Abstract:
An electronic height control system for a vehicle chassis air suspension is modified to provide bimodal operation. In the second mode of operation the chassis air suspension system is maintain a stationary in a stable, level position to serve as a base for a mobile aerial lift unit installed on the vehicle. Operation of the suspension system in its stabilization and leveling mode is triggered by activation of a vehicle power take-off unit (“PTO”) used to position the aerial lift unit.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The invention relates to motor vehicle suspension systems and more particularly to a suspension system for motor vehicles which carry and support positionable loads such as aerial lift units. 
   2. Description of the Problem 
   Aerial lift units, used for accessing equipment mounted on utility poles or for tree trimming, are conventionally mounted on medium duty trucks. The truck provides a base for the aerial lift unit, and is subjected to differing rotational moments as the aerial lift unit is extended and positioned. Off center loads on the vehicle suspension obviously tend to cause the vehicle to tip to one side or to lean forward or backwards. To stabilize vehicles either outriggers have been used or the vehicles have been equipped with torsion bars. Outriggers are in effect struts which extend outwardly from the vehicle into contact with the ground to stabilize the vehicle. Outriggers though require side to side clearance which may or may not be available. The general alternative is to stiffen the vehicle&#39;s suspension with torsion bars. The torsion bars are oversized and vehicles so equipped require large amounts of ballast (up to 4500 lbs) to stabilize the chassis and allow movement of the aerial lift. Torsion bar stabilizing systems are very stiff. Some newer torsion bar designs use both front and rear stabilizing bars. With such suspension systems ride and comfort of the driver are sacrificed in the service of providing a base for the aerial lift unit. Torsion bar systems are preferred many times because the vehicles must operate in tight locations that don&#39;t allow outriggers to be deployed. A second advantage of a torsion bar system is the operators do not have to worry about putting the outriggers down before using the aerial device. Torsion bars also tend to lift wheels on the opposite side of the vehicle from the direction of deployment of the aerial lift. This is, up to a point, an advantage. With the wheels lifted, the spring suspension for those wheels does not continue pushing the vehicle in the direction in which the vehicle is leaning. While torsion bar systems offer some unique design solutions they bring with them some real trade-offs. A torsion bar chassis may be unable to level the chassis at the job site; ANSI regulations require that the chassis be level to within 5 degrees before use. This means that in some situations, when the operator cannot position the chassis within the 5-degree allowance, he must call in an aerial device with outriggers or perform the work in an unsafe condition. 
   Contemporary vehicles used to support aerial lift units that provide stabilization without outriggers use torsion bar systems. The ballast required by these systems is heavy and the torsion bars are expensive and dramatically affect the drivability of the vehicle. The torsion bars are up to three inches in diameter and can be difficult to install. The large quantities of ballast required dramatically reduce the carrying capacity and fuel economy of the chassis. 
   SUMMARY OF THE INVENTION 
   According to the invention an electronic height control system for a vehicle chassis air suspension is modified to provide a second mode of operation. In the second mode of operation a stationary vehicle equipped with such a suspension system provides a stable, level platform as a base for a mobile aerial lift unit carried by the vehicle. Operation of the suspension system in its stabilization and leveling mode is triggered by activation of a vehicle power take-off (“PTO”) facilities used to position the aerial lift unit. The bimodal suspension control system utilizes cooperation between a vehicle electrical system controller (“ESC”), an electronic hydraulic control module which controls a hydraulic system providing positioning of the aerial lift unit, a suspension control module (“SCM”) for a vehicle air suspension system and one or more data links, such as one provided by a controller area network (CAN), which directly or indirectly link the three control modules. The suspension control module gains an input from a two axis (biaxial) chassis level sensor while retaining inputs from suspension height measurement sensors (spring extension/compression measurement). An auxiliary switch package used for control of PTO operation is typically connected to the ESC by a secondary, low baud rate datalink. 
