Controller Development for a Separate Meter-In Separate Meter-Out Fluid Power Valve for Mobile Applications

Research output: ResearchPh.D. thesis

Abstract

In most mobile vehicles which are used within construction, agriculture, material handling, forestry, garbage handling etc. a fluid power system is used for power transport and power distribution. The transported/distributed power is usually generated by a diesel engine or from an electrical battery. The largest advantages of the fluid power system are its high energy density and its robustness. Currently there is no cost effective and robust alternative to using a fluid power system for the power transport in the kilo- watt range necessary to establishing a linear motion of tools in mobile machinery. For a rotary motion electrical motors controlled by using power electronics is a competing technology because of their high energy efficiency. Additionally, the energy density of electrical devices is still increasing.

In fluid power systems where more consumers (cylinders or motors) are supplied by a single pump the fluid is distributed through valves. A valve works by controlling a fluid stream through the valve by varying the opening of an orifice. The disadvantage by this is that when controlling the fluid flow rate a pressure drop is created across the orifice. This results in a throttle loss equal to the controlled flow rate times the pressure drop across the orifice. By a constant flow rate the best energy efficiency is therefore obtained by keeping the pressure drop across the orifice as low as possible. More orifices are commonly included in a single valve.

A specific type of valve, which is commonly used in many types of mobile applications, is a 4-way proportional valve. In this type of valve two fluid streams are controlled: One fluid stream from a pump to a fluid consumer and one fluid stream from the fluid consumer to a fluid reservoir. In a 4-way proportional valve it is necessary to use a separate control of the two fluid streams to minimise the throttling losses. The purpose of the research documented in this dissertation is to investigate how a 4-way proportional valve may be build to fulfil the increasing demands with regard to energy efficiency and functionality. And to develop controllers for a valve prototype whereby the two mentioned fluid streams may be controlled separately.

First an introduction to mobile fluid power systems is given. It is explained that the future trend within mobile fluid power systems goes towards integration of sensors and microprocessors into the components. The particular research area is motivated by the use of two examples. They explain how a separate control of the meter-in and the meter-out flow of proportional valves, together with integration of sensors, may minimise throttling losses and give increased functionality of the fluid power system.

Next the hydraulic functionality, which is anticipated to be integrated into proportional valves in the future, and additionally also influences on the layout of a future valve, is described. Existing valve concepts are evaluated with regard to their functionality, compared to the number of degrees of freedom which must be controlled. New valve concepts are also suggested and evaluated. A single valve concept is selected for further study.

A parametrised model of the selected valve concept is derived and verified experimentally by means of a prototype valve. A linearised model, which is to be used in the subsequent development of controllers, is derived from the verified non-linear. The demands for compensation of varying load pressure across the valve is examined by measuring the static performance of state of the art valves. Next the dynamical demands for valve are examined qualitatively.

Controllers for the individual control of the spools of the prototype valve are designed. The design involves two different methods for pilot operation of the spools. The pilot control method by which the best performance is at first obtained, compared to the static demands put forward, is rejected. This is because it has a low relative stability due to design restrictions. Robust controllers for the valve using the remaining pilot control method are developed. The robustness is evaluated by simulations and afterwards the controllers are tested experimentally.

A model of a hydraulic actuator system with a flexible load structure is derived. For the establishment of the equivalent parameters for the model, basis is taking in a real life loader crane. Subsequently, the multivariable interactions of the actuator system are analysed by means of a linearised model. Two controllers based on separate meter- in separate meter-out are developed and subsequently tested by using the non-linear simulation models of both the valve and the loader crane.

The research documented in this dissertation has contributed to the identification of valve concepts which are suitable as a platform for future proportional valves. It has contributed to the development of a parametrised model of a pilot operated spool valve, which may be used by engineers in design work involving spool valves. Additionally, by the research different controllers for the control of a pilot operated spool valve have been developed and tested. Because pilot operation of spools is generic in fluid power systems, it is anticipated that by a modification of parameters the controllers may also be used for the control of other fluid power components. Finally, the research has contributed with two new methods whereby a decoupled control of the velocity and the pressure level of a hydraulic actuator may be obtained.

