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DOTX-45 Dissolved Oxygen System Part 8 –PID Controller

DOTX-45 52 -













++=

∫

tde

Dtdte

tePoutput

)(

)()(

)(

Where:

output = controller output

P = proportional gain

I = integral gain

D = derivative gain

t = time

e(t) = controller error (e=measured variable – setpoint)

Figure 8-1 ISA (Ideal) PID Equation

The most notable feature of the algorithm is the fact the proportional gain term

affects all components directly (unlike some other algorithms - like the “series”

form.) If a pre-existing controller utilizes the same form of the algorithm shown

above, it is likely similar settings can for made if the units on the settings are

exactly the same. Be careful of this, as many times the units are the reciprocals

of each other (i.e. reps-per-min, sec-per-rep.)

PID stands for “proportional, integral, derivative.” These terms describe the three

elements of the complete controller action, and each contributes a specific

reaction in the control process. The PID controller is designed to be primarily

used in a “closed-loop” control scheme, where the output of the controller directly

affects the input through some control device, such as a pump, valve, etc.

Although the three components of the PID are described in the setting area

(section 6.25), here are more general descriptions of what each of the PID

elements contribute to the overall action of the controller.

P Proportional gain. With no “I” or “D” contribution, the controller output is

simply a factor of the proportional gain multiplied by the input error

(difference between the measured input and the controller setpoint.)

Because a typical chemical control loop cannot react instantaneously to a

correction signal, proportional gain is typically not efficient by itself – it

must be combined with some integral action to be useful. Set the P term to

a number between 2-4 to start. Higher numbers will cause the controller

action to be quicker.

I Integral gain. Integral gain is what allows the controller to eventually drive

the input error to zero – providing accuracy to the control loop. It must be

used to affect the accuracy in the servo action of the controller. Like

proportional gain, increasing integral gain results in the control action

happening quicker. Set the I term to a number between 3-5 to start (1-2

more than P). Like proportional gain, increasing the integral term will

cause the controller action to be quicker.

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