Patent Publication Number: US-2012023924-A1

Title: Variable hydraulic system

Description:
TECHNICAL FIELD 
     The following disclosure relates generally to hydraulic systems with variable flow. In particular, the following disclosure relates to hydraulic transmissions and includes use and modes of operation for vehicle traction and auxiliary systems. 
     BACKGROUND 
     A conventional hydraulics system sometimes employs variable displacement pumps, such as load sensing pumps, when higher efficiency is desired. The variable displacement pumps control flow continuously from zero flow to maximum flow but are more costly than a fixed displacement pumps. In many hydraulic systems, it is also common to use two separate pumps for redundancy purposes. 
     If a fixed displacement, variable speed pump is used in the conventional hydraulic system, the pump exhibits accelerated wear if operated at low flow rates with high pressures. To prevent wear, the rotational speed of the pumps is maintained above approximately ⅓ of the maximum pump speed. For example, if a pump is capable of 10 gallons-per-minute (GPM), it is operated such that it does not produce less than 3.3 GPM. If flow in the conventional system is required at less than 3.3 GPM, the flow goes through a relief valve to dissipate the difference between 3.3 GPM and the required flow, which wastes the energy by heating the hydraulic fluid. 
     SUMMARY 
     An embodiment of the invention includes a hydraulic system having a prime mover connected to a first fixed displacement pump having a maximum flow A, and a hydraulic loop connected to a system of valves and actuators, with the loop in fluid communication with the first pump with a first check valve. A second fixed displacement pump is in fluid communication with the loop, and the second pump has a maximum flow B. The second pump is arranged in parallel with the first pump. An electric machine is connected to the second pump and electrically coupled to a battery. The electric machine is operable as one of a motor and a generator. A controller is connected to the prime mover and the electric machine. 
     Another embodiment of the invention includes a vehicle having a prime mover connected to a first fixed displacement pump having a maximum flow A. A hydraulic loop is connected to a system of valves and actuators, and the loop is in fluid communication with the first pump with a first check valve. A second fixed displacement pump is in fluid communication with the loop, and the second pump has a maximum flow B. The second pump is arranged in parallel with the first pump. An electric machine is connected to the second pump, and electrically coupled to a battery. The electric machine is operable as a motor and a generator. A controller is connected to the prime mover and the electric machine. A plurality of traction devices supports a chassis, at least one of plurality of traction devices driven by a hydrostatic motor, the hydrostatic motor in fluid communication with the system of valves and actuators. 
     Yet another embodiment of the invention includes a hydraulic system having a hydraulic loop connected to a system of valves and actuators using a valve and a first fixed displacement pump in fluid communication with the hydraulic loop. The first pump has a maximum flow A and is connected to a first prime mover. A second fixed displacement pump is in fluid communication with the system of valves and actuators and has a maximum flow B. The second pump is connected to an electric machine operable as a motor to output mechanical power, and operable as a generator to output electrical power to a battery. The first pump and the second pump operate to provide a net flow to the system of valves and actuators, the net flow between negative B flow and positive A+B flow, and the first and second pump each comply with a respective minimum speed requirement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a variable hydraulic system according to an embodiment; and 
         FIG. 2  is a schematic of a variable hydraulic system according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for the claims and/or as a representative basis for teaching one skilled in the art to variously employ the present invention. 
       FIG. 1  depicts a variable hydraulic system  100 . The system  100  may be used on a vehicle or in a stationary system, and may be used to drive various hydraulic work functions, such as lift and propulsion. In one embodiment, the system of valves and actuators includes hydraulic traction circuit for a vehicle. In another embodiment, the system of valves and actuators includes an aerial lift function for a platform on a vehicle. The drive system  100  may be hybrid powered or electric powered. The drive system  100  has a hydraulic loop  102  in fluid connection with a manifold  104 . The manifold  104  contains valves and actuators to provide pressurized fluid for the work functions provided by the system. 
     A first prime mover  106  is connected to a first pump  108  to provide pressurized fluid to the hydraulic loop  102 . The first prime mover  106  may be an internal combustion engine, such as a gasoline or diesel engine, or an electric machine, such as an AC or DC electric motor. If the prime mover  106  is an electric machine, the prime mover  106  is electrically connected to a battery  116 . An electric machine  110  is connected to a second pump  112  also in fluid communication with the hydraulic loop  102 . The electric machine  110  is connected to an inverter/controller  114 , which is in turn connected to a battery  116 . In one embodiment, the electric machine  110  is an AC motor and operates as a motor to output power or torque, or as a generator to generate electricity using a power or torque input. The inverter/controller  114  is bi-directional to power the electric machine  110  for operation as a motor, or as a generator to recharge the battery  116 . 
