HEIGHT ADJUSTMENT SYSTEM FOR A VEHICLE WITH AIR SUSPENSION

Abstract: In a height adjustment system for a vehicle with an air spring bellows (16), an oscillation damper (18) and a height measuring sensor (20), each arranged between the vehicle body (10) and an axle (12), the valve arrangement (24) for supplying compressed air to and discharging air from the air spring bellows (16) is closed in advance of the vehicle body reaching its adjusted height in anticipation of the effect of frictional forces in the damper (18). This is achieved in accordance with a characteristic curve stored in a memory zone (52) which delivers a correcting threshold in dependence upon the velocity (v) of correction and in accordance with the stored characteristic curve.

Description: HEIGHT ADJUSTMENT SYSTEM FOR A VEHICLE WITH AIR SUSPENSION State of the art The present invention relates to a height adjustment system of the kind described in the pre- characterising clause of claim 1. Such a system is known from EP-A-0091017 wherein compressed air is admitted to the air spring when a distance sensor senses that the height of the vehicle body is below a lower threshold and air is released from the air spring when the distance sensor senses that the body height is above an upper threshold. As a result of frictional forces in the damper, the adjustment travel extends beyond that at closure of the solenoid valve arrangement. An attempt to compensate for this overshoot is made by appropriate choice of the upper and lower thresholds. However, this does not take into account the fact that the amount of overshoot depends also on the velocity of vertical movement of the vehicle body relative to the axle at the instant of closure of the valve arrangement and possibly other factors. It is an object of the invention to avoid this di sadvantage . Advantages of the invention The above object is achieved in accordance with the invention by a height adjustment system having the features of the characterising clause of claim 1. By an appropriate choice of parameters, the range of error in the adjusted height can be substantially reduced. The invention is readily realized by the use of a characteristic curve stored in a memory in accordance with claim 2. Sufficient accuracy can be obtained by adopting the measures of claims 3 and 4. The characteristic curve can be determined empirically or by trial and error in accordance with claim 5 or 6. However, in a particularly advantageous embodiment, the stored characteristic can be corrected or a choice out of a range of stored characteristics can be made, in accordance with claim 7. This has the advantage that compensation can be made for changes which occur in the friction and damping characteristics while the vehicle is in use. Drawings The invention is further described, by way of example, with reference to the accompanying drawings, in w ic : - Fig. 1 is a diagram of an air suspension system for a vehicle fitted with a height adjustment system, Fig. 2 is a block diagram of the height adjustment system according to the invention, and Fig. 3 is a graph showing the characteristic stored in a memory of the height adjustment system. Description of the Exemplary Embodiments Fig. 1 shows diagrammatical ly 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, an oscillation damper 18 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. 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 non- return valve 28. The valve arrangement 24 comprises a 3-port, 2-position valve 30 and a 2-port, 2-position valve 32 in a line 34 between the reservoir 22 and the suspension bellows 16. The valves are spring-biased to their positions shown and are operated by solenoids 36, 38 with the assistance of air pressure from the reservoir 22. Thus, upon failure of the compressed air supply, the valve arrangement 24 cannot be opened. 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 A. 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 n. of the vehicle body 10. Referring to Fig. 2, the height sensor 20 is connected to a comparator 40 for raising the vehicle body and to a comparator 42 for lowering the body. The outputs of the comparators 40, 42 are connected to a control computer 44 which energises the valve solenoids 36, 38 via end stage amplifiers 46, 48; respectively. The output A, of the height sensor 20 is differentiated in a differentiator 50 to obtain the velocity V of vertical movement of the vehicle body and this velocity V" is fed to a memory zone 52 (which may be part of the computer 44) in which is stored a characteristic curve. This characteristic curve, which is shown in Fig. 3, gives the amount oC by which the switching thresholds for the comparators 40 and 42 must be adjusted to compensate for the frictional forces in the oscillation damper 18. As a first and sufficient approximation it is supposed that the required threshold adjustment^ is directly proportional to the velocity of vertical movement of the vehicle body so that the majority of the characteristic curve is represented by a straight line of suitable slope. There is a minimum constant adjustment °t min at velocities below V min which is determined by the expected errors of measurement and calculation. There is a maximum constant adjustment < ~ C maχ at velocities above V max which is determined by the air pressure in the reservoir 22, the capacity of the air spring bellows 16 and the flow cross sections of the valves 30, 32 and the air line 34. At a subtractor 54, the height adjustment ^ is subtracted from a height value Λ^ which it is desired to obtain when increasing the body height - from a too low a value. At an adder 56, the height adjustmento is added to a height value -.£ which it is desired to obtain when the body height Λ. is decreasing from a too high a value. The outputs of the subtractor 54 and the adder 56 are applied to the reference inputs of the comparators 40 and 42, respectively. Ideally ' x 1 should equal Λ_ 2 but, in practice, it is necessary to build some hysteresis into the system to avoid an excessive number of readjustment operations. Nevertheless, it is possible to feed the same desired height value 0 to both the subtractor 54 and the adder 56 if the minimum threshold adjustment is increased to establish the hysteresis. Should the sensor 20 measure a vehicle body height λ. which is too low, the comparator 40 supplies a signal to the computer 44 which energises both valve solenoids 36, 38, whereby to admit compressed air to the air spring bellows 16. The velocity γ~ of the consequent height adjustment is fed to the memory zone 52 which delivers a threshold adjustment determined by the stored characteristic curve. The switching off threshold of the comparator 44 is thus adjusted to < [ - ° , whereby the computer 44 closes the valves 30, 32 before the desired body height Λ. j (or/i 0 ) is reached. The stored characteristic curve is so chosen that the body reaches the desired height /i 1 (or^~ 0 ) when the frictional forces in the damper 18 have dissipated. Similarly, when the vehicle body height Λ is measured as being too high, the comparator 42 signals the comparator 44 to energise only the solenoid 38 of the valve 32, whereby to discharge air from the air spring bellows 16. The valve 32 is closed when the height the switching off threshold ^ 2 +° to establish the desired body height .^2 (orΛ^ 0 ) when the frictional forces have dissipated. Typically, the adjustment velocity can vary between V min = 5 mm /sec and V maχ = 50 mm /sec. In the absence of the invention, it would be necessary to choose an average velocity between these values and to determine the switch-off thresholds accordingly, in order to achieve a usable compromise in the event of various values. As a result, the height adjustment falls short of the desired value in the case of low adjustment velocities and overshoots the desired value in the of high adjustment velocities. As a result, the ac adjustment may depart by ± 15 mm from the desired v By a suitable choice of characteristic curve, this departure can be reduced to ± 2 mm when the vehicle stationary. Due to vehicle oscillations when the vehicle is moving, the deviation from the desired va is increased both with and without the invention, in spite of filtering of the measured height signals fr the sensor. The characteristic curve of Fig. 3 can be established empirically by making measurements of th actual adjustment velocity v and the consequent va of the eventual overshoot when the frictional forces have been overcome or dissipated. Another way is by trial and error. An initial characteristic curve is established arbitrarily and the error in the eventua height adjustment actually produced by the system is noted. The characteristic curve stored in the memor zone is then changed in a sense to reduce this error The process is then repeated as often as is necessar bring this error down to an acceptable value. In th trial and error method, the corrected characteristic curve can be stored in a non-volatile memory, to avo having to correct a fixed characteristic curve each the system is switched on. There is also the possibility of obtaining suitably accurate characteristic curves for individual systems, e.g., to compensate for differences in flow cross sections of valves and conduits and aging of the damper. Thus, the invention avoids the need for a compromise which would result from an adjustment to give optimum height regulation at half load and the consequent poorer regulation at smaller and larger loads. Another possibility is for the comparator 44 to be programmed to adjust the characteristic curve stored in the memory zone 52. To this end, as shown in dotted lines, the measured height -t is compared with the desired value . Q in a further comparator 60 and the error is fed to the computer 44. If this error is over a predetermined value in the steady state, i.e. in the absence of any signals from either of the comparators 40, 42, a corresponding correcting signal is fed to the memory zone 52 to alter the characteristic curve in the sense that the error will be reduced the next time an adjusting operation takes place. In practice, two oppositely poled comparators 60 would be needed for making corrections in either direction as may be necessary. If a volatile memory 52 is used, the one characteristic curve can be corrected as necessary. An alternative would be to use a non-volatile memory in which a series of characteristic curves is stored, of which only one is used at any given time. In such case, the correcting signal from the computer 44 changes the characteristic curve in use. The above measures have the advantages that only a single characteristic curve or a single range of characteristic curves must be determined in the first instance and it is possible to compensate for changes which occur in the damping and friction characteristics while the vehicle is in use.

