CN101111684A - hydraulic drive system - Google Patents

Abstract

A hydraulic drive system for an actuator using a pair of pressure compensated hydraulic machines to control flow to or from the drive chamber of the actuator by varying the control pressure of one of the machines. The machine is mechanically linked to allow energy recovery and to replenish the accumulator to store the supplied energy. The drive system can be combined with other servos including transmissions for integration in vehicles. The transmission uses pressure compensated supply and torque control of the wheels.

Description

hydraulic drive system

technical field

The present invention relates to energy transfer systems, and more particularly to systems utilizing hydraulic fluid as the energy transfer medium.

Background technique

It is known to transfer energy to a load from a power source such as a motor or an internal combustion engine through the intermediary of a hydraulic drive system. Such systems typically have a pump driven by a power source and a motor connected to a load. By adjusting the hydraulic flow between the pump and motor, it is possible to impart motion to the load, hold it in a fixed position or affect its configuration.

Control of fluid flow is usually accomplished by a valve mechanism, in its simplest form, a valve that simply opens or closes the flow between the pump and the motor and thereby regulates the motion of the load. Such valve systems are relatively inefficient in terms of the energy dissipated across the valve. In a typical installation, the valve may be closed, requiring a pump in the center to deliver pressure against the relief valve. The pressure supplied to the fluid is thus lost as heat. In an open center configuration, careful valve fabrication is required to obtain transfer between zero flow and full flow, while load hold control and flow metering across the valve result in energy losses.

The valves for controlling the flow are thus relatively complex and produced with high precision in order to obtain the necessary control functions. As such, valves tend to be specialized and do not provide flexibility in implementing different control strategies. Most notably, since control is achieved by metering the flow across the entire orifice, there are inherently significant energy losses when controlling fluid flow. Control valves regulate motion by controlling flow over a restricted port at the inlet of the device. Because, control valves are usually a one-piece cartridge, providing similarly restricted ports to vent flow and result in significant energy losses.

In order to reduce the operating force required for the valve, it is known to use servo valves in which a pilot operation is used to control the fluid flow. In this configuration, the pivot valve balances a pair of pilot flows and can be moved to increase one flow and decrease the other. Changes in flow are used to control valves and operate hydraulic devices. The force required to move the pilot valve is less than that used to control the valve, and thus enhanced control is obtained. However, continuous flow through the pilot valve at high pressure results in significant losses. Control valves themselves also suffer from the inadequacy of energy losses due to metering flow across the restricted ports, and thus, while providing enhanced control, the energy losses are significant.

Contents of the invention

It is therefore an object of the present invention to eliminate or reduce the above-mentioned disadvantages.

In general, the present invention provides a hydraulic drive in which the flow from the actuator is controlled by a variable capacity hydraulic machine. The machine is pressure compensated in order to maintain the pressure in the actuator. A control signal is provided to adjust the maintained pressure and thereby the flow from the actuator.

Thus according to one aspect of the invention there is provided a hydraulic drive system comprising: an actuator having a pair of chambers arranged to apply a motive force derived from fluid in the chambers to move the drive member in opposite directions. Each chamber is connected to a respective one of a pair of variable capacity hydraulic machines, each machine having a pressure compensating control operable to adjust the capacity of the machine to maintain a predetermined pressure in the chamber. An overspeed control is operable on at least one of the machines to vary its capacity and allow flow of fluid from one of the chambers and corresponding movement of drive components.

Description of drawings

The present invention is described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a hydraulic drive for a linear actuator.

FIG. 2 is a detailed view of components used in the drive of FIG. 1 .

Figure 3 is a schematic diagram similar to Figure 1 of a linear actuator with improved control.

Figure 4 is a schematic diagram of a linear actuator similar to Figure 1 implementing a further control function.

Figure 5 is a schematic diagram of a rotary drive.

Figure 6 is a schematic diagram of another embodiment of a driver with enhanced energy recovery capability.

Figure 7 is a view of a vehicle incorporating hydraulic transmission.

FIG. 8 is a schematic diagram of the hydraulic transmission utilized in FIG. 7 .

Figure 9 is a response curve for different responses under different operating conditions.

Detailed ways

Referring to FIG. 1 , a hydraulic drive system 10 includes an actuator 11 having a cylinder 12 with a piston 14 supported therein. Piston 14 is connected to a piston rod 16 extending from opposite ends of cylinder 12 . Piston 14 subdivides cylinder 12 into chambers 18 and 20, which are connected to supply lines 22, 24 via ports 26, 28, respectively. Rod 16 is connected to load 30, shown schematically as a horizontally sliding mass.

The supply lines 22 , 24 are connected to the outlets of a pair of variable capacity hydraulic machines 32 , 34 . The machines 32, 34 are typically swash plate assemblies, where the angle of inclination of the swash plate determines the capacity of the machine. Alternatively, the device is a radial piston pump, where the variation in eccentricity of the control ring determines the capacity of the machine. The machines 32, 34 are reversible to allow each to operate in pump mode and motor mode. The details of these machines are well known and no further description is necessary. A particular preferred embodiment of such a machine is disclosed in co-pending application PCT/US2005/004723, the entire contents of which are hereby incorporated by reference.

