PID Control : control tuning

The performance of a PID control loop depends on the following:

• The quality of the measuring and control devices.
• The effect of process upsets.
• The control stability as manifested in the ability of the measured variable to return to its set point after a disturbance (see figure 1). This ability is dependent on the correct controller PID settings, which is accomplished through good tuning.

figure 1

Tuning means finding the ideal combination of P, I, and D to provide the optimum performance for the loop under operating conditions. Keep in mind that “ideal control” must be determined for a specific application.

Loops can be tuned either for minimum area, minimum cycling, or minimum deviation (see figure 2).

• “Minimum area” produces a longer-lasting deviation from the set point. It is used for applications in which overshoot is detrimental (e.g., a defective product would result).
• “Minimum cycling” produces minimum disturbances with a minimum time duration. Applications with a number of loops in series benefit from this setup because it provides overall process stability.
• “Minimum deviation” maintains close control with small deviations and is the most commonly used. However, there is cycling around the set point. The amplitude should be kept at minimum.

figure 2

Controller tuning is generally done automatically, manually, or through adjustments based on experience. In all cases, a few simple rules will minimize problems.

• Check with the operator before starting.
• Before retuning an existing controller, note the old settings (just in case you need to go back to them in a hurry).
• If you are on manual, and the process is steady, take note of the output signal to the valve (in case you need to go back to manual in a hurry).
• On cascade loops, tune the secondary controller first, with its set point in local mode.

Automatic Tuning

In automatic controller tuning, the software/hardware vendor has included a feature in the equipment to perform the tuning function.

Manual Tuning

Manual tuning is a combination of art, science, and experience. In addition, two elements are required for good tuning. First, a good understanding of the loop being tuned is required; second, lots of patience is essential, since some loops may take a long time to properly tune.

There are two basic methods for manual tuning: open loop and closed loop. Open loop tuning may be used to tune loops that have long delays such as analysis and temperature loops, and closed loop tuning may be used to tune fast loops such as flow, pressure, and level loops.

Open Loop
The open loop method (see figure 3) consists of the following steps:

• Putting the controller on manual (open loop)
• Making a step change to the output (X) (5 to 10%)
• Recording the resulting action (PV) from the feedback element
• Finding the reaction rate R (= B/A)
• Finding the unit reaction rate Ru (= R/X)
• Finding the effective lag L (time intercept)
• Setting the controller PID values

gain = 1.2 / (Ru x L)
integral = 0.5 / L in repeats/minute
derivative = (0.5) L in minutes

• Where only P and I values are required, the settings are

gain = 0.9 / (Ru x L)
integral = 0.3 / L in repeats/minute

• Testing and fine tuning, if required

figure 3

Closed Loop
The Ziegler-Nichols closed loop method (see figure 4) consists of the following steps:

• Putting the process on auto control using “P only” mode (set I and D to minimum)
• Moving the controller set point 10 percent and holding until PV begins to move
• Returning the set point to its original value
• Adjusting gain until a stable continuous cycle is obtained (i.e., critical gain, Gc)
• Measuring period of cycle (Pc)
• Setting the controller PID values

gain = (0.6) Gc
integral = 2 / Pc in repeats/minute
derivative = (0.125) Pc in minutes

• Where only P and I values are required, the settings are

gain = (0.45) Gc
integral = 1.2 / Pc in repeats/minute

• Testing and fine tuning, if required

figure 4

Reference : The Condensed Handbook of Measurement and Control