   Air spring based suspension systems have been advantageously equipped with height control systems which individually vary the air pressure in the springs or air bladders to keep the extension of the springs equal to one another notwithstanding changes in or shifts of the vehicle&#39;s load. Height control though does not level the vehicle when the vehicle is stopped on uneven ground. Level control in the second mode of operation is based on readings from a biaxial level sensor mounted to the chassis. Response to this input may be limited by inputs from the extension measurement sensors but level control does not operate to make spring extension equal unless the vehicle is on level ground. The chassis is leveled automatically every time the aerial device is used. When the operator arrives at the job site and activates a PTO enable switch, the chassis is first leveled with the unit still stowed to provide the best possible stability. When the mobile aerial lift unit is in operation the system functions in a manner analogous to torsion bar stabilization, adjusting the axle loading to optimize stability. The electrical system controller coupled with a controller area network interlinking control modules provides integration of chassis suspension, and ESC provides additional safety interlocks for the operator. When the work conditions are out of the safe five-degree allowable grade, the system does not work or operates only in a de-rated safe zone. This system ties together the chassis suspension control with the ESC, user inputs, level sensor outputs and hydraulic controls. 
   Additional effects, features and advantages will be apparent in the written description that follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a simplified illustration of a truck mounted aerial lift assembly for locating an operator in various raised positions. 
       FIG. 2  is a high level schematic of a vehicle suspension system providing height control. 
       FIG. 3  is a flow chart illustrating a program for utilizing the vehicle height control system for leveling and stabilization. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings, and particularly to  FIG. 1 , an example of a mobile aerial lift unit is illustrated in simplified presentation for clarity of illustration. The mobile aerial lift apparatus includes a truck  1  with an aerial lift unit  2  mounted to the bed thereof. The aerial lift unit  2  includes a lower boom  3  and an upper boom  4  pivotally interconnected to each other and to the truck bed through support  6  and rotatable support bracket  7 . A basket  5  is shown secured to the outer end of the upper boom  4  within which the operating personnel are located during the lifting to and locating within a selected work area in accordance with known practice. Basket  5  is typically pivotally attached to the out end of the boom  4  to maintain a horizontal (level) orientation at all times. The aerial lift unit is mounted to the truck bed through support  6 . A rotatable support bracket  7  is secured to the support  6  and projects upwardly. The lower boom  3  is pivotally connected as at pivot  8 , to the rotatable support bracket  7 . A lifting lower boom cylinder unit  9  is interconnected between bracket  7  and the lower boom  3 . In the illustrated embodiment, a pivot connection  10  connects the lower boom cylinder  11  of unit  9  to the bracket  7 . A cylinder rod  12  extends from the cylinder  11  and is pivotally connected to the boom  3  through a pivot  13 . The cylinder units are connected to supplies of a suitable pressurized hydraulic fluid, to lift and lower the assembly as desired. The rotatable support bracket is rotated using pressurized hydraulic fluid as well. 
   The outer end of the lower boom  3  is interconnected to the lower and pivot end of the upper boom  4 . A pivot  16  interconnects the outer end of the lower boom  3  to the pivot end of upper boom. An upper boom/compensating cylinder unit or assembly  17  is connected between the lower boom  3  and the upper boom for pivoting the upper boom about pivot  16  for positioning of the upper boom relative to the lower boom. The upper boom/compensating cylinder unit  17  is constructed to permit independent movement of the upper boom  4  relative to lower boom  3  and to provide a compensating motion between the booms to maintain the upper boom raising with the lower boom and is similarly connected to the sources of pressurized hydraulic fluid. Conventionally, aerial lift unit  2  requires positive hydraulic pressure to support operation of lower boom cylinder  11  or the upper boom cylinder  17  for lifting or lowering. The hydraulic positioning system for aerial lift unit  2  is a conventional hydraulic power take off system providing raising, lowering and rotation. Its actuation is controlled by valves which in turn are electronically controlled. 
