Electrohydrostatic Actuation: An Energy-Efficient Option

Figure 1. Depicted is the general layout of an electrohydraulic actuator and actual image of a Moog EHA.

Electrohydrostatic actuators (EHAs), widely used in the aerospace industry, are now putting wind beneath the design wings of machine builders by combining the best of electromechanical and electrohydraulic technologies. The EHA converts power from electric to hydraulic to mechanical. An electric servomotor drives a bidirectional, variable-speed pump connected to the two chambers of a hydraulic cylinder (Fig. 1). Depending on the flow, the axis extends or retracts.

In contrast to a conventional hydraulic system, power is controlled by the pump instead of a servovalve. Varying the speed of the pump varies the flow, and, as a result, varies the hydraulic power. Also, the pressure in the chambers is load-dependent. This lets the electrohydrostatic actuator efficiently use energy and deliver power on demand.

The Benefits of EHA

The hydraulic oil in an EHA absorbs little heat—typically around 40°C—because the system is so efficient. Convection cooling is usually sufficient for an EHA, which allows designers to build compact, modular units with a self-contained hydraulic system. Because the actuator’s oil ages very little, users typically don’t have to change it during the system’s life. The EHA hydraulic fluid shows less stiffness than an electromechanical system. However, various layouts are possible, and machine operators can adjust the EHA to carry out fail-safe options or serial movements of several axes.

Industrial machines that employ EHAs benefit immediately (Fig. 2). There’s no reason for concern about working with first-generation products, because the systems are well-established.

Figure 2. Here, a deep-drawing press uses EHA for the die cushion axis. (Courtesy of Dresden University)

Thanks to the continual price decline in power electronics and servomotors, engineers are better equipped to develop an EHA that’s ideal for many industrial applications and competitive with traditional hydraulic solutions. Eliminating hydraulic plumbing, auxiliary pumps, servovalves, and maintenance of filters and valves gives the EHA great appeal. The self-contained systems approach translates into much higher reliability, since it mitigates, or even eliminates, failure and maintenance associated with individual components of traditional electrohydraulic solutions.

So, why should machine builders evaluate EHA? For those motion-control axes that require higher forces unattainable without large gearbox reductions with electromechanical (EM) solutions, EHA becomes a solution. And machine builders can use EHA if they want designs that combine EM actuation with hydraulic actuators to provide all-electric interfaces (Fig. 3).

Figure 3. These layouts present a comparison of the three actuation technologies.

As for the cost of ownership, the EHA doesn’t require a hydraulic infrastructure to incorporate into a machine. EHA allows machine builders to evaluate combining electric and hydraulic actuation technology without consideration of the fixed cost of the hydraulic-system infrastructure.

EHA Delivers EM Benefits

The industrial machinery market is shifting toward machine electrification to save energy, lower maintenance costs, and improve performance. EM solutions can’t compete with the high forces available in hydraulic systems. To achieve these forces, it’s impractical to use a servomotor combined with gearboxes and rotary-to-linear conversion. EHA, however, has a wider range of force available in a compact form factor while eliminating mechanical gearing. The lack of ball screws or gearing is compelling to machine builders who seek simplicity in design.

An automation engineer will see that an EHA looks and performs like a servomotor-based EM solution. An EHA is a closed-loop servo system requiring the same mechatronic skills to size an actuator and tune the dynamic performance. The tools for determining loop gains and nonlinear compensation for EM solutions are employed when integrating the EHA into a system. Furthermore, the control interfaces for EM and EHA are alike if Moog supplies the servo drives and EHA. In fact, the underlying digital servo drive technology used in Moog’s EHA stems from the technology in the company’s electric drives. Thus, digital interfaces (e.g., EtherCat) or a traditional analog command interface are available.

EHA technology offers machine builders a way to individualize the functionality of a machine, a process, or a movement, which is a step forward for hydraulic or EM architectures. Conventional systems have a large hydraulic power supply and servovalves that control axis movements or functions, which inherently have a lot of energy losses. With EHA, machine builders can eliminate these losses because the system employs individualized axis architecture, where an electric interface provides (via the electric servomotor pump combination) only the power needed, hydraulically, for specific functions and movements.

EHA Application Suitability

Evaluating if an EHA suits a machine-control application requires assessing the economics and performance. From a “black box perspective” an EHA functions like an EM solution. From the perspective of the automation controller (PLC in most cases), the interfaces are identical—motion profiles are planned through digital interfaces. Two scenarios offer the ideal application for an EHA:

1. Electromechanical machine conversions that need the force capability or power density of hydraulics for a small number of axes.

2. All-hydraulic machines that have only one or two axes of motion.

Machines that require a relatively small number of hydraulic axes often are excellent candidates for EHA. This is an economic justification and doesn’t factor in any of the dynamic performance issues, which designers must evaluate.

Conclusion

To help machine builders incorporate EHA into a new generation of machines, Moog is combining standard building blocks, such as servodrives and a servomotor/pump combination, in typical sizes with a manifold and cylinder that designers can customize to an application’s exact needs (Fig. 4).

Figure 4. A wide range of building blocks combine with customized products to deliver EHA solutions for a customer's needs.

Using EHA technology saves a substantial amount of energy via power on demand, while the dynamic performance is equal to conventional EH or EM systems. Such a self-contained axis leads to a decentralized machine axis design, which optimizes axis performance. EHA is environmentally friendly, easy to install, and the interfaces are identical to EM solutions; the functionality is far more flexible and the achievable forces are greater.

The building-block components are adapted to the specific requirements of an EHA application. For example, the motor-pump unit includes a mechanical interface that allows a direct mounting to a cylinder or manifold, and the pump can operate in the four-quadrant operation mode. And with EHA, machine builders can choose the right technology for an application without incurring integration hurdles. It’s no wonder EHA is helping machine builders’ designs soar.

Achim Helbig, Ph.D., is Innovation Projects Manager at Moog Inc., East Aurora, N.Y.