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 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 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.
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.
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.
More than a decade of research has placed the Xpac in the forefront of actuation. Considered by many as the best in the world, the Xpac offers control resolution as low as 0.05%, compact size, hard positioning and wide-ranging analog input. The separate control enclosure for the electronics allows the unit to be installed in locations that exhibit temperature extremes, vibration or a hazardous environment.
REXA has developed a unique operating system for pulse signals. Optimum use of the Xpac's capabilities is accomplished by keeping its microprocessor (the PCP) in control. Pulses are not motor power, but a true low-level (24-120 vac, 10 mA) signal that changes the internal target position. The actuator will continue moving until the actual position satisfies the target position. The motion (speed, acceleration, resolution) of the unit will be in accordance with the user values set during calibration.
Designed as a cost-effective actuator and drive for routine control applications, the Mpac offers 2% resolution, hard positioning and 4-20 mA input signal. The self-contained configuration and digital calibration make the Mpac quick to install and simple to wire with a single-step pushbutton setup. The no-maintenance Electraulic™ power module, total solid state controls and optional spring failure bring a dimension of safety and reliability not found in other electric units.
For two position operation, the Mpac utilizes a single, low level (24-115 vac. 10 mA or dry contact) control signal. When the signal is present, the actuator is at one end. Withdraw the signal and the unit moves to the opposing limit. The position for a signal may be easily reversed during setup.
A batch process will usually utilize two position valves. Consistent, repeatable and immediate response is the key to in-flight material content and successful batching. The Mpac is the ideal actuator for this type of operation. The hydraulic circuit provides exacting motion throughout the stroke regardless of material composition or load variation.