Interpreting PID Output

[buck8pe]

Hi, first time poster - so go easy and I'm relatively new to control systems and electronic engineering, so noddy answers are actually appreciated.
I'm going to post a link to the question I posted on another site to save me typing in all the detail again. I've decided to ask here because the site is more specialized.

Link:

http://electronics.stackexchange.co...nterpret-pid-output-in-a-slow-response-system

The specific question I have is in the update section, although it's worth reading the entire post to get a sense of what I'm trying to achieve. As a further update, I've disabled the I contribution from the controller and the error response is more stable, which I would expect.
I'm actually OK on PID theory, in that I understand enough to get by. My question really relates to how the PID affects the system itself (Position Vs Velocity). I didn't get a good intuitive understanding of either of these approaches from the contributors and my efforts to find good entry level descriptions on the net was poor. I'd appreciate a decent (intuitive) description of either in relation to my particular project. Regards.

 

[waross]

Have you ever used Cruise Control when driving?
The older units used Proportional control. The speed dropped slightly climbing grades as an expression of the proportional control. Very few drivers flt the need to constantly correct for a speed error of 1 or 2 kilometers per hour.
I regret that the world of industrial control doesn't work the way you seem to think that it should.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[buck8pe]

Guys, many thanks for your answers and I admire your patience. My thinking got twisted after a response about position vs velocity back in the other forum.

@IRStuff, your comment has straightened all this out: " Your problem seems to stem from the concept, "whenever the PID gives an output of 0;" the error is zero, not the command. The command is whatever it takes to get the error to be zero."
Believe it or not, that is how I understood it at the very start (before posting on SE).

It hasn't been a waste of time, I've received a few alternatives from waross and I've started reading "Application Control Theory for Embedded Systems" which is suitably entry-level. Regards.

 

[roydm]

This thread reminds me of the control system used in a crushing plant I worked.
The 14 crusher motors were 400 HP and the power was controlled by increasing or decreasing the speed of a belt feeding rock into the crushers, the setpoint would be 395-400 HP and that's where they would run 24/7 sometimes for weeks on end.

The controllers were Ramsey designated PEIC type.
At a fixed time the controller would compare the setpoint and process then depending on the error output a long or short positive or negative pulse to a motorized actuator controlling the belt speed. For the next little while the controller would wait and see what the outcome was before making the next change.
The controller Gain was set by changing the Error to pulse duration ratio.
The wait and see time was also adjustable, I don't know what you would call that certainly not Gain, perhaps Integral but not quite because normally integral action doesn't have a delay.

At one time the Instrument gurus tried to replace one of these controllers with a 3 term Foxboro, although they could get it to work it would never come close to controlling the crusher at 100% power.

I mention this because the actuators had no feedback loop as such, they were very simple just forward /reverse.