VEHICLE AIR SUSPENSION APPARATUS WITH ACCURATE SIDE TO SIDE LEVELLING

Abstract: A vehicle suspension apparatus has at least one levelling device (11) with a level sensor on each side of the vehicle. Levelling action is initiated when a level sensor indicates height out of a dead band around the calibrated level for the level sensor. Once initiated, levelling action occurs independently for all levelling devices in closed loop to target levels, different for intake and exhaust, which allow overshoot to a common intermediate level, the intermediate level allowing settling to the calibrated level. Order of levelling is rear first with intake before exhaust and then front with exhaust before intake to prevent the rear from falling below the front. Levelling is prevented when the vehicle is not moving but has moved since the last opening of one or more selected vehicle doors.

Description: This invention relates to suspension apparatus of the type which automatically respond to one or more level sensors to maintain a predetermined level of the sprung mass of the vehicle relative to the unsprung road wheels as the loads on various portions of the vehicle change. Although such vehicle levelling suspension apparatus have been known for some time and the prior art is crowded with devices and controls (as, for example, shown in US Patent No 4,466,625), there has been little attention paid to the side to side levelling of the vehicle under static conditions. Such levelling is controllable by apparatus of the three or four corner variety, wherein at one or both ends of the vehicle the levels of the sides are independently adjustable. Side to side levelling is important primarily for appearance purposes. Most vehicles have a shorter distance from side to side between wheels than from front axle to rear axle. Thus, differences in level from side to side produce a slightly greater slant in the vehicle body relative to the ground than the same differences from front to rear. In addition, the shape of most vehicles when viewed from the front or back, and especially the low, straight bumper line, appears to accentuate slight differences in level between the sides of the vehicle that would not be apparent in front to back level when viewed from the side of the vehicle. Finally, people are more inclined to accept differences in level between front and rear as opposed to such differences from side to side, since the former may be viewed as a positive design feature such as a "rake" for streamlining while the latter is generally not so construed. Therefore, even though there is actually nothing wrong with a vehicle that may have 2.5 cm (one inch) or so difference in side to side level, it is desirable to reduce this difference when possible to produce a quality appearance for customer satisfaction. This calls for a suspension apparatus with more accuracy than prior art arrangements have shown. On the other hand, it is not desirable for the suspension apparatus to be continually cycling as it responds to every tiny difference in level at each corner. The sound of the compressor and other fluid components can be annoying, especially when the vehicle is stationary so that there is no masking of the sound by other, more expected sounds. To avoid this, suspension apparatus generally use dead bands and/or hysteresis in responding to the height sensor signals. Once again, the issue is customer satisfaction; and the two somewhat conflicting concerns must be reconciled. The object of the present invention is to provide a suspension apparatus which has more accurate side-to-side levelling. To this end, a suspension apparatus in accordance with the present invention is characterised by the features specified in the characterising portion of Claim 1. The suspension apparatus of this invention greatly reduces the problem in a way that causes levelling to proceed to a more accurate degree sufficient to produce level appearance from side to side while retaining the advantages of dead band and/or hysteresis in reducing the annoyance of apparatus operation. It is of the type having a fluid pressure operated levelling device and level sensor on each side of the vehicle and control apparatus comprising intake means responsive to a low signal from one of the level sensors to increase the pressure fluid to the associated levelling device and thus raise the level of the sprung mass adjacent the associated road wheel and exhaust means responsive to a high signal from one of the level sensors to decrease the pressure fluid to the associated levelling device and thus lower the level of the sprung mass adjacent the associated road wheel. It comprises control means effective to control the response of the intake and exhaust means to the signals from the level sensors to initiate levelling activity in closed loop operation for all levelling devices as required to individual target levels when a level sensor indicates one of the levelling devices outside a dead band and discontinue levelling activity when all level sensors have indicated that the target levels have been reached. It further comprises first memory means for each of said levelling devices storing a calibrated level, an upper trim level above the calibrated level and a lower trim level below the calibrated level for that levelling device, the upper and lower trim levels defining the dead band for that levelling device and second memory means for each of said