Measuring and testing – Vibration – Sensing apparatus
Reexamination Certificate
2000-06-06
2001-12-04
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
Sensing apparatus
Reexamination Certificate
active
06324913
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for determining system parameters, such as the operating speed, working pressure and pivot angle of an axial piston unit for hydrostatic drives.
Increasing complexity, the comfort desired by customers and users, and new fields of use, lead to the increased use of sensors in hydrostatic drives having axial piston units. These sensors permit the drive to be regulated in an appropriately optimum manner. So that the operating state can be estimated reliably as the starting point for optimum regulation, the operating speed, working pressure or operating pressure and pivot angle of the piston units are important characteristic quantities for the hydrostatic drive. These system parameters for the piston units have hitherto been determined by means of special speed, working-pressure or pivot-angle sensors. A disadvantage of this is that a certain number of sensors of different design and different operating principle are used. This has disadvantages in terms of the costs and the simplification of the hydrostatic drives and also in terms of maintenance and ease of repair.
Therefore, the principal object of the invention is to provide a method for determining the operating speed, working pressure and pivot angle of an axial piston unit, which works with uniform sensors, has a high degree of accuracy in determining the system parameters and makes it possible to have a cost-effective and easily repairable axial piston unit.
These and other objects will be apparent to those skilled in the art.
SUMMARY OF THE INVENTION
By virtue of their discontinuous operation, axial piston units for use in hydrostatic drive systems generate alternating forces which subject the entire axial piston unit to vibrations. Structural vibrations of this kind are referred to as solid-borne sound. When the solid-borne sound is discharged into the air by the piston unit, the solid-borne sound gives rise to airborne sound which, in the case of a corresponding frequency position, is perceptible to the human ear. When the solid-borne sound is discharged, for example, into a hydraulic fluid, the solid-borne sound gives rise to liquid-borne sound.
Since the solid-borne sound is generated on the basis of the alternating forces of the piston unit, and since the alternating forces, in turn, depend on the operating speed, working pressure and pivot angle of the piston unit, the solid-borne sound contains information on these system parameters and therefore makes it possible to evaluate these accordingly. The solid-borne sound is in a fixed functional relation to the alternating forces causing the vibrations, as long as the sound transmission distance from the point of generation of the alternating force to the position of the solid-borne sound sensor mounted on the piston unit does not change. The solid-borne sound signal therefore contains all the information which is also present in the alternating forces.
Drive units of axial piston units usually have an odd number of cylinders or displacement chambers. Nine pistons is a typical number in this case. Since axial piston units can work both as a pump or as a motor, the invention relates, in general, to determining the operating speed, working pressure and pivot angle in axial piston units.
In the preferred embodiment of this invention, the operating speed is determined from the basic frequency of the piston force profile (alternating force) by a frequency analysis of the basic frequency. The basic frequency of the piston force profile is determined from the frequency analysis being divided by the number of pistons of the axial piston unit.
During one complete revolution of an axial piston unit operated as a pump, each displacement chamber is connected to the high-pressure side during half of the revolution and to the low-pressure side during the other half of the revolution. When the corresponding displacement chamber is connected to the high-pressure side, the piston executes a feed stroke. The change-over from high pressure to low pressure, and vice versa, takes place in the dead center positions. Due to compensating flows as a result of hydraulic capacities, the change-over operation lasts for a certain amount of time, i.e., it does not take place infinitely quickly, so that the pressure build-up and pressure reduction in the respective displacement chamber likewise take place at a finite speed. This pressure profile acts on the displacement piston and leads to a dynamic load on the structure of the axial piston unit, thus resulting in a defined profile of the piston force or alternating force. Each individual piston leads to such a piston force profile or profile of the alternating force. If there are a plurality of pistons, the individual piston force profiles induced by the pistons are superposed. Since, the number of pistons of an axial piston unit is known, the piston force or its profile has, during one complete revolution, a maximum number in the piston force profile corresponding to the number of pistons. This piston force leads to a defined solid-borne sound profile, from which these maximum numbers can likewise be derived. It is possible, for example, to determine the operating speed of the axial piston unit from the solid-borne sound profile.
If, however, the axial piston unit runs in the pressureless state, the operating speed of the axial piston unit is determined from vibration components which are determined from the measured unbalances of the drive unit of the axial piston unit. It is thus possible to determine the operating speed both when the pistons are loaded with working pressure or when they are in the pressureless running state.
When the operating speed, as one of the system parameters, is determined, then, the solid-borne sound signal dependent on the piston force is generated, and the solid-borne sound is measured by means of displacement, speed or acceleration pick-ups. A harmonic analysis of this solid-borne sound signal is then carried out, from which its basic frequency is determined.
According to a second embodiment of the invention, the working pressure, as one of the system parameters, is determined from the amplitude of the harmonics which correspond to the number of pistons and which are determined from the frequency analysis. This procedure is possible, since the piston force profile is determined essentially by the profile of the working pressure in the axial piston unit. In this case, a transmission function is set up, which, as a function of the frequency, takes into account amplifications and attenuations of the solid-borne sound signal. That signal results from the structural resonances of the solid-borne signal on its way through structural parts/subassemblies of the axial piston unit to the sensor which is picking up the solid-borne sound. This transmission function therefore likewise ensures that influences of the structure of the parts or of the subassemblies of the axial piston unit on the solid-borne sound signal are taken into account. This increases the accuracy of the method. This weighting result, taking into account the transmission or correction value function, makes it possible to determine the working pressure of the axial piston unit. Preferably, the transmission function is determined empirically. It takes into account various influences, not accurately detectable mathematically, in the material structure or the structural make-up of the components of the axial piston units.
In a third embodiment of the invention, the pivot angle of the axial piston unit is determined from the amplitude ratio of the even harmonic and odd harmonic of the basic frequency of the piston force profile (alternating force) of the axial piston unit. The piston force profile is directly dependent on the solid-borne sound signal and is derived therefrom.
The pivot angle of the axial piston unit is determined through the following steps. First, a solid-borne sound signal dependent on the piston force is generated, specifically by the solid-borne sound signal being measured by means
Saint-Surin Jacques
Sauer-Danfoss Inc.
Williams Hezron
Zarley McKee Thomte Voorhees & Sease
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