Abstract:
An interlock for a vehicle platform lift meets the Federal Motor Vehicle Safety Standards (FMVSS 403) effective in late 2004 with minimum modification of vehicle electronic control arrangements. The interlock prevents forward and rearward mobility of vehicles unless the platform lift is stowed. The interlock operates to prevent accidental release but does not affect vehicle movement when the platform lift is stowed. Operation of the lift is allowed by placing the transmission in park or by placing the transmission in neutral and actuating the parking brake. A vehicle body computer controls operation when the vehicle is running. A bypass enable relay allows use when the vehicle&#39;s engine is not running.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The invention relates to interlocks for vehicle platform lifts. 
   2. Description of the Problem 
   Federal Motor Vehicle Safety Standards (FMVSS 403) which come into effect in late 2004 require interlocks to prevent forward and rearward mobility of vehicles unless the platform lift is stowed. These interlocks must discourage accidental release and not affect vehicle movement when the platform lift is stowed. Operation of the lift may be permitted with the interlock inhibiting forward and rearward movement of the vehicle, which is to be done by placing the transmission in park or by placing the transmission in neutral and actuating the parking brake or vehicle service brakes by means other than the operator depressing the vehicle&#39;s service brake pedal. 
   SUMMARY OF THE INVENTION 
   According to the invention there is provided an interlock for a vehicle platform lift. The interlock is based on logic implemented through a body computer and a set of enable and power relays. A wheelchair door switch having a state controlled by the position of a wheelchair door on the vehicle side is also provided. The body computer is coupled to sense the state of the wheelchair door switch. The body computer is further connected to a data network which transmits data to the body computer from a plurality of controllers coupled to the data network. The controllers report states for a parking brake, a transmission and the engine ignition status. The body computer is programmed to respond to an open state of the wheelchair door, the parking brake being set and the transmission being in park or neutral for generating an enable signal. The enable signal is applied to a first enable relay for energizing the first enable relay. The interlock further includes a vehicle ignition switch having keyon and keyoff positions. A second enable relay is connected to the ignition switch for energization in response to placement of the ignition switch in the keyoff position. The vehicle platform lift is coupled to a power relay for receiving vehicle power. The power relay state is in turn controlled by either of the enable relays. The power relay has a high side input connected to the first and second enable relays and a low side input connectable to ground through the wheelchair door switch. 
   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 perspective view of a bus to which the present invention is advantageously applied. 
       FIG. 2  is a block diagram of the interlock circuit for a platform lift system. 
       FIG. 3  is a flow chart illustrating the logic implemented using a body computer interacting with the circuitry of the interlock system. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , a school bus  10  having a body  12  and a side or wheelchair access door  14  through the body is shown. A conventional platform lift (not shown) is used to load and unload wheelchairs from school bus  10 . The lift may be extended from side door  14  when the side door is opened. Side door  14  must be opened to use the platform lift and the interlock of the present invention includes a switch operated by movement of the door. 
   Referring now to  FIG. 2 , a block diagram of the vehicle platform lift interlock  20  is illustrated. In its preferred form, platform lift interlock  20  incorporates intelligent control capabilities which have become increasingly popular in motor vehicles of the early 21 st  century. The term “intelligent control capabilities” refers to the use of control modules/controllers associated with major body systems and a network to link various control modules with one another and with a programmable body computer. As illustrated here, an SAE J1939 controller area network (CAN)  25 , comprising a twisted pair of wires and built in CAN controllers for various controllers, provides for data exchange between the controllers and a body computer  30 . Only those controllers involved in implementing the interlock are illustrated. These include an engine controller  32 , a transmission controller  34 , an intelligent brake system or brake system controller  36  (typically an anti-lock brake system (ABS) controller) and a instrument and gauge system cluster controller  38 . Intelligent brake system controller  36  monitors the status of parking brake  40 , which reports the status as a standard message type over the bus  25  for detection by body computer  30 . Transmission controller  34  similarly reports the status of transmission  42  over the J1939 bus  25 . This signal is typically intended for the engine controller  32 , which passes it by way of the bus to body computer  30 . Body computer  30  also transmits messages over bus  25 , particularly a signal to instrument and gauge cluster controller  38  for activating an alarm under certain circumstances as spelled out below. 
   Body computer  30  includes output ports which allow the direct attachment of the body computer to devices requiring power and the body computer includes input ports allowing it to detect the state of switches connected to the body computers input ports. One such switch connected directly to a body computer  30  input port is a wheelchair door switch  44  which pulls the port to ground when the switch is closed. Body computer  30  is programmed to associate a low or grounded signal on the port with a door open condition. 
