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.