Today the most widely used actuators are electric and pneumatic: electric ones are compact, convenient and modulate well; pneumatic ones are simple where air is available. But both have inherent weaknesses — electric actuators struggle to deliver large drive forces and face life and maintenance issues, while pneumatic ones lack stability and control accuracy.
Electro-hydraulic actuators (including high-precision electro-hydraulic servo actuators) readily achieve large drive force, high-precision modulation and fast open/close. Why are they then little used? Because traditionally they needed a hydraulic power unit or servo control system — bulky and leak-prone — and were costly to run (energy and maintenance), so they appeared only where large force or precision was unavoidable.
The REXA integrated electro-hydraulic actuator, made by KOSO of the USA, has now entered the Chinese market and found wide application. REXA is an intelligent mechano-electro-hydraulic actuator in two series: Xpac — a modulating, microprocessor-controlled integrated electro-hydraulic actuator whose control performance rivals common electro-hydraulic servo actuators — and Mpac, dedicated to on-off duty with excellent stability and reliability. REXA's highly integrated design eliminates the bulk of traditional electro-hydraulic actuators while preserving and improving their superb control. Below is a short tour of the Xpac series: composition, principle and features.
REXA actuators consist of two parts: the actuator (hydraulic cylinder and electro-hydraulic power module integrated together) and the control box. In use the actuator mounts on the driven device; the control box may be near or remote, connected by cable — power, control and feedback cables up to 150 m — for convenient distance control.
The heart of the actuator is the electro-hydraulic power module: motor, gear pump, flow-match valves, oil reservoir, heater and bypass solenoid (spring-fail units only). Five module sizes exist — B, C, 1/4D, 1/2D and D — all delivering oil to the cylinder at 2,000 psi (13.78 MPa); different pump/motor combinations give different maximum flows, hence different maximum stroke speeds. Higher speeds come from combining modules or REXA's unique booster-pump system. The modular structure raises versatility, stabilises product performance and cuts spare-part stock.
REXA Xpac Flow Match System
How does the Xpac achieve high-performance modulation? The key is its patented Flow Match System — the latest in hydraulic control. Internal fluid pressure stays constant regardless of the magnitude or direction of the external load, so the actuator responds rapidly to control signals with smooth, precise strokes, achieving positioning other methods can hardly match. The Flow Match System lets Xpac position incrementally: the motor-pump runs only when movement is needed; once the cylinder reaches the target the pump stops, and the system locks Xpac in place — modulation is unaffected by pump starts, stops or reversals.
The control box houses the position control processor (PCP), power supply, motor driver, fuses, filter and a terminal. The PCP contains an 8-bit 80C52 microprocessor, a 32 KB EEPROM for programs, a 2 KB EEPROM for data, a 12-bit A/D converter and a 3-key, 5-digit display, plus two connectors for optional I/O boards.
Xpac is governed by the PCP with three operating modes — automatic, set-up and field. In modulating service the PCP runs in automatic. In set-up mode the user sets speed, travel, dead band, control signal and other parameters through simple key steps. In field mode the PCP keyboard allows manual operation.
Operation: from the commissioned travel and signal range the PCP converts the incoming control signal into a target position; the current position comes from the feedback unit on the actuator; their difference is the control error. If the error exceeds the (user-set) dead band, the PCP starts the motor to correct.
The bidirectional gear pump pressurises either side of the double-acting cylinder through one of two flow-match valves (FMV1/FMV2). To move the piston left, FMV2 is pressurised via port A; the spool shifts left under the pressure difference, opening the check valve; oil flows from port D to B and from A to E, and the piston moves left under the pressure at D. Where the piston moves against the load, ports B and C sit at near-atmospheric pressure and FMV2 also throttles, keeping a suitable back-pressure in chamber D for smooth running. On reaching position the pump stops, both check valves close, the oil is trapped, and the cylinder holds without motor operation.
Xpac output is linear or quarter-turn. Linear thrust runs from 0.9 to 53 tonnes; quarter-turn torque from 68 to 45,194 Nm — enough for most control duties. Linear speed varies with module and output — up to 101 mm/s at 8,896 Nm thrust; quarter-turn speed likewise — 0.55 s/90° at 282 Nm. Dead band 0.1%, adjustable 0.1–0.5% via the PCP; positioning accuracy better than 0.15% of stroke; linearity better than 0.05%; repeatability better than 0.10%; normal ambient -12 to 90℃. For special, demanding duties — minimum control point setting, flow characterization, water-hammer suppression (two-speed) and surge control — REXA offers optional software or hardware.
Despite such sophisticated control, the REXA actuator installs and operates with striking simplicity. It pairs easily with any valve, can replace complex servo mechanisms and delivers precise control with ease. REXA actuators increasingly serve power plants, petrochemical pipelines, metallurgical equipment, waterworks control valves and gas pipeline valves.
In short, REXA integrates the simplicity of electric operation, the speed and power of hydraulics, the reliability of solid-state electronics and the flexibility of user configuration — a revolutionary technology that simplifies the use, adjustment, diagnosis and maintenance of hydraulic systems, well worth wide adoption.
