SOME PEOPLE will tell you there hasn’t been anything new in world of industrial hydraulics in years. These might be the same people who would contend that there haven’t been advances in the internal combustion engine either. Just as the fundamental principles of the internal combustion engine still apply today, so do the fundamentals of hydraulics. What has changed is the refinement these products have undergone, and the dramatic increases in performance that have been realized.
Not Your Father’s Hydraulic System
“When I started in this business, hydraulics was an easy discipline,” says Brian Lee, a seasoned veteran in the controls industry now working for Machine Tool Builders Inc., Machesney Park, Ill., a retrofitter and upgrader of machine tools systems. “Forward and back was all it did, and it wasn’t hard to figure out. Nowadays, you have to double check if it’s a hydraulic axis, or an electric servo axis, because you can’t tell them apart by the way they perform.”
The fundamentals of the hydraulic valve haven’t changed in more than 40 years, states Mike Panebianco, territory manager for Parker Hydraulics, but what has changed is the technology that controls the valves. “We used to have design centers filled with mechanical engineers to develop our valves,” he recalls. “Now, we have all electrical engineers to develop our new products.”
Panebianco refers to the amount of electronic control that now is available for hydraulic applications. The advent of these electronic advances comes on two fronts: in the overall system control, as well as the control of the individual valve.
“We’ve given our hydraulic motion controllers the ability to control pressure and force, in addition to controlling position, with the ability to smoothly transition between position and pressure control,” states Peter Nachtwey, president of Delta Computer Systems. “Such capabilities enable hydraulic systems to become more productive and decrease lifecycle costs.”
Most engineers are familiar with the “bang-bang” approach that has been used for years, where a directional control valve drives a cylinder to mechanical extremes with little or no regard for finesse. However, with the precision resulting from advances in electronic control of the valves, the performance of a hydraulic system can exceed that of an equivalent servo system for some applications.
“The control electronics compensate for the effect of the differential cylinder, linearize flow characteristics and solenoid characteristics of the valves, and raise the damping level of the drive through feedback of status variables,” says Johannes Grobe, head of product management of industrial controls for Rexroth Industrial Hydraulics. “We see applications developing acceleration to 80 g. We have aluminum die-casting machines with electro-hydraulic axes that decelerate from 600 m/min to 0 in 30–40 msec because of the performance that comes from combining the power of hydraulics with the control of electronics.”
More Sophisticated Basics
Before selecting a valve to meet the application requirements, system designers need to know exactly what the application calls for, as well as what they will get when asking for various different valve types. The terms “servo valve” and “proportional valve” are discussed a lot, but it is important to understand the nature of each, as well as their benefits and shortcomings prior to making a selection.
Servo valve technology has been around since the 1930s, and received a lot of attention in the 1960s as a means of addressing space and weight issues while trying to get the space program off the ground. With a servo valve (See Figure 1), a linear change in coil current results in a linear change in spool position, as well as a linear oil flow change through the valve. Servo valves are precision components, manufactured to exacting tolerances in order to get the desired performance, especially in flow characteristics.
|FIGURE 1: THE VENERABLE SERVO VALVE|
|A linear change in a servo valve’s coil current results in a linear change in spool position, as well as a linear oil flow change through the valve. Source: Rexroth Hydraulics|
Proportional valves (See Figure 2) on the other hand typically are less expensive valves built to less stringent tolerances. Contrary to what you might expect, the proportional valve contains built-in feedback, while the servo valve often times does not. The proportional valve uses feedback on the spool, typically in the form of a linear variable-displacement transducer (LVDT), to measure spool position. The spool position then is fed back into the valve controller, where the controller adjusts accordingly to linearize flow.
|FIGURE 2: A PROPORTIONAL RESPONSE|
The valve spools (1) of these proportional valves are actuated by a moveable coil (2) over a friction-free permanent magnet cylinder (3). This direct actuation of the spool, coupled with high resolution feedback, allows the valves to be positioned more precisely, in less time. The end result is high speed precision. Source: Rexroth Hydraulics