There has been a tremendous resurgence in robotics, particularly legged robotics in the last few years. Maturity in actuator technologies has catapulted a new generation of lightweight, high performance, mobile and agile robots. Standard actuation technologies used in mobile robotic platforms include hydraulics, electro-hydraulics and electro-mechanical. A renewed interest in hydraulic actuation for robotics stems from its characteristic of having a large power to mass ratio that dramatically increases the operational capability of anthrobots and powered exoskeletons. When there is a need for these robots to move heavy loads and place them with precision, no power source can compete with hydraulics in terms of cost and reliability (ref. Hydraulic Robot Provides Precise Positioning).
Hydraulics is an ideal choice for highly dynamic articulated robot applications for a number of reasons (Ref: Hydraulic Control):
High specific power density
Centralization of power source
Greater accuracy in frequency control
Robustness against impact load
Adjustability of the compliance of locomotion
Built-in heat-transfer and cooling through fluid flow
The power-saving benefits of hydraulics in robotics application stem from the fact that a single electric motor and hydraulic pump can power many motion axes. And each axis, in turn, is a fraction of the weight of an electric motor-driven actuator of equivalent power. In addition, the hydraulic pump’s motor can be sized to meet the average load the system must carry. This can be significantly smaller than in electromechanical systems, where the motor driving each axis must be sized for peak loads. Hydraulic systems smooth out energy requirements with accumulators. These simple devices store excess energy in a hydraulic circuit and re-deliver them rapidly on demand. As a result, accumulators are called upon to supply the potent power that energizes the hydraulics of these heavy duty mobile robots. A high specific power density (power per unit mass or KW/kg) of these accumulators allows for more efficient harnessing of the energy captured and delivered, thereby increasing the efficiency of operation.
Conventional wisdom holds that electric drives are more efficient than their fluid-power counterparts. But for certain tasks, hydraulics holds the upper hand.
Actuators are the ‘muscles’ of a robot, the parts which convert stored energy into movement (Ref. Robot Evolution: The Development of Anthrobotics). Actuation devices typically convert electrical command to mechanical power. A comparison of the different actuation devices and their relative pros and cons are displayed in the plot (courtesy Lonnie J Love, “Is there Sizzle in the Fluid Power Industry?“). As shown in the plot, the hydraulic actuation system has the advantage of maximum specific power density (KW/kg) and therefore, is the actuation system of choice where a sudden burst of power is essential. Consequently, reduction of the mass (kgs) of the entire actuation mechanism is key to improving its power density and hence its value proposition over other competing actuation mechanisms.
Steelhead Composites is developing and testing ultra-lightweight diaphragm accumulators, in the size range of 250 cc to 1 L that can enable advanced, autonomous robotic applications of the near future. The expected availability of these accumulators is Q3 of 2016. The all-composite construction of these accumulators reduces weight to a very small fraction of their steel counterparts while maintaining operating pressure in the range of 200-350 bar . The result is extremely high power density for the hydraulic actuation systems and the added potential of less weight to haul around for power hungry robots.