Abstract:
An electronic transmission control system that can achieve a transmission ratio based on the operator inputs and the current vehicle operating conditions. The transmission constantly connects the engine to the load, and the transmission ratio is only varied by a change in command from the present invention. The transmission&#39;s mechanical function is solely to vary the ratio between its input and output. In using the present invention, an operator must select an operating mode, either automatic or manual, using a two-position switch. While in the automatic mode, the present invention determines the vehicle speed by considering the position of the throttle and the operator&#39;s use of brakes. In the manual mode, the present invention further considers the operator&#39;s selection of a gear condition.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation of application Ser. No. 10/435,892 filed May 12, 2003 now U.S. Pat. No. 6,852,064. 
   This application is based upon Applicants&#39; Provisional Application Ser. No. 60/396,653 filed Jul. 18, 2002. 

   BACKGROUND OF THE INVENTION 
   This invention relates generally to hydromechanical transmissions and, more particularly, to electronic control systems for hydromechanical transmissions. 
   Hydromechanical transmissions (HMTs) have been developed for vehicles to replace conventional automatic belt drive transmissions. In particular, HMTs have been developed for use with all-terrain vehicles (ATVs). The advantages of HMTs include increased power capacity, greater durability, and protection from environmental degradation. Even though the mechanical implementation and functionality of HMTs is very different from conventional belt-driven units, consumers prefer that vehicles drive and feel like conventional belt-driven units while still offering the advantages of HMTs. 
   Conventional belt drive transmissions use a centrifugal clutch or slipping belt to smoothly accelerate the vehicle from rest. Smooth startup conditions, however, are difficult to achieve with HMTs. 
   Another disadvantage of HMTs is the inability to react quickly to a dynamic operating environment. ATVs operate at a wide range of speeds, from creeping speeds to as fast as 90 km/hr. In addition, ATVs are used for a variety of functions, from racing to pulling heavy loads. Further, ATVs often are used on a wide variety of ground surfaces. HMTs often have difficulty reacting quickly to these factors, producing a harsher ride than conventional belt-driven units. 
   Yet another disadvantage of HMTs is the inability to react to operator-controlled braking systems. HMTs typically provide very little dynamic braking capability and therefore must be protected from overspeed during vehicle deceleration. 
   It is therefore a principal object of this invention to provide an electronic control system for HMTs that allows for smooth startup conditions. 
   A further object of this invention is to provide an electronic control system for HMTs that allows for quicker reaction to a dynamic operating environment. 
   Still a further object of this invention is to provide an electronic control system for HMTs that allows for an improved reaction to operator-controlled braking systems. 
   These and other objects will be apparent to those skilled in the art. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention comprises an electronic transmission control system designed to achieve a transmission ratio based on the operator inputs and the current vehicle operating conditions. The invention is intended for HMTs; however, the present invention also may be used with pure hydrostatic transmissions or any other transmission system that provides an infinitely variable transmission ratio from zero to maximum output speeds. 
   Because of the present invention&#39;s ability to provide a smooth startup condition, the present invention is best suited for use with dynamic operating conditions. In particular, the present invention reacts quickly to rapidly changing load and operation characteristics. Further, the present invention is best suited for use with high speed vehicles. The invention is intended for use with ATVs; however, the present invention also may be used with other types of vehicles, both large and small. 
   The present invention is optimized for ratio-controlled HMTs. In such an arrangement, the transmission constantly connects the engine to the load, and the transmission ratio is only varied by a change in command from the electronic control system. The transmission&#39;s mechanical function is solely to vary the ratio between its input and output. This is different from conventional transmissions, which use a torque or load-sensitive device, such as a slipping belt, centrifugal clutch, pressure-modulated clutch, or torque converter, to achieve a smooth startup condition. 
   In using the present invention, an operator must select an operating mode, either automatic or manual, using a two-position switch. While in the automatic mode, the present invention determines the vehicle speed by considering the position of the throttle and the operator&#39;s use of brakes. In the manual mode, the present invention further considers the operator&#39;s selection of a gear condition. Both modes of operation require the operator to select a range gearbox condition, such as forward low, forward high, reverse, neutral, or park. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an overall system diagram of the invention; 
       FIG. 2  is a more detailed diagram showing the swashplate position control, setpoint calculation, and engine load monitor; 
       FIG. 3  is a diagram illustrating the automatic mode; 
       FIG. 4  is a diagram illustrating the manual mode; and 
       FIG. 5  is a graph for manual mode startup plotting setpoint versus engine command. 
   