levelling devices storing an intake trim level between the calibrated level and the upper trim level and an exhaust trim level between the intake trim level and the upper trim level. The intake trim level is the target level for intake levelling activity, and the exhaust trim level is the target level for exhaust levelling activity, the intake and exhaust trim levels being set to provide for overshoot during intake and exhaust, respectively, to a common intermediate level therebetween. The common intermediate level is set to provide for settling to the calibrated level after levelling activity is discontinued. The result of the suspension apparatus of the invention is that, levelling action is initiated only when a corner is out of the dead band but once initiated, it continues until target levels are reached by all corners with great accuracy, including allowance for overshoot and settling. Once the accurate level is obtained, the dead band prevents a new initiation of levelling until a corner once again goes significantly out of level. The present invention is now described, by way of example, with reference to the following description of a preferred embodiment, and the accompanying drawings, in which:- Figure 1 is a schematic diagram of a vehicle with a suspension apparatus according to the invention; Figure 2 defines various levels of a single levelling device and level sensor; Figure 3 is a flow chart describing the basic levelling algorithm for the suspension apparatus of the invention when initiated by a sensed out of level condition; and Figure 4 is a flow chart describing the overall levelling operation initiation of the suspension apparatus shown in Figure 1. Referring to Figure 1, a motor vehicle is represented by four road wheels 10, one at each corner of the vehicle. Associated with each road wheel 10 is a levelling device 11. Levelling device 11 may be an air spring or similar device or an air controlled shock absorber. It may particularly be a combination air spring and shock absorber such as shown in US Patent No. 4,635,909, or published European patent application no. 0257779. In any case, each such levelling device is responsive to a pressure fluid supplied thereto or exhausted therefrom to raise or lower, respectively, the adjacent portion of the sprung mass of the vehicle. Each levelling device 11 includes a level (position) sensor with the capability of generating a continuous analogue or high resolution digital level position signal. The outputs of these level sensors are provided to a control computer 12. Control computer 12 also receives and IGN signal from the vehicle ignition switch, a vehicle speed signal SPD, a disable signal DIS, door switch signals from selected vehicle doors and may include an operator control switch input for operator selection of level, such as higher level for off road operation or lower level for high speed operation. A compressor/exhaust apparatus 13 selectively provides or vents a pressurized fluid such as air through an air drier 15 and pressure switch 16 which also has a signal output to control computer 12. The air line runs from pressure switch 16 to a control valve assembly 17 which controls the communication of compressor/exhaust apparatus 13 with the individual levelling devices 11 in response to signals from control computer 12. An air line 18 runs from each valve of control valve assembly 17 to one of the levelling devices 11. Signal lines 20 run from the level sensors in three of the four levelling devices 11 to control computer 12. Pressure switch 16 provides a signal to control computer 12 when the pressure to any levelling device 11 falls below 241 kPa (35 psi) to automatically provide air to that levelling device. This is especially important after service in which an air spring levelling device is removed and reinstalled. The repressurization of the levelling device is accomplished automatically. The levelling operation of the apparatus will now be described, with reference to Figures 2 and 3. Referring to Figure 2, a calibrated trim for each level sensor is stored in the memory of control computer 12. This may be accomplished during the manufacturing process by physically levelling the vehicle and causing each level sensor to be read, with the resulting readings stored in non-volatile EEPROM. From each calibrated trim value, upper and lower trim levels may be determined by adding and subtracting a predetermined number. In the case shown in Figure 2, the upper and lower trim levels are 8mm above and below the calibrated trim level. The upper and lower trim levels establish the dead band, with no levelling action initiated unless a level sensor is out of its particular dead band. Still referring to Figure 2, an intake trim level is established 2mm above the calibrated trim level; and an exhaust trim level is established 4mm above the calibrated trim level. The latter is above the former, and both are between the calibrated trim level and the upper trim level. Once a levelling operation is initiated, the intake or exhaust trim level will be used as the target level in closed loop control for each levelling device 11 until it is achieved. The intake trim level is used for intake or compression levelling activity requiring raising a corner of the vehicle; and the exhaust trim level is used for exhaust levelling activity required lower a corner of the vehicle. Once levelling activity is initiated, the upper and lower