Proportional and Servo Valve Technology

8 • Fluid Power Journal March/April 2003 The ability to achieve automated stepless control of pressure and flow rate in fluid power systems has undergone major development in the past twenty-five years. Electrohydraulic servo valves were invented in the late 1930s as a high tech, though high cost, solution to motion control needs. The mid-1980s saw the practical introduction of proportional valves as a viable and reasonably priced alternative to servo valves. This article will explore the technology used in these proportional and servo valves as well as attempting to shed some light as to what type of valve may be most appropriate for a given application. If true, stepless control of pressure or flow wasn’t that critical to the operation of a machine, preset pressure or flow control valves could be achieved by having a bank of preset valves. The appropriate valve would be connected into the circuit via the actuation of a solenoid valve. For example, three discrete pressures could be achieved by having two pilot relief valves connected in parallel to the vent port of a ventable pilot operated relief valve. These two valves would be isolated from the pilot operated relief valve by two-way normally-closed solenoid valves. By individually actuating the 2-way valves, three different pressures could be achieved. But what if stepless pressure control was required? What if the pressure increase or decrease needed to follow a specific rate? What if that rate change wasn’t constant? Then what could the machine designer do? Until the invention of servo valves, if the need existed to achieve varying pressure to an actuator in an effort to control force or torque, one needed to either have the machine operator turn an adjustment knob, stroke a lever, or a mechanical means needed to be designed to have a mechanical input device or linkage vary the setting of the valve. The same need held true if flow needed to be varied. Human control of a valve could be fairly inconsistent. Mechanical control of the valve, while possibly being more consistent and repeatable, might not offer much flexibility for different adjustment rates. Most electrical machine control systems were not all that well developed until the introduction of the microprocessor in the early 1980s. As most machines that had electrical automation used relay logic, the sequence of operations of the machine was not easily revised. Relays are digital, or on-off, devices. The introduction of the microprocessor, and hence, the PLC (programmable logic controller) opened the door to a great amount of control options to machine designers. The operation sequence of a machine was no longer hardwired in relay logic. While Boolean operations were possible in relay logic, it was inconvenient to do so, as well as difficult, expensive, and time consuming to accomplish. The common introduction of PLCs and proportional valves greatly expanded the control options available to machine designers. The initial proportional valves that appeared on the market are what are now commonly termed “open-loop” valves. In contrast to mechanical feedback (MFB) servo valves, a feedback link does not exist between the coil assembly and the valve spool. Since a feedback loop between the input command and the valve output does not exist, the feedback loop is “open” rather than “closed.” In an effort to improve the performance of proportional valves, relative to the performance of servo valves, manufacturers added linear variable displacement transducers (LVDTs) to proportional valves in order to sense the spool (or poppet) position. The output signal from the LVDT was fed back to the amplifier card. A summing amplifier on the amplifier card calculated the difference between where the spool was supposed to be and where it actually was, and the output to the coil was changed in an effort to position the spool to achieve the desired output based on the input. These enhanced proportional valves are termed “closed loop” proportional valves. Since the means of feedback is electrical, not mechanical, this gave rise to the term “electrical feedback” (EFB).