   The simplest case of operation does not involve body computer  30 . A wheelchair lift mechanism  60  incorporates the motor, motor and valve control circuitry and lift mechanism. Power is available to the wheelchair lift mechanism  60  if the ignition  54  is in its keyoff position. Moving ignition  54  to the keyoff position results in the high side input of single pole double throw relay  52  going low, which in turn results in relay  52  closing and allowing power to be coupled from battery  48  through relay  52  to the high side input of power relay  50 . If switch  44  is simultaneously closed, then the low side input of power relay  50  is pulled to ground through diode  56 , which is connected between the low side input and switch  44 . This energizes relay  50  resulting in battery power being available to wheelchair lift mechanism  60  through relay  50 . Opening door switch  44  (which occurs when the door  14  is closed) deenergizes power relay  50  and opens the power relay, which interrupts power to the wheelchair lift mechanism  60 . 
   When the ignition  54  is in the keyon position body computer  30  controls the application of power to wheelchair lift mechanism  60  through relays  46  and  50 . Body computer  30  determines from signals received over J1939 compliant CAN bus  25  and an input port connected to wheelchair door switch  44  if the conditions required for permitting operation of the wheelchair lift  60  are met. Firstly, transmission  42  must be in neutral. The status of transmission  42  is reported over CAN bus  25  by transmission controller  34 , picked up by the engine controller  32  and reformatted for receipt by the body computer  30 . Those skilled in the art will of course realize that the body computer  30  may be programmed to detect the output of the transmission controller  34  directly, however, in some vehicles, the transmission controller may communicate over a private bus to the engine controller rather than transmitting over a public bus  25 . Engine controller  32  also reports engine  33  ignition status to body computer  30 . Parking brake  40  is required to beset. In the preferred embodiment the status of the parking brake  40  is determined by the intelligent brake system controller  36 , and is reported over CAN bus  25 . In some vehicles parking brake status is fed directly to a body computer  30 . Finally, the body computer input from switch  44  must be pulled low by closure of the switch. With all of the conditions met body computer  30  will connect its relay driver output port to ground energizing relay  46 . Body computer  30  is also programmed to activate an alarm  144  under the control of instrument and gauge cluster controller  38  should an attempt be made to operate the wheelchair lift mechanism  60  without first meeting the selected conditions. 
   Relay  46  is connected with its high side input connected to battery  48  and its low side input to the relay driver output port of body computer  30 . Energization of relay  46  results in the relay connecting battery  48  to the high side input of power relay  50 . Operation of the power relay  50  does not depend upon whether its high side is connected to battery power through relay  46  or relay  52 . Power relay  52  operates to connect the input of wheelchair lift mechanism  60  to vehicle power as already described. 
   Referring to  FIG. 3  a flow chart is used to describe the interlock routine  70  implemented by the programming of body computer  30  and the associated relay circuitry. Interlock routine  70  is involved when the vehicle is powered. An attempt to operate wheelchair lift mechanism  60  invokes decision step  72 , which looks the status of the wheelchair door switch  44 . It the door  14  is closed (i.e. switch  44  open) the NO branch is followed to step  86  which provides for deactivating the relay driver output of body computer  30  and exit from the routine. Where the door  14  is opened and switch  44  closed, the YES branch is followed from decision step  72  to decision step  74 . Step  74  looks for the presence of an acceptable status report from the intelligent brake system controller  36 . It the report is not present, the routine is exited. If the report is present the YES branch is followed to step  76  where ignition status as reported by the engine controller is checked. If the engine is not running the status of the transmission is unimportant and steps  78  and  80  relating to transmission status may be omitted. If the ignition status is high, steps  78  and  80  are executed to determine if the transmission controller is transmitting good data and is it reporting the status of the transmission to be either in park or in neutral. If transmission data is not good the routine is exited. If the transmission is not in the proper state an alarm is sounded (step  80 ) before the routine is exited. Following the YES branch from step  80  (transmission status in park/neutral) or the NO branch from step  76  (ignition not active) step  82  is executed to determine if the park brake has been set. If not, step  88  is executed to issue an alarm and the routine is exited. If the park brake has been set the relay driver output of body computer  30  is coupled to ground before the routine is exited. 
   The invention meets the interlock requirements of the federal motor vehicle safety standards with a minimum of modification of existing vehicle control electronics. 
   While the invention is shown in only one 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.