Abstract:
A total fluid power system for an engine powered vehicle includes an engine driven, variable displacement pump, a pair of variable displacement wheel motors and auxiliary functions. Transducers sense engine speed, pump output pressure and motor speeds to supply signals to pump and motor controls which process the signals with operator input signals to maintain pump and motor displacements for optimum engine power usage and desired vehicle performance.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to hydraulic systems and more particularly to a regulated pressure hydraulic system using variable displacement motors and pump, with a closed loop control system. 
     In the past, where many power outlets were desired from a single engine in a vehicle, the practice was to include a number of mechanical transmissions in addition to a number of hydraulic pumps to supply power to remote hydraulic motors. This was true even though there were only one engine and only 100% of its horsepower could ever be transmitted at any one time. 
     Further, extremely complicated control systems were required when many functions with varying requirements were included in the system or when the speed of a motor had to be regulated or where maximum performance from limited power was required. 
     SUMMARY OF THE INVENTION 
     The present invention provides a regulated pressure hydraulic system utilizing a closed loop variable displacement pump and a plurality of closed loop variable displacement motors in which the displacement of the pump is varied so as to maintain a regulated pressure to the motors which are responsive to a control signal, inlet pressure, loads and engine power. The pump output flow is controlled to regulate system pressure and the displacement of the motors are controlled to regulate the output torques and speed of the motors. 
     The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description taken in conjunction with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a portion of the total fluid power system in which the present invention is embodied; and 
     FIG. 2 is a schematic illustration of the remainder of the total fluid power system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, therein is shown a total fluid power system 10 which is generally incorporated in a hydrostatic drive vehicle (not shown). The total fluid power system 10 of the preferred embodiment includes a hydraulic system generally designated by the numeral 12 and an electrical control system generally designated by the numeral 14. 
     The hydraulic system 12 includes a fluid reservoir 16 which supplies fluid to a conventional charge pump 18 which is driven by the vehicle&#39;s prime power source, an engine 20. The engine 20 further drives a variable displacement main pump 22, the inlet of which is connected to the outlet of the charge pump 18. The outlet of the main pump 22 is connected to an outlet line 24 which in turn is connected to a relief valve 26, first and second inlet lines 28 and 30, and a flow control valve 32. The inlet lines 28 and 30 are connected respectively to first and second motors 34 and 36. The motors 34 and 36 are variable displacement, variable speed fluid motors which have output shafts drivingly connected to the drive wheels (not shown). 
     The first and second motors 34 and 36 can vary displacement from plus 100% to minus 100% and can go through zero displacement and reverse direction without reversing the direction of fluid flow therethrough. Th flow control valve 32 is connected to a conventional intermittent operation fluid function generally designated by the numeral 38. The relief valve 26, the first and second motors 34 and 36, and the function 38 are connected to a reservoir line 40 which is connected through a check valve 42 between the charge pump 18 and the main pump 22, and to a reservoir 16 through a pressure regulating valve 44. 
     Referring now to the electrical control system 14, therein is shown a motor control 50 and a main pump control 54. The motor control 50 includes an operator input control 56 containing a non-linear transducer 60 and rate limiter circuitry 64. The rate limiter circuitry 64 is connected to first inputs of speed error summing junctions 68 and 70. The second inputs of the speed error summing junctions 68 and 70 are connected respectively to motor speed transducers 72 and 74 which sense the speed and direction of rotation of the output shafts of the first and second motors 34 and 36. The outputs of the speed error summing junctions 68 and 70 are connected to speed error amplifiers 76 and 78, respectively, and thence to conventional multipliers 80 and 82. The multipliers 80 and 82 are connected, respectively, to first and second motor displacement controls 84 and 86. 
     The first and second motor displacement controls 84 and 86 are closed loop controls containing speed error displacement summing junctions 88 and 90 whose first inputs are connected to the multipliers 80 and 82, respectively, and whose outputs are connected respectively to motor displacement error amplifiers 92 and 94. The amplifiers 92 and 94 drive conventional valve and actuator systems 96 and 98 which, respectively, operate to change the displacement of the first and second motors 34 and 36, respectively. The motor displacement controls 84 and 86 further include motor displacement transducers 100 and 102, respectively, which are positioned to sense the actual displacement of the first and second motors 34 and 36, respectively. The displacement transducers 100 and 102 are further connected to the second inputs of the speed error-displacement summing junctions 88 and 90 to provide the feedback signal for closed loop control. 
     Referring now to the main pump controls 54, therein is shown a high torque control 103 which includes selector circuitry 104 which is connected to the outputs of the speed error summing junctions 68 and 70 and which passes on the larger of the two inputs. The output of the selector circuitry 104 is connected to a speed error amplifier 106 and thence to the first input of a gate 108. The second and third inputs of the gate 108 are connected respectively to the displacement transducers 100 and 102 in the first and second motor displacement controls 84 and 86. The output of the gate 108 is connected to rate limiter circuitry 110. 