The machines 32, 34 are connected by a common drive shaft 36 to a prime mover 38, typically an electric motor or an internal combustion engine. The machines 32 , 34 receive fluid from and return fluid to the sump 40 . Each machine has a capacity adjustment mechanism 42 , 44 whose configuration can be adjusted by swash plate adjustment motors 46 , 48 . The motors 46 , 48 are independently operable and controlled by respective control units 47 , 49 . As can be seen in more detail in FIG. 2 , each control unit 47 , 49 receives a control signal from a control module 50 as a result of manipulation of a manual control 51 . The control module 50 communicates with the control units 47, 49 via signal lines 52, 54, respectively. Each of the signal lines 52 , 54 includes a reference pressure signal 61 and a swash plate position feedback signal 57 . Input to the control module is provided by the controller 51, which control is shown as manual control, although obviously this could be generated automatically by other control functions or as part of a programmed sequence.

The control units 47, 49 are similar and therefore only one of them will be described in detail. The control units 47 , 49 receive pressure feedback signals from the supply lines 22 , 24 via internal signal lines 56 , respectively. A feedback signal for the displacement of the swash plate is also available via signal line 57 and a feedback signal for the machine speed via signal line 58 .

Pressure reference signal 61 and pressure feedback signal 56 are compared at pressure control driver 63 which is connected by control line 65 to swash plate drive 67 . Swash plate drive 67 produces output error signal 68 . This error signal 68 is applied to a valve driver 69 whose output is the drive signal 62 .

The drive signal 62 is applied to an actuation coil 64 of a closed central valve 66 that controls the movement of the motor 46 and thus the capacity of each machine 32 . Valve 66 has a valve position feedback signal 70 which is fed to valve driver 69 such that drive signal 62 is the difference between error signal 68 and valve position signal 70 .

In operation, the load 30 is initially stationary and the capacity adjustment members 42, 44 are initially positioned relative to the machines 32, 34 at substantially zero capacity with maximum system pressure, typically on the order of 5000 psi at each port 26, 28 . The machines 32, 34 acquire this state when the reference signal 61 is applied to the pressure control actuator 63, and any pressure loss will provide a signal to the swash plate 67 to move the swash plate 67 to supply fluid. This can result in an increase in the sensed pressure in the feedback line 56 and a net zero sum at the driver 67 . In this state, the drive shaft 36 simply rotates the machines 32 , 34 without producing output at the supply lines 22 , 24 . The fluid is essentially locked in the chambers 18, 20 and thus movement of the piston 14 relative to the cylinder 12 is inhibited. Any system leaks will cause the pressure in each line 22, 24 to drop and generate a corresponding error signal from the pressure control driver 63 to adjust the respective components 42, 44 to maintain the pressure.

In order to move the load 30, the manual control 51 is moved in the direction in which the load is to be moved, indicated by the arrow X in FIG. 1 . For purposes of this initial description, it is assumed that control 51 provides a simple fixed value step function, ie "on" or "off" control module 50 . Subsequent examples will describe alternative control strategies. Upon movement of the manual control 51, a signal 33 is provided to the control module 50, which generates a corresponding signal in the control lines 53, 54, in the case of 52, to affect the movement in the desired direction.

The pressure reference signal 61 is set to require a nominal minimum pressure, eg 100 psi at port 26 , so that the signal on control line 65 also indicates an increase in machine 32 capacity in motor mode to reduce the pressure at port 26 . Swashplate 67 thereby provides an output error signal to valve driver 69 indicative of the desired valve position that would place machine 32 in motor mode to reduce pressure in port 26 and allow fluid flow from chamber 18 . Valve position feedback signal 70 is indicative of the neutral position of valve 66 , so valve actuation signal 62 is applied to detent 64 to reduce error and thereby open valve 66 .

Initially, the capacity of the machine 32 will increase enough for the drop in pressure at the port 26 and the signal 57 corresponds to the reference signal 61 from the controller 50 . The control signal 65 is thus reduced to zero. Valve position feedback signal 70 is thereby acted upon through valve driver 69 to close valve 66 and inhibit further movement of swash plate 42 . Any increase in machine capacity will reduce the pressure at port 26 below the pressure set by reference pressure 61 and control signal 65 will act to reduce capacity and bring the pressure back to reference pressure 61 .

Due to the reduced pressure at port 26, the pressure in chamber 20 is maintained at the maximum set value when reference signal 61 associated with control unit 49 is not modified. The pressure differential acting across piston 14 causes initial movement of piston 14 which in turn reduces the pressure at port 28 . The pressure control drive 63 of the control unit 49 thereby generates a control signal 65 which can generate an output error to the swash plate drive 67 and cause the machine 34 to increase capacity in pump mode to maintain the reference pressure. Movement of the piston 14 induces flow from port 26 and the pressure at port 26 will increase there above the nominal set pressure. The pressure control signal 65 is then operable through the swash plate drive 67 to increase the capacity of the machine 32 in motor mode while maintaining the desired nominal pressure. The pressure difference across the piston 14 will thus accelerate the mass 30 . As mass 30 accelerates, the capacity of machine 34 in pump mode continues to increase to provide fluid to maintain the reference pressure, and the capacity of machine 32 in motor mode will likewise increase to maintain the nominal set pressure. Acceleration of the mass 30 will continue to accelerate and the capacities of the machines 32, 34 are adjusted under pressure compensating control to maintain their respective set pressures at the ports 26, 28. When the machine 34 attains maximum capacity, the mass no longer accelerates but attains a steady velocity with the pressure at port 28 maintained at the maximum reference pressure and the pressure at port 26 maintained at the nominal low pressure.

