Electrohydrostatic Actuation Captures the Best of Hydraulic and Electromechanical

Hydraulic actuation has existed for a long time and continues to provide the power density required of many applications. But in recent years there has been a push toward electromechanical actuation options because they are considered to be more energy efficient and there are no issues with oil leakage – but this technology has its drawbacks as well.   

Electrohydrostatic actuation, on the other hand, brings together the best of both technologies said Dave Geiger, Engineering Manager, New Business Capture at Moog Inc. “It offers you all the benefits that electromechanical [provides and] it also offers you all the benefits of hydraulic actuation.”

While more costly than electromechanical options, he said that if you need to capture the best of both worlds, electrohydrostatic is your best choice.

Electrohydrostatic Actuation Ticks all the Design Boxes

Geiger explained that electrohydrostatic actuation technology works similarly to electromechanical actuation in that a bidirectional motor controls the linear or rotary motion of the output device (i.e., an actuator). The difference is that electrohydrostatic technology uses a hydraulic transmission to transfer motion versus a mechanical transmission.

“So, it's a hydraulic transmission between the output of the actuator and the motor itself,” he said.

Compared to purely hydraulic or electromechanical actuation, Geiger said electrohydrostatic technology ticks all the boxes in the five main categories designers grapple with when developing motion control systems: 

• energy efficiency
• environmental cleanliness
• low noise emissions
• high forces and
• zero backlash.

Hydraulics are not known for being particularly energy efficient – though there is a lot of development effort in this area currently – and the potential for oil leaks tends to negate their environmental cleanliness. They are also noisy, but again there are developments in progress to reduce their noise levels. But hydraulics are very good at providing high forces and do not create backlash which is not a good thing to have when trying to create precise motion control, said Geiger.

The shift to electromechanical systems came about because hydraulics are lacking in the first three categories, especially energy efficiency and environmental cleanliness which have become more important factors today than they were in the past. While electromechanical actuation offers the benefits of energy efficiency, low noise emissions and environmental cleanliness, this technology lacks the ability to provide high forces and not have backlash in the system.

Geiger said this is where electrohydrostatic comes in because it performs well in all five categories as it brings the beneficial aspects of electromechanical technology together with those of hydraulics.

READ MORE - Electrohydrostatic Actuation: An Energy-Efficient Option  

How to Choose the Right Actuation Method

Understanding your cost parameters and application requirements are key factors to consider when determining which type of actuation technology to use.

Market Adoption Continues to Grow for Electrohydrostatics 

Electrohydrostatic actuation has been proving itself in various industries for many years, and continues to gain market share in those looking for a combination of high forces, energy efficiency and reduced environmental impact.

Geiger said Moog has been using the technology since the 1980s when the aircraft industry wanted to move to fly-by-wire – essentially running wires from where the actuation is happening – for its flight controls. Historically, hydraulic systems were used but the industry wanted to eliminate all the hydraulic lines involved due to their potential to leak and the efficiency gains possible by having fewer lines through which to transfer energy.

Electromechanical was not an option because while the required forces are low, the industry did not want there to be any risk of primary flight controls failing. He explained they wanted a by-wire (essentially an electric) solution but did not want mechanical devices such as screws or gearboxes to be part of it due to their potential to fail.

This left electrohydrostatic actuation as the option because from a transmission point of view, the technology works the same as a hydraulic system, said Geiger. “We [Moog] ended up moving that industry into electrohydrostatic for primary flight controls.”

Industrial machinery is another sector shifting away from hydraulic actuation technology, due in part to greater environmental pressures to reduce energy consumption and emissions. Geiger said the move from hydraulics to electromechanical or electrohydrostatic – depending on the force needs of the machine – started slowly but has picked up the pace in recent years. “Now there’s fairly quick movement,” he said.

Because of the energy efficiency and high force capabilities possible with electrohydrostatic actuation, he anticipates rapid movement toward the technology over the next 10 years in the industrial machinery sector.  

READ MORE - Industrial Hydraulic Concept Offers Best of Both Worlds

Geiger also sees opportunity for the use of electrohydrostatic actuation in the mobile construction equipment market as it transitions to battery-electric power systems. Often considered to be a slow adopter of new technology, the construction machinery sector has increased development of battery-electric options over the past 5 years.

He said these machines require the use of highly efficient actuation to minimize energy drawn from the battery which could otherwise shorten its operating time between charges. There are examples of compact machines using electromechanical actuation, such as the Bobcat T7X all-electric compact track loader developed in collaboration with Moog Construction.

The challenge, however, is the larger size machines with higher force and durability requirements which cannot be met with currently available electromechanical technology. Electrohydrostatic actuation might be the right solution though for these machines because it can provide the high levels of force and efficiency required.

“I think that's the big future right now for electrohydrostatic because it's such a big market and that industry is changing,” said Geiger. “From a volume point of view and from a movement, I think that's going to be where we're going to see a lot of traction.”

He noted that other heavy-duty mobile machinery like agricultural machinery and mining equipment could also benefit from this technology. And if fuel cells become the power source of choice in the future, electrohydrostatic systems will prove beneficial in these applications as well.

As with any technology, cost is always a factor. Geiger said certain industries are very cost sensitive which can cause hesitation when moving to a different actuation technology.

Construction is one of these industries which he said is extremely cost sensitive. “That’s one of the reasons that industry has stuck with classic hydraulics for such a long time because it was so cheap.”

Cost pressures will be a huge challenge to overcome because electrohydrostatic systems are not cheap, said Geiger. But construction is an industry with high-volume potential “and we're hoping that through volume we can get those costs down,” he concluded.