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.

Fluid power is the original motion control technology and has been used for everything from pressing and packaging to injection moulding and handling. As manufacturers’ requirements for precision and control grew, fluid power evolved to meet this challenge with servo and proportional control, offering cost-effective and high-performance solutions.

However, the development of servo motor technology was a direct competitor to fluid power motion control and has, over the years, replaced fluid power systems in many applications. Having said that, fluid power still has a key role to play in motion control and should always be considered as an option. The following is a list of key areas to consider when looking at your next motion control application.

High loads and cycle rates
In general, the electric option, either a ball screw and servo motor or electro-mechanical cylinders, typically have a shorter life span than a positioning cylinder when moving a large mass at the same velocity/acceleration. Also, when exceeding a 15 to 20 HP threshold, the size of the components becomes an issue; a 25 to 30 HP servo motor is relatively large if required for each axis of movement. One of the advantages of hydraulics is that the power source can be located away from the actuator and therefore space requirements on the axis itself are reduced.

Capital cost also can be reduced if the axis movements are not required at the same time since fluid power would use one power unit for all actuators. In addition, depending on the overall system layout, it’s possible to drive multiple axes with limited input HP. Servo motor drives almost always need dedicated motors and drives for each axis.

Another consideration is the power utility service and the demand charge levied by the number of electric motors in a plant. Servo motors typically have higher peak-power demands than a hydraulic power unit. And depending on the duty cycle, accumulators can be used to average power demand by storing fluid under pressure when system flow demand is low. So with proper design, savings can easily be realized in situations where actuators do not run at the same time or have intermittent use.

Finally, with current technology, there are things that simply can not be done with servo drives. For example, force control is easy to do with fluid power. In a modern paper machine with a large roll with a fixed shaft and rotating outer shell (driven by a servo drive), the shell can be rotated at a high speed. Using hydraulics, the roll shell can be precisely loaded at roughly 60 individual points inside the shell. This is done to control paper surface finish to produce magazine quality paper.

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.

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.