   Upon extension and positioning of the aerial lift boom  2  the suspension system for truck  1  can subject the vehicle to destabilizing loads. In addition, truck  1  may have to be parked at a location which is not level. The present invention provides an alternative control scheme for a height control suspension system to provide chassis leveling and which can operate to prevent operation of the aerial lift unit if the vehicle is positioned and oriented on ground that is excessively sloped, or when a selected position could destabilize the truck  1 . 
   Referring to  FIG. 2 , a high level schematic of a suspension control system  29  providing bimodal chassis leveling is illustrated. An electrical system controller  30  is a body computer and is linked by a datalink  31  to a suspension control module  38  which controls pressure in the air bags  52 ,  54 ,  56 ,  58  to maintain vehicle height or to level the vehicle depending on the operational mode selected. Right front and rear suspension air bags  54 ,  56  support the vehicle at points under right side chassis frame rail  23 . Left front and rear air bags  52 ,  58  support the vehicle at points under left side frame rail  25 . Datalink  31  is preferably part of a public controller area network (CAN) conforming to the SAE J1939 standard. Electrical system controller  30  is also linked to an auxiliary gauge switch pack (“AGSP”)  34  by a datalink  35 . It is possible that datalinks  31 ,  35  operate on the same CAN, but more typically datalink  35  will be a lower speed datalink conforming to the SAE J1708 standard. Electrical system controller  30  further communicates with a PTO controller such as an electronic hydraulic control module (EHCM)  32  over datalink  33 . Datalinks  33 ,  31  may coexist on the public CAN or datalink  33  may exist on a private CAN. Where the datalinks occur on physically separate busses, the electrical system controller  30  is readily programmed to pass messages appearing on one datalink to another datalink. 
   Air spring suspension systems conventionally utilize valves admitting compressed air to the air bags and exhaust valves discharging air from the air bags to adjust the pressure in air bags installed between vehicle wheels and the vehicle chassis. Overall suspension spring rates can be adjusted to compensate for vehicle load and air spring pressure can be adjusted location by location to compensate for changes in a vehicle&#39;s load. In the conventional control regime, the pressure in individual air springs is adjusted to keep the air springs at a constant extension, equal to the extension of the other springs. In the present patent this is termed “driving suspension” mode. 
   An air spring suspension system  29  for a vehicle  1  equipped with a mobile aerial unit  2  is illustrated as having four air springs including a left front suspension air bag  52 , a right front suspension air bag  54 , a right rear suspension air bag  56  and a left rear suspension air bag  58 . The flow of compressed air to each of left front suspension air bag  52 , a right front suspension air bag  54 , a right rear suspension air bag  56  and a left rear suspension air bag  58  from air supply tank  51  is regulated by an air flow control manifold  50 . The release of air from each of left front suspension air bag  52 , right front suspension air bag  54 , right rear suspension air bag  56  and left rear suspension air bag  58  is controlled by height control valves  42 ,  44 ,  46  and  48 , respectively. The air flow control manifold  50  and the height control valves  42 ,  44 ,  46  and  48  are all under the control of a suspension control module  38 . In driving suspension mode control by the suspension control module is responsive to four air bag extension sensors  43 ,  45 ,  47  and  49 , which are associated with left front suspension air bag  52 , a right front suspension air bag  54 , a right rear suspension air bag  56  and a left rear suspension air bag  58 , respectively. Driving suspension mode provides maintaining pressures in each of left front suspension air bag  52 , right front suspension air bag  54 , right rear suspension air bag  56  and left rear suspension air bag  58  such that the vehicle is kept at a constant, predetermined height above the ground at all four corners. In other words each of the springs is kept at a constant average extension, ignoring temporary variation due to road noise and cornering. Although such systems are often called “self-leveling” suspensions, they do not level the chassis in an absolute sense, but rather even it. Thus the suspension supports the vehicle at equal heights above the ground at the four points of support. Such a control scheme does not provide a “level” chassis in an absolute sense unless the vehicle is on level ground. Right and left side frame rails  23 ,  25  (shown in phantom) represent the chassis or spring section of truck  1 . 