     In one embodiment, the prime mover  106  is a series direct current (DC) motor and the second electric machine  110  is an alternating current (AC) induction motor. In another embodiment, the prime mover and the electric machine  106 ,  110  are both AC motors, and the prime mover  106  is connected to the battery  116  via an additional inverter/controller (not shown). Alternatively, the prime mover  106  and the electric machine  110  are AC or DC permanent magnet motors, or any four quadrant motor. In another embodiment, the prime mover  106  is an internal combustion engine, such as a spark ignition, diesel, turbine, or others as are known in the art; while the electric machine  110  is any four quadrant motor. 
     The first and second pump  108 ,  112  are axial piston pumps, vane pumps, other fixed displacement pumps as are known in the art, or a combination thereof. The fixed displacement pumps  108 ,  112  may be configured for operation as either a pump to provide pressurized fluid (pumping) or as a turbine, or other device for converting fluid energy to mechanical energy (motoring). 
     In an embodiment, a valve  118  is provided and switches the pumps  108 ,  112  between a parallel orientation in the hydraulic loop  102 , and disconnecting the second pump  112  from the hydraulic loop  102  when not required for use (as shown), or when running the first pump  110  alone. The parallel orientation, with both pumps pumping, acts to increase the pressure of the pressurized fluid in the hydraulic loop  102 . A check valve  120  is located downstream of the outlet of the first pump  108  to prevent backflow into the pump  108  when the system  100  is in the parallel orientation and operating. The hydraulic loop  102  connects to a reservoir  122  which contains the hydraulic fluid. 
     In an embodiment, the pumps  108 ,  112  are fixed displacement pumps having variable speeds to produce the required hydraulic flow in the loop  102 . A controller  124  controls the speeds of the pumps  108 ,  112 , thereby controlling the flow in the loop  102  between large negative and large positive flows and across zero net flow to the manifold  104 . The controller  124  meters low flow in the loop  102  without running the pumps  108 ,  112  below their minimum speed requirements. The controller  124  also determines the operational modes of the hydraulic system  100 . 
     The loop  102  may be connected to the manifold  104  using a valve  126 , or other load connector such as a blocking valve, controlled check valve, or valve attached to an actuator within the manifold  104 . The valve  126  acts to control the flow to the manifold  104  as well as provide a load on the flow in the loop  102 . 
     The drive system  100  has several operating modes for the vehicle  100 . The modes are controlled using the electronic control module  124 . The control module may provide for a user interface, maintenance interface, system control, and the like. 
     In an embodiment, the first pump  108  has a maximum flow of A gallons per minute when pumping. The second pump has a maximum flow of B gallons per minute when pumping or negative (−) B gallons per minute when motoring, where the positive or negative flow relates to the direction of the fluid. The pumps  108 ,  112  each have a minimum pump speed, either pumping or motoring, which corresponds to a minimum flow. For example, pump  112  may pump between minimum speed flow, b, and maximum flow B. Pump  112  may also motor between minimum speed flow −b, and maximum flow −B. Zero flow through pump  112  lies between b and −b flow. Any references made to flow below from a pump lie within the possible ranges outlined. References to an increase or decrease in flow are made to the magnitude of the flow, i.e. flow “increases” from −b to −B. 
     The controller  124  controls the speed and operation of the pumps  108 ,  112  to create a net flow from the hydraulic loop  102  to the system of valves and actuators  104 . The controller  124  varies the speed of the first pump  108  and the speed of the second pump  112  to create a variable net flow between negative B flow and positive A+B flow. 
     Valve  118  also serves to unload the second pump  112  from the hydraulic loop  102  during a transition operation. A transition operation is when the second pump is changing from a pumping mode to a motoring mode or vice versa. Alternatively, transition operations are when the second pump  112  is changing between a negative and a positive second pump speed, or vice versa. Unloading the second pump  112  during the transition helps to maintain pump life and minimize wear or damage from cycling from a positive pump direction to a negative pump direction. 
