【正文】 de and the Pentium will do diagnostics and provide a graphical user interface. Figure 2 Separate Meter In / Separate Meter Out Setup 4 CURRENT WORK Flow Control by Direct Actuation of the Spool Most flow control valves on the market today work with a pressure pensator (Andersen。 Feigel 1990). This is not a new idea but has not been implemented mercially because of the high cost of pressure transducers and micro controllers. However, with the current drop in cost of micro controllers and pressure transducers this idea is now mercially feasible. The concept is very simple, spool position is calculated from the Bernoulli equation using the pressure drop across the spool and a reference flow. Even though this is a simple equation, it is not easy to implement. The accuracy of the flow control is dependent on the precision of the position sensors and of the pressure on the pressure or the position signals can cause stability problems. Filtering the noise,introduces delays in the control which can also affect stability. In addition the Bernoulli equation is not followed exactly over the entire operating range of the valve, so it may be necessary to store the valve characteristics as a data table or develop a more plex equation. Cylinder Control Strategy To control a hydraulic cylinder, the strategy has to be able to handle four different situations depending on the directions of the load and the velocity of the cylinder. Refer to Figure 3 The control strategies that have appeared in the literature are usually quite plex and depend on measurements of the cylinder position and velocity (Elfving,Palmberg 1997。Virvalo 1997). They are also based on rather plex control algorithms. It is the goal of this thesis to start with a control strategy which is based on simple PI controllers and makes no demands for position and velocity of the cylinder. The performance of this system will be lower than a plex control strategy, but it may be easier to implement mercially because it has no need for special sensors and is easier to understand for the average engineer. Figure 3 Different Situations in Crane Operation Another feature which needs to be acknowledged when designing a control strategy, is thetype of valve used. Mobile hydraulic valves demand low leakage and since most mobilevalves are spool valves, they usually have large overlaps. In addition, to make the cost of thevalve acceptable to industry, the actuation stage on the spool is usually quite slow. This bination of large overlap and slow actuation makes it hard to implement many of the strategies that have been presented. Pressure control especially bees difficult when there is an overlap and a slow actuator. One example of a new strategy which is simple and robust is described as follows. Flow control is implemented on the inlet side and pressure control is implemented on the outlet flow control is based on the Bernoulli equation. Pressure control is done by a PI controller which maintains a low constant pressure to increase the efficiency and prevent work around large overlaps and slow actuation stage, the pressure controller only does meter out control. This means that if the controller wishes to raise the pressure, it can’t add flow to the cylinder, it can only decrease the opening of the meter out port. The benefit of this is that the only time that the spool has to cross the zero position is when the operator wishes to change the direction of motion of the cylinder. For the case where the load force and the velocity are in the same direction, this strategy has to be modified. In this case, the pressure reference of the pressure controller at the outlet is increased to a value which opposes the load force. The pressure reference is increased when it is noticed that the pressure of the inlet side is dropping. The pressure reference is also controlled by a PI controller. A schematic model of the controller system for the load lowering case is shown in Figure 4. At the time of writing this paper the initial experimental tests had been performed on the real crane shown in Figure 1 . Stability was not achieved because the crane is equipped with a load holding valve. However, the load