   Suspension control module  38  shifts to chassis leveling mode in response to indication from the electrical system controller  30 , or possibly direct indication, that a power take-off controller  32  has been engaged. A necessary condition for power take-off operation is that a PTO engagement switch has been activated on the auxiliary gauge switch pack  34  as registered by the ESC  30  over datalink  35 . Other conditions may be required before ESC  30  indicates that PTO operation has been requested, e.g. that the transmission be in park (or out of gear) and that a park brake be set (not shown). Failure of any of these other conditions may cancel PTO operation. But for purposes of this discussion, the narrow sense may be taken as operative where only engagement of the PTO switch is strictly required. Upon indication that PTO operation has been requested the suspension control module  38  provisionally enters chassis leveling mode. The inputs from extension sensors  43 ,  45 ,  47  and  49  become subsidiary to a 2-axis level sensor  40  mounted on the chassis. The measurements from extension sensors  43 ,  45 ,  47  and  49  may remain of secondary interest for indicating approach to out of limits operation or for indicating an attempt to operate the aerial lift unit with the vehicle on excessively sloping terrain. However, suspension control module  38  operates primarily in response to the indications of 2-axis level sensor  40  that vehicle  1  is not level. Equalization in the extension (or compression) of left front suspension air bag  52 , right front suspension air bag  54 , right rear suspension air bag  56  or left rear suspension air bag  58 , relative to one another is no longer a criteria, although it may be desirable to keep the average extension close to the usual norm for all of the springs and to limit extension/compression of air bags  52 ,  54 ,  56 ,  58  to maximum/minimum limits. Where the chassis, or spring section of the vehicle cannot be leveled, with aerial lift unit  2  stowed, ESC  30  can cancel PTO operation. 
   If the vehicle&#39;s sprung section can be leveled, chassis leveling is no longer provisional and PTO operation is engaged. Upon extension of aerial lift unit  2  to one side of the vehicle it should be clear that the suspension on the same side of the vehicle will tend to be compressed under the off center load. Suspension control system  38  will direct air to the suspension air bags on the same side of the vehicle to keep the vehicle level. Suspension control system  38  also releases air from air bags on the opposite side from the side to which the aerial lift unit  2  is positioned to keep the average extension of all four air bags  52 ,  54 ,  56 ,  58  close to the usual norm and to reduce the amount of force generated by the air springs located on the side of the vehicle distal to the direction of extension to the aerial lift unit. 
   Continued leveling of the vehicle chassis (in the absolute sense relative to the earth&#39;s gravitational field) occurs in response to events. It will be recognized that aerial lift unit  2  is periodically repositioned. Operation of the aerial lift unit  2  occurs in response to user inputs through, for example through auxiliary gauge switch pack  34  and is effected through operation of hydraulic control solenoids  36  by EHCM  32 . These events are reported by the EHCM  32  to ESC  30  and in turn to the suspension control module  38 , triggering releveling of the vehicle chassis based on output of the 2-axis level sensor  40 . Suspension control module  38  recognizes that limits to leveling are reached when extension sensors  43 ,  45 ,  47  and  49  indicate that the corresponding air bags have reached the limits of their operation. When such limits are reached the suspension control module  38  indicates such to the electrical system controller  30  which can in turn impose limits on the freedom of operation for the EHCM  32 . It should be recognized that the limits for extension of hydraulic lifts cannot be predetermined without foreknowledge of the slope on which the vehicle is to be used. Obviously the limits for extension of the aerial lift will change depending upon the horizontal direction in which the unit is aimed. Indeed ESC  30  may require retraction of the aerial lift unit  2  if an attempt to rotate the unit is made after extension. ESC  30  indicates limits to the EHCM  32  disabling movement in certain directions, or further extension of the aerial lift, as possibilities. Alternatively retraction of the aerial lift  2  concurrent with movement may be required at the expense of added software complexity. 
