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.

Xpac: 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 contact input. The separate control enclosure for the electronics allows the unit to be installed in locations that exhibit temperature extremes, vibration or 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.

Complete specifications are listed in the product bulletins:

• Xpac, Linear
• Xpac, Rotary
• Xpac, Drive

To augment actuator operation, see a list of <OPTIONAL EQUIPMENT>.

The Literature Room has a complete list of available Technical Memos. We recommend some specific topics of general interest.

• STROKING SPEED- Xpac, Mpac
  The tortoise or the hare, both are winners withREXA/koso america inc
• SPRING FAILURE
  Security in the event of a power outage (also see PM21)
• HAZARDOUS AREA APPROVALS CODE: Div2 | Div1
  Available for any location (see TM19 for ambient temperatures)
• MANUAL OVERRIDE
  When electric power is unavailable, but the hydraulic system is on-line
• PULSE OPERATING SYSTEM
  Contact control from the DCS
• AUXILIARY CONTROL
  Pushbutton override on the front of the enclosure or at a remote location

Mpac: Designed as a cost-effective actuator and drive for routine control applications, the Mpac offers 2% resolution, hard positioning and multiple modes of operation. 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.

The manual mode of operation offers external pushbuttons to move the unit to any position within its calibrated stroke. The pushbuttons may be located either on the Mpac control compartment or on a remote manual station. An electronic controller may also provide input to manual mode, but duty cycle must be limited to less than 25%.

Complete specifications are listed in the product bulletins:
Mpac, Linear, Rotary & Drive

Additional topics concerning the Mpac:

• STROKING SPEED- Xpac, Mpac
  The tortoise or the hare, both are winners with REXA/koso america inc
• SPRING FAILURE
  Security in the event of a power outage (also see PM21)
• POSITION TRANSMITTER
  Self calibrating, two-wire, 4-20 mA, passive or active
• AUXILIARY CONTROL
  Pushbutton override on the front of the enclosure or at a remote location