The development during the last decades in mobile fluid power control has been directed to achieve better controllability and better energy utilization. However, most valve concepts still use mechanical or hydro-mechanical control of the main control valves, but the use of electro-hydraulic control systems is increasing. The main valves are with few exceptions of spool type. Control demands such as load sensing and pressure compensation are normally fulfilled by adding attachment valve units for each purpose. Unfortunately, the additional valves make the system more complex. If the main valves can be made more intelligent, the number of attachment valves can be reduced.
A major disadvantage (and an advantage) of the spool valve is that meter-in and meter-out orifices are mechanically linked to each other and thus giving a less flexible control. It would definitely increase flexibility if one could separate the controls of meter-in and meter-out orifices. A natural way to do this is to exchange the spool valve with seat valves. This is a well-known technique which has been used in heavy industrial applications for several years. However, in mobile fluid power systems the use of seat valves has normally been restricted to pressure relief valves and check valves, as reasonably cheap proportionally controlled seat valves did not earlier exist.
This paper deals with the ways to control (using a computer) an actuator with four electrohydraulic proportionally controlled seat valves in a system with an electrohydraulic controlled variable pump. The analysis is made by using both simulation and experiments on a test set-up controlled by a personal computer.
The main task of this investigation is to examine the control strategies offered by separate controls of meter-in and meter-out orifices. A discussion is also made of demands requested by this control strategy on the components, e.g the valves.