difference between on/off and feedback control

[garimella]

On/off control also takes feedback for processing and a PID also uses feedback to achieve its goal. Can we say that on/off is special type of feedback control where output hovers around a range of values.

 

[KlausST]

Hi,

I'd say ON/OFF has nothing to do with feedback. It is the control method....how the load is beeing controlled.

ON/OFF is some kind of digital control method... In opposite to an analog control mechanism.
PWM is a fast ON/OFF control, withing the load with a triac or SCR is ON/OFF control, too.

******
PID is the calculation method, the algorithm...how to calculate the new control value.
The control value for an ON/OFF control can be the duty cycle.

Klaus

 

[FvM]

Systematically, on-off control is clearly a kind of closed loop respectively feedback control. It can be considered as P control with infinite gain.

An on-off closed loop shows permanent oscillations with a frequency determined by the control process.

In my view, pulse width modulation isn't on-off control. It's continuous, e.g. PID, control combined with a modulator.

 

[garimella]

can we just say that there are only 2 classification open and closed loop and in closed loop we classify them as either on/off, PID, feedforward, ratio or cascade based on requirement?

 

[FvM]

All control topologies where the controller output depends on the process value belong to the closed loop category.

 

[CataM]

Differences in control are made based on if they are linear or not.
There is the ON/OFF control, linear control (involves the standard transfer function and known regulators e.g. PID) and non linear control.

 

[KlausST]

Hi,

In my view, pulse width modulation isn't on-off control. It's continuous, e.g. PID, control combined with a modulator.

Isn't ON/OFF the same as PWM, just with much lower freqeuncy?

Klaus

 

[FvM]

Isn't ON/OFF the same as PWM, just with much lower frequency?

Not the same, I think. Consider e.g. an on-off controlled heater operating with low duty cycle. The average temperature will be considerably above the set-point (and vary with duty cycle respectively heat demand). A PI controller with PWM keeps the average process value at the set-point temperature.