In the simplest form of control provided by manual control 51 , actuator 10 will continue to move mass 30 in the direction set by controller 50 . When the desired position of the mass 30 is obtained, as observed by the operator, the manual actuator 51 is returned to the center position, increasing the reference pressure 61 to a maximum pressure. To achieve the pressure indicated by reference signal 61, the capacity of machine 32 will be reduced to increase the pressure in port 28 to the set value. The pressure differential across the piston 14 is removed and the mass 30 is decelerated. The capacity of the machine 34 is thus also reduced to maintain the pressure at the set value and both machines 32, 34 are gradually reduced to minimum capacity as the mass decelerates. The pressure at the ports 26, 28 is then the same and holds the load 30 at rest. It should be noted that during movement, the modulation of the reference pressure 61 is only applied to the machine 32 and the machine 34 simply operates in a pressure compensation mode to follow the movement of the piston 14 .

Movement of the manual control 51 in the opposite direction likewise applies a control signal through the signal line 54 to generate an actuation signal at the valve 66 of the control unit 49 and the reduction in the required pressure increases the capacity of the machine 34 and in the opposite direction produces Responsive movement.

During motion of load 30 , swash plate position feedback signal 57 is applied to control module 50 to provide an indication of the machine operating mode, ie pump or motor mode, and for predictive control in modifying reference pressure signal 61 .

To adjust for different operating conditions, as shown in Figure 9, a rotational feedback signal 58 is used to vary the initial conditions of the ramp function and obtain an optimal response in terms of pressure control. As the pressure supply increases in response to the increasing pressure of the reference signal 61 , as sensed in the pressure sense line 56 , the ramp initial point T reaches the controller 63 at which point the controller 50 changes the pressure signal. The controller 50 also receives a speed feedback signal 58 and changes the initial value as indicated by T ₁ and T ₂ which are inversely proportional to the sensed speed. At low rotational speeds, the pressure gain (rate of pressure increase) is low because the time for the system to respond is lengthened due to the relatively low rates of pump and motor modes in the machine. However, at higher rotational speeds, the rate of pressure gain is higher. Thus, at higher RPMs, the initial value _T1 is a lower pressure value, while at lower RPMs, the initial value _T2 is a higher pressure value. In this way, system response can be matched to changing operating conditions of the system.

The rotational speed of the machine, provided by feedback 58 , can be used to vary the response of the machine and thus the reference pressure signal 61 . In order to provide optimum response, ie suppress overshoot and minimize undershoot, the control signal to valve 66 is modified by a ramp function.

Alternatively, the angular configuration of the swash plates 42, 44 may be used to vary the onset of modification. In this case, when the supplied pressure is increased in response to an increase in the reference signal 61, as sensed at the pressure sense line 56, the initial point T of the ramp reaches the controller 63, at which point the controller 50 Change stress signals. Controller 50 also receives swash plate feedback signal 57 and changes the initial point as shown by T1 and T2 which are inversely proportional to the sensed position. At low swash plate angles, the pressure gain (rate of pressure increase) is low because the time for the system to respond is increased due to the relatively low rates of pump and motor modes in the machines 34,32. However, at higher swash plate angles, the rate of pressure gain is higher. Thus, at higher swash plate angles, the initial value _T1 is a lower pressure value, and at lower swash plate angles, the initial value _T2 is a higher pressure value. In this way, system response can be matched to changing operating conditions of the system.

The rotational speed of the machine, provided by feedback 58 , can be used to vary the response of the machine to vary the reference pressure signal 61 . In order to provide optimum response, ie suppress overshoot and minimize undershoot, the control signal to valve 66 is modified by a ramp function.

It should be understood that by utilizing the variable capacity of the machines 32, 34 on a common drive, the fluid energy discharged from the crushing chamber can be redirected through the shaft 36 to the main rotor, the machine under pump conditions or otherwise will be hereinafter A detailed description.

Fluid flow from the crush chamber ( 18 in the example above) generates torque as it flows through each machine 32 . The torque generated depends in part on the capacity of the machine and is applied to the drive shaft 36 to supplement the torque applied through the main rotor 38 . In some cases, such as gravity-assisted motion of the load 30, the torque obtained from one machine is sufficient to maintain the set pressure in the other machine, while in other cases, in addition to the recovered torque, additional torque from the main rotor 38 is required. coming energy. Where additional torque is required, active sub-control will sense the increased need (for example, by reducing speed in the case of a compression ignition internal combustion engine) and respond accordingly.

The deceleration of the mass 30 also provides a source of energy which can be recovered by the mechanical linkage of the machines 32 , 34 . As stated above, when the control 51 is returned to the neutral position, the condition of the machine 32 maintains the maximum reference pressure. The continuous movement of the mass 30 due to its kinetic energy must therefore be acted against the maximum pressure by the machine 32 , which is still in motor mode. The machine 32 is thereby driven by the fluid expelled from the chamber 18 and significant torque is exerted on the drive shaft 36 . Torque is applied until both the mass and the swash plate return to substantially zero capacity.

