An occasional topic of conversation in the control industry is understanding what is meant by coordinated motion. The topic is fairly ambiguous since we have many examples of complex pieces of machinery with high axis counts and which definitely give the impression of coordinated motion. The illusion of coordination is based on the fact that we observe control system behavior in real time.
Large, complex systems with many axes of motion can be achieved with a simple sequencer like a Programmable Logic Controller (PLC). The pneumatics industry has been doing it for years. Even animatronic systems for creating sophisticated entertainment systems at theme parks run using simple sequencer logic.
Is there a way to distinguish coordinated motion from simple sequenced motion? Think about the difference between playing a game of “Tic Tac Toe” and “drawing a circle”. When you move around the board in Tic Tac Toe or checkers, it’s point to point. Even though there are two axes and they are “loosely” coordinated, it is possible to create the desired motion using two linear axes and sequencing. In this scenario, there is no interaction between the axes while the motion is executing.
When a machine like a water jet cutter has to cut a circle, every pulse from the feedback of the axes has to be coordinated. The accuracy of the circular motion becomes a product of the feedback accuracy and the speed requirement. This gets further complicated because the control system update rate, velocity control loop, current loop, all become part of the actual performance. In mechanical systems, circularity error becomes measurable as the feedback becomes more precise. Error around the perfect circle is generated by accumulated control error at every level in this architecture, which makes getting it “right” a little trickier than it looks.
This doesn’t mean the PLC’s can’t do true coordinated motion. Many PLC systems have dedicated motion modules that use separate processors to do true coordinated motion. 4 Axis motion processors usually used a high end Digital Signal Processor to do the math and control high performance applications. But if the requirement is for 5 or 6 axes of coordinated motion, a PLC can be problematic. The coordination requirement between multiple motion processors become dependent on backplane updates between the modules. Not good for high performance motion unless the updates are faster than the clock speed on the digital signal processor.
So, telling the difference between coordinated motion and non-coordinated motion is not very difficult, but what to do about controlling coordinated motion is a lot more difficult.