By the way, since I have mentioned Omron: the PTO cards are called "positioning modules", while the analog ones - "motion modules". Feel the difference?
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Yes. It is a marketing thing. However a high quailty module will always be doing position control if it wants to do camming or gearing or fancy motion profiles. The better modules will have a trajectory generator so the ideal postion, velocity and acceleration is always known. I do not get hung up on position versus motion module. Motion sounds arbitrary to me. Position does not.
So the answer is: it all depends on the application. For simple "point-to-point" moves one may do perfectly well with PTO module. For serious motion you need analog. I did not mention the ever more popular network-based systems (SERCOS, Mechatrolink, Profibus etc.) - these are expensive and fancy versions of "analog" systems.
You forgot PWM. It is different than a pulse train.
Isochronous (constant time) bus systems are interesting. Profibus is not ready for prime time yet. At least not in a isochronous application. I havn't heard of Maechatrolink. Sercos, USB, and Firewire can be isochronous and provide ways of making an expandable system without analog.
Well, that is what I think. I might be wrong here
I think you got it right. I just want everyone to know there is a difference between a PTO and a PWM. A pulse train does not imply a duty cycle.