1. Technical Field
The disclosed embodiments are directed generally to fluid power systems employing over-center motors, and, in particular, to fail-safe operations that are configured to remove output torque from a fluid motor in the event of a malfunction in the associated system.
2. Description of the Related Art
In recent years, significant interest has been generated in hybrid vehicle technology as a way to improve fuel economy and reduce the environmental impact of the large number of vehicles in operation. The term hybrid is used in reference to vehicles employing two or more power sources to provide motive energy to the vehicle. For example, hybrid electric vehicles are currently available that employ an internal combustion engine and a generator, which generates electricity to be stored in a battery of storage cells. This stored power is then used, as necessary, to drive an electric motor coupled to the drive-train of the vehicle.
There is also interest in the development of hydraulic hybrid vehicles, due to the potential for greater fuel economy, and a lower environmental impact than hybrid electric vehicles. According to one configuration, a hydraulic hybrid vehicle employs an internal combustion engine (ICE) to drive a hydraulic pump, which pressurizes hydraulic fluid. The pressurized fluid is then either used to drive a hydraulic motor coupled to the drive wheels of the vehicle, or stored in an accumulator for later use.
There is a class of hydraulic machines that may be employed in hybrid operation that includes a rotating barrel having a plurality of cylinders, and pistons reciprocating within the cylinders. The barrel is configured to rotate over a valve plate having inlet and outlet ports. As the barrel rotates over the valve plate, fluid passes into, and out of, the cylinders of the barrel. In a hydraulic pump, fluid is drawn into each cylinder from a low-pressure inlet port and forced out of the cylinder to a high-pressure outlet port. In a hydraulic motor, fluid from a high-pressure inlet enters each cylinder in turn and vents to a low-pressure outlet. Some machines, commonly referred to as pump/motors, are configured to operate as pumps and as motors, according to how fluid is applied to the machine. To operate the device as a pump, a mechanical shaft is driven by a motive source such as an engine, causing the barrel to rotate so as to pump fluid from the low pressure port to the high pressure port. To operate the device as a motor, fluid is allowed to travel through the device from the high pressure port to the low pressure port, causing the barrel to rotate, and in turn rotating the mechanical shaft from which mechanical power may be taken.
One type of pump/motor is a bent-axis pump/motor. The term “bent-axis” refers to an angle between the rotational axis of the barrel and the rotational axis of the mechanical shaft, commonly known as the stroke angle. The stroke angle determines the amount of fluid displaced by the machine per revolution of the shaft or barrel, with a larger angle corresponding to a larger displacement. In a variable-displacement bent-axis pump/motor, the stroke angle can be adjusted so as to vary the displacement of the device while it is in operation. This allows the output of the device to be varied from a maximum output at a maximum stroke angle (perhaps 45 degrees), to zero output at a zero stroke angle. Several methods are commonly employed to vary the stroke angle. In some devices, the barrel is carried on a back plate structure that slides along a bearing surface that defines the arc of angular travel and provides access to ports by which fluid enters and exits the barrel. In others, the barrel is carried on a structure known as a yoke, which defines the arc of angular travel by pivoting about a central trunnion, and carries fluid to the barrel via fluid ports originating in the trunnion and extending along one or two yoke legs to the barrel.
In what is commonly known as an over-center pump/motor, the stroke angle may be further stroked past the zero stroke angle into an angular range generally referred to as a negative-stroke angle. These pump/motors therefore have two distinct angular ranges, a positive-angle range and a negative-angle range, one of which will, by the specific configuration of the hydraulic circuit, correspond to a pump mode and the other to a motor mode. Over-center pump/motors can thereby act as drive motors or as engine pumps on a hydraulic hybrid vehicle. In the role of a drive motor, an over-center pump/motor will operate as a motor to drive the vehicle and as a pump to perform regenerative braking. As an engine pump, it operates primarily as a pump driven by the engine, but can also operate as a motor to start the engine.
