Abstract:
A method for warning a driver of a moving vehicle of potential imminent unintended application of a park brake because of an indication that a motorized lift ( 12 ) is out-of-stow. A processor ( 68 ) executes an algorithm for issuing an alarm to signal the driver when air pressure in a line ( 46 ) to a device ( 50 ) that is holding the park brake released has decreased to pressure somewhat greater than that which will cause the park brake to be automatically applied, allowing the driver some measure of time to park the vehicle before the park brake is applied.

Description:
FIELD OF THE INVENTION 
   This invention relates to a motor vehicle having a lift platform that when the vehicle is parked, can be deployed and then operated to lift and lower an object, such as a person seated in a wheelchair, between the vehicle floor and ground level, and that thereafter is placed in stowed position when the vehicle is ready to be driven. 
   BACKGROUND OF THE INVENTION 
   Certain vehicles have motorized lifts for raising and lowering large objects to facilitate loading them into and unloading them from space inside the vehicle. One example of such a device is a wheelchair lift. Various models of wheelchair lifts are commercially available. One type comprises a platform that when placed generally horizontally either on the floor of a vehicle or on ground adjacent the vehicle allows a wheelchair to be rolled onto and off it. With a wheelchair having been placed on the platform, the platform can be raised and/or lowered to move the wheelchair to and from floor- and/or ground-level. The lift is motorized, comprising a prime mover and associated mechanism for raising and lowering the platform. When unoccupied, the platform can be operated to a stowed position in preparation for the vehicle to be driven. 
   A motor vehicle having a wheelchair lift is subject to U.S. government regulations, as specified in FMVSS. One requirement mandates that the vehicle be rendered immovable when the lift is out of stow. One means for compliance with that requirement comprises automatically operating a vehicle&#39;s park brake to apply a holding or locking force to vehicle wheels when the vehicle is stopped thereby preventing the vehicle from moving. 
   In certain vehicles the park brake uses an on-board pressurized air supply to keep the park brake at each wheel from otherwise being automatically applied by a device, sometimes called a SAAR (meaning spring actuated, air released) for short. Application of a park brake occurs when the air supply to the SAAR is shut off, allowing the spring force of the SAAR to be effective to lock the wheels. Pressurized air must be applied to the SAAR in order to release the park brake. 
   The driver can apply and release the park brake by operating a “push-pull-double-check” (PPDC) valve, sometimes referred to as a “park brake knob”. For compliance with applicable wheelchair lift regulations, a vehicle may have a solenoid-operated interlock valve for causing the compressed air supply to the park brake knob to be shut off when the wheelchair lift is out of stow. A supply valve is caused to open by compressed air pressure being applied through the interlock valve to a pilot port when a switch or sensor signals that the wheelchair lift is in stow, thereby enabling the supply valve to open and pass compressed air from a supply port to a delivery port so that the park brake knob can apply and release the park brake. 
   SUMMARY OF THE INVENTION 
   The present invention arises in consequence of the recognition of the possibility that the supply valve could, for any of several reasons, cause the compressed air supply to be shut off to the park brake knob while the vehicle is being driven. Were that to happen, compressed air would become trapped in the line leading to the SAAR. As long as that pressure were to be maintained, the park brake would continue to be held released. Failure to maintain that pressure, such as through leakage, could however lead to unintended release of the SAAR, and hence possible application of the park brake while the vehicle is moving. 
   The present invention provides a solution that is intended to alert the driver to the possible incipiency of unintended application of the park brake in such a situation. 
   Possible reasons for unintended trapping of compressed air in the line to the SAAR while the vehicle is in motion include failure of a component or wiring in the portion of the electrical system associated with the lift, and loss of proper adjustment of a switch or switches in that portion of the electrical system for sensing that the lift has been stowed and/or any associated door has been fully closed. 
   Leakage from air lines in a vehicle like a truck or bus can occur at joints and connections along the lines, and normally small amounts of leakage, which may be virtually unnoticeable, are tolerable when all components of a particular system are fully functional. That is because the pressurized air source can make up for the leakage loss. In the case of the park brake system that has been described above, the potential exists for connections in the line to the SAAR to leak. 
   When the lift is indicated to be out-of-stow, resulting in compressed air becoming trapped in the line to the SAAR, and if a leak is present in that line, pressure loss from such a leak cannot be made up because the supply valve is not kept open by pilot pressure. A sufficiently large pressure loss in the air line to the SAAR will cause the park brake to be automatically applied. If the leak is small, the pressure loss may take a long time to occur, but in any event, trapping compressed air in the line to the SAAR while the vehicle is being driven is considered undesirable. 