     FIG. 3  is a flow chart illustrating at a high level, operation of the leveling mode for a suspension control system. Upon PTO enable (step  62 ) the leveling mode is entered as indicated by engagement of the active leveling system (step  64 ). At step  66  it is determined from readings taken from the 2-directional level sensor  40  whether the chassis is level. If not, suspension control module  38  adjusts air pressure in the air bags until the chassis is level or a limit is reached with respect to one of the air bags. This is indicated at step  70  as a query relating to whether the system has been able to level the chassis (to within 5% limits). If not, then step  72  is executed indicating an out of level warning. If the system is functioning properly this warning essentially indicates that the ground is not level enough to allow operation of the aerial lift. Although it is conceivable that aerial lift unit  2  could be extended safely in a particular direction (for example in a direction opposite to the down slope of a hill) even though the vehicle is initially too far out of level, the program does not at this time contemplate determination as to whether such extension could be safely made. Instead PTO operation is not enabled and suspension control is returned to driving suspension mode. 
   If at step  70  it was determined that the chassis was leveled, then operation is advanced to step  74  where it is determined if PTO enable remains on. This, as discussed above, requires that the PTO enable switch be set to on and may, in addition, require certain other conditions be met such as the park brake being set or that the vehicle transmission be in park. If these conditions are not met then PTO enable is not “ON” and the program returns the suspension control module to its default, driving suspension mode. It should be kept in mind that other factors, outside of the scope of the present invention, may have to be dealt with. For example, it is possible that an operator may disengage PTO operation while the aerial lift unit  2  is extended. Programming control may provide for automatic retraction of the unit under these circumstances or not. A customer may not want a vehicle to be driveable without the aerial lift unit  2  being in a fully stowed condition. In such cases a return to driving suspension mode operation should not be taken as indication that a transmission controller (not shown) is enabled to permit putting the vehicle in gear. Such considerations are outside of the scope of the present discussion and specific provisions may depend on customer specifications, e.g. an operator may wish to allow the vehicle to be able to creep forward with the aerial lift unit  2  extended. If PTO enable remains enabled as determined at step  74  then execution advances to step  78 . Otherwise the routine is escaped along the execution path to step  76 . 
   Step  78 , following a YES indication from decision step  74 , confirms PTO operation enablement. The reason for this instruction will be clearer in context of the discussion below. Next, at step  80  it is determined if the aerial lift unit  2  has been moved, as reported by EHCM  32 . If not, execution loops back to step  74  to reverify that PTO remains enabled and to confirm PTO operation. This provides an exit from the routine once the aerial lift unit  2  has been stowed and the operator has disabled PTO operation along the program execution route to step  76 . If aerial lift unit  2  is determined to have been moved then it is determined if the movement has put the chassis in an out of level condition at step  82 . If the chassis is not level, an iterative chassis level operation  84  is performed following along the NO branch from step  82 . Successful completion or failure of the iterative chassis leveling operation is determined at step  86 . If the chassis cannot be leveled the NO branch from step  86  leads to execution of step  88  which limits operation of the PTO controller EHCM  32  so that the aerial lift unit  2  cannot be moved in certain directions or further extended in a given direction. From step  88  program execution returns to step  80  to detect further movement of the aerial lift unit  2 . Along the YES branches from both step  82  and step  86 , indicating that the chassis is level, program execution advances to step  90  which releases limits on EHCM  32  if previously applied. The need to allow release of the limits may be required if, for example, in response to limits on operation the operators of the aerial lift unit  2  changed workers in a lift cage to reduce the weight borne by the aerial lift unit. After execution of step  90  program execution loops back to step  74 . 
   The present invention provides the best possible leveling of a vehicle to support aerial lift operation. The very heavy and limited torsion bar system it functionally emulates is no longer required. Ride and drivability of the vehicle are not compromised for the sake of stability of the vehicle as a base for an aerial lift unit. The removal of ballast increases the cargo-carrying payload of the vehicle. The system allows operator implement safety interlocks and stability monitoring that increases the usefulness of the complete unit. 
   While the invention is shown in only two of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.