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REXA Electraulic™ Actuators & Drives
Available in Linear, Rotary and Drive configurations, REXA’s two models, Xpac and Mpac, can fulfill virtually any positioning application.
Modulating
Modulating control is thought to be constant movement of the positioning device. This is only true in a few instances. A motor operating 25 percent of the time would be considered excessive. The more important characteristic of modulating service is the ability to start, stop and reverse direction. Continuous duty motors without limitations on start, stop or reverse cycles provide REXA actuators with the ability to meet both criteria.
Pulse Signal or Manual Pushbuttons
Pulse control was developed to operate gearmotor actuators (MOV). In its classic form, a low voltage relay would power a contactor to turn a 440 vac motor in the appropriate direction. When the relay was activated, the motor would turn. With the relay off, all power to the unit was cut and the gearmotor would remain in place. Crude positioning was caused by low duty cycle, motor run-on, relay cycle time and gear wear. Resolution in excess of 2% could be expected.
As modern electronic controllers were developed, provisions to continue with pulse control were included. Innovative methods to turn pulses into true control signals by varying the frequency and duration were tried. Although potentially effective, the real stumbling block, the gearmotor actuator, remained in the control loop. Increased pulses of short duration only served to exacerbate the low duty cycle and to compound the maintenance problems.
Pushbuttons are a manual pulse signal. These applications will position the valve anywhere within its stroke. Smaller gearmotors can be powered directly through the buttons or larger units may require the intercession of a contactor. In many cases, the actuators are upgrades from a manual handwheel operator.
Two Position
The majority of automated applications are two position. The driven device is either at one or the other extreme of travel. In general, this is mild duty for an actuator. Conditions such as rapid cycling, temperature extremes and vibration will place additional consideration upon the selection of a product.
Emergency Shut Down (ESD)
Most safety-related applications are simple and straightforward, but require the utmost reliability and surety of closure. An actuator must have the ability to remain open for long periods without binding and then move quickly to the closed position. Although no device is 100% foolproof, a spring is the most desirable closure mechanism.
