Production of hydrostatic axial piston machines by means of stepper motors

A description is given of an adjustable hydrostatic axial piston machine whose angle can be varied by means of a servo-piston (9) to which actuating pressure can be applied and which is connected to an electrohydraulic control valve. The control valve has a control piston (15) and a control sleeve (22) which can be moved axially relative to one another. The control piston (15) can be axially adjusted with a stepper motor (10), and the control sleeve (22) is coupled mechanically or hydraulically to an adjusting mechanism (9, 7) for the angle for the purpose of feeding back the angle.

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.degree. 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 control sleeve in the case of axial displacement in such a way
 that control edges present on the control piston uncover openings in the
 control sleeve for a flow of oil to the servo-piston, as a result of which
 the position of the servo-piston, and thus of the swash angle, is changed
 until the control edges are closed again by the movement of the control
 sleeve in the same direction. This provides the desired proportionality
 between an electronically prescribed step of the stepper motor and the
 change in the swash angle. The control piston is preferably supported on
 the stepper motor in a spring-loaded or pressure-loaded fashion.
 The system of the invention exhibits a proportional behavior of the change
 in the swash angle and the electronically prescribed step of the stepper
 motor. This operating principle has substantial advantages with regard to
 the adaptability to today's axial piston machines of bent axis design.
 Substantial advantages consist, inter alia, in that there is no need for
 extensive changes to the mass-produced components of the respective axial
 piston machines. Rather, all that is required is a mechanical adaptation
 of the stepper motor to the endcap of the variable displacement motor.
 This renders it possible for known axial piston machines also to be easily
 retrofitted with the adjusting mechanism according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 FIG. 1 shows a cross-sectional view of an axial piston motor according to
 the invention in accordance with a first exemplary embodiment of the
 invention, in which the adjustment of the angle is implemented by driving
 by means of a stepper motor. This adjustment is an electrically
 proportional servo control.
 The adjustable axial piston motor is supplied by means of pressurized
 hydraulic oil which is led by a delivery pump (not shown) via appropriate
 connecting lines to the endcap 6 of the variable displacement motor. The
 hydraulic oil is connected via a valve segment 7 to axial pistons 2
 arranged in a cylinder block 4. A shaft 1 of the variable axial piston
 motor is mechanically connected to the axial pistons 2 of the rotating
 group. When pressurized hydraulic oil is applied to them, these axial
 pistons 2 exert a torque on the shaft 1 of the axial piston motor.
 The swash angle is varied by means of a servo-piston 9 which is connected
 to the cylinder block 4 via a spindle 8 and the valve segment 7. The
 position of the servo-piston 9, and thus the magnitude of the angle, is
 set by means of a control valve which has a control sleeve 22 and a
 control piston 15 which is also denoted as a control slide valve. A
 stepper motor 10 is driven via an electronic drive circuit such that it
 carries out a desired number of steps or also half steps depending on the
 design of the drive circuit. The stepper motor 10 has a rotor 11 which
 rotates in a fashion proportional to the number of steps (see FIG. 2).
 Depending on the type and design of the stepper motor, this is a range of
 60 degrees or 1.8 degrees per full step. The angle of rotation,
 corresponding to one step, of the rotor 11 of the stepper motor 10 depends
 on the design of the stepper motor respectively used. It is also possible
 to execute, for example, half steps or quarter steps by means of tailored
 drive sequences which are provided by the electronic system. At its output
 end, the rotor 11 of the stepper motor has a motion transmitting screw
 thread 12. A rod 13 is coupled in a rotationally secure fashion to the
 housing of the stepper motor 10 by means of a slot/key connection, and is
 designed as a nut 14 (FIG. 2). As a result, rotation of the rotor 11 of
 the stepper motor 10 produces an axial movement of the rod 13.
 FIG. 3 shows a detailed view in accordance with the embodiment of FIG. 1.
 The position control with displacement feedback for adjusting the angle of
 an axial piston motor is accomplished with feedback via a control sleeve.
 The control piston 15 is permanently pressed by a spring 16 against the
 ram 13 of the stepper motor. As a result, the control piston 15 is axially
 displaced when the rotor 11 of the stepper motor 10 rotates. Along its
 length, the control piston 15 has a plurality of control edges 17, 18, 19
 and 20. By means of the volumetric flow over the control edges, which
 cooperate with openings or channels in the control sleeve 22, the
 servo-piston 9 is displaced to control sleeve 22. The control sleeve 22 is
 pressed by means of a compression spring 23 against a cone of the
 servo-piston 9, and is displaced in the axial direction which is in the
 same direction as the control piston 15, and until the control edges 17,
 18, 19 and 20 reclose. The control sleeve 22 has a projection which is
 seated on the cone of the servo-piston 9. Axial displacement of the
 control piston 15 as a consequence of the rotary movement of the rotor 11
 of the stepper motor 10 displaces the servo-piston 9. The longitudinal
 axis of the servo-piston is essentially aligned perpendicular to the
 longitudinal axis of the control piston 15, along its longitudinal axis.
 As a result thereof, the adjusting mechanism for the swash angle is
 correspondingly acted upon to change the swash angle. Thus, this control
 valve, which is controlled by control edges, produces proportionality
 between a step of the stepper motor 10 and a change in the angle. This
 principle of the adjustment is termed electric position control with
 displacement feedback.
 In this arrangement, the control sleeve is preferably supported in a
 spring-loaded fashion on the adjusting mechanism for the swash angle in
 such a way that it executes a corresponding axial stroke movement when the
 swash angle changes. The control piston is supported in a spring-loaded
 fashion on a stepper motor or is connected via a spring to the output side
 of the stepper motor, and can thereby be adjusted in the axial direction.
