Contrary to what some people think, fluid power systems are capable of very high-performance motion control. And a well-designed, closed-loop hydraulic actuator can do things that motors can't do economically and reliably, such as positioning very heavy loads with a 10-micron resolution or better. Furthermore, oil pressure can be stored easily for use when power demand is high.
Pumps in fluid power systems can be sized to meet the needs of average loads, whereas electric motors that drive motion axes directly must be sized to handle the maximum loads that the system must handle. Plus, a single fluid pump can serve many actuators, whereas each motion axis in a motor-driven system must typically be powered by its own motor.
However, gaining the benefits requires close attention to good system design principles. Fast, precise motion requires a controller with specialized fluid power capabilities. A well-designed hydraulic system, with components that are selected for precision and sized correctly, is equally important. For example, old two-position bang-bang valves should be replaced by servo-quality proportional valves that can make smooth and precise changes to oil flow. This allows trial-and-error tuning of systems to be replaced or augmented by the use of automated tuning tools that can optimize system performance and reduce energy waste.
Peter Nachtwey has more than 30 years of experience developing hydraulic, pneumatic, electronic and vision systems for industrial applications. He graduated from Oregon State University in 1975 with a BSEE and served in the US Navy until 1980. He became president of Delta Computer Systems, Inc. in 1992. In addition to leading Delta’s engineering and R&D programs, he has presented technical papers for IFPE, NFPA, FPDA and various technical conferences.
It is possible to get very good accuracy with servo pneumatics. A lot depends on the load, changes in the load, the quality of the the components and the desired speeds. I have a video where I move about a 5 lb load pretty quickly doing point to point moves and a sine wave where the frequency and amplitude change.
Here is an example of what can be done.
This cannot be done using a normal PID controller with just velocity and acceleration feed forwards.
Interesting article, Peter. I recall years ago there was some talk about high-resolution positioning with servopneumatics. Can you get good positioning accuracy with pneumatics and do those systems still exist?
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