Patent Description:
An axial piston pump generally comprises a plurality of pistons arranged within a cylinder block. The cylinder block may be driven to rotate about its axis by a shaft, which is typically connected to an internal combustion engine, or other mechanical drive means.

A diagram of an axial piston pump known in the art is shown in <FIG>. The axial piston pump comprises a plurality of pistons <NUM> which are located in a circular array within a piston barrel <NUM>. The pistons <NUM> are rotated around a longitudinal axis by rotational shaft <NUM> which is located at a longitudinal centre of the piston barrel <NUM>.

Each piston <NUM> is connected to the swash plate <NUM> via a connector, typically a ball and socket joint. The swash plate <NUM> is moveable about a pivot point such that the angle of inclination of the swash plate <NUM> can be varied. In <FIG>, the angle of inclination of the swashplate is <NUM>° such that the axial piston pump has zero displacement. The angle of inclination of the swash plate <NUM> is controlled by some form of actuator, for example a servo piston <NUM>.

The pistons <NUM> within the piston barrel <NUM> are arranged to bear against a swashplate.

The variable displacement of the cylinders within the piston barrel <NUM> is typically provided by variation in an angle of a swash plate. The angle of the swash plate may be controlled by a solenoid valve, which in turn controls the displacement of the cylinders.

As the piston barrel <NUM> rotates, the pistons reciprocate within the piston barrel <NUM>. A valve plate provided on the opposite end of the piston barrel <NUM> to the swashplate defines an at least one inlet <NUM> and at least one outlet <NUM> for fluid being pumped through the axial piston pump.

In some known axial piston pumps, the rotational position of the valve plate inlets <NUM> and outlets <NUM> may be adjusted by providing an adjustable valve plate. An example of the adjustment of an adjustable valve plate using timing screws <NUM>, <NUM> is shown in <FIG>. As shown in <FIG>, the timing screws <NUM>, <NUM> may be used to rotate the position of each of the valve plate inlets <NUM> and outlets <NUM> about an axis at the centre of the adjustable valve pate. Such adjustments to the rotational positon of the adjustable valve plate affects the timing of the pump, that is the point within the rotational cycle of the pistons where hydraulic fluid is being drawn into the pistons, and also the point at which hydraulic fluid is being expelled from the pistons.

The change in timing brought by adjustment of an adjustable valve plate in turn affects the "stiffness" of the axial piston pump. The stiffness of the axial piston pump reflects the relationship between the pump displacement and the output pressure. Axial piston pumps with increased stiffness require a greater pressure to destroke the pump. By adjusting the timing of an axial piston pump (via an adjustable valve plate) the stiffness of the axial piston pump can be calibrated mechanically.

Against this background, the present disclosure aims to provide an improved, or at least commercially relevant alternative axial piston pump or axial piston pump controller.

<CIT> discloses a system for estimating a displacement of a pump. Pump stiffness control map and adjustment factor are not mentioned in this document.

<CIT> (D2) discloses a dielectric sensor arrangement and method for swashplate angular position detection.

<CIT> (D3) discloses a discharge capacity control system for a variable capacity compressor.

According to the invention, which is defined by the appended claims, an axial piston pump controller for an axial piston pump having a fixed valve plate and a variable displacement is provided. The axial piston pump controller is configured to:.

The controller is configured to control an axial piston pump having a fixed valve plate. The controller calculates a pump stiffness adjustment factor which is used to modify the nominal value for the pump displacement current determined based on the pump rotational speed. In effect, the pump stiffness adjustment factor can increase or decrease the stiffness by increasing or decreasing the pump displacement control current output with respect to the nominal value calculated based on the pump rotational speed. Thus, rather than determining the pump displacement control current using a one dimensional control map based on engine speed, the controller uses a three dimensional control strategy (pump rotational speed, pump output pressure, and pump displacement). The effect of changing the stiffness of the axial piston pump is similar to the effect achieved by adjusting the timing of the pump based on the positon of an adjustable valve plate. Accordingly, the controller allows an axial piston pump having a fixed valve plate to be controlled as if it had an adjustable stiffness similar to an axial piston pump having a variable-position valve plate.

