Method and device for determining a wear condition in a hydrostatic pump

Abstract

A method for determining a current wear (w) of a hydrostatic pump, particularly of a radial piston pump, with a variable-speed drive, where the pump is connected to a fluid passage, in which a fluid is pumped by the pump to create a current actual volume flow in the fluid passage. A current actual volume flow (Q.sub.act) is determined, by measuring the volume flow in the fluid passage at a predetermined drive-vector, a computed volume flow (Q.sub.comp) is determined, by a first computational method, at the predetermined drive-vector, and the current wear (w) of the pump is determined, by a second computational method, which relates the current actual volume flow (Q.sub.act) to the computed volume flow (Q.sub.comp).

Claims

1. A method for determining a current wear of a hydrostatic pump, the method comprising: providing a variable-speed electric motor operable to drive the hydrostatic pump; providing a fluid passage connected with the hydrostatic pump, wherein a fluid is communicated through the fluid passage by the hydrostatic pump, the hydrostatic pump creating a current actual volume flow in the fluid passage; determining the current actual volume flow (Qact) by measuring the volume flow in the fluid passage at a predetermined drive-vector, wherein the pre-determined drive-vector includes a first pressure and a second pressure of the fluid; determining a computed volume flow (Qcomp) by a first computational method, at the predetermined drive-vector; and determining the current wear of the hydrostatic pump by a second computational method, which relates the current actual volume flow (Qact) to the computed volume flow (Qcomp), wherein the first pressure is a working pressure of the fluid, and the second pressure is a preload pressure of the fluid.

2. The method of claim 1, wherein the second computational method determines a ratio, which is a quotient of the actual volume flow (Qact) at the predetermined drive-vector to the computed volume flow (Qcomp) at the predetermined drive-vector.

3. The method of claim 1, wherein the second computational method determines a ratio, which is an average of a set of quotients, wherein each of the quotients is the quotient of the actual volume flow (Qact) at the predetermined drive-vector to the computed volume flow (Qcomp) at the predetermined drive-vector.

4. The method of claim 3, wherein the average of the set of quotients is a weighted average.

5. The method of claim 1, wherein the drive-vector comprises: a rotational speed of the variable-speed drive.

6. The method of claim 1, wherein the drive-vector comprises: a viscosity of the fluid.

7. The method of claim 1, wherein the drive-vector comprises: a temperature of the fluid.

8. The method of claim 1, wherein the first computational method comprises a linear function or a polynomial function of values of the drive-vector.

9. The method of claim 1, wherein the first computational method comprises an n-dimensional matrix of sampling points.

10. The method of claim 9, wherein the matrix of sampling points is determined by several measurements.

11. The method of claim 10, wherein the several measurements are weighted.

12. The method of claim 9, wherein the matrix of sampling points is stored locally and/or centrally.

13. The method of claim 1, wherein determining the wear is used for a prediction of the wear of the hydrostatic pump.

14. The method of claim 1, wherein the hydrostatic pump is a radial piston pump.

15. The method of claim 1, further comprising providing a cylinder in fluid connection with the hydrostatic pump, wherein the working pressure of the fluid is operable to move the cylinder.

16. An electro-hydrostatic pump device, comprising: a radial piston pump; a variable-speed motor operable to drive the radial piston pump; and an electronic control unit; wherein the radial piston pump is connected to a fluid passage in which a fluid is communicated by the radial piston pump, the radial piston pump operable to create a current actual volume flow in the fluid passage, wherein the electronic control unit is operable to measure the volume flow in the fluid passage at a predetermined drive-vector including a first pressure and a second pressure of the fluid, respectively, to determine the current actual volume flow (Qact); wherein the electronic control unit is operable to determine a computed volume flow (Qcomp) by a first computational method, at the predetermined drive-vector including the first pressure and the second pressure of the fluid, respectively; and wherein the electronic control unit is operable to determine the current wear (w) of the pump by a second computational method, which relates the current actual volume flow (Qact) to the computed volume flow (Qcomp), wherein the first pressure is a working pressure of the fluid, and the second pressure is a preload pressure of the fluid.

