【文章內(nèi)容簡介】 er in its response to electronic input signals. Electronic controls have given proportional valves accuracy and signalresponse times that are adequate in most industrial applications, Stavrou said. When a proportional valve is operating, its spool moves through a distance directly proportional to the current received by a solenoid. In one of Rexroth39。s proportional valves, for example, an electric amplifier receives a signal of up to 9 volts, which it converts into a current of up to amps. The current reaches an electric solenoid, which has a plunger that moves back and forth. The plunger moves the valve spool through a distance defined by the measure of current sent to the solenoid. By changing the volume of flow over time, the valve variably controls the speed and force of a driven unit. Similarly, a proportional valve that changes the direction of flow also controls the direction of output ponents on a driven unit. Proportional valves are often used in an openloop system that works without sensor feedback. However, they are sometimes integrated into closedloop systems in which a sensor, usually a linear variable differential transformer, sends signals that tell the controller where the solenoid plunger is during each stroke. The sensor can also send signals that measure the output of the driven unit. Closedloop systems, which now incorporate proportional valves, were once the domain of servo valves. The servo devices are machined more precisely than proportional valves and usually have two or three stages. Typically, in the firststage a solenoid or torque motor controls the flow of fluid in a pilot circuit. (Some torque motors are powerful enough to directly control valves without a pilot circuit.) In the second stage, the pilot circuit controls the movement of the valve spool, which itself regulates fluid flowing to the driven unit. When a torque motor is used, an electrical current is sent to its coils to move an armature. The armature is connected to a flapper plate that moves back and forth between orifices. These orifices are entry ports to two separate channels within the pilot circuit. Separately, the channels run to the opposite ends of a spool that controls the second stage of the valve. Upon receiving the electric current, the torque motor adjusts the position of the flapper plate. A difference in pressure emerges between the fluid flowing to opposite ends of the spool. Then the spool moves through a distance that is related to the pressure difference and electric current sent to the motor. Feedback on the position of the spool is often provided by a linear variable differential transformer. Proportional and servo valves are usually controlled from amplifier and control cards that slide into system control racks. The most sophisticated servo valves are often linked to programmable and puter numerical controllers, while some are controlled by custom software and hardware. In applications where time spent installing the valve needs to be kept to a minimum and machine space is tight, control cards are mounted directly into the valve39。s housing. USE AND ABUSE Servo valves hare higher performance than proportional valves, but they are more expensive. The average servo valve costs between $1000 and $2021, depending on its plexity according to the research firm Frost amp。 Sullivan Market Intelligence in New York. Electronic controllers can add more than $500 to the price. The average proportional valve, however, can cost half that much. The demand for electronic and conventional hydraulic valves is expected to grow at an annual rate of 5 percent in the United States through 1995, according to Frost amp。 Sullivan. Sales of proportional and servo valves are expected to increase in the three primary areas of application: mobile machinery, industrial machinery, and aerospace equipment. In the mobile machinery market, for example, electrohydraulic valves must be versatile and rugged. They have to fit in a range of machines, including garbage trucks, forklifts, and excavators. The valves are often used outdoors and in conditions that induce vibrations. One benefit of electrohydraulic valves is that they can increase safety in operating heavy equipment. Most large cranes, for example,