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

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 kilowatt 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.