Guidelines for sizing servohydraulics

The best controllers and software can't overcome a poorly designed hydraulic system. In typical servohydraulic motion systems, the valve mounts as close as possible to the cylinder. Experts also recommend motion controllers specifically designed for hydraulic applications. Cylinder-bore size simulations demonstrate relatively stable pressures for the larger cylinder. Typical Bode plots show amplitude response and phase lag of the spool as a function of control-signal frequency. The graph shows operating characteristics of various valves. Linear valves with servo-quality spools provide flow proportional to the control signal as long as pressure drop across the valve is constant. Global competition ensures that plant operators face the neverending task of making equipment more productive. This often demands higher speed and precision from their motion-control systems. But in the case of closedloop hydraulics, higher speeds and precision must start with good component design. No matter how much attention is given to electronic controllers and software, if the fluid-power cylinders and valves are sized or mounted incorrectly, system performance will suffer. With linear actuators, application requirements tend to set stroke length and cycle times. Designers then determine cylinder diameter and oil pressure. A common problem is attempting to increase actuator speed by undersizing cylinders. Engineers often assume that for a given oil flow, smaller cylinders accelerate quicker and run faster. However, this only works for light loads. For actuators moving moderate to heavy masses, available force — not oil flow — limits acceleration, velocity, and deceleration. Because piston diameter determines output force, a cylinder that is too small will never attain the required speeds and cycle times. A designer's first inclination may be to use the oversimplified equation /A. But it is only accurate if mass = 0. Only use the alternate form In fact, there is no simple tool for optimizing cylinder diameters. Two equations are shown here, each with advantages and limitations. We recommend calculating cylinder areas with both equations and using the larger of the two. The first equation is from Design of Electrohydraulic Systems for Industrial Motion