According to a second aspect of the disclosure an axial piston pump having a fixed valve plate and a variable displacement is provided. The axial piston pump comprises:.

The axial piston pump of the second aspect has a fixed valve plate. The controller of the axial piston pump includes a control map which can be used calculate a pump stiffness adjustment factor in order to effectively the stiffness of the axial piston pump. Accordingly, the axial piston pump of the second aspect can be controlled as if it had an adjustable stiffness similar to an axial piston pump having a variable-position valve plate. In contrast to an axial piston pump with a variable-position valve plate, the axial piston pump of the second aspect has an adjustable stiffness that does not require any mechanical adjustment of the axial piston pump.

A specific embodiment of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which:.

According to an embodiment of the disclosure, an axial piston pump is provided. A schematic diagram of the axial piston pump <NUM> is shown in <FIG>. As shown in <FIG>, the axial piston pump <NUM> comprises a housing <NUM>, a servo piston <NUM>, a piston barrel <NUM>, a piston barrel housing <NUM>, a pump control valve <NUM>, a fixed valve plate <NUM>, a pump head <NUM>, a rotational shaft <NUM>, and a swashplate <NUM>, a plurality of pistons <NUM>, and a controller.

The axial piston pump <NUM> shown in <FIG> may be a non-feedback axial piston pump. In this context, non-feedback refers to the absence of mechanical feedback which may be configured to mechanically feedback changes in the output pressure to the control of the axial piston pump displacement.

The axial piston pump <NUM> may be installed in a closed-loop hydraulic system. As such, the hydraulic fluid pumped through the axial piston pump <NUM> is pumped though a closed circuit (ignoring any hydraulic fluid losses or leakages from the closed loop) and essentially returns back to the axial piston pump <NUM>.

The plurality of pistons <NUM> of the axial piston pump <NUM> are located in a circular array within the piston barrel <NUM>. The pistons <NUM> may be spaced at equal intervals about the rotational shaft <NUM> which is located at a longitudinal centre of the piston barrel <NUM>. The piston barrel <NUM> is compressed against the fixed valve plate <NUM> by a spring <NUM>. The spring <NUM> is shown in a cut-away portion of <FIG>.

Each piston <NUM> is connected to the swash plate <NUM> via a connector, typically a ball and socket joint. The swash plate <NUM> is moveable about a pivot point such that the angle of inclination of the swash plate <NUM> can be varied. In <FIG>, the angle of inclination of the swashplate is <NUM>° such that the axial piston pump has zero displacement. The angle of inclination of the swash plate <NUM> is controlled by the servo piston <NUM>, as discussed further below.

The fixed valve plate <NUM> comprises at least one arcuate inlet port (not shown) and at least one arcuate outlet port (not shown). For example, the fixed valve plate <NUM> may be provided with similar inlet and outlet ports to the valve plate shown in <FIG> (although the valve plate <NUM> of <FIG> does not include rotational adjustment features). The arcuate inlet port is configured to receive hydraulic fluid at a relatively low pressure. Hydraulic fluid is discharged from the pistons <NUM> at a relative high pressure through the arcuate outlet port.

During operation of the axial piston pump <NUM>, the piston barrel <NUM> rotates so that each piston <NUM> periodically passes over the each of the arcuate inlet port and the arcuate outlet port of the fixed valve plate <NUM>. The rotation of the piston barrel is driven by rotation of the rotation shaft <NUM>, which in turn may be connected to a source of motive power. For example, in the embodiment of <FIG>, the rotation shaft may be driven (rotated) by an internal combustion engine or an electric motor connected to a battery.