   The present invention employs an efficient use of components and materials in conjunction with existing vehicle systems to provide a signal for alerting the driver to the possible incipiency of unintended application of the park brake due to loss of pressure in the line to the SAAR. 
   The invention contemplates that the driver, upon being alerted, will have sufficient time to drive to a suitable stopping place where the vehicle can be stopped and parked. 
   In a specific embodiment to be described here, a tee is connected into the air line to the SAAR, and a pressure switch is connected to the third port of the tee. The pressure switch is electrically connected to the vehicle electrical system. Advantageous use is also made both of existing information in the electrical system controller and of existing warning devices. 
   According to one generic aspect, the invention relates to a motor vehicle comprising a park brake for wheels on which the vehicle travels, a pressurized air source, and an air circuit for controlling an operating device for the park brake to unlock the wheels when pressurized air from the source is acting on the operating device through an air line leading to the device with nominal pressure that assures unlocking of the wheels and to lock the wheels when air pressure in the air line is less than some minimum pressure for keeping the wheels locked. 
   The vehicle has a lift that is selectively operable to an out-of-stow position that allows the lift to be used when the vehicle is parked and to a stowed position that does not allow use of the lift. A selectively operable lift interlock valve is connected in the air circuit for allowing pressurized air from the source to act on the operating device whenever the lift is indicated to be in stowed position and for blocking the air line leading to the device whenever the lift is indicated to be in out-of-stow position. 
   A pressure-sensitive device senses air pressure in the air line leading to the operating device. An indicator is operated by the pressure-sensitive device when air pressure in the air line leading to the operating device has decreased significantly from the nominal pressure, but is still greater than the minimum pressure, to indicate the possibility of imminent locking of the wheels. 
   The foregoing, along with further features and advantages of the invention, will be seen in the following disclosure of a presently preferred embodiment of the invention depicting the best mode contemplated at this time for carrying out the invention. This specification includes drawings, now briefly described as follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a fragmentary perspective view of a side of a vehicle, such as a passenger bus, showing a wheelchair lift in a deployed position. 
       FIG. 2  is a schematic diagram of an air circuit that is associated with a park brake knob and park brake, that has an interlock with the wheelchair lift shown in  FIG. 1 , and that embodies principles of the present invention. 
       FIG. 3  is a strategy diagram that implements principles of the invention in the vehicle. 
       FIG. 4  is a schematic electrical diagram including an electrical system controller (ESC) that executes an algorithm in accordance with the strategy diagram of  FIG. 3 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows an entry side of a passenger bus  10  from which a wheelchair lift  12  has been deployed to place a lift platform  14  generally horizontally on the adjacent ground surface  16  where a wheelchair (not shown) can be rolled onto and off it. After the wheelchair has been rolled onto platform  14 , a lift mechanism  18  is operated by a prime mover, such as a motor, to raise platform  14  to the level of a floor  20  of the bus, allowing the wheelchair to be rolled into the bus interior through a door opening  22 . Unloading of the wheelchair is accomplished in the opposite manner. When platform  14  is unoccupied, it and the associate mechanism connected to it can be operated to a stowed position inside the bus, possibly with platform  14  covering door opening  22  itself, or at least assuming a position that allows a separate door (not shown) to close door opening  22 . 
   Bus  10  has hydraulic service brakes at its wheels. It also has a park brake  24  (see  FIG. 2 ) that is held released by pressure of compressed air being applied to a SAAR (spring apply, air release) device  50  associated with each wheel that has a park brake mechanism. With bus  10  parked, the pressurized air in device  50  can be exhausted to release a spring whose force is then applied to lock the respective wheel. When the park brake is to be released, compressed air is delivered to device  50  to overcome the spring force being applied to the park brake mechanism at the respective wheel, thereby releasing the spring and unlocking the wheel. 
     FIG. 2  further shows an air circuit  26  that is associated with park brake  24 . An air tank  28  provides a source of pressurized air that is delivered into an air line  30  via one port of a tee  32 . Line  30  extends from a second port of tee  32  through a manifold  33  and then a pressure transducer  34  to a supply port of a pilot-operated supply valve  36  that has a delivery port connected to one port of a double check valve  38  by an air line  40 . 