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In most mobile vehicles which are used within construction, agriculture, material handling, forestry, garbage handling etc. a fluid power system is used for power transport and power distribution. The transported/distributed power is usually generated by a diesel engine or from an electrical battery. The largest advantages of the fluid power system are its high energy density and its robustness. Currently there is no cost effective and robust alternative to using a fluid power system for the power transport in the kilo- watt range necessary to establishing a linear motion of tools in mobile machinery. For a rotary motion electrical motors controlled by using power electronics is a competing technology because of their high energy efficiency. Additionally, the energy density of electrical devices is still increasing.

In fluid power systems where more consumers (cylinders or motors) are supplied by a single pump the fluid is distributed through valves. A valve works by controlling a fluid stream through the valve by varying the opening of an orifice. The disadvantage by this is that when controlling the fluid flow rate a pressure drop is created across the orifice. This results in a throttle loss equal to the controlled flow rate times the pressure drop across the orifice. By a constant flow rate the best energy efficiency is therefore obtained by keeping the pressure drop across the orifice as low as possible. More orifices are commonly included in a single valve.

A specific type of valve, which is commonly used in many types of mobile applications, is a 4-way proportional valve. In this type of valve two fluid streams are controlled: One fluid stream from a pump to a fluid consumer and one fluid stream from the fluid consumer to a fluid reservoir. In a 4-way proportional valve it is necessary to use a separate control of the two fluid streams to minimise the throttling losses. The purpose of the research documented in this dissertation is to investigate how a 4-way proportional valve may be build to fulfil the increasing demands with regard to energy efficiency and functionality. And to develop controllers for a valve prototype whereby the two mentioned fluid streams may be controlled separately.

First an introduction to mobile fluid power systems is given. It is explained that the future trend within mobile fluid power systems goes towards integration of sensors and microprocessors into the components. The particular research area is motivated by the use of two examples. They explain how a separate control of the meter-in and the meter-out flow of proportional valves, together with integration of sensors, may minimise throttling losses and give increased functionality of the fluid power system.

Next the hydraulic functionality, which is anticipated to be integrated into proportional valves in the future, and additionally also influences on the layout of a future valve, is described. Existing valve concepts are evaluated with regard to their functionality, compared to the number of degrees of freedom which must be controlled. New valve concepts are also suggested and evaluated. A single valve concept is selected for further study.

A parametrised model of the selected valve concept is derived and verified experimentally by means of a prototype valve. A linearised model, which is to be used in the subsequent development of controllers, is derived from the verified non-linear. The demands for compensation of varying load pressure across the valve is examined by measuring the static performance of state of the art valves. Next the dynamical demands for valve are examined qualitatively.

Controllers for the individual control of the spools of the prototype valve are designed. The design involves two different methods for pilot operation of the spools. The pilot control method by which the best performance is at first obtained, compared to the static demands put forward, is rejected. This is because it has a low relative stability due to design restrictions. Robust controllers for the valve using the remaining pilot control method are developed. The robustness is evaluated by simulations and afterwards the controllers are tested experimentally.

A model of a hydraulic actuator system with a flexible load structure is derived. For the establishment of the equivalent parameters for the model, basis is taking in a real life loader crane. Subsequently, the multivariable interactions of the actuator system are analysed by means of a linearised model. Two controllers based on separate meter- in separate meter-out are developed and subsequently tested by using the non-linear simulation models of both the valve and the loader crane.

The research documented in this dissertation has contributed to the identification of valve concepts which are suitable as a platform for future proportional valves. It has contributed to the development of a parametrised model of a pilot operated spool valve, which may be used by engineers in design work involving spool valves. Additionally, by the research different controllers for the control of a pilot operated spool valve have been developed and tested. Because pilot operation of spools is generic in fluid power systems, it is anticipated that by a modification of parameters the controllers may also be used for the control of other fluid power components. Finally, the research has contributed with two new methods whereby a decoupled control of the velocity and the pressure level of a hydraulic actuator may be obtained.

Original languageEnglish
PublisherInstitut for Energiteknik, Aalborg Universitet
Number of pages253
StatePublished - 2005
Publication categoryResearch

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