Control system performance: Which figures should be considered?

Are the controls used?

• The method used is the system’s intended control method (automatic or cascade, not manual)
• The control element (valve, variable speed drive or other) does not reach its limits
• There is no interlock to prevent the controller from performing its functions.
• The equipment is operating

When all four of the statements are true, a system is being used. Incidentally, these statements should be true over 90% of the time for all control loops.

Are the controls functional?

Each control loop should lead the process to the set point on schedule. To do so, the loop must be stable, without oscillation. Thus, control systems are deemed functional provided:

• the oscillation index is below 0.4 (any oscillations disappear quickly)
• the stiction is weak
• the settling time is ± 50% of what is expected

Systems are functional when all three of the above conditions are met. These conditions should be met over 90% of the time for all control loops.

Are the controls efficient?

Controls are considered efficient if the systems can maintain the variable value at the set point. Performance is usually measured with one of the following methods:

• variability
• integral of absolute error (IAE)
• overshoot or maximum permissible error

The degree to which the objective is achieved must be measured for each loop. This value should exceed 90% for all loops.

Overall performance is obtained by multiplying the following:

%Use * %Operation * %Performance.

In fact, the longer the observation period, the higher targets should be.

For more information on this topic, contact our experts.