In some cases, by moving the control 51 in the opposite direction, the load 30 can be decelerated at a maximum rate, ie past the neutral position. This movement causes a signal to be applied via signal line 54 to indicate the desired nominal low pressure in port 28 and the maximum pressure in port 26 . Machine 32 thus reduces its capacity to maintain maximum pressure, and machine 34 similarly reduces its capacity but at a rate only to maintain the nominal low pressure in port 28 . A maximum pressure differential is then applied to decelerate the mass and bring it to rest. The swash plate is gradually moved to the zero displacement position, at which point control 51 can be released and equal pressure equalization is applied to each chamber. If control 51 is maintained in the reverse position, machine 34 will move to motor mode and machine 32 to pump mode, and the load begins to move in the opposite direction.

As above, the manual control 51 is either "on" or "off", but a proportional signal could be incorporated into the manual control 51 to obtain a progressive response so that the rate of movement of the load is proportional to the movement of the control 51 from the neutral position . In this case, the magnitude of the control signal 53 is proportional to the movement of the control 51 . Signal 52 will establish a reference pressure signal for pressure compensation that is proportional to the displacement of controller 50 . Assuming that a movement of the mass in the direction of the arrow X is required, the capacity of the machine 32 will be adjusted so that the pressure at the port 26 takes this value. The pressure at port 28 is maintained at a reference level so that the pressure differential across piston 14 can be modulated thereby and acceleration controlled.

The arrangement shown in Figure 1 provides a simple manual feedback, but the control signal can be modified to provide piston control of the actuator 18, as shown in Figure 3, where for clarity reference numbers are used to refer to the suffix Similar components to "a". In the embodiment of Figure 3, manual control 51a provides a proportional control signal to control module 50a. A position feedback signal 72a is obtained from the piston rod 16a of the actuator 11a and is also fed to the controller 50a to obtain an error signal indicative of the desired position represented by the manual control 51a and the true position represented by the signal 70a. The control module 50a generates a pressure reference signal 61a on the control signal line 52a which is applied to the respective control unit 47a of the motor 46a to adjust the machine 32a and move the piston 14a in the desired direction. Assuming that the load 30a moves in the direction of arrow X as shown in Figure 3, the machine 32a increases capacity to obtain a reduced pressure at port 26a, corresponding to the pressure set by the reference signal 61a, and fluid flows out of the chamber 18a . The machine 34a applies a maximum reference pressure to move the load 30a and changes capacity to maintain that pressure. When the desired position is achieved, the position signal 72a changes and the difference between the manual control 51a and the position signal 72a decreases to approximately zero. The swash plate gradually returns to the zero displacement position and any movement from this desired position generates an error signal at the control module 50a to adjust the appropriate pressure reference signal 61a and return the load to the desired position. The capacity of machine 32a is thus gradually reduced to increase the pressure, and the corresponding capacity of machine 34a is reduced until load 30a comes to rest in the desired orientation.

The control configuration of Figure 1 is also changed to provide speed control, where the maximum speed is limited. For clarity, like components are designated with like reference numerals with a "b" suffix. In the embodiment of Figure 4, rather than monitoring the position of the load as in Figure 3, the capacity of the machines 32b, 34b is monitored and used as an indication of speed. Referring to Figure 4, the manual control at 51b provides an output signal proportional to the achieved speed which produces a control signal 52b causing the machine 32b to move to motor mode and the reference pressure to drop to a nominally low value. In motor mode the capacity of the machine 32b is increased to reduce the pressure at port 26b to accelerate the load.

The capacity of the machines 34b, 32b is increased until the capacity indicated by the feedback signal 57b corresponds to the pressure set by the control 51b. The error signal is thereby removed and the capacity of machine 32b is reduced to establish a reference pressure. The reference pressure of machine 34b is at a maximum and the load is accelerated again until the capacity of machine 32b indicated by feedback signal 57b matches the output signal from control 51b. As the machine gradually reduces capacity, the swash plate position feedback 57b again introduces an error signal which can cause the machine 32b to increase capacity in order to reduce pressure. Thus, a stable speed is obtained, which is an intermediate value limited by the maximum capacity of the machine. This control is useful for automatic processes such as machine tool driving.

The linear actuators described above have been described as double sided actuators, but it will be understood that they can equally be used for single sided actuators, ie the piston rod protrudes from one side of the actuator and the chamber has a different area. The corresponding reference signal 61 can be adjusted proportionally to the area difference between the rod-side chamber and the piston-side chamber to control the movement of the cylinder in a manner similar to that described above in FIG. 1 .

Similar control structures can be used for rotary drives, as can be used for winches or similar applications. This configuration is shown in Figure 5, where like reference numerals are used to refer to like components, but with a "1" prefix for clarity of description. A pair of variable capacity hydraulic machines 132 , 134 are hydraulically connected to a fixed capacity rotary machine 180 by hydraulic lines 122 , 124 . The main rotor 138 is mechanically connected to each machine 132 , 134 and the winch assembly 130 is connected to the machine 180 . Machines 132 , 134 are controlled by motors 146 , 148 and control signals 152 , 154 are applied by control module 150 . The mass is at rest, each adjustment member 142 , 144 is set to approximately zero capacity, and a hydraulic lock is located in the supply line 122 , 124 . The machine's pressure compensation ensures that pressure is maintained in the system to lock the motor and inhibit rotation of the winch.