   DESCRIPTION OF THE INVENTION 
   With respect to  FIG. 1 , an electronic transmission control system  10  is disclosed that achieves a transmission ratio based on the operator inputs and the current vehicle operating conditions. The electronic transmission control system  10  works to control HMT  12 , which connects a vehicle engine  14  to the vehicle wheels  16 . 
   HMT  12  includes a pump  18  connected to a motor  20  by closed loop  22 . Pump  18  is connected to a driven gear  24  rotated by driving gear  26 , which is connected to a crank shaft  28 . Motor  20  is connected to gear  30 , which is connected to planetary gear set  32  and works to drive wheels  16 . 
   A glossary of terms for use in describing the control system  10  appears below: 
   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               Term 
               Description 
             
             
                 
                 
             
           
           
             
                 
               Automatic Mode 
               Electronic control system 
             
             
                 
                 
               automatically sets transmission 
             
             
                 
                 
               ratio. 
             
             
                 
               Brake Command 
               Sensed position of operator&#39;s 
             
             
                 
                 
               brake commanded (typically a 
             
             
                 
                 
               lever or pedal). 
             
             
                 
               Commanded Engine Speed 
               Throttle position that has been 
             
             
                 
                 
               converted to RPMs. This is an 
             
             
                 
                 
               approximate curve based on no 
             
             
                 
                 
               engine load. 
             
             
                 
               Current Engine Speed 
               Actual measured engine speed. 
             
             
                 
               Engine Load Monitor (ELM) 
               Control system block that 
             
             
                 
                 
               reduces swashplate command 
             
             
                 
                 
               during load conditions. 
             
             
                 
               Gear Command 
               Operator selected gear in 
             
             
                 
                 
               manual mode. 
             
             
                 
               Manual Mode 
               Electronic control system sets 
             
             
                 
                 
               transmission ratio based on the 
             
             
                 
                 
               Gear Command. Simulates a 
             
             
                 
                 
               transmission with a series of 
             
             
                 
                 
               discrete gear ratios. 
             
             
                 
               Set Point Calculation 
               Control system block that 
             
             
                 
               Block (SPCB) 
               calculates the desired 
             
             
                 
                 
               swashplate setpoint. 
             
             
                 
               Swashplate Setpoint 
               Calculated swashplate desired 
             
             
                 
                 
               position, determined by the 
             
             
                 
                 
               SPCB. 
             
             
                 
               Throttle Position 
               Sensed position of operator&#39;s 
             
             
                 
                 
               throttle commanded (typically a 
             
             
                 
                 
               lever or pedal). 
             
             
                 
               Vehicle Situation 
               Either accelerating or 
             
             
                 
                 
               decelerating. 
             
             
                 
               Vehicle Speed 
               Measured vehicle speed. 
             
             
                 
                 
             
           
        
       
     
