There’s a lot of cool technology available these days. And one of the problems of having a lot of options is “Navigating the Technology”. What’s the best solution for a particular mechatronic application? I think a good understanding of the application and associated constraints goes a long way to making good decisions.
Is the application a “prime mover”, running at a set speed for many hours or a positioning application? Prime movers run at constant speed for long periods like fans and pumps. These are best served by AC motors and across the line starters. When variable frequency drives are used, its usually to eliminate mechanical damping of the load and “tune” the electrical requirement to the mechanical requirement. This technology option can provide huge cost savings when properly applied.
Positioning applications are completely different. Most often, they are intermittent in operation and have varying loads. The goals are completely different and the solutions are different. But what can make positioning applications difficult is the myriad options for how to do it.
Linear motors are a great option with incredible speed, acceleration and precision. They are particularly popular in semiconductor manufacturing due to millionth-of-an-inch precision and design for clean room operation. But if you don’t have those requirements its overkill in performance and probably over cost for what’s needed.
There’s the venerable lead screw and stepping motor combination. Very cost effective and capable of impressive precision. Tow ten thousandths (0.0002 inches) accuracy is achievable at low cost.
But positioning applications need not be exclusively electronic. Pneumatics make great choices where speed and extended life cycles are required. One of the great attributes of modern pneumatic systems is the integration of the actuator and structural framing that is often a separate requirement in other systems. And with a wide range actuator options and styles available, the possibilities are endless.
Are there some guidelines that can help make the process more objective? In the positioning applications, one obvious differentiator is accuracy. And as a practical matter, it seems that there might be three logical categories; coarse (greater than 15 thousandths) medium (from 15 thousanthds to one ten-thousandth) and fine accuracy from 50 millionths to sub-micron). By considering a positioning requirement in terms of the accuracy requirement, a lot of technology choices become more clear.
Another attribute of the application is dynamics. The dynamic requirement considers how quickly the load is varying. This aspect directly impacts power electronics and control system performance. The relationship of current over time defines what power transistors will have to handle and correlates directly with variations in the load. After all, torque and current are the same in electrically based systems. Dynamic response in ac and dc drives is referred to as the inverse of time so typical performance is expressed in Hertz. A slow system response might be in the 2 Hertz range, meaning that the system must handle a known load variation and be able to stabilize itself in a half second. A closed loop ac drive may be capable of 100 Hertz performance. A high performance servo may have a speed regulation loops of 2 thousand hertz.
Armed with accuracy and dynamic response, we can sort out a lot of options on the way to better mechatronic solutions.