The capstan 130 is rotated with a signal from the actuator, and the signal to the motor 132 indicates the need to reduce pressure for increased capacity in motor mode. As fluid is conveyed in supply line 122 , the pressure compensating control of machine 134 adjusts to maintain the pressure at a controlled set pressure that causes capstan assembly 130 to rotate. Position and velocity controls as shown above can be used to control the movement of the load and maintain it in the desired position. Once this position is obtained, the error signal is removed by releasing the manual control 151 or releasing the feedback of the position or speed control, the swashplates 144, 142 are gradually returned to an approximately zero position where no power is transmitted through the system but load is passed through the motor sides pressure to maintain.

It can thus be seen that in each of the embodiments described above, a pair of pressure compensated variable capacity machines may be used to control the operation of the actuators.

Pressure compensation allows the use of minimal energy to hold the actuator and gain control over the movement of the actuator by disregarding the set pressure on the actuator discharge. Only one machine needs to be modulated, the other machine then maintains the set pressure and applies the motive force. The mechanical linkage of the machine can be used to recover energy from the fluid outflow from the actuator and can be applied to the machine, providing the motive force.

As noted above, the mechanical linkage of machines 32, 34 allows energy recovery under certain conditions. Energy recovery can be enhanced by adopting the configuration shown in Figure 6. The same reference numerals may be used to denote the same components, with the prefix "2" added for clarity. In the embodiment of FIG. 6 , a pair of variable capacity machines 232 , 234 are connected to actuator 211 , which is connected to load 230 . Each machine 232, 234 includes pressure compensated controls and is operated by manual controls 251 via controls 250, as described above. The machines 232, 234 are mechanically linked by a pair of meshing gears 236 so that they rotate in unison. Drive to the machine is provided by drive member 236 through a gear train 280 comprising gears 282,283.

Auxiliary hydraulic drive 284 is connected to gear 283 and provides fluid to auxiliary servo 276 . The drive 284 may be of fixed or variable capacity and may be controlled by the machines 232, 234 as appropriate. The gear train 280 also includes a gear 285 that drives an additional variable capacity hydraulic machine 286 . Machine 286 is connected to a hydraulic accumulator 288 operable to store and discharge fluid through machine 286 and thereby absorb energy from or distribute energy to gear train 280 . A speed sensor 290 is provided to monitor the speed of the gear train 280 and interfaces with the control module 250 .

In operation, the accumulator is initially empty and it is assumed that the auxiliary drive 284 provides a steady flow of fluid to the servo 276 . The mass 230 moves at a constant speed under the action of the machines 232, 234, and the main rotor 238 provides sufficient energy to the gear train 280 to meet the demand. If mass 230 decelerates at a maximum rate, as described above, machine 232 adjusts to maximum pressure in motor mode and generates significant torque to accelerate drive train 280 . Initially the active rotor removes fuel, assuming it is a compression ignition internal combustion engine, and the torque provided by machine 232 is used to drive machine 284 and provide fluid to auxiliary servo 276 . If torque will not be absorbed in this manner, the gear train will accelerate and the speed sensor 290 will provide a signal to the controller 250 to increase the capacity of the additional machine 286 in pump mode. The machine 286 thus provides fluid to the accumulator 288 under pressure at a rate capable of absorbing the torque present and maintaining the desired speed of the gear train 280 .

Since the mass 230 is stationary, the torque provided to the gear train is reduced and the speed is reduced. Due to the lack of induced energy via machine 232 , control 250 causes machine 286 to reduce pump action and return to approximately zero capacity, with energy stored in accumulator 288 . Similarly, if the auxiliary servo 276 requires more power during deceleration, the speed of the gear train 280 will be reduced and an adjustment will be made to the machine 286 . Available energy from the machine 232 is thereby redirected to the auxiliary servo 276 and the remaining energy, if present, can be used to pump the accumulator 288 .

If the load applied by the servo 276 continues to increase, the energy stored in the accumulator 288 can maintain the desired speed of the gear train 280 . Continued increasing loads will again cause the speed of gear train 280 to decrease and cause controller 250 to move additional machines 286 into motor mode. Pressurized fluid available in the accumulator is applied to the machine 286 to create pressure in the gear train and thereby maintain the desired speed. The swash plate of machine 286 is modulated to maintain speed at the desired level until all energy (or low threshold) in accumulator 288 is dissipated. At this point, further energy requirements are met by fueling the main rotor 238 . Mechanical linkage through the accumulator 288 of the machine 286 and modulation to maintain the speed of the gear train 280 within desired limits increases recovery energy utilization.

A system as described above can be integrated into the control strategy of a more complex machine, as shown in Figures 7 and 8, where like reference numerals are used with a prefix "4" to represent like components. Referring therefore to FIG. 7 , a vehicle V includes a chassis structure C supported on drive wheels W . The superstructure S rests on the chassis structure C and is rotatable about a vertical axis on a turntable T. The truss assembly B is pivotally mounted to the superstructure S for movement in the vertical plane. A truss actuator 411 is connected between the superstructure S and the truss assembly B and is operable to raise and lower the truss.