     In another embodiment, the controller  124  operates the first pump  108  (pumping) and the second pump  112  (motoring), such that the net flow from the loop  102  to the system of valves and actuators  104  is the difference between a flow provided by the first pump  108  and a flow received by the second pump  112 . The flows provided by pump  108  may be any value between that provided by the minimum pump speed and the maximum flow A. The flow provided by the second pump  112  may be any value between that provided by the minimum pump speed (in reverse or motoring) and the maximum motoring flow, −B. 
     In a further embodiment, the prime mover  106  drives the first pump  108  above the minimum pump speed to provide a flow to the hydraulic loop  102 . The second pump  112  receives flow from the hydraulic loop  102  and motors above minimum pump speed to drive the electric machine  110  as a generator to charge the battery  116 . The net flow from the hydraulic loop  102  passes through the valve  126  and to the system of valves and actuators  104 . The net flow to the system of valves and actuators  104  is metered by the controller  124  varying the speed of at least one of the first pump  108  and the second pump  112 . 
     In another embodiment, the prime mover  106  drives the first pump  108  to pump to provide flow to the hydraulic loop  102  and system of valves and actuators  104 . The electric machine  110  drives the second pump  112  as a pump to also provide flow to the hydraulic loop  102  and system of valves and actuators  104 . The pumps  108 ,  112  may be metered by the controller  124  to each provide flow between that provided by their respective minimum pump speeds and maximum flow, A or B respectively. 
     In one example, the first pump  108  is pumping at or above its minimum pump speed requirement. The second pump  112  is motoring at or above its minimum pump speed requirement. The flow pumped from the first pump  108  is offset by the flow required by the second pump  112 , and the net flow to the system of valves and actuators  104  is at or near zero flow. This may be used when the battery  116  needs charging and no flow is required by the system of valves and actuators  104 . 
     In another embodiment, the valve  118  is closed such that only the first pump  108  can provide flow from the hydraulic loop  102  to the system of valves and actuators  104 . The prime mover  106  drives the first pump  108  to provide flow to the hydraulic loop  102  and the system of valves and actuators  104 . Flow through the second pump is prevented due to the valve  118  closure. The first pump  108  may provide flow from its minimum pump speed up to the maximum flow for the pump, A. 
     In an embodiment, the first pump  108  is inactive, and the valve  118  is open such that the first pump  108  and second pump  112  are in parallel. The electric machine  110  drives the second pump  112  (pumping) between the minimum pump speed of the pump, and up to the maximum flow of the pump, B. The pump  112  provides flow to the hydraulic loop  102  and the system of valves and actuators  104 . Flow from the second pump  112  is prevented from backflowing into the first pump  108  by the check valve  120 . 
     In another embodiment, the system of valves and actuators  104  has an external load, such as a lift function fully raised or a propulsion system at the top of a hill, or in other words the system  104  has an amount of stored potential energy. When the system of valves and actuators  104  releases the potential energy into flow energy, the system  104  provides flow to the hydraulic loop  102 , which thereby drives the second pump  112  (motoring) to drive the electric machine  110  as a generator to charge the battery  116 . The first pump  108  is inactive as the flow from the system of valves and actuators  104  cannot flow through the pump  108  due to the check valve  120 . 
     In the case that the prime mover  106  is an engine, the engine  106  may run at one of a plurality of constant speeds, run at varying speeds, or run at a constant speed, or power output, or torque output, such as one that would maximize fuel efficiency for example. 
     The engine  106  is often operated at an approximately steady output to increase engine efficiency. When there is excess power output by the engine  106  that is not required as flow by the system of valves and actuators  104 , the excess power may be transferred through the hydraulic loop  102 . The engine  106  drives the first pump  108 , which provides flow to the loop  102 . Any excess flow not required by the system of valves and actuators is directed to the second pump  112  which motors to power the electric machine  110  as a generator to charge the battery  116 . 
     Alternatively, when there is insufficient power from the engine  106  to provide flow from the first pump  108  to the system of valves and actuators  104 , additional power may be provided by the electric machine  110  driving the second pump  112  (pumping) to augment the flow in the hydraulic loop  102  and maintain a generally steady engine output. This ability to augment the flow with the second pump  112  allows for a smaller engine  106  than is typical. The changes in required flow may be managed by the electric machine  110  acting as a motor or a generator in concert with the second pump  112 , while the engine  106  runs at a generally stabilized power output within a desired range. 
