PID Auto Mode/Manual Mode Settings

HI

My question is, can controller not be used for varying SP as well as for regulation? I mean let the large step got through the controller ( instead of bypassing) and when the desired set point is reached, let is regulate

No reason why not. If you want the new set point, that would be the way to do it. Here is an example:

Your car has cruise control. If you want a new speed, just bump it up or down. You changed the set point.

If you want to pass a slow driver, you step on the gas and speed up (manual control). When you release the accelerator, the car returns to the old set point. This would be an example of manual override.

Hi

This is my point as well. Then no need to switch between auto or manual mode. Only one mode could do. Change the set point at will and then regulate when done.

But why PID controllers come with two options?

Because PID units without both modes do not sell as well as those that do have both modes.

Also manual mode can be very useful when troubleshooting a failing system.

"Because PID units without both modes do not sell as well as those that do have both modes."

This is commercial concern and does not address the need for two modes. Finally can SP be changed in auto mode? is yes then manual mode is unnecessary

"Also manual mode can be very useful when troubleshooting a failing system."

If the controller tuning is slow due to process characteristic, or due to an upstream or downstream failure, an alert operator can override the tuning to correct for an upset before the tank overflows, for example.

For instance, the fill valve for a tank is wide open, trying to keep up with a rapidly falling tank level. The downstream process stops taking the liquid out of the tank suddenly, the pump trips off or the valve closes. The process gain of the tank level is slow, lots of gallons of fluid for a very small change in level, for example.

The operator sees the high level alarm on the tank, sees the fill valve is at 100%, slowly, slowly, slowly closing, and puts the level control PI controller in manual and sets 0% for the valve, stopping the flow into the high level tank.

While you could do the same thing perhaps almost as fast response by setting 0% for the level set point, you then have to reset the level, and hope you remember where it should be, among all your other duties.

Also, some processes are complex feedback with long delays, they are easier to start up in Manual, until close to your final quality product, then once all processes stabilized, put the process in Auto control. Bumpless transfer is best for this, the operator just pops it in Auto and can then attend to other things, and not have to compensate for control induced upsets due to controller integration windup, for instance.

The more appropriate question is "When", not <...Why...>.

The answer to this question can be found in the plant's Control Philosophy and Functional Design Specification documents. As every application is different, the forum cannot answer the question better than the individual user.

"Horses for courses" _{- Anonymous Poster #0}

Automatic bumpless transfer refers to a centralized computer system that is controlling a process via a field located PID controller.

If the computer fails,the controller reverts to the manual mode,and uses the same setpoint and parameters as was used in the auto mode.Thus,the "bumpless" description.

It also allows the operator to change setpoints in the field if required when the need arises without having to change a program plan in the main computer.

The manual mode is also used for calibration check of the controller by injection of inputs and outputs that do not interfere with the centralized computer.

bumpless also goes in the other direction, manual to auto, or from one regulator mode to another, such as between speed and torque control for rotating machinery, as an example. Setpoint tracking, PV tracking, MV tracking are enabled to facilitate the various transfer modes.