FEATURE: The future of electro-hydraulics | Industrial Vehicle Technology International

Over the past three decades off-highway vehicle designers have moved away from building machines that have hydraulics controlled via mechanical systems and hydraulic pilot lines to utilising electronic control – electro-hydraulics. But, while such systems are well established, the full possibilities of this architecture are still being explored.

Even though hydraulic movements are still very much analogue in nature, the controls that operators use are almost wholly now in the digital age, though the change did not come about overnight, rather via a gradual evolution.

This is borne out by component specifications. Danfoss, for example, after years of designing components that could be used with either mechanical or digital interfaces, has now completely transitioned its new components to electro-hydraulics.

“The interface of our hydraulic products has gone through three stages of evolution,” says Jeff Herrin, senior vice president of R&D at Danfoss Power Solutions. “The first stage, dating back many years, involved hydraulic and mechanical interfaces. For example, a hydraulic pump controlled by a lever that an operator could use to create the desired flow.

“The second stage introduced designs with both lever access and digital control. This transitional phase was necessary because industry shifts don’t happen overnight. We needed a 10-year plan where customers who wanted the legacy solution could still get it, while we also built in the modern, future solution of a digital actuator that they could control.

“Once we reached a certain level of adoption, we moved to the final stage: products specifically designed for digital control. These don’t carry the baggage of mechanical devices, which add cost and compromise designs.

“For most of our products we’re already at this last stage so we no longer design future products with legacy interfaces. If customers still need a mechanical interface, they can buy the older generation, which we still produce. They have different performance and safety levels, but they remain available.

“Our focus now is on designing hydraulics with digital interfaces because we know the future lies in electro-hydraulic systems. In fact, 100% of our new product development programs are targeted in that direction.”

New ways to move

Recently Volvo Construction Equipment has used electro-hydraulic architecture in its new Independent Metering Valve Technology (IMVT), available on 50-tonne EC550 and EC530 excavators.

“IMVT marks a departure from the traditional mechanically coupled hydraulic systems and introduces intelligent electronic control with numerous variables,” says Sejong Ko, Volvo CE’s product manager for large excavators. “The results are up to 25% improved fuel efficiency and better operator control.”

IMVT enables extra features using a closed-centre valve-control system, which unlike traditional open-centre systems, can stop or minimize hydraulic flow through the main control valve when there’s no demand for a hydraulic actuator. This significantly reduces fuel consumption and eliminates the need for an eco mode.

It also uses independent cylinder control, sensing and controlling each side of a cylinder’s chamber independently, without using hydraulic spools and pilot lines. This results in more precise control of the excavator’s movements.

“IMVT replaces traditional spools with poppet and electro-hydraulic valves in the main control valve (MCV), reducing pressure losses and optimizing valve control based on load conditions,” says Sejong Ko. “This leads to faster cycle times, improved regeneration and reduced flow loss, with analysis showing a 20% boost in productivity.”

Hydraulic hybridisation

Despite potential energy recovery levels in off-highway vehicle hydraulics being relatively small when compared to the well-established hybrid systems used in hydraulic braking for automotive; in recent years, tighter emissions targets and rising energy costs have provided greater demand, particularly in excavators, which generate the most carbon emissions of any vehicle class in the off-highway world.

“But it’s not a trivial thing to do in hydraulics,” says Herrin. “You have to add more complexity with additional components and circuits, and to do it, you absolutely have to have an electronic system.”

The value proposition for that additional complexity has historically been fairly poor, with OEMs having to make the trade off on complexity versus benefit. But the growing push for sustainability is driving innovation. “We did a refresh on our industrial pump product line,” says Herrin. “It was designed 30 years ago and is now being refreshed and updated to modern specifications. The spec for this new pump was, in fact, to be able to recover energy as well as pump energy.”

In 2020 Volvo CE first announced that it had developed a Hybrid system for its excavators which recovers energy directly from hydraulic systems, not primarily by using pumps, but instead by means of a hydraulic accumulator. It captures and reuses hydraulic power, resulting in up to 17% increase in fuel efficiency and 15% reduction in CO2 emissions. Recently the OEM announced this Hybrid technology would be added to its large EC400 and EC500 40 and 50 tonne machines, having already been available on four smaller vehicles in the range – the EC380E Hybrid, EC350E Hybrid, EC300E Hybrid and EC250E Hybrid excavators.

