March 2008

Power to the fluids

By Peter Nachtwey

Fluid power refers to energy transmitted via a fluid under pressure. 

Pneumatic fluids are typically compressed air or inert gas. In hydraulic systems, that fluid is a liquid, like oil or water. 

Fluid power's motive force results from the principle that pressure applied to a confined fluid is transferred uniformly and undiminished to every portion of that fluid and to the walls of its container. 

A surface (e.g., a cylinder piston) will move if the difference in force across the surface is larger than the total load plus frictional forces. The resulting net force can then accelerate the load proportionately to the ratio of the force divided by the mass.

Although hydraulic and pneumatic power have common characteristics, there are some key differences. For example, because hydraulic fluid is much less compressible than gas, hydraulics are preferable to pneumatics when precise position control is required. 

Conversely, pneumatic power has an edge in applications where hydraulic oil's presence could cause problems, like in food processing machines. Pneumatic systems are also typically less expensive  to build.

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Fluid power advantages

Fluid power can work in diverse applications ranging from mobile construction and aerospace equipment to powering industrial machinery, and it offers several advantages over other types of motive force.

In fluid power systems, a single source of fluid pressure (e.g., a compressor or pump) can power many motion axes or fluid power devices. The power source can sit where space is not critical. Because much of the system's size and weight off-loads onto the power unit, the individual actuators can be small vis-à-vis the power they produce. 

In addition, they are often quieter and generate less heat than electric actuators. Fluid power actuators can also serve in hazardous environments where electric sparks cannot happen.

By using accumulators to store energy, a hydraulic power unit needs to provide only slightly more than the average demand, increasing efficiencies for machines with varying load cycles.

In applications requiring a constant holding pressure or torque (e.g., presses), hydraulic actuators have a big advantage because they use no energy while they are stationary.

In contrast, electric motors draw considerable current to maintain torque, even while stopped. Most motors will overheat and fail under these conditions.

Hydraulic cylinders are very smooth and efficient for linear movement. There are no poles that cause cogging, and there is no need for backlash compensation.

Controlled variable valves 

The oldest and least sophisticated hydraulic or pneumatic systems employed an on/off pressure valve control, sometimes called "bang-bang" because of the "jerkiness" that discrete control causes. 

New systems designed for smooth, accurate motion use electronically controlled variable valves. These devices can employ sophisticated predictive control algorithms, using inputs from position and pressure sensors to provide tighter control and more flexibility than was possible with hydraulic control elements.

By including special smart functions, controller manufacturers can offer higher productivity to machine builders.

Hydraulic and pneumatic power offer many advantages over electric motors, especially for systems that require high-speed linear travel, moving or holding heavy loads, or very smooth position or pressure control. 

In comparison, hydraulic and pneumatic actuators are smaller and quieter. They also produce less heat and electromagnetic interference at the machine than electric actuators do. In many cases-particularly in high-performance hydraulic or pneumatic systems-they offer machine builders a considerable cost savings compared with similar machines employing purely electrical or mechanical motion.

ABOUT THE AUTHOR

Peter Nachtwey has an electrical engineering degree and 26 years of experience developing hydraulic,  pneumatic, and vision systems for industrial applications. He is president of Delta Computer Systems (www.delta compsys.com) in Vancouver.