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.

Cost competitiveness
When looking at a machine’s total costs, there are four key areas to consider: (1) components, (2) installation, (3) maintenance and (4) power. A fluid power system should win out for the total of areas (1) and (2) depending on the number of axes; however, due to fluid/filter maintenance costs and hydraulic systems’ lower efficiency, it will be more expensive for (3) and (4).

For life cycle costs, the deciding factor will be the machine’s duty cycle and life expectancy. It would certainly make sense to use fluid power motion control for a one-off system, for say a movie, that will be scrapped or reconfigured for the next production. But for something like a flight simulator, which has a high duty cycle and long life expectancy, servo drives will probably make the most sense. A detailed analysis of duty cycle and system efficiency is required to find the true winner.
Another factor is if the end user is not familiar with proper hydraulic system maintenance techniques, then maintenance costs can be significantly higher. However, this can easily be dealt with through proper training.

As always, a complete understanding of the machine will lead to the best results, and the use of fluid power components have to be predetermined in the design stage; simply replacing an axis or two on a machine may be counterproductive given that an HPU will be required to drive even one axis. But considering its large power output and the small envelope size at the actuator, fluid power is a very viable source of motion control.

Advanced electronic controls
In addition to costs, fluid power is also competitive in terms of its sophistication, ease of use, interoperability and response. Modern electro-hydraulic controls, for example, are highly evolved with on-board electronics and spool position sensors to provide linear characteristics as well as electronic valve tuning to suit the application. In many cases, control software provided by manufacturers allows valve parameters to be loaded and changed via connection to a laptop or network.

In addition, the modern electro-hydraulic control valve with on-board electronics can be as easy to use as supplying it with power and a command signal. These valves can also be specified to interface with the popular communication protocols such as Ethernet, PROFIBUS, field bus and CANbus. The latest generation of these valves can also incorporate the motion control function and allow the closed loop feedback to be connected directly to the onboard valve electronics.