The Final Say
Why is good control important?
By Dick Caro
It takes a lot of effort to attain "good control." Certainly, loop tuning is taught at ISA, but few control/instrument engineers actually practice loop tuning. Most of the distributed control systems (DCSs) have some type of automated loop tuning, and suppliers provide loop tuning software that can be added to any system, including programmable logic controllers (PLCs). Some processes, however, make it almost impossible to achieve optimal loop tuning. Understanding ways to improve control makes engineers more valuable to their company and improves operations.
First, what do we mean by "good control?" A control loop is said to be "in tune" when the process does not deviate from the setpoint. A poorly tuned control loop has excessive deviations from setpoint, which is referred to as noise, often caused by improperly set tuning constants. Sometimes, the process is nonlinear, so one set of optimal tuning constants to have good control is not possible. Processes that have large dead times such that the process reaction time to setpoint changes or load changes are greater than the process time constant cannot be tuned to improve performance using simple feedback control are another challenge. In these cases, improvements can be achieved by selecting some type of advanced control.
Aside from simple feedback loop control concepts, "good control" also means the process setpoints are set at the values that will make the process economic. Every process has some economics that should govern the values for all of the setpoints. The objectives of running the process will determine those setpoints. For example, the setpoints for achieving maximum product production rate are quite different from those to achieve minimal consumption of energy. Often, the product purity is the governing factor, while in other processes, it is more economical to assure that the least amount of a product component winds up in the waste stream or in a secondary product flow. Many times, the primary economic factor is product yield.
In many processes, the operating conditions must be held within certain constraints. Very often, the process may be operated anywhere between those constraints, but some type of penalty will result if those constraints are violated. Processes with a high amount of variability must have their setpoints adjusted such that the constraint will not be violated.
Control loops that are out of tune or do not have some type of advanced control to eliminate or compensate for long dead time, appear to be "noisy," as illustrated by the left-hand recording chart in the figure. When optimal tuning or advanced control is implemented, the recording chart might appear more like that illustrated by the right-hand chart in the figure.
Notice the process constraint remains the same, but the reduced process variance now allows us to raise the setpoint much closer to that constraint. This assumes that by raising the setpoint closer to the constraint, some economic advantage will be attained, which here is called "setpoint improvement." The setpoint improvement might be to increase production rate or to improve product yield, for example.
Note it is not enough just to tune the control loop to gain this economic advantage. You must adjust the setpoints. Of course, it was the reduction in control loop variability that enabled you to adjust the setpoint. Now you know why good control is important.
ABOUT THE AUTHOR
Dick Caro is an industrial automation consultant for CMC Associates, a Certified Automation Professional, and a Life Fellow of ISA. Caro chairs the ISA50 (fieldbus) standards committee and two ISA100 (wireless) subcommittees.