   An air line  42  that branches at a tee  44  connects a second port of double check valve  38  to primary and secondary supply ports of a PPDC valve, or park brake knob,  45 . An air line  46  connects a third port of double check valve  38  to both a delivery port of PPDC valve  45  and a port of a quick release valve  48 , passing to the latter through manifold  33 . An air line  49  connects another port of valve  48  to a SAAR device  50 . A tee  52  in air line  46  provides for line pressure to be communicated to a park brake indication switch  54  that is normally closed (NC) and that is connected to the vehicle electrical system. Supply valve  36 , double check valve  38 , and park brake knob  45  collectively provide one example of a means for applying and releasing the park brake when the lift is in stow. 
   From a third port of tee  32 , air line  30  extends to an inlet port of a park brake interlock solenoid valve  56  that has a normally closed valve element  56 V and a solenoid  56 S that when energized operates valve element  56 V to open valve  56 . Solenoid  56 S is fed from the ignition terminal IGN of the vehicle ignition switch through a switch  37  that distinguishes between the lift being in stow and out-of-stow. With the vehicle engine running, the energization/de-energization of solenoid  56 S is controlled by switch  37 . 
   An air line  58  connects an outlet port of valve  56  through manifold  33  to a pilot port of supply valve  36 . An exhaust line  60  provides for venting from an exhaust port of valve  36 , as does an exhaust line  62  from an exhaust port of PPDC valve  45 . A park brake monitor switch  64  is communicated to pressure in air line  58  via a tee  66 . 
   Switch  54  is associated with an electrical system controller (ESC)  68  of the vehicle electrical system as shown in  FIG. 4 . ESC  68  comprises a processor that is programmed in accordance with an algorithm  70 , shown in  FIG. 3 , that iterates from time to time beginning at a start point  72 . 
   A step  74  of the algorithm monitors the condition of switch  54 . If the park brake is indicated as being released due to pressure in air line  46  being substantially nominal pressure provided by air tank  28 , a step  76  of the algorithm monitors vehicle speed, as broadcast on a data link of the electrical system. 
   If the vehicle is indicated to be moving, a step  78  of the algorithm monitors switch  54  to ascertain if pressure has been lost in air line  46  in an amount large enough to indicate a possible imminent unintended application of the park brake. A decrease from the nominal air tank pressure that is present in air line  46  when the lift is in stow due to valve  56  being open, to a pressure that while still greater than the minimum pressure needed to keep the park brake from being applied, is indicative of the possibility of imminent unintended application of the park brake. Such a decrease in pressure can occur in the following way. 
   When the park brake is released and the lift is in stow, air line  46  is pressurized from the air tank through supply valve  36  and valve  45 . If switch  37  is closed, thereby indicating the lift being in stow, solenoid  56 S is energized to keep valve element  56 V open for supplying tank air pressure to the pilot port of supply valve  36 . That allows the park brake to be applied and released by operation of park brake knob  45 . When switch  37  opens to indicate the lift having come out of stow, valve element  56 V closes, causing line  58  to exhaust through an exhaust port  56 E. Pilot pressure to supply valve  36  is thereby lost, and so it closes with line  40  being exhausted through valve  36  in the process. 
   Air pressure present in air line  46  then shuttles double check valve  38  to block it from air line  40 . With line  40  blocked, a closed loop comprising double check valve  38 , line  44 , valve  45 , and the portion of line  46  between the delivery port of valve  45  and valve  38  is created. In other words the entire portion of air circuit  26  from valve  38  to valve  48  is now shut off from air tank  28 . Air leakage from that shut off portion will cause pressure in air line  46  to decrease. When pressure reaches the low pressure threshold of PPDC valve  45 , it will “pop” and exhaust line  46 , thereby applying the park brake. 
   If pressure in line  46  decreases significantly from the nominal tank pressure to some predetermined pressure above the low pressure threshold of PPDV valve  45 , a step  80  activates an alarm to alert the driver. If not, the algorithm returns to step  72  via a step  82  that in the absence of any alarm being given, has no effect, but would be effective to de-activate an alarm that is being given. 
   Had step  74  indicated that the park brake was being applied, the algorithm would have not performed steps  76  and  78 , and instead would have returned to step  72  via step  82 . 
   Had step  76  indicated that the vehicle was not moving, the algorithm would have not performed step  78 , and instead would have returned directly to step  72 . 
   By using an existing alarm device or devices in the bus a separate additional alarm device is not required. Existing alarm devices are often capable of giving distinctive alarms for particular conditions, and it is preferred that such a device be used to give a unique alarm to warning of potential impending application of the park brake. 
   The invention can be implemented with minimal additional hardware and the connections into pre-existing electrical and air systems. Algorithm  70  can be implemented by suitable programming in a existing processor ESC  68 . 
   While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles of the invention apply to all embodiments falling within the scope of the following claims.