Over-center pump/motors have several advantages over pump/motors that are restricted to only a positive angle. In particular, fluid switching is much simpler in comparison to a positive-angle pump/motor. In order to reverse the torque of an over-center motor, it is merely necessary to stroke from a positive angle to a negative angle, or vice-versa, while in a positive-angle motor, the polarity of the fluid ports must be reversed, which requires a fluid switching valve (commonly known as a mode valve) capable of high-speed switching of high-pressure fluids at very high flow rates. Such valves are a significant source of pressure drop in fluid supply due to the convoluted channels the fluid must pass through. They may also create undesirable noise when switched from one mode to another.
It will be recognized that, in most fluid power systems employing variable displacement machines, it is important that the motor be easily returnable to a zero-displacement condition in order to remove torque from the output shaft of the motor. Of course, in a positive-angle motor, the zero-stroke angle position is easily placed at one extreme of the range of motion of the machine; thus it only requires that an actuator controlling the angle of the motor be charged to move toward that extreme as far as possible. When the motor will not travel further, it is at zero. On the other hand, in the case of an over-center motor, such an action would drive the motor to its maximum displacement in either a positive-angle or negative-angle direction, rather than to a displacement of zero, which lies in between. Instead, to reach the relatively indefinite zero-displacement angle, the stroke angle must be controlled quite accurately, and deliberately held in this position once it is attained. Therefore, with an over-center pump/motor, reaching a zero displacement position with sufficient speed and precision inherently requires a greater degree of control.
In the design of hybrid vehicle systems, the safety of the occupants of the vehicle is of significant concern. It must be assumed that, over the lifetime of the vehicle, there will be malfunctions in the mechanical and electrical systems. It is therefore desirable to minimize the potential danger associated with such malfunctions.
Of particular concern is a loss of control over displacement of a pump/motor. Because the displacement determines the power being transmitted by the device, a loss of control over displacement could have the effect of accelerating or braking the vehicle in an uncontrolled manner, potentially causing harm to the occupants and/or to the vehicle and its components.
In a vehicular application, it is commonly known in the art to control the displacement of pump/motors by means of an electronic vehicle controller that issues electronic displacement commands to a displacement control. The displacement control commonly includes a fluid switching valve operated by one or more solenoids that respond to the electronic commands from the vehicle controller. The switching valve then actuates the displacement by directing hydraulic fluid to one or more hydraulic displacement actuators, which then would mechanically stroke the angle of the pump/motor toward the desired displacement.
Therefore it can be seen that a loss of control over displacement could be caused by an electronic control failure, or by a physical failure such as a hydraulic or mechanical malfunction.
An electronic control failure can result from a loss of electrical power or loss of electronic command signals. For example, if the vehicle controller were to lose electrical power and was no longer issuing commands to the displacement control, or if the circuit relaying the commands were to fail, or if the electrical coil of a solenoid controlling the fluid switching valve were to fail, then control over displacement would be lost. One potential provision for such an event might be to provide for the displacement control to take on a default position actuating the pump/motors to a default mode and displacement setting which the system has been designed to accommodate in a safe and stable manner. Clearly, a zero displacement position is a desirable default position because it removes the capability of transmitting torque. However, because the zero displacement position of an over-center pump/motor is mechanically indefinite, there may be some concern about the reliability of reaching this position, given that one form of failure has already occurred. For this reason it is also appropriate to consider an alternative default position that is more mechanically definite and perhaps more reliably attained. In the case of an over-center pump/motor, this mechanically definite position can only be a maximum displacement at the extreme of either the positive-stroke or negative-stroke range, that is, a maximum displacement in either pumping or motoring mode. While it may be counterintuitive to suggest that a maximum-power position be commanded in the case of a failure, it is possible to render even this situation safe by providing for appropriate measures in the design of the circuit so that it operates in a safe and stable manner in this condition.