trim bands will be ignored and all levelling devices 11 activated as required to their individual target levels. Regardless of which direction levelling activity is moving for a particular levelling device 11, the apparatus will coast or overshoot the target level to a common intermediate level; and the apparatus will gradually settle after levelling activity is complete from the common intermediate level to the calibrated level. The levelling control algorithm for the embodiment of Figure 1 is shown in the flow chart of Figure 3. It reaches the title block "LEVEL" 25 if a corner is detected outside the upper or lower trim band. Once a corner is detected out of trim, all four corners will be sent to their target levels, which are the intake or exhaust trim points, depending on whether intake or exhaust is needed for that level. The order is intake before exhaust and, for either, front and rear in the order that prevents the rear from falling below the front. For either front or rear, if both sides need the same levelling activity (intake vs. exhaust), they are adjusted together. From the title block 25, the program asks at decision step 26 if a corner is below the lower trim level. If the answer is yes the program asks at decision step 27 if the rear levelling devices 11 have reached the intake trim level. If not, intake action is initiated or continued for the rear levelling devices in step 28; and the program returns to decision step 27. If the answer is yes, the program asks at decision step 30 if the front levelling devices 11 have reached the intake trim level. If not, intake action is initiated or continued with the front levelling devices; and the program returns to decision step 27. If the answer is yes or if the answer is no at decision step 26, the program asks at decision step 31 if the front levelling devices 11 have reached the exhaust trim level. If not, the program initiates or continues exhaust for the levelling device 11 at step 32 and returns to decision step 31. If the answer is yes, the program asks at decision step 33 if the rear levelling devices 11 have reached exhaust trim level. If not, exhaust action is initiated or continued at the levelling device 11; and the program returns to decision step 31. If the answer is yes, levelling is completed; and the program returns from the routine until the next time a corner is detected out of the dead band defined by the upper and lower trim levels. The overall level control, in which the necessity of levelling activity initiation is determined, is shown in the flow chart of Figure 4. The apparatus starts at the "RESTART" title block and enters the START mode. A short height averaging period of, for example, 1 second is selected at step 40. This height averaging period is a common transient eliminating feature of vehicle levelling apparatus. In this embodiment, the input samples from each level sensor are processed in a software low pass filter routine with for a time corresponding to the averaging period. The short period is chosen for the start to get immediate levelling activity if required. At decision step 41, the program asks if the vehicle is moving, as determined by the vehicle speed signal SPD. If so, the program shifts to the DRIVE mode, yet to be described. If not, the program asks at decision step 42 if the average time is over. If it is not, the program returns to decision step 41. If it is, the program asks at decision step 43 if the vehicle is in trim - that is, if no corners are out of trim. If not, the LEVEL routine described in Figure 3 is performed. If so, however, this LEVEL routine is skipped; and the program asks at decision step 45 if there has been a change in the ignition status as indicated by the IGN signal. If so, the program returns to RESTART. If not the program asks at decision step 46 if the IGN signal has indicated ignition off for more than 10 seconds. If so the program enters the POWER DOWN routine to turn off the apparatus. This 10 second period allows some levelling action after the ignition is turned off. If not the program asks at decision step 47 if the vehicle is moving. If so the program enters the DRIVE mode. If not the program asks at decision step 48 if it has been in the START mode for more than 30 seconds. If not, the program re-enters the START mode for another 1 second averaging period. If so, however, the program asks at decision step 50 if the vehicle has moved since the last door was opened, as indicated by the SPD signal and door sensors. If not the program enters the PARK mode. If so the program enters the STOP mode. The PARK mode begins by setting the height averaging period to 3 seconds at step 51, although it could be set to another short time or eliminated, if desired, in favour of the START mode. The program next asks at decision step 52 if the vehicle is moving. If so, the program enters the DRIVE mode. If not, the program asks at decision step 53 if the averaging time (3 seconds) is over. If not, the program returns to decision step 52. If so, the program proceeds to decision step 43, already described. The DRIVE mode begins by setting the averaging period to 20 seconds at step 55. Next, the program asks at decision step 56 if the vehicle is accelerating or decelerating. If so the program re-enters the DRIVE mode to start a new 20 second averaging period, since the acceleration probably took one or more level sensors out of the dead band but no levelling