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Old April 22nd, 2005, 10:24 PM   #12
Peter Nachtwey
United States

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Join Date: Apr 2002
Location: Vancouver, WA, US
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That isn't proportional only.

Originally Posted by marksji
Well done Ron!
I'd like to add one thing though. In the real world there are times when proportinal only control does work. Any time you have cascaded control loops it is possible that a proportional only loop can be the best fit for one of the loops, ie:

P Loop -> PID Loop
I see an integrator in your control system.

I would prefer to change the term 'sealed' to 'lossless' or 'integrating'. Integrating? WHAT IS THAT? Yes, Ron the text books are right. A system with just a proportional gain will not reach the set point. Those sealed systems are lossless and are therefore perfect integrators. Yes, the integrator can be part of the physical system.

I like to think in terms of position and velocity control. It should be obvious that a proportional term only will not allow one to control the actual velocity at the target velocity because losses will cause the system to slow down till the kenetic energy reaches zero.

A position system will the target position if there is no friction ( losses ). This is because the change in position is cause equal to the integration of velocity.

The control nerds call these systems type 0 and type 1 systems. A type 0 system has no integrator ( velocity control ). A type 1 system has one integrator (position control). An example of a type 2 system is an inertial guidance system. Here an accelerometer is integrated once to provide speed and integrated again to provide position. The fact that this system integrates twice makes it a type two system.

In short, the text book is correct because the integrator can be part of the process or system itself.
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