A physical failure is an even more fundamental concern, as it would result in total loss of control over displacement, even if the electronic controller and electrical command circuits were fully functional. For example, if the yoke of a pump/motor were to become physically immobilized by a particle of debris interfering with the yoke pivot joint, then no electronic command, nor the default actuation measure described above, would have any effect on the displacement. The pump/motor would remain frozen at whatever displacement it is at, and accordingly would continue to transmit power. Similarly, if the fluid switching valve controlling a displacement actuator were to freeze due to a particle of grit in the valve, the pump/motor would either remain hydraulically locked at its current displacement (if the valve froze in a locking position), or stroke to maximum displacement (if the valve froze in a stroking position). The inability to change displacement would either cause the vehicle to continue accelerating (if the failure occurs in a drive pump/motor while in motor mode), or brake rapidly to a stop (if it is a drive pump/motor in pump mode for regenerative braking). Either behavior would present an unacceptable safety hazard. Therefore it is important to provide for safe behavior of the vehicle even in the case where no control over displacement can be exerted and no default displacement can be attained.
In common practice, it is known to address these and similar concerns by providing each pump/motor with an isolating means by which it may be hydraulically isolated from the rest of the circuit if it becomes unresponsive to control. For example, this isolating feature might be built into the mode valve of a positive-angle pump/motor. This valve also provides a convenient way to isolate the device in case of other types of failure, such as a blow-off failure event in which the case of the pump/motor is at risk of over-pressurization due to the cylinder barrel momentarily losing its seating and allowing high pressure fluid to escape into the case. However, over-center pump/motors do not require a mode valve for mode switching, meaning that this isolation function would require the addition of a dedicated valve. Providing such a valve for each pump/motor adds to the overall cost of the system, reduces its efficiency by presenting additional flow restrictions, and increases the number of controls that the controller must manage. To reduce the cost and complexity of the system it would be preferable to omit individual isolating valves and instead rely on shutoff of the high pressure fluid source, and possibly also the low pressure reservoir, in order to de-energize the entire system when control over any pump/motor has failed.
In addition to providing for failsafe operation, there are also safety considerations related to shutting down and powering up a vehicle that utilizes over-center pump/motors. On shutdown, it is preferable that all pump/motors should be actuated to zero displacement and the high pressure accumulator hydrostatically disconnected from the circuit. While the system thus resides in a depressurized state, it is conceivable that the yoke of a pump/motor may move away from the zero position to a positive or negative angle, due to the action of gravity on the yoke, or due to any movement of the vehicle while it is shut down (for example, if the vehicle is jostled by another vehicle while parked, or if the vehicle has been towed). Therefore on startup, high pressure must only be restored to the circuit if it is certain that the drive pump/motor has remained at zero displacement; otherwise the vehicle may begin to accelerate unexpectedly when pressure is restored, or the engine pump may begin to turn the engine. While a parking pawl and a parking brake may be provided to help prevent the vehicle from moving in such a case, a better solution would ensure that each pump/motor is actually at zero displacement before restoring high pressure to the circuit; and if not, to restore it to that position. For this reason the procedure for starting up and shutting down such a vehicle is an important factor in its safety.
Additionally, the interest of minimizing the cost of a hydraulic hybrid vehicle suggests that the displacement control should be as simple as possible. By defaulting to a mechanically limited, extreme angular position rather than to a central position, the control valve may be configured with fewer control ports, and the actuator cylinders may have a simpler design. Additionally, the elimination of individual isolating valves on each pump/motor reduces cost and reduces the number of components that must be controlled by the controller.
In applicant's co-pending patent application Ser. No. 11/540,089, entitled SAFE OVER-CENTER PUMP/MOTOR, various embodiments are directed to fail-safe devices and systems that are configured to automatically command an over-center pump/motor to zero displacement and/or shut off the high-pressure fluid supply to the pump/motor in the case of a malfunction. The present invention includes several additional and alternative approaches oriented toward similar goals.
Embodiments of the invention are directed toward (a) providing for safe response to both electronic and physical loss of displacement control in a hydraulic hybrid vehicle, and (b) safe shutdown and startup procedures for such a vehicle.
It is noted that many of the disclosed provisions are also effective at addressing a blow-off failure mode, in which the cylinder barrel of a pump/motor has become unseated causing high-pressure fluid to escape into the pump/motor case. In this situation, catastrophic over-pressurization of the pump/motor case can be prevented by the measures disclosed herein.