The vehicle V includes a main rotor 438 connected to a hydraulic drive system 410 by a gear train 480 , as shown in greater detail in FIG. 8 . As can be seen in FIG. 8, a main rotor 438, which may be in the form of an electric motor or an internal combustion engine, provides input to a mechanical gear train 480 which transmits drive to a plurality of variable capacity hydraulic machines 432, 432a, 434, 434a , 484 and 486. Each hydraulic machine 432, 432a, 434, 434a, 484, and 486 is variable capacity and has a capacity adjustment member 442, 443, 444, 445, respectively. Machines 432, 432a, 434, 434a, 484, and 486 are generally adjustable swash-plate machines having a tiltable swash plate acting on axially reciprocating pistons within rotating barrels, as referenced above described in the preceding examples.

Drive to the truss actuator 411 is provided by a pair of machines 432, 434 through a manual control 451a capable of controlling flow on either side of the piston 414 as described above with reference to FIGS. 1 and 2 . Similarly, the turntable T can be driven by a rotary motor 480 via a manual control 451b that can control a pair of machines 432a, 434a in the manner as described with respect to FIG. 5 . An additional machine 486 transfers power between an accumulator 488 and a gear train 480 as described with respect to FIG. 6 .

Hydraulic machine 484 is pressure compensated, as described with respect to FIG. 2 , and auxiliary servo 476 is connected to wheel drives 502 , 504 , 506 and 508 through supply conduit 500 . Each wheel drive 502 , 504 , 506 and 508 drives a wheel W respectively and is each a variable capacity reversible hydraulic machine with a control unit 547 as described with reference to FIG. 2 . Each has a tuning member 510, 512, 514 and 516 controlled by a corresponding valve. Hydraulic machines 510-516 are of similar construction to machines 32, 34 and need not be described in detail.

The capacity of each drive 502 - 508 is controlled by swash plate position signal 461 , which is generated by control module 450 . Each drive 502-508 also provides a rotational speed signal 458 on signal line 452 for monitoring the operation of each machine.

Operator control of the delivery is provided to the control module 450 via manual controls 451c, 451d, 451e. Manual control 451c controls the direction and speed of the vehicle V's propulsion, control 451d controls the braking of the vehicle V, and control 451e steers the vehicle V. These are typical controls and it is understood that other common interfaces may be used.

The operation of the hydraulic drive system is described under the assumption that initially the vehicle is stationary and the truss is locked in the lower position. With the vehicle at rest, each machine 432, 432a, 434, 434a is at essentially zero capacity and maintains a maximum set pressure. Wheel drives 502 - 508 are similarly set at minimum capacity to deliver zero torque, and machine 484 is at essentially zero capacity maintaining maximum pressure in conduit 500 . Essentially, this setting is simply sufficient to compensate for any leaks in the system, but does not generate vehicle motion.

The actuator 488 is fully vented and the capacity of the additional machine 486 is minimal. Each machine 432, 432a, 434, 434a, 484, and 486 is minimal, the main rotor 30 simply turns the machine, does not produce any output, and is therefore at minimum power demand.

To initiate motion of the vehicle V, the operator moves the control 451c in the desired direction of motion and provides an appropriate control signal 453c to the control module 450 . Typically, this is a proportional signal that not only indicates direction but also a torque input at the wheels that will determine the vehicle's speed of motion. The control module 450 provides control signals 452 to the wheel drives 502 - 508 to obtain a torque setting (displacement), corresponding to the input signal from the control 450 . This will be the proportional torque setting, indicating the corresponding proportional machine capacity. For maximum acceleration, this would correspond to maximum displacement. As the capacity of the wheel drives 502-508 increases under the control of the respective swash plates 510-516, the pressure in the supply line 500 decreases, causing pressure compensation of the machine 484 to increase the capacity of the machine. The resulting torque from drivers 502 - 508 caused by fluid flow through conduit 500 rotates wheels W and propels the vehicle.

The capacity of the wheel drives 502-508 continues to increase until the swash plate position feedback 457 indicates that the required capacity has been achieved and the required torque is delivered to each wheel. During this time, the pressure in conduit 500 will be maintained by increasing the capacity of machine 484 under pressure compensating control. Unless interrupted, by adjusting the control 451c or increasing the vehicle's load, the vehicle V will accelerate until the machine 484 reaches equilibrium when the external load matches the available torque.

When the vehicle has achieved the desired speed, the operator releases control 451c to reduce the capacity and then torque of the wheel drives 502-508 to inhibit further acceleration and maintain the desired speed. Machine 484 reduces the capacity of the vessel to maintain pressure at a maximum while maintaining flow through the wheel motors. A steady state is reached where the torque supplied to the wheels W matches the load on the vehicle V. Under certain conditions, such as traveling downhill, no torque is needed to maintain the desired speed and wheel drives 502-508 and machine 484 return to essentially zero capacity. Under such conditions, the vehicle simply travels with no net power supplied to the wheels 14 .

To brake the vehicle V, brake control 451d is activated (which may be integrated into control 451c if appropriate). Application of the brake control 451d produces a signal 453d proportional to the control 450 that can adjust each wheel drive to a pump mode with a selected capacity. The swash plates 510 - 516 thereby move from motor mode to focus on pump mode and result in an increase in pressure in conduit 500 . Machine 484 initially reduces its capacity and then collectively enters motor mode under pressure control to maintain maximum set point. The swash plate feedback signal 457 holds the wheel drives at the capacity commanded by the brake control 451d and pumps fluid through the machine 484 at maximum pressure. The torque required to do this is delivered by the momentum of the vehicle and thus brakes the vehicle V . Adjusting machine 484 to motor mode will cause power to be supplied from machine 484 to gear train 480 .