activity is desired for a transient vehicle manoeuvre. If not the program asks at decision step 57 if the vehicle is moving. If not, the program enters the STOP mode. If so, the program asks at decision step 58 if the average time is over. If not the program returns to decision step 56. If so the program asks at decision step 60 if there have been 3 consecutive 20 second averaging periods with a corner out of the dead band. If so, the LEVEL routine of Figure 3 is called. If not the program proceeds to decision step 45, already described. The STOP mode begins by disabling correction at step 61. The program next asks at decision step 62 if the vehicle is moving. If so the program enters the DRIVE mode. If not the program asks at decision step 63 if a door is open as indicated by the door sensors. If not the STOP mode is re-entered. If so the program returns to RESTART. The STOP mode is thus entered only when the vehicle is not moving but has moved since the last time a door was opened. As soon as a door opens, the program leaves the STOP mode, since levelling activity is anticipated. There are several possible features of the apparatus which are not shown in the flow charts. The apparatus may be programmed to change the target levels, either automatically in response to vehicle speed SPD or by operator switch to lower the vehicle body for better handling and/or fuel economy at high speeds. The lowering may be identical for all four corners or may be greater in front for a streamlined "rake". Similarly, the target levels may be raised in response to an operator switch, at least below a predetermined vehicle speed, for greater clearance in off road operation. In addition, because of the accurate side to side levelling of the apparatus, when the car is moving a difference between the levels of the sides may be used as an indication of a turn to prevent levelling. This gives a good indication of a vehicle turn without the need for an additional sensor.

Claims: A suspension apparatus for a vehicle having a sprung mass supported relative to road wheels (10) by pressure fluid activated levelling devices (11), including at least one such levelling device (11) on each side of the vehicle activatable to vary the level of the sprung mass of the vehicle adjacent the associated road wheel (10), the suspension apparatus further comprising a level sensor for each of the levelling devices (11) and control apparatus comprising intake means (13-18) responsive to a low signal from one of the level sensors to provide increased pressure fluid to the associated levelling device (11) to raise the level of the sprung mass adjacent the associated road wheel (10); exhaust means (13-18) responsive to a high signal from one of the level sensors to provide decreased pressure fluid to the associated levelling device (11) to lower the level of the sprung mass adjacent the associated road wheel (10); and control means (12, 25-35) effective to control the response of the intake and exhaust means (13-18) to the signals from the level sensors to initiate levelling activity in closed loop operation for all levelling devices (11) as required to a target level when a level sensor indicates one of the levelling devices outside a dead band and discontinue levelling activity when all level sensors have indicated that the target level has been reached; characterised by first memory means (12) for each of the levelling devices (11) storing a calibrated level, an upper trim level above the calibrated level and a lower trim level below the calibrated level for that levelling device (11), the upper and lower trim levels defining the dead band therebetween; and second memory means (12) for each of the levelling devices (11) storing an intake trim level between the calibrated level and the upper trim level and an exhaust trim level between the intake trim level and the upper trim level, the intake trim level being the target level for intake levelling activity and the exhaust trim level being the target level for exhaust levelling activity, the intake and exhaust trim levels being set to provide for overshoot during intake and exhaust, respectively, to a common intermediate level therebetween, the common intermediate level being set to provide for settling to the calibrated level after levelling activity is discontinued, whereby accurate side to side levelling is attained. A suspension apparatus as claimed in Claim 1 comprising four levelling devices (11), one adjacent each corner of the vehicle, wherein the control means (12, 25-35) further comprises means (25-35) for determining the order of levelling activity by first servicing levelling devices (11) requiring intake to reach their target levels with service at least initiated for the rear intake before the front intake and, when exhaust levelling is initiated, at least initiating front exhaust service before rear exhaust service, whereby the rear of the vehicle does not fall below the front of the vehicle during levelling. A suspension apparatus as claimed in Claim 1 or Claim 2, in which the control means (12, 25-35) further comprises means (47-50) for preventing activation of the intake and exhaust means (13-18) while the vehicle is not moving but has moved since the last opening of one or more selected vehicle doors.

Inventors: KOWALIK, JAMES J., MAJEED KAMAL N.
Applicants: GENERAL MOTORS CORPORATION
Application year: 1988
Main classification: B60G17
IPC codes: B60G17