     The rate limiter circuitry 110 is connected to the first input of pressure error summing junction 112. The second input of the summing junction 112 is connected to a conventional pressure transducer 114. The pressure transducer 114 is fluidly connected to the outlet line 24 and electrically connected through a function generator 116 to the multipliers 80 and 82 in the first and second motor controls 50 and 52, respectively. The third input of the summing junction 112 is connected to pressure reference circuitry 118. The output of the summing junction 112 is connected to the input of a pressure error amplifier 120 and thence to a multiplier 122. The multiplier 122 is connected in turn to a pump displacement control generally designated by the numeral 124. 
     The pump displacement control 124 includes a pressure error-displacement summing junction 126, the first input of which is connected to the multiplier 122 and the output of which is connected to a pump displacement error amplifier 128. The pump displacement error amplifier 128 is thence connected to a conventional valve and actuator system 130 for changing the displacement of the variable displacement main pump 22. A pump displacement transducer 132 is connected to the pump 22 to sense its displacement and provide displacement feedback to the second input of the pump displacement error summing junction 126. 
     The electrical control system 14 further includes an engine feedback system generally designated by the numeral 134 which includes reference engine speed circuitry 136 which is connected to the first input of a function generator 138. A second input of the function generator 138 is connected to an engine speed transducer 137. The output of the function generator 138 is connected to the multiplier 122. 
     In operation, the speed is changed by the operator providing manual speed command inputs to the operator input control 56. The input to the non-linear transducer 60 provides a forward signal therefrom which follows a predetermined curve shaped to give better resolution at low speeds, i.e. for the same amount of adjustment, there is less of a speed change at lower speeds. The non-linear transducer 60 output signal then goes to the rate limiter 64 where the rate of change of the signal is limited to prevent undesirable acceleration or decelerations of the vehicle, irregardless of how fast the operator input control 56 is moved, but which does not provide an undesirable lag. The resulting forward input signal is then summed in the speed error summing junction 68 with a feedback signal proportional to the actual motor speed from the speed transducer 72. 
     Where the operator input was intended to speed up the first motor 34, a speed error signal will be produced at the speed error summing junction 68 and will be amplified by the speed error amplifier 76, processed by the multiplier 80 as will later be described, and inputted into the speed error-displacement summing junction 88. The summing junction 88, when summing the speed error signal from the multiplier 80 and the unchanged displacement signal from the displacement transducer 100, will provide an increase displacement signal which will be amplified by the displacement error amplifier 92 and supplied to the valve and actuator 96 so as to increase the displacement of the first motor 34. As the displacement of the first motor 34 increases, the displacement transducer 100 will feed back the displacement signal thus causing the motor displacement to be proportional to the amplified speed error signal. 
     It is well known that the torque of a variable displacement motor is proportional to its displacement and input pressure. However, the first effect of the increased displacement is to reduce the pressure in the outlet line 24. The pressure reduction is sensed by the pressure transducer 114 which produces and feeds back a pressure signal proportional to the reduction in pressure to the pressure error summing junction 112 where it will be compared with a pressure reference signal from the pressure reference circuitry 118 which provides a predetermined standby operating pressure in the outlet line 24. A pressure error signal indicating the lack of sufficient operating pressure will be sent from the pressure error summing junction 112 to the pressure error amplifier 120 and thence to the multiplier 122. Provided that the difference between the actual engine speed and the engine speed reference signal is within limits, as will be later described, the pressure error signal will be further inputted to the pressure error-displacement summing junction 126 where it will be summed with the unchanged displacement signal from the displacement transducer 132 to produce an increase displacement signal. The increase displacement signal will be sent to the pump displacement error amplifier 128 and thence to the valve and actuator system 130 to cause the displacement of the main pump 122 to increase so as to increase the pressure and fluid flow in the outlet line 24 until the increase displacement signal is eliminated by the feedback signal from the displacement transducer 132. The resulting increased torque of the first motor 34 causes the speed of the motor to increase. The speed of the first motor 34 is fed back from the speed transducer 72 in the form of an increasing signal to the speed error summing junction 68 to reduce the speed error signal and establish the new pump and motor displacements for the new speed. 
     Conversely, the first motor 34 is slowed by having the operator input control 56 cause the first motor displacement control 84 to decrease the displacement of the first motor 34. A decrease in the displacement of the first motor 34 will cause the speed of the motor to decrease. This in turn will cause an increase in pressure which will be fed back to the pump displacement control 124 to cause the main pump 22 to decrease its displacement and output flow, to maintain the system pressure desired. 
     In operation, when the first and second motors 34 and 36 are operating at a constant speed, the imposition of a load requiring increased torque from either or both of the motors 34 and 36 will cause a change in the speed of the motors. 
