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Aug
08

Commissioning Control Systems – Installed Characteristics

From a control perspective, it is highly desirable that the process gain be constant. If the process gain is constant, then the same proportional gain may be used over the entire operating range of the control loop. However, if for example the valve characteristic has not been selected based on the process requirements, then the installed characteristic could be non-linear as illustrated below.

From this plot, the process gain may be determined at any operating point by dividing the percent change in the controlled parameter by the percent change in the manipulated parameter. As illustrated in this example, the process gain varies from 0.5 to 4, that is, the process gain changes by a factor of eight. Even if the loop tuning is selected to have a high gain margin, such a large range in process gain can cause slow response in the low gain region and potentially unstable operation in the high gain region. If the process gain is known to change over the operating range of the final control element, then loop tuning should be established in the high gain region to ensure that stable operation will be achieved over the entire operating region. From a control perspective, the product of controller gain and process gain should be a constant value over the entire operating region of the final control element. If the process gain is changing, then the impact is the same as changing the control loop gain of a process with constant gain. Thus, changes in process gain can have a large impact on control performance.

As addressed in Chapter 12 of Control Loop Foundation – Batch and Continuous Processes, changing the valve characteristics during control system commissioning to provide a linear gain over the operating region may not be an option because of the time and expense associated with changing valve characteristics. To ensure stable operation, the tuning may be established at the operating point of maximum process gain. However, in the low gain region, control performance will be sluggish. When you are confronted with this situation, a better solution is to compensate for the changes in process gain by installing a characterizer block between the PID and Analog output blocks as illustrated below.

If the control system does not support this block, then often it is possible to construct equivalent capability using other tools of the system provided for doing calculations.

Characterizer blocks are available in many Foundation Fieldbus devices and in some control systems. The relationship between the primary inputs and the output of the characterizer block may be defined by 21 x,y pairs over the final control element operating range. Input values that fall between these points are automatically determined by the characterizer block using linear interpolation. The objective is to define the characterizer relationship such that the product of the characterizer gain and process gain is constant over the entire operating range of the final control element. The curve defined by the characterizer points appears as the inverse of the plot of the final control element installed characteristic, as illustrated below.

Through the use of a characterizer, a consistent process response to setpoint changes and load disturbances may be achieved even if the installed characteristic of the final control element is non-linear.