Matti Pietola

Applicability of Pump Models for Varying Operational Conditions

It is commonly known that the characteristics of a fluid power pump depend on pump type, pressure, rotational speed and displacement. But in addition to these, also all the other parameters or factors associated with the operating conditions may have a significant effect on the characteristics. The most important of these are the pump construction and size, operating point temperature and the characteristics of the oil, which also depend on temperature and pressure. The aim of this study is to show the effects that the varying operational conditions have on the characteristics of a axial piston pump, to compare the measured characteristics with other published characteristics of axial piston pumps and to study the capability of pump models to represent these characteristics. The results include information of the effects of fluid temperature, type of fluid and the setting value of the displacement on the pump characteristics along with the effects of pressure and rotational speed. The sensitivity of the pump to each of the parameters is discussed. The effect of limited information of pump characteristics on the reliability of simulation results is studied using the Schlosser models.© 2003 ASME

The Effect of Parameter Uncertainty on the Reliability of Pressure Accumulator Simulations

The aim of this study is to demonstrate how the various parameters and the uncertainty associated with them affect the simulation results of a pressure accumulator. The parameters to be studied are related mainly to the pre-charging procedure of a membrane accumulator and cover the constants for the pressure and temperature of the nitrogen gas and the efficient volume of accumulator. These parameters are included in the non-linear model which is suited especially for large amplitude and low frequency transients. The background of the work is related to the fact that simulation is already an important tool in product development work. To be able to design the control of the system and predict the performance of it an estimate of the accuracy of the calculations is needed. The usability of simulation is determined by the fact how reliable the information is. To gain full benefit of simulation more attention has to be paid to the validity of the models, the accuracy of the parameter values needed in the models and the sensitivity of these parameters. The most sensitive parameters have to be recognized and paid special attention to the accuracy of the values given to them. Also the changes of these values in time due to wear or other modification in the system have to be noticed. Numerous factors have an influence on the accumulator operation. The most important of these are the ratio of the operation pressure to the pre-charge pressure, the amplitude of the flow rate disturbances, the temperature of the hydraulic fluid and the gas, the viscosity of the hydraulic fluid, the thermodynamic process of the nitrogen gas in the accumulator and the changes in the speed of sound. Also the mounting of the accumulator, the fittings used and the connecting pipes may have a significant role in the dynamics of an accumulator. The methods used in the study include measurements in time domain, modeling, simulations, and analytical work. The accumulator dynamics may alter due to remarkable changes in operating points like pressure levels. These are possible because of large amplitudes of flow rates particularly at the low frequency area where the presented study is focused on. The results include model analysis and information of the importance of the most fundamental parameters of the models and suggestions for future research work.Copyright

Effect of parameter uncertainty on reliability of hydraulic transmission system simulation

Simulation of fluid power systems has become a tool widely used for designing and testing purposes. The usefulness of this approach is however highly dependent on the results that have uncertainty to some extent. In the theoretical approach uncertainty depends mainly on the simplifications made in the modeling, algorithms used and parameters. In the experimental approach uncertainty depends on the transducers and the construction of the measurement system and data transfer. This paper discusses the effect of parameter uncertainty on the reliability of virtual testing of hydrostatic transmission systems in general. The definition of the total uncertainty by using a sum of variances is presented and applied to basic equations of fluid power. As an application a simplified hydrostatic transmission circuit is investigated. Importance of grade of models in reducing the computational inaccuracy is discussed.Copyright

Usefulness of High Resolution Thermography in Fault Diagnosis of Fluid Power Components and Systems

Infrared thermography has been used routinely in industrial applications for quite a long time. For example, the condition of electric power lines, district heating networks, electric circuits and components, heat exchangers, pipes and its insulations, cooling towers, and various machines and motors is monitored using infrared imaging techniques. Also the usage of this technology in predictive maintenance has proved successful, mainly because of effective computers and tailored softwares available. However, the usage of thermal sensing technique in fluid power systems and components (or other automation systems in fact) is not as common. One apparent reason is that a fluid power circuit is not (and nor is a hydraulic component) an easy object of making thermal image analyses. Especially the high flow speed, fast pressure changes and fast movements make the diagnosis complex and difficult. Also the number of people whose knowledge is good both in thermography and fluid power systems is not significant. In this paper a preliminary study of how thermography could be used in the condition monitoring, fault diagnosis and predictive maintenance of fluid power components and systems is presented. The shortages and limitations of thermal imaging in the condition monitoring of fluid power are also discussed. Among many other cases the following is discussed: (1) pressure valves (leakage, wrong settings), (2) check valves (leakage); (3) cylinders (leakage and other damages); (4) directional valves and valve assemblies; (5) pumps and motors (leakage in piston or control plate, bearings). The biggest advantage of using thermography in the predictive maintenance and fault diagnosis of fluid power components and systems could be achieved in the process industry and perhaps in the commissioning of fluid power systems in the industry. In the industry the predictive maintenance of fluid power with the aid of an infrared camera could be done as part of a condition monitoring of other systems, for instance bearings.