     For example, when the load is imposed on the first motor 34, the motor 34 will be slowed and the pressure in the inlet line 28 and thus the outlet line 24 will be increased. As the pressure increases in the outlet line 24, the pressure transducer 114 increases the signal to the pressure error summing junction 112 causing the pump displacement control 124 to decrease the displacement of and fluid flow from the pump 22. With the pump fluid flow decreased, the speed of the first motor 34 decreases further which causes a decrease in the feedback signal from the speed transducer 72 to the speed error summing junction 68. The speed error summing junction 68 will then cause the first motor displacement control 84 to increase the displacement of the first motor 34 which results in a pressure decrease thereat. This pressure decrease is then fed back from the pressure transducer 114 to the pressure error summing amplifier 112 which causes the pump displacement control 124 to increase the displacement of the pump 22 and thus speed up the first motor 34 to its previous speed with increased torque output. 
     Conversely when the loads drops off, the first motor 34 will increase in speed and the pressure in the outlet line 24 will decrease. As the pressure in the outlet line 24 decreases, the pressure transducer 114 decreases the pressure signal to the pressure error summing junction 112 causing the pump displacement control 124 to increase the displacement of the pump 22. With the pump displacement and output flow increased, the speed of the first motor 34 increases further and increases the signal from the speed transducer 72 to the speed error summing junction 68. The speed error summing amplifier 68 will then cause the first motor displacement control 84 to decrease the displacement of the first motor 34 which results in a pressure increase thereat. This pressure increase is fed back to cause the displacement of the pump 22 to decrease and thus slow the first motor 34. 
     As would be evident to those skilled in the art, although the load may be imposed only on one, the other motor will change displacement in accordance with the pressure fluctuations in the outlet line 24 so as to maintain an actual speed equal to the operator input speed. Further, as is evident, the motor inlet pressure and displacement, which determine motor torque, are always balanced to match the load and the pump displacement, which determines pump output pressure and flow, is varied to maintain the desired motor speed. Any number of power outlets, such as additional motors, can be introduced without affecting the others so long as the total power requirement does not exceed the total engine horsepower. 
     Under normal operating conditions, the standby pressure as established by the pressure reference circuitry 118 is pre-set to a selected value, however, when extreme loads are imposed on the motors 34 and 36, a high torque mode of operation is available. As the limit of normal operation is approached, the displacement of one or the other or both of the motors 34 and 36 will increase to a predetermined maximum (approximately 95% of maximum motor displacement) which activates the gate 108 so as to allow the selector 104 to provide the larger of the two speed error signals through the gate 108 to the rate limiter circuitry 110. The rate limiter circuitry 110 provides a signal having a predetermined rate of increase to the pressure error summing junction 112 which provides a signal to pressure error amplifier 120 and thence to the pump displacement control 124 to cause the displacement of the main pump 22 to increase and force the pressure in the outlet line 24 to higher levels than that established by the pressure reference signal from the pressure reference circuitry 118 with the upper level being established by the maximum output of the engine 20. With the pressure in the outlet line 24 increased, the displacements of the first and second motors 34 and 36 will be unchanged or will decrease to provide the torque required to maintain vehicle speed. 
     Similarly, when the high torque mode is no longer required, the selected operating standby pressure will be re-established. When the pressure in the outlet line 24 drops below a predetermined minimum, it is often desired that the functions other than the motors 34 and 36, such as the power steering fluid function 38, have priority use of the remaining pressure. In this situation, the pressure transducer 114 through the function generator 116 will provide a pressure signal according to a predetermined curve which is in part linear which will cause the multipliers 80 and 82 in the motor control 50 to nonlinearly reduce the speed error signals until the speed error signal is completely blocked at a predetermined low pressure in the outlet line 24. If the signals from the speed error summing junction 68 and 70 are reduced to zero, the first and second motor displacement controls 84 and 86 will reduce the displacements and thus the flow requirements of the first and second motors 34 and 36 to zero. This pressure priority operation is reversible as the pressure in the outlet line 24 again increases. 
     When the total power requirement of the hydraulic system 12 begins to approach the total engine horsepower, the engine 20 will begin to lug down in speed and there will be an increased difference between the actual engine speed and the reference engine speed. This difference is processed in the engine feedback system 134 by the function generator 138. The signal from the function generator 138 will follow a predetermined curve which is in part linear so as to selectively modify the pressure error signal in the multiplier 122 to gradually cause a reduction in the pressure error signal as the error between the actual and reference engine speeds increases. As a signal from the multiplier 122 decreases, the pump control 124 will cause the displacement of the pump 22 to decrease so as to reduce the burden on the engine 20 and allow the engine 20 to run at optimum speed. 
     While the invention has been described in conjunction with a specific embodiment, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the aforegoing description. For example, the motors could be used to drive auxilliary functions other than the drive wheels of the vehicle. Accordingly, it is intended to embrace all such alternatives, modifications, and variations which fall within the spirit and scope of the appended claims.