ON/OFF AND PID CONTROLS HELP A MAJOR NEWBIE!!! (basic questions i think)?

[greeK_god_HirYu]

Can anybody help me wih these I just don't fully understand this yet thank you!!!

1.) What is the difference between on/off control and continuous PID control?

2.) What happens to a feedback control loop when the set point is changed?

3.) What happens to a feedback loop when a disturbance occurs?

my guesses are for the first the on/off only has two configurations 0% and 100% control while for pid you have 0-100% am i right?

for the second i don't have anything on that and for the third i think the loop will be closed...

please help, we have a preliminary exam tomorrow and i need to do good at this thanks!!!
XD

 

[mike1942f]

1. An on/off control doesn't shut off until the set point is reached, (like opening of a thermostat at room temperature) the heat in the element or other heat source plus that already radiated to the walls causes the temperature to continue to rise. When off and starting to cool, the sensor "discovers" the temperature is below the set point after the walls are below the set point - so the temp continues to fall below the set point until it rises - past the set point - and so on. And this doesn't mention hysteresis the (at least slight) difference in temps between the off point and on point (due to springiness in the bimetallic strip if nothing else) needed to keep the temp from chattering.
A PID control can be running an ON/OFF device (like a relay or SSR) but it doesn't wait for the set point to be reached - it adjusts the time on for the heater so that as the set point is approached it is on less and less time, slowing the rise - this is Proportional power. The electronics also Integrate the heat supplied and compute the Derivative (thus PID) so the temp cruises right up to the temp and doesn't over shoot.
Modern electronics (like the controller on my annealer in my backyard) "learn" the PID settings (which can also be set to specific values) and you can watch as it turns off the heater on the way up to "see" how the temp keeps rising so that by the set point is reached it goes right there and stops.
2. When the set point is changed, the ON/OFF jumps or drops to the new point and waves back and forth the new point (oscillates) The PID simply cruises to the new setting
3. When it is "bumped" - the lid is opened to look at the contents - the temperature drops and the behavior when the lid is closed is the same.