Marine Engineering Study Materials

PID Controller

Setting Knob

Set the setting pointer (red) to the desired graduation on the scale by setting knob.

Proportional Band Control Dial

As we have discussed before, shifting position of the nozzle with respect to flapper changes proportional band. This is achieved by turning proportional band dial. The smaller the value on the dial (i.e., Proportional band becomes narrower), it becomes sensitive. But if it becomes too sharp, hunting occurs. When the value on the dial becomes too dull, deviation of measured value from set value, in the case of load change, becomes excessive.

Reset Time Dial

Reset time dial actually controls opening and closing of integral action valve. Reducing reset time means opening the valve while increasing means closing the valve. When reset time is shortened, the time required for balance point is shortened. But when it becomes too short, stability is lost and it apt to cause hunting. When reset time is made too long, it consumes too long time before balance is established at the set value.

Rate Time Dial (This dial is not given in the case of proportional + integral action)

Rate time dial varies opening and closing of derivative action valve. It is used for the process which involves much time lag (delay). When rate time is made too long it causes hunting. Whereas if it is too short, satisfactory effect is not obtained.

Change-over of Direct-Reverse Action

Direction of operation of control valve is determined by the nature of the process. It is determined by whether the control valve is a direct operation or a reverse. For example, in the case of the figure shown above, it acts in reverse. To change the set up into direct action (i.e., the action where increase of input causes increase in output), turn the proportional dial to the right for ¾ turns.

Optimum Adjustment of the Controller

The optimum adjustment of the controller can be obtained by the following steps.

1. Set the reset time dial at max. (20 min.) and when there is a rate time dial, set it at minimum value (0.05min).
2. Starting from 250%, gradually reduce the proportional dial from 250% to 10%, while checking the result of adjustment. It causes hunting when it comes to a certain proportional zone. The optimum value is at 2-4 times of such proportional zone.
3. Gradually reduce the reset time by turning reset time dial. When it comes less than the marginal value, it causes hunting. So reverse it slightly from such point and then fix it.
4. Fix the rate time dial at about ¼ of the ultimate reset time or about ½ of the time delay of the process and control device. In this case, proportional zone can be made slightly smaller.