A Universal Definition
Too many readers, I fear, will interpret the above definition to mean that only electrical actuation is suited for industrial motion control. Such is simply not the case. Therefore, we need a definition with which all engineers and technical specialists can identify, one that does not exclude any type of actuation medium.
If we define motion control in terms of what it contains, the list of possible constituent parts becomes endless—and so does the definition. But, more importantly, Mr. Ross’s article goes on to list the “industry leaders” in the constituent parts, including fluid power component manufacturers. I suspect Mr. Ross’s background is in military or aerospace because his list, it seems to me, is slanted in that direction. If true, this is unfortunate because the future of America lies not in its military and aerospace hardware, but in the commercial, industrial, and manufacturing sectors.
I prefer to define motion control in terms of its objective, and in my book, Design of Electrohydraulic Systems for Industrial Motion Control, I offer this definition:
“Simultaneous control of acceleration, velocity, and position.”
Indeed, this is what fluid power engineers are in the business of providing. The degree of precision is a function of the application and the job that must be done. Sometimes, a proportional valve will suffice. In other cases, only a servovalve will work. In fact, at specification time, it is difficult to determine whether a given set of system requirements is “precise” or “imprecise.” Only an engineering evaluation will reveal that.
For example, obtaining a positioning error in steady state of, say 0.020 in. with a total travel of 60 in. can represent a formidable design challenge, whereas, 0.001 in. of error with only 4 in. of stroke sometimes is easily accomplished. On the other hand, if the load becomes 50,000 lb and cycle time must be less than one second, the likelihood of success is reduced. These are simple examples of the motion control specification process. Ultimately, however, a large number of usually conflicting requirements and hardware parameters must be brought together to design a workable system.
The components and adjustments (tuning) needed for a successful application are not always obvious at the outset. Most systems require extensive calculation and evaluation. Thus, the application engineer must proceed from specifications to system design by assuming that a solution to the problem at hand lies in motion control. Subsequent component evaluation then will yield the degree of precision needed and reveal whether closed-loop servo control is needed. Only those engineers trained in designing the highest level of system sophistication are in a position to decide if a lesser level of sophistication is acceptable.