   
   The control system  10  has two modes of operation, automatic  34  and manual  36 . The operator selects the mode of operation using a two-position switch (not shown). 
   With respect to  FIG. 3 , the automatic mode  34  of operation is shown. In the automatic mode  34 , the operator adjusts the throttle position  38  to achieve a desired commanded engine speed  40 . In addition, the operator may apply the vehicle brakes  42  either to slow down or completely stop the vehicle. Further, the operator adjusts the range selection control  44  to select the range gearbox  46  condition, including forward high, forward low, reverse, neutral, and park (FIG.  1 ). The range gearbox  46  also may include reverse low and reverse high conditions. A reverse creep condition may be achieved by stroking the swashplate further into the stroke. 
   The electronic control unit  48  ( FIG. 1 ) takes the operator inputs and uses them to achieve a transmission ratio. Specifically, the throttle position  38 , which is converted into a digital or electrical signal by a sensor (not shown), is translated into the commanded engine speed  40  by comparing the throttle position to a predicted no-load engine RPM. The electronic control unit  48  determines the throttle position  38  and then estimates what the engine speed would be in an unloaded condition. The relationship between the throttle position  38  and the predicted no-load engine RPM is typically non-linear and is defined in the Position vs. RPM Profile software module  50  (FIG.  2 ). The electronic control unit  48  also considers the vehicle speed  52  in addition to the brake command  42  discussed above. 
   The electronic control unit  48  includes a setpoint calculation block (SPCB)  54 , which takes the commanded engine speed  40 , brake command  42 , and vehicle speed  52  as inputs. The SPCB  54  determines the vehicle situation  56 , which is either accelerating or decelerating. The SPCB  54  then uses an algorithm  58  ( FIG. 2 ) to calculate the swashplate setpoint  60  based on the vehicle situation  56 . In either an accelerating or decelerating vehicle situation  56 , the swashplate setpoint  60  can be modified through a time-based dynamic ramp within the SPCB  54 . 
   If the SPCB  54  determines the vehicle situation  56  to be accelerating, then the electronic control unit  48  also uses the swashplate position control  62  in determining the swashplate setpoint  60 . The swashplate position control  62  uses the swashplate setpoint  60  and the actual swashplate position  64  to generate a signal for the swashplate control  66 , which provides closed loop swashplate position feedback. The swashplate position control  62  takes the engine speed  68  and brake command  42  as inputs and compares them against a Commanded Engine Speed vs. Swashplate Setpoint Profile. When the brakes are applied, the brake command  42  overrides the requested setpoint  60  to slow the vehicle. 
   If the SPCB  54  determines the vehicle situation  56  to be decelerating, then the swashplate setpoint  60  is based on the actual vehicle speed  52 . In this situation, a Vehicle Speed vs. Swashpoint Setpoint Profile is used. When the brakes are applied, the brake command  42  overrides the requested setpoint  60  to slow the vehicle. 
   The electronic control unit  48  also includes an engine load monitor (ELM)  70 . ELM  70  takes the commanded engine speed  40 , current engine speed  68 , and the vehicle speed  52  as inputs to determine the engine load condition. The output of the ELM  70  reduces the raw setpoint  60  in the case of excessive load. ELM  60  also produces a downshift behavior during re-acceleration. Because of ELM  60 , the engine speed  68  increases with the vehicle speed  52 . This creates a desirable feel to the vehicle, whereby the operator perceives that the vehicle speed  52  is increasing as a function of the increasing engine speed  68 . 
   With respect to  FIG. 4 , the manual mode  36  of operation is shown. Similar to the automatic mode, the operator adjusts the throttle position  38  to achieve a desired commanded engine speed  40 . In addition, the operator may apply the vehicle brakes  42  either to slow down or completely stop the vehicle. Further, the operator adjusts the range selection control  44  to select the range gearbox  46  condition, including forward high, forward low, reverse, neutral, and park (FIG.  1 ). In the manual mode, the operator also adjusts a gear selector  72  to limit or set the gear ratio. There are typically between four and six simulated gear ratios from which the operator may choose by selecting the shift up  74  or shift down  76  condition (FIG.  2 ). 
   As with the automatic mode, the SPCB  54  takes the commanded engine speed  40 , brake command  42 , and vehicle speed  52  as inputs. In the manual mode, the SPCB  54  also takes the gear command  72  as an input. The SPCB  54  determines the vehicle situation  56 , which is either accelerating or decelerating. The SPCB  54  then uses an algorithm  58  ( FIG. 2 ) to calculate the swashplate setpoint  60  based on the vehicle situation  56 . In either an accelerating or decelerating vehicle situation  56 , the swashplate setpoint  60  can be modified through a time-based dynamic ramp within the SPCB  54 . 
