Fluid Power: The Original Motion Control

Fluid power may be "mature" but its pure horse power makes it still relevant in an increasingly electric field

Comments Off September 12, 2008
by Pat Jones, P. Eng.

As a result of these advancements, good design can easily achieve high response and accuracy of 0.001 in., which may be acceptable in a lot of applications.

While these components are high performance and reliable, it should be noted that they do need to be maintained properly, fluid cleanliness being the biggest issue. Proper maintenance is not rocket science, but these systems must be dealt with a certain way to achieve design reliability.

Natural frequency
Another key issue with fluid power is its low natural frequency, which is due primarily to fluid compressibility. Therefore, it may not offer the same response as an electric drive, which is inherently stiffer.

Also, hydraulic spool valve and differential area hydraulic cylinder characteristics make them non-linear and therefore must be compensated for in the valve/system controls. Great gains have been made in this area by designing spool valves (high-performance proportional valves) with spool position feedback loops allowing the valve characteristics to be programmed.

In addition, the use of a true hydraulic axis controller allows for smoother, faster and more precise control methods such as Feed Forward and State Feedback.
The bottom line is that although servo drives have rightly replaced fluid power motion control in many applications, fluid power is the choice when large loads need to be controlled at high accelerations. Also, a fluid power motion control system will typically be cheaper to purchase and install, with life cycle costs being determined by duty cycle and maintenance.

Sources for this article include: “Life Cycle Motion Base Cost Comparison: Electric vs. Hydraulic” by Charles Bartel and Dan Foster, Moog Inc. and “Fluid Power vs. Electromechanical Power” by Peter Nachtwey, Delta Computer Systems.

Pat Jones, P.Eng., is the owner of Consolidated Fluid Power (CFP) Ltd., where he is an instructor and consultant.