Description

(1) The figures show:

(2) FIG. 1: An example of the performance curves of a radial piston pump;

(3) FIG. 2: An example of variations of volume flows, depending on viscosity and temperature;

(4) FIG. 3: Parts of a simplified hydraulic system comprising a pump and a cylinder;

(5) FIG. 4: An example of variations of volume flows, measured for selected rotational speeds.

(6) FIG. 1 depicts an example of the performance curves of an arbitrary radial piston pump, as typically shown on datasheets of hydraulic pumps. One curve, labelled with “P”, shows the relation between power P consumed by the pump's electric motor (right y-axis) and the pressure p provided by the pump. Another curve, labelled with “Q”, shows the relation between volume flow Q (left y-axis) and the pressure p. It is clearly visible that the volume flow Q decreases—at least slightly—for higher pressures p. This is mainly caused by a higher leakage flow at higher pressures. The leakage—and thus the steepness of this curve labelled “Q”—may be lower for pumps with high-density seals and/or cylinders. For worn-out pumps, both the values of this curve decrease and the steepness of this curve increases.

(7) FIG. 2 depicts another example of the performance curves of the pump of FIG. 1, but it shows examples of the dependency of the curve “Q” on viscosity and temperature, using an arbitrary example-fluid. In this FIG. 2 it is clearly visible that the values of this (bright grey) curve decrease and the steepness of this curve increases for lower viscosity v and/or higher temperature T of the fluid. Also, the values of this curve increase and the steepness of this curve decreases for higher viscosity v and/or lower temperature T of the fluid.

(8) FIG. 3 depicts some parts of a simplified hydraulic system comprising a pump apparatus 10, a cylinder 20, and fluid passages 31, 32. (Further necessary components of a hydraulic system, which are of lower relevance for this invention, are not shown.) The pump apparatus 10 comprises a pump 11, which is driven by a variable-speed electric motor 10 via shaft 14, which has during operation a rotational speed n. The pump 11 is connected to a differential cylinder 20 via fluid passages 31, 32. The differential cylinder 20 comprises piston 23, piston rod 24, and two chambers 21, 22. The pump 11 pumps the hydraulic fluid via passages 31, 32 to said cylinder 20. The upper passage 31 of the cylinder 20 is connected to a first pressure chamber 21, and the lower passage 32 is connected to a second pressure chamber or annular chamber 22. By pumping the hydraulic fluid into the first 21 or the second 22 pressure chamber, the piston 23 and the piston rod 24 are moved down or up, respectively, as shown by the arrow 26 with dotted line. The piston rod 24 is moved with velocity or speed s. There are several methods to measure the actual volume flow Q.sub.act: It can be measured by a flow meter in at least one of the passages 31 or 32. Or the velocity s of piston rod 24 can be measured and multiplied with a factor that expresses the piston areas of the first 21 or the second 22 pressure chamber, depending on the direction of the movement.

(9) FIG. 4 depicts an example of variations of volume flows, measured for selected rotational speeds. The diagram shows several sample points of measurements of the volume flows with leakage flows. The measurements are taken for several pressures, i.e. comprising equidistant values of pressures with p=(25, 50, 75, . . . 275) [bar]. The measurements are also taken for several rotational speeds n, e.g. for n=(300, 500, 1000, 1500, . . . ) [rpm]. In this example, a linear curve through these measuring points is constructed, by using the mean squared error (MSE) method.

LIST OF REFERENCE SIGNS

(10) 1 electro-hydrostatic drive 10 electric motor 11 pump apparatus 12 electric motor 14 shaft 20 cylinder 21 first pressure chamber 22 second pressure chamber 23 piston 24 piston rod 26 arrow with dotted line 31, 32 passage n rotational speed p pressure Q volume flow Q.sub.act current actual volume flow Q.sub.comp computed volume flow s speed of piston rod T fluid temperature v fluid viscosity w current wear