Fluid power: the hidden giant

Fluid-power systems: almost as fast as a speeding bullet, definitely more powerful than a locomotive, and possibly able to leap tall buildings in a single bound. They should sport a Superman logo. This pervasive, hidden technology employs a simple principle that leads people to think that implementation of fluid power is just a select-assemble-and-run task. Nothing could be further from the truth.

At the end of the 19th century, in response to the need to more effectively transmit power from one point to another, hydraulic systems replaced traditional mechanical systems. Developers during the Industrial Revolution emphasized fluid power, but most applications were steady in nature and required only static considerations for design. At the beginning of the 20th century, hydraulic-control-system development experienced a major setback, and decades of stagnation followed, with the phenomenal growth of electrical power. The stagnation ended as World War II drove the need for power transmission requiring high effort and fast response, which only hydraulic systems could provide because of their superior power density over electrical devices. Over the next 40 years, industry was the steward of technology for the fluid-power world. A resurgence of interest in fluid-power-control systems is occurring at universities, and industry/university collaboration is growing.

Unfortunately, some people consider fluid power a specialist subject. Hydraulic-control systems are essential, however, in applications requiring large forces or torques, with a fast response and high accuracy. They have a better power-to-weight ratio than electrically actuated systems, which are limited by magnetic saturation, and they excel in environmentally difficult applications. In addition, the hydraulic medium is mechanically stiffer than the electromagnetic medium. Self-lubrication and inherent heat transfer are also advantages. Fluid-power applications are numerous. They include vehicle steering, braking, and suspension systems; industrial-mechanical manipulators and robots; and actuators for aircraft and marine vessels. They are all multidisciplinary systems and require a systems approach to design and implement. The required engineering background includes fluid mechanics, electromechanics, system dynamics, computer-control systems, and electronics—in other words, a mechatronics background.

Power conversions occur throughout a basic hydraulic-control system (Figure 1). A variable-speed-motor-driven pump pressurizes the hydraulic fluid. A relief valve and an accumulator (not shown) regulate and stabilize the pressure of the fluid. A servovalve, driven by an electric valve actuator, provides a controlled supply of fluid into the actuator, which is either a piston-cylinder device or a hydraulic motor, controlling both flow rate and pressure. Low-pressure fluid from the servovalve is filtered and returned to the reservoir. A feedback digital-control system completes the system.


Today, the competitive advantage of fluid power is greatest in mobile applications, such as mobile heavy equipment, human-assistance devices, mobile human-scale equipment, and hydraulic hybrid passenger vehicles. In these applications, fluid power can be a compact, efficient, and effective source of energy transmission. The opportunities are great for mechatronics engineers in this re-emerging field.