The energy supplied to the gear train 480 causes the gear train including the main rotor to accelerate. The rotational speed of the gear train is monitored by a speed sensor 490 and an increase in this speed is detected by the control module 450 . This can condition the machine 486 associated with the accumulator 488 to move into pump mode and supply fluid to the accumulator 488 under pressure. The displacement of the machine 486 is controlled to maintain the speed of the gear train 480 at a set speed. The accumulator is thus charged by energy recovery due to vehicle braking.

The storage of energy depends on the effectiveness of the brakes, and the machine 486 modulates capacity to maintain the speed of the gear train 480 at the desired level.

When brake control 451d is removed and speed control 451c is reapplied, wheel drives 502-508 are again adjusted to motor mode and machine 484 reverts to pump mode to maintain pressure in line 500.

As the machine 484 moves and supplies power to the pipeline 500 , an initial reduction in the rotational speed of the gear train 480 is sensed and the machine 486 is adjusted to motor mode to supply power from the accumulator 488 to the gear train 480 . The energy stored in accumulator 488 during braking is thus available for vehicle transmission during further acceleration cycles. When the accumulator 488 is depleted, a decrease in engine speed will be noticed and the fuel supply to the engine modulated to maintain the speed constant.

Truss B is operated by modulation of machines 432 , 434 . To extend the truss actuator 411, a control signal is sent from the operator 451a to the control 450, indicating pressure and direction. Control 450 then adjusts reference signal 461 applied to pressure control 463 associated with machine 432 . This causes the machine 432 to increase capacity in motor mode and thus reduce the pressure to the low reference pressure. The machine 434 responds with its pressure control, increasing its capacity in pump mode and extending the cylinder 411, as described above. The rate of motion can be adjusted by modulation of the adjustment member 451a to obtain a desired rate of motion.

When lowering the truss B, there is also the opposite operation, where the capacity of the machine 434 is increased in motor mode. When the truss B is lowered, there is also positive energy recovery, available from the fluid expelled through the machine 434 , and this is transferred to the gear train 480 . Again, the accumulator 488 may be supplied by operation of the machine 486 if the energy transfer is sufficient to increase the rotational speed of the gear train, and conversely, during the lift cycle, the fluid stored in the accumulator 486 may be passed through the machine 486 Supply to gear train 480 to assist in rotation of machine 434 or machine 484 .

A similar energy transfer can be obtained from the rotation of the superstructure S, where the inertia of the superstructure can be used to store energy in an accumulator for subsequent use. In its basic operation, therefore, it should be noted that the hydraulic transmission 410 is operable to transfer energy from different points of consumption and to conserve energy when required through the use of the accumulator 488 . Although a rotary drive 480 has been shown for the turntable T, a drive similar to 502 could be used in the same manner.

Individual control of wheels W also allows individual wheel control via signal line 458 by monitoring the rotational speed of individual wheels 14 . When one of the wheels W engages a lower friction surface such as ice or mud, its speed will be different from the other wheels W during acceleration or braking. The speed difference is noted by control 450 and the capacity of the machine is reduced accordingly to reduce the torque applied to a particular wheel. In extreme conditions, the capacity of the machine is reduced to zero so that a particular wheel can be considered to travel without torque application. However, under this condition, the pressure in the conduit 500 is maintained to achieve wheel balancing, thereby maintaining the traction or braking effect on these wheels. Once the wheel is decelerated, torque is reapplied. This allows traction control and ABS to be implemented.

Individual drive to the wheels can also be incorporated into the steering of the vehicle by adjusting the torque applied to each wheel on the same axle. Rotation of control 451e generates a signal that the pair of wheels are required to rotate at different speeds from each other. Thus, the capacity and torque increase of the outer wheel, which requires a higher rotational speed, is provided by a corresponding lowering of the inner wheel. Due to the pressure compensation of the machine 484 , the pressure applied to each wheel remains constant and thus the acceleration of the outer wheel occurs, resulting in the driving action of the vehicle without generating energy via the machine 484 .

Claims (38)