   If the SPCB  54  determines the vehicle situation  56  to be accelerating, then the electronic control unit  48  uses the engine speed  68 , brake command  42 , and gear command  72  to calculate the swashplate setpoint  60 . In this case, the Commanded Engine Speed vs. Swashplate Setpoint Profile is used. When the brakes are applied, the brake command  42  overrides the requested setpoint  60 , thereby limiting the maximum transmission ratio and vehicle speed. 
   If the SPCB  54  determines the vehicle situation  56  to be decelerating, then the electronic control unit  48  uses the actual vehicle speed  52  and the gear command  72  to calculate the swashplate setpoint  60 . In this case, the Vehicle Speed vs. Swashplate Setpoint Profile is used. When the brakes are applied, the brake command  42  overrides the requested setpoint  60  to slow the vehicle. The gear command  72  limits the swashplate setpoint  60 . 
   The electronic control unit  48  also uses the ELM  70  to determine the engine load condition. ELM  70  takes the commanded engine speed  40 , current engine speed  68 , and the vehicle speed  52  as inputs. The output of the ELM  70  reduces the raw setpoint  60  in the case of excessive load and produces a downshift behavior during re-acceleration. The swashplate position control  62  uses the output of the ELM  70  as well as the actual swashplate position  64  to generate a signal for the swashplate control  66  (FIG.  2 ), which provides closed-loop swashplate position feedback. 
   In operation, the electronic transmission control system  10  quickly reacts to a wide variety of vehicle dynamics and operating conditions. The electronic transmission control system  10  can operate from creeping speeds up to a maximum vehicle speed  52  of 90 km/hr. without changing transmission modes. 
   Further, the elimination of a centrifugal clutch allows the electronic transmission control system  10  to achieve zero vehicle speed. HMT  12  can be designed to achieve zero output speed by the selection and arrangement of planetary ratios and hydrostatic component sizing. The swashplate position control  62  then uses a zero speed offset to command the HMT  12  to zero speed. Holding zero speed also can be accomplished by measuring the speed and direction of the control leg  30  of the planetary gear set  32  (FIG.  1 ). This offers an advantage over a centrifugal clutch because the electronic transmission control system  10  can hold the vehicle at zero speed independent of the load, even on steep slopes. 
   Because the electronic transmission control system  10  does not use a centrifugal clutch, the system  10  does not have inherent mechanical or hydraulic characteristics to provide a smooth startup condition. The smooth startup condition is achieved through use of the time-based dynamic ramp within the SPCB  54 . In the automatic mode  34 , the electronic transmission control system  10  can achieve a smooth startup condition using a dynamic ramp based on the vehicle speed  52 . In the manual mode  36 , the control system  10  can achieve a smooth startup condition by using a short automotive curve  78  combined with a fixed ratio  80 , as shown in FIG.  5 . 
   In another embodiment, the electronic transmission control system  10  can achieve a smooth startup condition by using a hydro loop variable bypass valve  82  (FIG.  1 ). A hydro loop variable bypass valve  82  connects the two sides of the hydrostatic power loop  22  together only when commanded. This interconnection reduces the torque transmitting capacity of the hydrostatic units, and therefore can help modulate the vehicle startup condition. The bypass valve  82  may be infinitely variable or may operate in an on/off arrangement. The bypass valve  82  also may be used to quickly reduce engine load when the brakes  42  are applied. This provides smoother deceleration and reduces engine lug-down and stalling during hard braking. Alternatively, the control system  10  may also be adapted to use a brake sensor (not shown) to help prevent stalling during hard braking. Such a sensor may be used to synchronize the HMT  12  with the brakes  42  to avoid fighting between them. 
   The electronic transmission control system  10  also provides very little engine dynamic braking. Some engines  14 , particularly low power recreational and utility vehicles, have very little capacity to absorb power during vehicle deceleration. If the transmission ratio is decreased too quickly, excessive torque might be applied to the engine  14  resulting in overspeed and damage. Because the SPCB  54  determines the vehicle situation  356 , the control system  10  recognizes when the vehicle is decelerating. The ELM  70  inputs the actual vehicle speed  52  and uses the Vehicle Speed vs. Setpoint Profile to continually adjust the transmission ratio to decelerate the vehicle without over-speeding the engine. 
   From the foregoing, it is seen that this invention will accomplish at least all of its stated objectives.