“We do it by mounting a hydraulic accumulator just behind the boom, and as the boom goes down, the high- pressure oil that you use to retard the boom dropping goes into the accumulator instead of being routed to the tank,” says Dr Ray Gallant of Volvo CE. “This gives the accumulator a high-pressure charge that’s then routed through assist motors and put back into the cylinders on the upswing. The entire process is controlled automatically – the operator isn’t required to do anything. He simply sees a small symbol on the dash that tells him when his accumulator is charging and discharging.”

Volvo CE’s hybrid hydraulic technology doesn’t generate electricity or require batteries for storing it. “It requires minimal components – there are no electronics to program and no control systems other than system monitoring,” says Gallant. “Maintenance is also simple as the system is a essentially a combination of hydraulic flow and valves.

There are other solutions that are more reliant on pumps being capable of operating in a bi-directional manner. For example, pump- controlled actuators manage motion primarily through speed and/or displacement control of individual pumps, making the pump a central component in the system. It is in this domain that promising technologies include Danfoss’s Digital Displacement pumps, floating cup technology, and high-speed pumps. “We expect these solutions to reach the market over time,” says Hongsuk Kim, head of excavator medium and hydraulic platform at Volvo CE. “Particularly for electrically powered machines, where bi-directional speed- controlled pumps can recover energy back to the electrical power system.”

In the future additional steps could be taken with hydraulic energy recovery technologies. For example, dead-weight compensation systems. “These use additional cylinders and hydraulic accumulators operating in parallel with the main hydraulic motion control system and could offer advantages in specific applications,” says Hongsuk Kim. “These systems are relatively simple and should not face significant hurdles to reaching market readiness.

“A more advanced solution is the common pressure rail architecture, which utilizes one or several semi- constant pressure levels combined with hydraulic accumulators. In this setup, force and speed are controlled through hydraulic transformers and multi-chamber cylinders, which represent state-of-the-art technology. Even these advanced systems require multiple independently controlled valves, much like IMVT, demonstrating that our development efforts in IMVT are paving the way for future innovations in hydraulic energy recovery.”

Electromechanical actuators

A number of prototype vehicles, as well as Bobcat’s production all-electric T7X and S7X compact track loaders and skid steers, have done away with traditional hydraulics altogether and replaced them with electromechanical linear actuators. Herrin sees continued evolution in the field. He notes that Danfoss Power Solutions is investing in electromechanical actuators as

a complementary product line to their electrification portfolio. While the use cases for this technology are currently modest, Herrin anticipates that, “as electrification takes a bigger hold in off-highway equipment, ultimately electrified linear actuation will eliminate hydraulic fluid on machines. Rotary actuators (wheel drives, work functions) will be first, steering next, and linear actuators last.

Volvo CE is already making use of electric-motor driven actuators in some of its excavators, and reports positive customer feedback. “It is important to note that electric motor-driven actuators can work in conjunction with hydraulics as an intermediate power transmission medium or, in some cases, without hydraulics altogether,” says Hongsuk Kim. “However, both hydraulic and electric systems have their respective advantages and limitations. We do not believe that one technology will fully dominate across all applications.

At Volvo, we are not committed to phasing out hydraulics but instead focus on adopting the technology that best suits our customers’ needs. We acknowledge the challenges associated with electro-mechanical drives, especially for linear motion applications. These challenges are less pronounced for rotary drives, making them more feasible to replace hydraulics now.”

Enabling automation

One of the key advantages of electro- hydraulics is that is has enabled machine control – whether that be semi-autonomous driver assistance systems or full automation, being able to control movements via digital systems is a critical enabler.

“One of our flagship innovation programmes is around autonomy,” says Herrin. “There are very few use cases for fully autonomous machines right now, but there’s a whole lot for semi-autonomous, which mean operators can be less skilled, because the machine can operate itself in cycles that are repeatable.”

This trend towards automation is driving the need for more advanced technologies. As Herrin points out, “When you get into that space, AI and machine learning are required. It’s the core digital algorithm technology that enables automation.”

This article first appeared in the October issue of iVT