1. A hydraulic drive system comprising: an actuator having a pair of chambers separated by a drive member and arranged to apply motive force to said drive member in opposite directions, each member are connected to each of a pair of variable volume machines, at least one of which is operable in pump mode to supply fluid into the chamber and at least one other variable volume machine is operable in motor mode to consume fluid expelled from the other of said chambers, each said machine having a pressure compensating control to regulate said machine to maintain a respective predetermined pressure in said chamber, and operable to modulate the Said predetermined pressure in at least one, and thereby varying external control of the motive force acting on said drive member. 2. A hydraulic drive system as claimed in claim 1, wherein each of said machines is reversible so as to operate in pump mode or in motor mode, and when the other of said machines is in motor mode The control is operable to regulate one of the machines in pump mode. 3. The hydraulic drive system of claim 2, wherein the control modulates the predetermined pressure by reducing the pressure at the machine that is regulated in the motor mode. 4. The hydraulic drive system of claim 3, wherein said control maintains a predetermined pressure in said machine regulated in said pump mode while reducing the pressure of said machine in said motor mode. 5. The hydraulic drive system of claim 3, wherein each of said machines is a variable capacity swash plate machine. 6. The hydraulic drive system of claim 1, wherein the machines are mechanically coupled to transfer energy therebetween. 7. The hydraulic drive system of claim 1, wherein said another machine operable in motor mode is mechanically connected to an additional hydraulic machine to supply fluid to the accumulator. 8. The hydraulic drive system of claim 7, wherein the additional hydraulic machine is a variable capacity machine. 9. The hydraulic drive system of claim 8, wherein the capacity of the additional hydraulic machine is adjustable to absorb energy from the other machine and maintain a predetermined operating condition of the other machine. 10. The hydraulic drive system of claim 9, wherein the capacity of the additional machine is adjustable to deliver energy from the accumulator to the mechanical connection to maintain the operating condition . 11. The hydraulic drive system of claim 10, wherein said one machine, said other machine and said additional machine are all mechanically linked to allow energy to be transferred between them. 12. The hydraulic drive system of claim 11, wherein each of said machines is a reversible variable capacity machine. 13. The hydraulic drive system of claim 12, wherein an auxiliary machine is mechanically connected to the machine to provide an auxiliary servo. 14. The hydraulic drive system of claim 12, wherein the auxiliary servo is a transmission. 15. The hydraulic drive system of claim 10, wherein said transmission comprises a reversible variable capacity machine mechanically connected to said additional machine, and a hydraulic drive unit hydraulically connected to said machine. 16. The hydraulic drive system of claim 15, wherein the reversible variable capacity machine is pressure compensated to maintain pressure in a hydraulic connection between the machine and the hydraulic drive unit. 17. A hydraulic drive system as claimed in claim 16, wherein said drive unit is of variable capacity to vary the torque delivered at said predetermined pressure and said control varies its capacity. 18. The hydraulic drive system of claim 17, wherein the drive unit is reversible. 19. The hydraulic drive system of claim 18, wherein a plurality of drive units are connected to the reversible variable capacity machine. 20. The hydraulic drive system of claim 19, wherein each of said drive units provides a rotational output, and said control monitors the relative rotational speeds of said outputs and adjusts said capacity to move said output to said The speed is maintained at the defined ratio. 21. A hydraulic drive system as claimed in claim 20, wherein the output speed is reduced by reducing the capacity of the corresponding drive unit to reduce the torque produced. 22. The hydraulic drive system of claim 20, wherein said ratio is varied by said control. 23. The hydraulic drive system of claim 22, wherein said ratio is varied by an external input to said control. 24. The hydraulic drive system of claim 18, wherein the drive unit is reversible by applying an external output to the control. 25. The hydraulic drive system of claim 1, wherein a hydraulic motor is used to vary the volume of said machine by means of a valve responsive to a signal representative of the difference between said predetermined pressure and the pressure in said chamber. control the motor. 26. The hydraulic drive system of claim 25, wherein said valve applies feedback representative of a condition of said valve to modify said signal. 27. The hydraulic drive system of claim 25, wherein said control modifies said signal as said pressure increases and approaches said predetermined pressure. 28. The hydraulic drive system of claim 27, wherein said control modifies said signal to reduce the response of said valve. 29. The hydraulic drive system of claim 28, wherein the machine is a rotary machine and the control monitors the position of the swash plate and adjusts the modification of the signal accordingly. 30. The hydraulic drive system of claim 29, wherein onset of said modification occurs at a larger difference as said velocity increases. 31. The hydraulic drive system of claim 28, wherein said machine is a swash plate rotating machine and said control monitors swash plate position and adjusts modification of said signal accordingly. 32. The hydraulic drive system of claim 31, wherein onset of said modification occurs at a greater difference as said swash plate position increases. 33. A hydraulic transmission comprising a first variable capacity hydraulic machine driven by a main element, at least one variable capacity hydraulic drive unit, hydraulic conduits connected to said first machine and said drive unit, pressure compensating controls, and an external control operable on said first machine to vary its capacity and maintain a predetermined pressure in said conduit, and an external control operable on said drive unit and to vary its capacity to vary the torque. 34. The hydraulic drive system of claim 33, wherein said first machine and drive unit are reversible and said external control is operable to vary the capacity of said unit: from Conditions are changed to those delivering fluid to the conduit, the pressure compensating control thereby causing the first machine to reverse the mode of operation to maintain the predetermined pressure. 35. The hydraulic drive system of claim 34, wherein said first machine is mechanically connected to an additional variable capacity machine to provide fluid to the accumulator. 36. The hydraulic drive system of claim 35, wherein the capacity of the additional hydraulic machine is adjustable to absorb energy from the other machine and maintain a predetermined operating condition of the other machine. 37. The hydraulic drive system of claim 36, wherein the capacity of the additional machine is adjustable to deliver power from the accumulator to the mechanical connection to maintain the operating condition. 38. The hydraulic drive system of claim 34, wherein a plurality of drive units are connected to the conduit, and wherein each of the drive units provides a rotational output and the control monitors the relative rotational speed of the outputs and adjusts the the capacity to maintain the speed of the output at a defined ratio.

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