The angle of inclination of the swash plate <NUM> causes the pistons to undergo an oscillatory displacement in and out of the cylinder block, thus drawing the hydraulic fluid into the arcuate inlet port and subsequently expelling the hydraulic fluid out of the arcuate outlet port. The volume of hydraulic fluid expelled is related to the magnitude of the angle of inclination of the swash plate <NUM>. For small angles of inclination, the stroke of each piston <NUM> is relatively small, and thus the volume of hydraulic fluid discharged is relatively low. As the angle of inclination increases, the piston stroke increases, thus increasing the volume of hydraulic fluid expelled.

The angle of inclination of the swash plate <NUM> is controlled by a servo piston <NUM>. The servo piston <NUM> is configured to control the flow of hydraulic fluid for biasing the angle of inclination of the swash plate <NUM>. The flow of hydraulic fluid is proportional to the degree the servo piston <NUM> is opened. As such, the angle of inclination of the swash plate <NUM> is controlled based on the degree of opening of the servo piston <NUM>.

The degree of opening of the servo piston <NUM> is in turn controlled by pump control valve <NUM>. Pump control valve <NUM> comprises a solenoid actuator (not shown). The solenoid actuator controls a pilot pressure which in turn is used to control the degree of opening of the servo piston <NUM>. As such, a pump displacement control current supplied to the solenoid actuator of the pump control valve <NUM> controls the angle of inclination of the swash plate <NUM>, and thus the displacement of the axial piston pump.

The skilled person will appreciate that electro-hydraulic actuators for controlling the position of a swash plate <NUM> are well known to the skilled person. Accordingly, the skilled person will appreciate that the present disclosure may be applied to any axial piston pump having an electro-hydraulic actuator configured to control the variable displacement of the axial piston pump <NUM>.

The solenoid actuator of the pump control valve <NUM> is controlled by controller <NUM> which is configured to supply a pump displacement control current to the pump control valve <NUM>. The controller <NUM> may be a dedicated processor configured to perform the control scheme discussed below. In some embodiments, the controller <NUM> of this disclosure may be combined with other control functions. For example, an engine control unit (ECU) of a hydraulic machine may be used to provide the controller <NUM> according to this disclosure. As such, the controller <NUM> may be provided separately (i.e. not directly mounted on or incorporated into) from the axial piston pump <NUM>.

<FIG> shows a block diagram of a controller <NUM> according to an embodiment of the disclosure. As shown in <FIG>, the controller comprises a nominal current calculation module <NUM> that is configured to calculate a nominal pump current. The nominal pump current is calculated based on the pump rotational speed (i.e. the rotational speed of the rotation shaft <NUM>). In some embodiments, the controller may obtain this value, or a value representative of this value from the source of motive power that is connected to the rotation shaft <NUM>. For example, in some embodiments, the controller <NUM> may determine the pump rotational speed from the engine speed of an internal combustion engine that is driving the rotation shaft <NUM>. In some embodiments, the pump rotational speed may be in the range of about <NUM> revolutions per minute (rpm) to about <NUM> rpm.

In some axial piston pumps known in the art, the pump displacement control current provided to the axial piston pump is, essentially, the nominal pump current. That is to say, it is known in the art to calculate the pump displacement control current based on the pump rotational speed driving the pump. This calculation is typically performed using a one dimensional control map which provides a nominal pump current for different pump rotational speeds.

The controller according to the embodiment of <FIG> also calculates a pump stiffness adjustment factor. The pump stiffness adjustment factor, in combination with the nominal pump current value, is used by the controller to calculate the pump displacement control current. As such, the controller <NUM> according to the embodiment of <FIG> utilises further information of the operation of the axial piston pump in order to adjust the pump displacement control current provided to the axial piston pump. Specifically, the pump stiffness adjustment factor provides a means for the controller to effectively change the stiffness of the axial piston pump in response to a change in the displacement or pressure of the axial piston pump whilst operating at a constant engine speed.

