What are the advantages and disadvantages of an ONOFF and PI

What are the advantages and disadvantages of an ON-OFF and PID controller? When would you use one over the other?

Solution

PID controller advantages include:

1.Feasibility

2.Easy Implementation

3.The PID gains can be designed based upon the system parameters if they can be achieved or estimated precisely.

4. The PID gain can be designed just based on the system tracking error and treats the system to be \"blackbox\" if the system parameters are unknown.

Disadvantages of PID:

1. PID controller generally has to balance all three-gains impact to the whole system and may compromise the transient response, such as settling time, overshoots, oscillations.

2. If the system parameters can not be precisely estimated or achieved, the designed PID gains may not resist the uncertainties and disturbances, and thus present low robustness.

3.Though the PID gains can be well-designed, the PID controller still has low robust ability compared with the robust controller when the system encounters to multiple challenges from the operating environment of the system, such as temperature, weather, power surge etc.

Advantages of On off controller:

1. It is the simplest form (or mode) of feedback control.

2. On-Off controllers with On-Off or two-position valves were employed on the steam, spray, and damper operators.

3. Its principal advantage is that it\'s low in cost, that is the controller and valve are usually inexpensive.

Disadvantages of ON OFF controller:

1. The principal disadvantage of On-Off control is that its normal condition is constant cycling.

2. Constant open-to-shut cycling of steam valves, no matter what their characteristics, is disruptive to boiler houses and steam headers, not to mention damaging to the valves themselves.

Usage of one over other:

The user must tune a proportional controller to match the characteristics of the process, which can be difficult in some cases. On-off control is chosen for simplicity. The only parameters that need to be set are the switching differential and setpoint. On-off control is suitable if the response delay is short when the output is switched and the maximum rate of rise is small, or if a very stable process isn’t needed.

A proportional controller output isn’t just on or off. Its output can be adjusted to any value between 0% and 100% of power. Just as with on-off, the control direction is important. Reverse or inverse acting changes the output from 100% to 0% as the process rises if heating. Direct acting would go from 0% to 100% if cooling. When correctly tuned, the controller can adjust its output to match the power that is required to keep the process stable at the setpoint. This is something an On-Off controller cannot achieve.