantaneous actual value h of the height of the vehicle body varies with respect to time t. Fig. 3C shows the state of the delay stage 54 and Fig. 3D shows height regulating operations taking place.

The adjusted value of the desired height ho is also shown in Figs. 3A and 3B. Dotted lines above and below the desired height ho show tolerance bands or hysteresis in the comparators 41 and 43, such that the filtered sensor outputs NIV, and NIV2 must go outside the respective tolerance bands before the comparators 41 and Z 43 yield respective output signals.

In Fig. 3 it is assumed that the desired height h 0 is adjusted from a lower value to a higher value at an instant TO9 and that, prior to this instant, the delay circuit 54 is effective and no adjustment operation is taking place because the difference between NIV1 and h. lies within the tolerance band. The immediate effects of the change in the desired height ho are that the signal 11 sets the delay time of the delay stage 54 at zero as shown at Fig. 3C and NIVI.4=ho to cause a height adjustment operation to take place as shown at Fig. 3D. This state of the delay stage 54 continues as shown in Fig. 3C so long as NIV2:lho.

BY way of example, the filtered signal NIV, is shown as reaching the lower edge of the tolerance band about the newly adjusted desired height h. at the instant T1, e.g, upon encountering a pothole in the road surface, whereupon the height adjustment operation is immediately switched off. However, NIV2---'ho so that the delay by the delay circuit 54 remains at zero, whereby the height regulating operation is immediately switched on again at the instant T2 when NIV, again falls below the tolerance band of ho The same happens at instants T3 and T4. Shortly after the instant T5 when NIV, reaches h. for the third time and again switches off the height regulating operation, the signal NIV2 smoothed with the larger time constant also reaches h. (or rather the lower edge of the tolerance band thereof) and the time delay td Of 15 to 25 secs. of the delay stage 54 is brought into effect. Thereafter, no signal from the comparator 41 can reach the control device 44 unless it exists continuously for at least the delay period td. Thus, for example, at instant T7 NIV, exceeds the upper tolerance of h. and at instant T14 falls below the lower tolerance of ho and in each case a height regulating operation cannot immediately commence and the delay stage 54 starts to "count" the time. At the instants T8 and T15, before the delay time td has elapsed, the signal NIV7 returns in each case to h. and the delay stage is re-set without any height regulation having taken place.

At the instant TI, the signal NIV1, smoothed with the small time constant t, of 0.4 sec, falls below ho and remains below this value for longer than the delay time td. This can happen, for example, due to a change in the load or due to an air leakage in the suspension system. At the instant T12 at the expiry of this delay time td the signal from the comparator 41 is passed on to the control device 44 to effect the height regulating operation, which continues up to the instant T13. At the instant T13 NIV1 returns to ho and the height regulating operation is switched off. At the same instant the delay stage 54 is re-set.

Fig. 3A shows a reduction in the value of NIV, from an instant T9, e.g. due to a load change but at the instant TIO, before the time delay td has elapsed, a substantial transient increase in the actual height h causes NIV, to return to ho. Consequently the delay stage 54 is re-set and the time count starts again from Ti, as described above.

Thus there are two modes of operation. The first mode is commenced as a consequence of an adjustment in the desired height h. (instant to). During this mode, the height regulating operation is carried out without any delay in accordance with NIV1 but under the supervision of NIV2. The first mode is terminated when NIV2 first reaches the newly adjusted ho (instant t6). Thereupon the second mode commences, in which the height i regulating operation is carried out in accordance with NIV, but with the delay td. In the second mode NIV2 is of no influence. Thus, the fall of NIV2 below the lower threshold of ho shortly after T9 and the return of NIV2 across the lower threshold ho shortly after T14 have no consequence.

In another embodiment (not shown), it is arranged that, in the second mode, the height regulating operation is carried out with the delay time td but in accordance with NIV2 smoothed with the longer time constant t2 instead of NIVi. This can be achieved by using another delay stage or by switching the input of the delay stage 54 from the comparator 41 to the comparator 43 at the instant NIV2 first reaches ho following a height adjustment. Referring again to Fig. 3, it can be seen that the re-setting of the delay. stage due to a transient fluctuation in NIV, (T1O to T11) would be avoided by using NIV2 in the second mode.

On the other hand, and in the prior art, the switching over from the first to the second mode at the first instant (T1) NIV1 reaches h. would result in a premature introduction of the delay time td and would mean that a long time would elapse, probably with several height regulating operations, before the desired height h. is reached properly.

A process opposite to that described with reference to Fig.3 when the driver adjusts the potentiometer 50 in a direction to reduce the desired height hC).

The control device 44 is programmed so that, in the first mode following a height adjustment, a temporary overshoot of NIV, does not cause a height regulating operation in the opposite direction. Thus, the short overshoot of NIV, above the upper threshold of ho between instants T3 and T4 does not cause a height regulating operation in a direction to reduce the vehicle height h. This does not apply.in the second mode when the time delay td is operative.

The control device 44 is preferably a microcomputer in which the components 40 to 43, 52 and 54 are incorporated as software.

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Claims

1. Height regulating system for a vehicle having an air spring suspension, in which the actual height (h) of the vehicle body (10) is measured by a sensor (20) disposed between the vehicle body (10) and a respective axle (12) and, before or after filtering in a smoothing filter (40), the resulting signal NIV, is compared with a desired height (ho), and after a relatively long delay (td) in a delay stage (54), a control device (44) controls the admission of compressed air to and release of air from the air spring (16) accordingly until the actual value (h) substantially equals the desired value (ho), and in which the delay (td) by the delay stage (54) is omitted in the event of adjustment of the desired height (ho), characterised in that a second smoothing filter (42) having a time constant (t2) greater than the time constant (tl) of the first filter (40) is provided and, before or after filtering in the second smoothing filter (42), a corresponding signal (NIV2) is compared with the desired value (ho) and the resulting difference signal is used to render the delay (td) by the delay stage (54) operative once more, whereby in the event of an adjustment of the desired value (ho), the height regulating operation is effected in a first mode via the comparator (41), which compares the first signal (NIVI) with the desired value (h.) without the delay (td), until the second signal (NIV2) smoothed with the longer time constant (t2) first reaches the desired value (ho)g whereafter in a second mode the height regulation is effected with the delay (td) until such time as the desired value is again adjusted.

2. Height regulating system as claimed in claim 1, in which, in the event of the first signal (NIV1) overshooting a newly adjusted desired value (ho) in the first mode before the second signal (NIV2) smoothed with the longer time constant (t2) reaches the desired value (ho), a height regulating operation in an opposite sense is inhibited.

3. Height regulating system as claimed in claim 1 or 2, in which, in the second mode in which the delay (td) is operative, any height regulating operations are carried out in accordance with the second signal (NIV2) smoothed with the longer time constant (t2).

4. Height regulating system for a vehicle, constructed and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.

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