Adcanced Control: OT, A line follower

[Peter Nachtwey]

Why can't you do motion control with a standard plc?
It's not fast enough.

Can you do motion control with a fast plc? Yes, but this would make this article obsolete.

Another example:
http://elm-chan.org/works/ltc/report.html
Can you program a small line follower robot with a standard plc? No, it's not fast enough. But a 8 Bit Microcontroller can do this.

First, I want to point out that the line follower does not need to generate the target position, velocity and acceleration. This is all predefined by the line on the table.
The target generator in a motion controller requires about hundreds or thousands times more code than a simple PID. The target generator is the most difficult part of a motion controller by far.


Just following a line using a PID should be simple but....
Read the section about tracking. The integrator was not used.

Why?
It is obvious to me that they are doing it wrong. Most people that do line followers are doing it wrong.



The people on this forum have talked many times about the sample time being the same as the loop update time of the PLC.
This is a common error with Rockwell PLC. I think it is unforgivable that this should happen BUT..
executing a PID as a function time is NOT the only option.
PIDs can be executed as a function of other things. In the case of a line follower it should be distance along the line, NOT TIME!


I will let you all think about this.