<FIG> shows a graph of which shows the relationship between the pump displacement control current and the resulting percentage pump displacement of the axial piston pump (wherein <NUM> % pump displacement is a swash plate angle of inclination of <NUM>° and <NUM> % pump displacement is the maximum angle of inclination). As shown in <FIG>, a plurality of lines are shown representing the relationship at different constant pump output pressures under a constant pump rotational speed (e.g. <NUM> rpm).

It will be appreciated from <FIG> that for axial piston pumps operating with a pump displacement control current controlled based only on pump rotational speed, the displacement of the pump will vary depending on the output pressure of the axial piston pump. For example, for a pump displacement current of <NUM> mA and a pump output pressure of <NUM> bar, the axial piston pump will have a displacement of about <NUM> %. In the event that the output pressure of the axial piston pump increases to about <NUM> bar with no change in pump rotational speed and thus no change in pump displacement current, the axial piston pump would destroke itself to a displacement of about <NUM> %.

According to the embodiment of <FIG>, the pump stiffness adjustment control map provides a pump stiffness adjustment factor which effectively increases the stiffness of the pump in response to such a change in pressure for a fixed engine speed. That is to say, the pump stiffness adjustment factor can increase the pump displacement control current in response to an increase in system pressure to try to reduce or prevent the pump from destroking in response to an increase in output pressure.

As shown in <FIG>, the pump stiffness control map has inputs: output pressure and pump displacement. The output pressure of the axial piston pump may be measured using a pressure sensor located at, or proximal to, arcuate outlet port of the axial piston pump. The pump displacement may, in some embodiments, be measured using a dedicated sensor (e.g. a sensor configured to determine the angle of inclination of the swashplate), or may be estimated using a pump displacement estimation module as shown in the embodiment of <FIG>. As such, the controller of <FIG> is configured to adjust the nominal pump current based on the pump stiffness adjustment factor in order to determine the pump displacement control current to be provided to the axial piston pump.

In the embodiment of <FIG>, the displacement of the pump (percentage displacement) is estimated using the pump displacement estimation module. The pump displacement estimation module may be configured to estimate the pump displacement based on a relationship between the hydraulic fluid volume output by the axial piston pump and the output of a motor driven by the axial piston pump.

In such a case, the pump displacement (DP), pump rotational speed (SP), motor rotational speed (SM) and motor displacement (DM) are related by the following equation: <MAT>.

The motor rotational speed SM can be measured using a suitable sensor, the output of which is provided to the controller <NUM>. The pump rotational speed SP may also be measured and provided to the controller <NUM>. The motor displacement can be inferred from the motor speed based on a calibration of the motor at a range of different motor speeds. As such, a control map for estimating the pump displacement can be generated having as inputs: motor rotational speed and pump rotational speed which allows the pump displacement to be estimated. The estimated pump displacement can then be provided to the pump stiffness control map in order to determine the pump stiffness adjustment factor.

A graph showing the effect of the pump stiffness adjustment factor is shown in <FIG>. The solid black line in <FIG> shows the relationship between the pump output pressure and the percentage pump displacement were the axial piston pump to be controlled based on the nominal current only (i.e. a fixed current of <NUM> mA) when operating at a constant pump rotational speed. As shown in <FIG>, once the pump output pressure exceeds about <NUM> bar, the force of the output pressure on the swashplate <NUM> causes the axial piston pump to destroke, reducing the percentage pump displacement.

The dashed line in <FIG> shows the effect of the pump stiffness adjustment factor on the resulting percentage pump displacement. As the output pressure increases above <NUM> bar, the pump stiffness adjustment factor can act to increase the amount of pump displacement current supplied to the axial piston pump, effectively increasing the stiffness of the swash plate in order to prevent the pump from destroking.

An example of a pump stiffness control map is shown in <FIG>.