 The supply of hydraulic oil to the servo-piston is controlled by means of
 the control piston so that the position of servo-piston, and thus of the
 swash angle, is varied until the supply of oil to the servo-piston is
 interrupted again by the movement of the control sleeve in the same
 direction. A substantial advantage of such a system consists in that it
 concerns an extremely simple, electrically proportional adjustment. In
 addition, there are substantial functional advantages by contrast with
 conventional systems such as elimination of hysteresis and temperature
 sensitivity, as well as an increase in precision.
 FIG. 4 represents a first alternate embodiment for adjusting the angle of
 an axial piston motor. Instead of the connection between the control
 sleeve 22 and the servo-piston 21 in accordance with the arrangement in
 FIGS. 1 to 3, a connection is provided between a control sleeve 22 and a
 valve segment 7. The principle of the functioning of the actual adjustment
 of the control piston 15 or control sleeve 22 corresponds to that
 described in connection with FIGS. 1 to 3, and so no further detail on
 this will be considered. In the embodiment of FIG. 4, an inclined surface
 24 is provided on a lateral surface of the valve segment 7. As the rotor
 11 of the stepper motor 10 rotates, this results in the production of a
 stroke movement of the control piston 15 and, subsequently, of the control
 sleeve 22, which is supported on the inclined surface 24 by means of a
 projection designed in a similar way to the arrangement of FIG. 3.
 A second embodiment of an adjusting mechanism for the angle designed as a
 yoke 45 and intended for an axial piston machine according to the
 invention is shown in FIG. 5. The feedback is implemented in this case via
 the yoke 45 of the adjusting system. Such an adjusting mechanism for the
 angle is useful and appropriate when there is a need for adjustment over
 very large angular ranges. For example, .+-.45 degrees. Thus, the desired
 adjustment can be implemented over large angular ranges via the yoke 45,
 to which a connecting element 44 is eccentrically pivoted with the control
 sleeve 22, via such a described crank mechanism. The actual functioning of
 the control valve, which has the control sleeve 22 and the control piston
 15 and is acted upon by the stepper motor 10, corresponds to that of the
 previously described exemplary embodiments, and will therefore not be
 described in more detail at this juncture. The control piston 15 is
 supported against a spring 16, while the control sleeve 22 is supported
 against a spring 23.
 A further third embodiment of the invention is represented in FIG. 6 in the
 form of position control with force feedback and driving by means of a
 stepper motor. The angle of this variable unit is transmitted to a spring
 26 via the connection 8 (see also FIG. 1). The spring force is indirectly
 proportional to the angle. A large angle signifies a small spring force,
 whereas a small angle signifies a large spring force. This force is
 transmitted onto a control piston 27, which is arranged in an axially
 movable fashion in a stationary control sleeve 30 and is provided with
 control edges (see FIG. 3), and is in equilibrium with the force of a
 further spring 28. In the event of a step by the stepper motor 10, as a
 result of which the ram 13 is moved in the axial direction, the spring
 force of the spring 28 changes. The control piston 27 is displaced, in
 order to maintain an equilibrium of forces between the two springs 26, 28.
 As a result, the control edges are opened with respect to the openings or
 channels provided in the control sleeve 30. This is attended by an
 adjustment of the servo-piston 25 and an increase in the spring forces 26
 and 28, until the control piston 27 reseals the openings in the control
 sleeve 30 by means of the control edges. In this system, the control
 sleeve 30 is fixed and cannot be displaced axially. Such a system likewise
 implements a proportional behavior between change in the angle of the
 axial piston motor and a step of the stepper motor 10. Particular
 advantages of this operating principle consist in the ease of adaptation
 to existing axial piston variable displacement motors of bent axis design.
 Consequently, mass-produced components of current axial piston motors
 require no substantial design changes or adaptation work. Only a
 mechanical adaptation of the stepper motor to the endcap 6 of the variable
 displacement motor is required.
 A fourth embodiment of the invention shown in FIG. 7 in which position
 control with displacement feedback and driving is accomplished by means of
 a stepper motor. It is shown in conjunction with an adjustable axial
 piston machine of a bent axis design. By contrast with the connection of
 the control sleeve to the servo-piston (see FIG. 1) or to the valve
 segment 7 (FIG. 4), a connection direct to the swash plate 29 is shown
 here. The control sleeve 36 is pressed against the flat surface of the
 swash plate 29 by a spring 34 via a ram 32. The control piston 33 is
 pressed against a ram 35 of the stepper motor 10 by a spring 31. The
 control sleeve 36 is hydraulically connected to adjusting pistons 37, 38,
 in order to produce an appropriate change in the swash angle of the swash
 plate 29. The cooperation between the stepper motor 10, the control piston
 33 and the control sleeve 36 is similar to that described in conjunction
 with the other embodiments (see FIGS. 1 to 3). As an alternative, the
 feedback of the swash angle can also be performed via the servo-piston 9,
 in a way similar to that represented in FIG. 1.
 A further embodiment of the invention is shown in FIG. 8. The functioning
 thereof is similar to that in FIG. 6. The control concept represented is
 electric position control of the adjustable axial piston motor with force
 feedback on the basis of a swash plate design 29. For this purpose, the
 control valve is connected on one side with respect to the stepper motor
 10, and on the other side with respect to the ram 32 by means of spring
 elements 40, 41.
 All the embodiments described for adjusting the angle are suitable equally
 for variable displacement motors and for variable displacement pumps of
 axial piston design. This holds for bent axis design and for swash plate
 design.