or

1 The plant didn’t blow up.

2 The process measurements stay close enough to the setpoint.

3 They say it’s OK and you can go home now.

1 Optimum decay ratio (1/4 wave decay).

2 Minimum overshoot.

3 Maximum disturbance rejection.

The choice of methods depends upon the loop’s place in the process and its relationship with other loops.

IAE - Integral of absolute value of error

ISE - Integral of error squared

ITAE - Integral of time times absolute value of error

ITSE - Integral of time times error squared:

• These mathematical methods are used primarily for academic purposes, together with process simulations, in the study of control algorithms.

or

1. Enter an initial set of tuning constants from experience. A conservative setting would be a gain of 1 or less and a reset of less than 0.1.

2. Put loop in automatic with process "lined out".

3. Make step changes (about 5%) in setpoint.

4. Compare response with diagrams and adjust.

Also known as the "reaction curve" method

The process must be "lined out"—not changing.

With the controller in manual, the output is changed by a small amount.

The process is monitored.

The following measurements are made from the reaction curve:

X     %            Change of output

R    %/min.     Rate of change at the point of inflection (POI)

D     min.        Time until the intercept of tangent line and original process value

The gain, reset, and Derivative are calculated using:

Another means of determining parameters based on the ZN open loop.

After "bumping" the output, watch for the point of inflection and note:

D is calculated using the equation:

D & X are then used in the equations on the previous page.

Mathematically derived from the reaction rate method.

Used only on processes that will stabilize after output step change.

The process must be "lined out"—not changing.

With the controller in manual, the output is changed by a small amount.

The process is monitored.

Steps

The gain, reset, and Derivative are calculated using:

The "controllability" of a process is depends upon the gain which can be used.

The higher the gain:

• the greater rejection of disturbance and

• the greater the response to setpoint changes.

The predominate lag L is based on the largest lag in the system.

The subordinate lag D is based on the deadtime and all other lags.

The maximum gain which can be used depends upon the ratio .

From this we can draw two conclusions:

• Decreasing the dead time increases the maximum gain and the controllability.

• Increasing the ratio of the longest to the second longest lag also increases the controllability.

Flow loops are too fast to use the standard methods of analysis and tuning.

Analog vs. Digital control:

• Some flow loops using analog controllers are tuned with high gain.
• This will not work with digital control.

With an analog controller, the flow loop has a predominate lag (L) of a few seconds and no subordinate lag.

With a digital controller, the scan rate of the controller can be considered dead time.

Although this dead time is small, it is large enough when compared to L to force a low gain.

Typical digital flow loop tuning: Gain= 0.5 to 0.7

Reset=15 to 20 repeats/min..

no derivative.