Claims: CLA I MS 1. Height adjustment system for a vehicle with air suspension having a damper (18) comprising a solenoid valve arrangement (24) for admitting compressed air to and releasing air from the air springs (16), a measuring device (20) responsive to the distance ( ) between a respective wheel axle and the vehicle body and a control device (44) for operating the valve arrangement (24) responsively to the distance measured by the measuring device to maintain such distance more or less constant despite changes in the vehicle loading, characterised in that the instant or the height (X,) at which the control device (44) closes the solenoid valve arrangement (24) is dependent upon the velocity (V) of vertical movement of the vehicle body (10) relative to the axle (12). 2. Height adjustment system according to claim 1, in which the control device (44) closes the solenoid valve arrangement at a desired height ( ) of the vehicle body adjusted by a threshold (O ) which Is determined from the velocity ( ) of vertical movement in accordance with a characteristic curve stored in a memory (52). 3. Height adjustment system according to claim 2, in which the threshold (oC) is proportional to the velocity (V) of vertical movement of the vehicle body (10), at least over a part of the stored characteristic curve. 4. Height adjustment system according to claim 2 or 3, in which the stored characteristic curve extends from a minimum threshold at wnicn tne velocity (YJni η ) is within the degree of accuracy of measurement and calculation and a maximum threshold ^ max at which the velocity ( maχ ) is the maximum that can be expected having regard to the available compressed air supply. 5. Height adjustment system according to claim 2, 3 or 4, in which the stored characteristic curve is determined empirically. 6. Height adjustment system according to claim 2, 3 or 4, in which the stored characteristic is determined by trial and error. 7. Height adjustment system according to claim 2, 3 or 4, in which the actual height ( k ) of the vehicle body (10) is compared with the desired height ( - 0 ) after an adjustment operation and, in the event of an error (L - - ) exceeding a predetermined value, the characteristic curve in use is changed or altered in a sense to reduce the error.

Inventors: FAHRNSCHON MATTHIAS, PISCHKE JUERGEN, TILLHON ENGELBERT, ZEHACZEK HELMUT, FAHRNSCHON, MATTHIAS, PISCHKE, JUERGEN, TILLHON, ENGELBERT, ZEHACZEK, HELMUT
Applicants: BOSCH GMBH ROBERT, ROBERT BOSCH GMBH
Application year: 1988
Main classification: B60G17
IPC codes: B60G17