Smart Actuator Propels Hydraulic ‘Beast of Burden’

A smart hydraulic actuator is enabling a powerful four-legged robot to negotiate harsh terrain without the aid of multiple cables, remote computers, or external devices.

Known as the Integrated Servo Actuator, the custom-designed component incorporates a two-stage servo control valve, position and pressure sensors, and onboard electronics. By doing so, it creates a simpler package for the legs that propel the robot.

"It dramatically simplifies the architecture of the quadruped," Gonzalo Rey, director of research and technology for Moog Inc. , told Design News . "By integrating all those functions into one piece of metal, it doesn't need any additional transducers, cables, or remote computers telling it what to do."

The HyQ (hydraulically powered quadruped) robot, created by researchers at the Italian Institute of Technology , was developed as a platform for the study of dynamic robotic motions, such as running and jumping, over rough terrain. From the outset, researchers hoped it could be applied to a variety of applications -- such as forestry, rescue, and construction trades -- where it could be used to haul equipment and supply power.

"We look at it as a power tool, a mechanical beast of burden that can negotiate rough terrain and deliver payloads and power when it gets there," Rey told us.

To accomplish that, however, the 200-lb robot needed to control the torque of 12 of its leg joints. Torque control is key to the robot's ability to maintain balance over uneven surfaces, Moog engineers said. Using position control without it, they said, would likely cause the robot to topple when it encountered unexpected obstacles.

"Torque control allows the designers to stabilize the robot, even in less structured environments," noted George Small, principal engineer for Moog. "Position control alone is just not robust enough for those kinds of applications."

To solve the problem, Moog engineers endowed the hydraulic actuator with a two-stage hydraulic servo valve accompanied by pressure sensing, position sensing, and closed-loop control via the unit's onboard electronics. All of the components are incorporated into a custom-designed, printed titanium actuator body made by additive manufacturing. During operation, the actuator's hydraulics typically operate at 2,300 psi, although they are capable of pressures in excess of 3,000 psi, Small said.

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Moog engineers said the key to achieving torque control is the bandwidth of the hydraulic valve. The HyQ's valves operate between 150-300 Hz, enabling the system to control joint-level torque in the range of 40 Hz. Such speed is critical, engineers say, because the robot must be able to quickly react when its "foot" strikes an uneven surface.

"With each step, it stays on the ground for only a fraction of a second," Rey said. "And in that fraction of a second, you have to control the torque and generate a force to balance it."

Robots have long been able to accomplish such feats, but the Integrated Servo Actuator enables designers to do it in a compact way. "Traditionally, you would have scattered all the various functions into individual components," Small told us. "Here, it's easier

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