Abstract:
A power take off unit for a motor vehicle equipped with a standard transmission is clutched using compressed air available on the vehicle. Available air pressure is monitored and a clutch interlock implemented through the vehicle electrical control system is made responsive thereto to prevent engagement of the clutch if the load is too heavy for the available air pressure.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to power take off units for motor vehicles and more particularly to compressed air actuated clutch type power take off units. 
   2. Description of the Problem 
   Trucks and other commercial vehicles often come equipped with power take off (PTO) units operated at the election of the driver or operator. A (PTO) unit is coupled to the vehicle drive train using a clutch which is pressure actuated. On vehicles equipped with standard (manual) transmissions, compressed air from the vehicle&#39;s brake system is often used to close and clamp the PTO clutch. The air pressure available limits the clamping force generated. As a result, the maximum torque available through the clutch is limited by available air pressure. If the clamping force is too low for the PTO load, the clutch will slip resulting in damage to the clutch and a failure to carry the load on the PTO system. In the prior art pressure sensitive cut off valves have been incorporated into an air line between the brake system, compressed air system and the PTO unit to prevent operation of the PTO unit at low pressures. 
   SUMMARY OF THE INVENTION 
   According to the invention an air actuated power take off unit is inhibited from operation in response to detection of air pressure in the compressed air supply system falling below a minimum level. The invention is advantageously applied to a motor vehicle equipped with a standard transmission, air brakes, an air brake system pressure monitoring system, and a power take off unit employing a compressed air activated clutch. The clutch is actuated using compressed air from the vehicle&#39;s brake system. Pressure sensors for the brake system provide a standard method of monitoring air pressure which is required for the operation of such brakes. Clutch clamping force in the PTO unit is directly related to available air pressure. The interlock monitors air pressure and if air pressure fails to meet a minimum value, PTO operation is inhibited, or if the PTO unit is already engaged, it is disengaged. Under some circumstances the invention may be implemented through modification of body controller software and without hardware modification of the vehicle. Optionally another data link device may be used as a slave of the body controller to implement the invention. 
   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 side elevation of a truck equipped with a power take off system. 
       FIG. 2  is a schematic of a truck chassis illustrating some major truck systems. 
       FIG. 3  is a high level block diagram of a vehicle electronic control system. 
       FIG. 4  is a block diagram of an air actuated PTO clutch control arrangement in accordance with a preferred embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the figures and particularly to  FIG. 1 , a preferred embodiment of the invention will be described. A conventional flat bed truck  12  rides on a plurality of wheels  14 . A driver usually controls the vehicle from a cab  16  positioned in the forward portion of the vehicle. An auxiliary system  20 , which may be a winch or other device, is positioned on the flat bed  22  over the rear wheels. The auxiliary systems may take a number of forms, but is one of a class of devices operated by a power take off device (PTO)  19 . PTO devices are often powered off of the vehicle drive train where they are engaged to the drive train using a clutch. PTO device  19  is controlled from a panel  18  mounted on the bed just behind cab  16 . Panel  18  includes a switch for requesting power take off operation to operate winch  20 . 
     FIG. 2  illustrates the major vehicle systems involved in implementing the present invention. A thermal engine  24  is longitudinally mounted on the forward end of a chassis  23 . Power from the engine  24  is coupled through a standard (i.e. manual) transmission  26  mounted to the rear end of the engine. Propeller shaft  28  is connected between the transmission  26  and a differential  30  in the rear portion of chassis  23 . Transmission  26  in turn drives a propeller shaft  28  to propel the vehicle. Engine  24  also powers an air compressor  34  which provides compressed air by an air line  33  to an air storage tank  32 . Air compression systems have long been used on trucks and other commercial vehicles to power air brakes  36  and, as described above, air actuated clutches for PTO devices  19 . In accord with this practice air lines  37  and  35  are shown connected between air storage tank  32  and the air brakes  36  and air actuated clutch  40 , respectively. Air line  35  includes a valve  38  which allows air to pass from tank  32  to clutch  40  or to be exhausted from the clutch. Air actuated clutch has a power input  21  off of standard transmission  26  and an output  23  to PTO device  19 . Typically clutch  40  and PTO device  19  are one unit which is referred to as an air clutched PTO. 
     FIG. 3  is a block diagram of an electronic control system for truck  12 . A body controller  130  includes a microprocessor  72 , associated memory  74  and a SAE compliant J1939 network interface  73 . Body controller  130  coordinates multiplexed transmissions of signals on serial bus  142 . Serial bus  142  interconnects an auxiliary instrument and switch bank  112 , a gauge cluster  114 , an engine controller  20 , and anti-lock brake system (ABS) controller  102 . Additionally, instrument and switch bank  12  may be connected to cab controller  16  by a private data link  44 . All system components are powered by a vehicle electrical power system  45 . 
   Body controller  130  collects data from and issues control signals to discreet devices including a valve controller  138 , used to release air from storage to the air actuated clutch to force engagement of the clutch, a pressure sensor  140 , used to measure air pressure in tank  32  and a PTO request switch  242 , which is typically operator actuated. In some vehicles pressure sensor  140  may be connected to the engine controller  120  and readings therefrom transferred to body controller  130  over serial bus  142 . A data link controller  134 , if available, may operate as a slave to body controller  130  to operate the PTO valve controller  138 . Such a data link controller might be provided by an air solenoid module. 
   Referring now to  FIG. 4  a schematic diagram of the preferred embodiment of the invention is illustrated. Body controller  130 , as already described receives a request for power take off operation from a PTO request switch  242 . Body controller also periodically determines the pressure in air storage tank  32  using pressure sensor and transmitter  140 . PTO clutch  40  is actuated by air from the vehicle&#39;s brake system, meaning air tank  32 . The air pressure applied to PTO clutch  40  is controlled by a valve  38 , which can be positioned to exhaust air from the clutch or to release air from tank  32  to the clutch. Valve  38  is positioned by a valve controller  138 , which is typically implemented using a solenoid. Valve controller  138  is in turn responsive to the state of a control signal from body controller  130 . Body controller  130  issues a control signal to valve controller  138  for opening the valve to PTO clutch  40  when the pressure sensor  140  signal indicates that sufficient pressure is available to prevent clutch  40  from slipping when PTO device  19  is under load  88 . The clamping force of PTO clutch  40  is dependent upon air pressure. PTO clutch  40  has a default disengaged state and thus releases when valve  38  cuts off air pressure. Body controller  38  may be programmed to move valve  38  from applying pressure to cut off should at any time pressure drops below the desired minimum level during PTO operation and body controller may be further programmed to reengage the clutch should air pressure in tank  32  recover to a point above the minimum level. 
   The present invention simplifies installation of air clutched PTO units on vehicles with standard transmissions, eliminating the need for a pressure sensitive cut off valve in the air line to the air clutched PTO unit. The minimum operating pressure is readily adjusted through changes in programming of the body computer. 
   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.