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Hydrostatic actuator energy from a self-contained package

Fluid power has been part of daily life for decades. Much of the high-powered machinery for building highways, constructing buildings and driving industries have been made possible only through the brute force that hydraulics technology provides. For many years, hydraulic technology has suffered a negative reputation because of external system leakage. Unfortunately, the problem has been approached under the concept of environmental concern, when in fact, leakage objections are much more local — as being unsightly and hazardous to workers. The mere adoption of bio-friendly fluids, although beneficial, is insufficient to neutralize the real objections to external leakage. Additionally, conventional hydraulic cylinders must also be powered from a remote hydraulic power source through piping systems, hoses and fittings, becoming a costly installation in a single force requirement.

Because of these complaints, the electrical drive industry made inroads into the otherwise traditional hydraulic markets with the electro-mechanical linear actuator, packaging together a motor-driven gearbox with a ball screw. This combination is mechanically intricate and thus suffers from unreliability — particularly in harsh environments. It is also limited in its force output density compared to what potentially could be achieved with a similar hydraulic unit. 

The increased demand for industrial automation in recent years has spurred development of a wide range of electronic devices for accurately controlling the position and speed of linear and rotary motion. The quality and performance of the actuators that produce this motion have improved, responding more reliably to the electronics that drive them. Subsequent advancements in hydraulics have created better, more compact valving.

M-mac Actuators Inc, North Vancouver, BC, applied these developments to the creation of a self-contained hydraulic actuator that would fill the gap between the limits of force at which a mechanical actuator is practical, to applications where the magnitude of force dictates a conventional hydraulic system. With a remote, centralized power supply, the actuator has the universal mounting options of electro-mechanical devices, but with double the force density, improved controllability, inherent lubrication and virtually maintenance free operation. The self-contained actuators combine a bi-directional, variable speed AC or DC electric motor coupled to a high efficiency, high pressure bi-directional pump that plugs into an integrated circuit block with an built-in facility to access a pre-charged expanding reservoir and volume compensator. This modular power cell fastens to the actuator cylinder and combines a fluid communication system that connects to each end of the cylinder chambers without using conventional hose and fittings. There is no need to vent to the atmosphere. The combined package is pre-filled with synthetic hydraulic fluid and sealed with zero-leak, O-ring plugs.

Installing the actuator simply requires mounting and an electrical interface to the motor. There are no directional solenoid valves — stopping and starting the actuator in any position is achieved by stopping and starting the motor. The motor controller and adjustable hydraulic valving can vary the actuator’s acceleration and deceleration. Closed loop position sensing capability can be added through co-axial linear transducers. M-mac says that their electric hydrostatic actuators (EHAs) have achievable positional resolution and repeatability within 0.002 in. (depending on peripheral controllers), and that actuator speed can be adjusted to a very low creep. Control of the actuator is achieved by direct drive between a high efficiency bi-directional pump and a standard reversing variable frequency motor. When stopped in any position, the actuator remains solidly locked, and it can hold any load within the relief stage setting of the integral load holding valve.

These stand alone actuators can replace electromechanical ball-screw actuators in many applications, such as precise modulation of flue dampeners in thermal energy applications, modulated control of large gate and globe valves, slide gates for bulk materials handling, and raising and lowering security barriers. The aerospace industry has recognized the advantages of stand-alone actuators for primary and secondary flight controls, and the EHA addresses the need for power-by-wire hydraulic actuators in a self-contained package — free of hydraulic lines and centralized hydraulic power supplies, this design reduces the possibility of leakage as well as the weight factor. In an industrial setting, a self-contained EHA would offer the same benefits of safety and cleaner operation, while providing high force densities where required, particularly in contaminated environments where ball screws are prone to failure without regular maintenance. 

—RM