Using Mode Changes to Uncover Tuning Issues

Mode Changes Provide Insight Into a Process’ Overall Performance and They’re Often Linked to Poor Controller Tuning.

Operator interventions and Mode Changes are often viewed negatively. When a PID loop is switched out of automatic there can be a sense that the controller wasn’t allowed to perform its job. With each change, goals related to production throughput and efficiency suffer a psychological hit. Since so much is invested in automating plant-wide production, interventions can feel like a step backwards. Viewed in a different light, however, those interventions provide potentially valuable insights that help production to move forward.

Operators are trained to oversee plant-wide production and to maintain safe, steady output. While operators rely on their facility’s control system, they hold primary responsibility for acting on information produced by that same system. Intervention is part of their job description. Still, rest assured that most operators would prefer to keep things in automatic mode rather than switch to manual control.

As a control loop performance metric Mode Changes keeps count of interventions. What’s more, Mode Changes provides particular insight into the effectiveness of a controller’s tuning parameters. Consider the following:

• Roomy Range
  

PID controllers can fail to maintain adequate control if the rate of production changes significantly – whether higher or lower – and the associated parameters remain static. Interventions in the form of Mode Changes are common in such situations, and they point to a common controller tuning issue.

Since parameters are designed for a general operating range, the solution may be as simple as adjusting the Gain to accommodate a broader range of control. Alternatively, an adaptive Gain strategy could be applied. Such an approach can accommodate the dynamics associated with different rates of production. Scheduling the use of different Gains for the various operating ranges should provide more effective control.

• Bad Behavior
  

Most Operators will admit that there are certain PID loops that they prefer to control in manual mode. Loops that operate at or near a constraint can give Operators the jitters. That’s especially true if the loop’s performance is critical or if the process is particularly dangerous. Switching to manual gives them a sense of control.

Intervention of this sort can often be attributed to poor tuning. A skilled Operator would know to intervene when the process is headed beyond a constraint or is losing stability. Tuning can reign in excessive variability and bring such a loop under control. However, Mode Changes of this type can also indicate a lack of Operator training. Whether a need for tuning or training, Mode Changes reveals a control-related problem that requires attention.

CLPM solutions like PlantESP offer trends of KPIs such as Mode Changes. Graphics such as the one shown above provide insight into the frequency at which interventions occur and can alert production staff to tuning and other performance-related issues.

Other Issues

For sure not everything is a problem and not every problem requires tuning. It’s been suggested that only 10% of performance issues can be corrected with tuning. The vast majority of issues are mechanical in nature. The biggest issue by far is Stiction.

There are any number of justifiable reasons why Mode Changes occur. Keep in mind that production facilities are dynamic and once in a while they undergo a change over, they’re idled temporarily, or they’re shut down for maintenance. All are Mode Changes that occur regularly and that have nothing to do with controller tuning.

Like the Oscillation Metric, Mode Changes is one of several KPIs that can be used to uncover performance issues and the need for PID controller tuning. Collectively they are a core component in any control loop performance monitoring (CLPM) solution.