In some embodiments, a single pump stiffness control map may be provided separately from the calculation of the nominal pump current based on the engine speed. As such, the pump stiffness adjustment to the pump displacement control current may be applied independently of the pump rotation speed. In some embodiments, the pump stiffness adjustment to the pump displacement current may also be dependent on pump rotation speed. As such, in some embodiments, a plurality of pump stiffness control maps may be provided. Each of the plurality of pump stiffness control maps may provide a map of values for the pump stiffness adjustment factor at a respective pump rotation speed. The controller <NUM> may be configured to select one of the pump stiffness control maps for calculating the pump stiffness adjustment factor based on the engine speed.

<FIG> shows an example of a further pump stiffness control map for a pump rotational speed of <NUM> rpm. As such, the pump stiffness control map of <FIG>(which is provided for a pump rotational speed of <NUM> rpm) and the pump stiffness control map of <FIG> may form a plurality of pump stiffness control maps. In other embodiments, at least three, five, or seven pump stiffness control maps may be provided across a range of operation pump rotational speeds. It will be appreciated that where any of the inputs: pump rotational speed, pump output pressure, or pump displacement percentage falls between values shown in the control maps, the controller may select the nearest value for use, or may use interpolation between the nearest points on the control map in order to calculate the pump stiffness adjustment factor.

<FIG> shows a graph of the variation in the relationship between pump displacement control current and pump displacement at <NUM> bar for different pump rotational speeds.

While the embodiment of <FIG> shows separate calculations for the nominal pump current and the pump adjustment factor, in some embodiments where the input to the plurality of pump displacement control maps includes the pump rotational speed, the calculation of the nominal pump current may be combined with the calculation of the pump stiffness adjustment factor in the pump stiffness control map.

<FIG> shows a block diagram of a controller <NUM> according to another embodiment of the disclosure. As shown in <FIG>, the pump rotational speed is used to select one of the plurality of control maps for use by the controller. Each pump rotational stiffness control map comprises values for the pump adjustment factor pre-combined with the nominal current value based on the pump rotational speed associated with the respective pump stiffness control map. As such, each pump stiffness control map is configured to directly output the pump displacement current to be output to the axial piston pump <NUM>.

Thus, according to this disclosure, the controller <NUM> may be configured to perform a method of controlling a displacement of an axial piston pump having a fixed valve plate and a variable displacement. In a first step of the method, a displacement of the axial piston pump is determined. As discussed above, the displacement may be determined by estimation using the pump displacement estimation module or by direct measurement using a suitable sensor.

A pump displacement control current to be supplied to the axial piston pump to control the displacement of the axial piston pump is also calculated. This step comprises calculating a nominal value for the pump displacement control current based on a rotational speed of the axial piston pump and calculating a pump stiffness adjustment factor based on a pump stiffness control map having as inputs: an output pressure of the axial piston pump; and the estimated pump displacement. The pump displacement control current to be supplied to the axial piston pump is then calculated based on the nominal value and the pump stiffness adjustment factor.

Once calculated, the controller <NUM> outputs an instruction to output the calculated pump displacement control current to the axial piston pump in order to control the displacement of the axial piston pump.

Thus, according to embodiments of this disclosure a controller <NUM> for controlling the displacement of an axial piston pump <NUM> is provided.

Claim 1:
An axial piston pump controller (<NUM>) configured to control an axial piston pump (<NUM>) having a fixed valve plate (<NUM>) and a variable displacement configured to:
determine a displacement of the axial piston pump (<NUM>);
calculate a pump displacement control current to be supplied to the axial piston pump (<NUM>) to control the displacement of the axial piston pump (<NUM>) comprising:
calculating a nominal value for the pump displacement control current based on a rotational speed of the axial piston pump (<NUM>);
calculating a pump stiffness adjustment factor based on a pump stiffness control map having as inputs: an output pressure of the axial piston pump (<NUM>), and the displacement of the axial piston pump (<NUM>); and
calculating the pump displacement control current to be supplied to the axial piston pump (<NUM>) based on the nominal value and the pump stiffness adjustment factor; and
output an instruction to output the calculated pump displacement control current to the axial piston pump (<NUM>) in order to control the displacement of the axial piston pump (<NUM>).