     In another embodiment, a third pump  128  is connected to the engine  106 . The third pump  128  is also hydraulically connected to a valve  130  via hydraulic line  132 . To start an inactive engine  106 , the electric machine  110  drives the first pump to provide flow. Valve  112  is open, while valve  118  may be opened or closed. The pump  128  is driven as a motor to start the engine  106 . The third pump  128  may be connected to the first pump  108  through a torque coupling such as a splined connection, a piggybacked connection, or the like. Alternatively, the third pump  128  may be driven directly by the engine  106 . 
       FIG. 2  illustrates another embodiment which includes a vehicle  150 . Types of vehicles  150  that may use a hydraulic drivetrain system include hydrostatic front end loaders, skid steer loaders, wheeled excavators, and the like. The vehicle  150  has a hydraulic system  100 , as described above with respect to  FIG. 1 , supported by a chassis  152 . The chassis  152  of the vehicle  150  is supported by traction devices  154  in contact with an underlying surface. 
     The traction devices  154  for the vehicle  150  may be any number of wheels or may be equipped with other fraction devices, such as tracks. The system of valves and actuators  104  provides hydraulic fluid to hydrostatic drive motors  156  connected to the fraction devices  154  to propel the vehicle  150  across the ground. The hydrostatic motors  120  may be arranged in series or in parallel. In one embodiment, each of the traction devices  154  is individually driven by a respective torque source, such as a hydrostatic drive motor  156 . In another embodiment, a portion of the traction devices  154  are driven using additional electric machines. In an alternate embodiment, only one hydrostatic drive motor  156  is connected to a pair of traction devices  154  using a differential or the like. 
     In a first operating mode, only pump  108  operates to provide pressurized fluid to the hydraulic loop  102  and the system of valves and actuators  104 , while the other pump  112  remains inactive, and the valve  118  remains closed. The system of valves and actuators  104  uses the pressurized fluid provided by one of the pumps  108 ,  112  to drive hydrostatic motors  156  attached to traction devices  154 , and/or provide a lifting or other work function for the vehicle  150 . 
     In a second operating mode, valve  118  switches to an open position, and both pumps  108 ,  112  operate in parallel within the drive system  100  to provide pressurized fluid to the hydraulic loop  102  and the valves and actuators  104 . 
     In a third operating mode, with valve  118  open, the first pump  108  provides pressurized fluid to the hydraulic loop  102  and system of valves or actuators  104 , while excess flow is used to motor the second pump  112  to rotate the electric machine  110  as a generator, and charge the battery  116 . The net flow in the hydraulic loop  102 , which may also be used by the valves and actuators  104  for drive and/or lift operations, is the difference between the first flow from the first pump  108  and the second flow from the second pump  112  motoring. If no flow is required by the valves and actuators  104 , the second pump  112  may use all of the first flow to motor and charge the battery  116  in a generation mode. 
     In a fourth operating mode, stored potential energy is recovered from either the lift or work function or the drive function of the system of valves and actuators  104 , for example when a platform is returning from a raised to a stowed position or the vehicle  150  is travelling down a sloped surface S. The second pump  112  is driven as a motor by flow returning from the valves and actuators  104 , through valve  118  in the loop  102  and before it reaches the reservoir  122 . The second pump  112  drives the electric machine  110  as a generator to charge the battery  116 . 
     For example, in operation, when a high flow rate through the loop  102  is required, one or both pumps  108 ,  112  provide flow. When low flow is required in the loop  102 , the first pump  108  operates above its minimum speed, while the second pump  112  motors at or above the minimum speed of pump  112  to generate mechanical energy. Since the first pump  108  is adding flow rate, pressure, or energy to the flow through the loop  102 , and the second pump  112  is removing flow rate, pressure, or energy from the flow in the loop  102 , the net flow to the manifold  104  is the difference between the two flows. 
     For example, if the first pump  108  pumps at 4 GPM and the second pump  112  motors at 3.3 GPM, the net flow is 0.7 GPM. Since the second pump  112  is motoring, the second pump  112  is acting as a turbine or generator, which supplies electric current charge to the battery  116  through the electric machine  110  acting as a generator. If the first pump  108  is powered by an engine  106 , the generated current charges the system battery  116 . If the first pump  108  is powered by an electric motor in place of the engine  106 , the generated current from the second pump  112  will reduce the amount of electric power the first pump  108  requires from the battery  116 . In this configuration, the second pump  112  functions as a variable set point pressure relief valve. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, features of various implementing embodiments may be combined to form further embodiments of the invention.