Pumps – Condition responsive control of drive transmission or pump... – Adjustable cam or linkage
Reexamination Certificate
1999-08-26
2001-09-04
Walberg, Teresa (Department: 3742)
Pumps
Condition responsive control of drive transmission or pump...
Adjustable cam or linkage
C091S506000, C092S012200
Reexamination Certificate
active
06283721
ABSTRACT:
BACKGROUND OF THE INVENTION
Variable hydrostatic axial piston machines of swash plate design have a cylinder block and a swash plate whose angle, and thus whose flow of hydraulic fluid, can be varied by means of a servo-piston to which actuating pressure can be applied. Typically, the servo-piston is hydraulically connected to an electrically driven valve. The valve is driven electrically, for which purpose it is customary to use proportional solenoids which are designed to act directly or as so-called nozzle-flapper valves which are designed to act as a pilot control. A version with pilot control is described in U.S. Pat. No. 5,205,201. In the case of variable displacement pumps, this is usually an angle of 0 degrees, whereas in the case of variable displacement motors it is either the maximum or minimum angle.
For special applications, in particular in the case of drive systems with variable displacement motors connected to fast driving vehicles, a good provision is not made in existing equipment to maintain the instantaneous angle in the case of failure of the electronic system or the electronic connection to the adjusting device.
An adjusting device with a stepper motor for a hydrostatic axial piston machine is described in DE 196 08 228 A1. In this known system, a rotary slide valve is operationally connected to a stepper motor. In this known system, the angle is fed back via levers to a rotary sleeve. The disadvantage with this system is that it is necessary to provide a complicated connection, which is free from play to the greatest possible extent, between the swash plate and the control sleeve, as well as to provide a gear between the control slide valve and the stepper motor, since the resolution of the stepper motor is insufficient. This means that the known system is of complicated design and expensive.
Furthermore, U.S. Pat. No. 4,290,447 describes an electrohydraulically proportional valve with an actuator comprising a linear power motor, a valve piston and an axially displaceable valve sleeve. Proportionality between current and force is characteristic of the linear power motor. Providing a spring in the linear motor means that the forces of the motor are in equilibrium with the spring force, as a result of which the force is proportional to the displacement. The tolerances of the springs and the tolerances of the magnetic air gaps are disadvantageous for these known systems. Moreover, the displacement of the linear power motor depends strongly on the friction of the valve piston, on the friction of the fit between the slide valve and the bore, as well as on the pollution in the gap. Furthermore, the magnetic hysteresis of the linear power motor influences the operational performance of such known axial piston machines.
Therefore, it is the object of the invention to create an adjustable hydrostatic axial piston machine in which the adjusting mechanism for the angle is of simple design and operates free from hysteresis to the greatest possible extent in an exactly reproducible fashion over the entire operating range.
SUMMARY OF THE INVENTION
A servo-piston to which actuating pressure can be applied can be used to vary the angle of a swash plate type of bent axis type hydrostatic axial piston machine. The servo-piston is connected to an electrohydraulic control valve which has a control piston and a control sleeve which can be moved axially relative to one another where the control valve implements an electrohydraulic proportional control system. The control piston is connected to a stepper motor so that the control piston and the control sleeve can be displaced axially relative to one another. The stepper motor is driven electronically. The control sleeve is mechanically coupled to the adjusting mechanism for the angle for the purpose of feeding back the swash angle.
In accordance with the preferred embodiment, the servo-piston is mechanically connected to a cylinder block of the axial piston machine via a spindle and a valve segment. The control piston/control sleeve assembly is supported in this case on the servo-piston which, for its part, is operationally connected to the spindle, which produces the connection to the cylinder block of the axial piston machine via a valve segment.
In accordance with a further aspect of the invention, in the case of the adjustable hydrostatic axial piston machine of bent axis design or swash plate design the angle thereof can be varied or adjusted by means of a servo-piston to which an actuating pressure can be applied. For its part, the servo-piston is connected to an electrohydraulic control valve whose control piston and control sleeve can be moved axially relative to one another. According to the invention, the control piston can be adjusted axially by means of a stepper motor, and the control sleeve is supported in a fashion loaded by fluid pressure on an adjusting mechanism for the swash angle, in order to feed back the angle, and executes an axial relative movement in the event of a change in the swash angle. A fluid pressure, preferably pressurized hydraulic oil, acts in this case on the control sleeve such that the latter is supported on the adjusting mechanism for the swash angle. Supporting the control sleeve on the adjusting mechanism for the swash angle means that the swash angle is fed back and that in the event of a change in the swash angle the control sleeve executes an axial stroke movement. However, it is also possible to apply a spring to the control sleeve.
It is preferable to use a yoke as the adjusting mechanism for the angle. The advantage of implementing the feedback of the swash angle by means of a yoke resides, in particular, in applications which require very large ranges of angles of adjustment such as, for example, ±45 degrees.
In accordance with a further embodiment, the adjusting mechanism for the angle is a valve segment connected to a cylinder block. The flow of hydraulic oil, which depends on the angle, to the axial pistons in the cylinder block of the axial piston machine (axial piston motor) and/or the hydraulic oil returning from the cylinder block (axial piston pump) are realized through the valve segment. The valve segment preferably has an inclined surface by means of which a stroke movement of the control sleeve is performed for the purpose of feeding back the position of the angle. This inclined surface represents a particularly simple and reliable design which permits an exactly reproducible adjustment, free from hysteresis to the greatest possible extent, of the angle by means of the adjusting mechanism for the angle, in conjunction with a simple design. The adjusting device with the valve segment permits the swash angle to be adjusted in an absolute fashion up to approximately 30° max.
In accordance with yet another embodiment of the invention, the adjusting mechanism for the angle is a swash plate. This is a position control with displacement feedback and driven by means of the stepper motor for the adjustable axial piston machine in swash plate design.
In a way known per se, the stepper motor has a rotor which preferably acts directly on a motion transmitting screw thread in such a way that when the stepper motor rotor rotates it is possible to generate axial movement which can be transmitted to the control piston. The electronic drive of the stepper motor in the form of steps, which can be performed very accurately and very precisely, thereby also effects a very precise, exactly reproducible axial movement of the control piston, which controls the adjusting mechanism for the swash angle so as to set desired and/or required angles exactly.
The control sleeve is preferably supported in a spring-loaded or pressure-loaded fashion on the adjusting mechanism for the swash angle in such a way that the control sleeve executes an axial stroke movement when the swash angle changes. That is to say, the control sleeve and control piston can be moved relative to one another and axially in each case.
The control piston is preferably designed in such a way and co-operates with the cont
Sauer-Danfoss Inc.
Van Quang
Walberg Teresa
Zarley McKee Thomte Voorhees & Sease
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