Commissioning Control Systems – Input Filtering

The method used to process analog inputs and outputs is unique to each control system.  Also, controller and IO bus design influence how often measurement and outputs are updated and thus dictate the control execution rate that is supported by the control system. Understanding the limitations imposed by the basic control system design (i.e., controller and bus design) may be important in achieving the desired control performance when commissioning the control system.

As discuss in Chapter 11 of Control Loop Foundation – Batch and Continuous Processes , the analog input cards used to access field measurements typically contains a hardware filter. This hardware filter is designed to attenuate any high frequency noise from the measurement before it is processed by the A/D converter. An example of the hardware filtering provided on an analog input card is shown below.

The analog cards of some control systems may also contain an adjustable software filter that may be used to prevent sample aliasing i.e. measurement distortion introduced by undersampling an analog value. To avoid aliasing, it is necessary to sample at least two times as fast as the highest-frequency content in the signal. If a signal is sampled at a lower rate then the signal can be distorted by higher frequencies being incorrectly interpreted as lower frequencies as illustrated below.

The hardware filter on an analog input card may be designed to ensure that no aliasing occurs if control is executed at a specific rate, for example, once per 100 milliseconds. However, if control is executed at a slower rate to reduce controller loading, then added filtering may be required to avoid aliasing. A software filter that executes at the same rate as the A/D converter may be set to provide additional filtering to ensure that aliasing does not occur when control is executed at a slower rate than is supported by the hardware filter. When commissioning a system that does not provide this capability then it may be necessary to apply damping at the transmitter to avoid aliasing.

The lag introduced by damping and filtering is usually insignificant compared to the other process dynamics. However, on very fast liquid flow or pressure processes, filtering and damping can degrade control performance if the control execution rate is slow. Thus, anti-aliasing software filtering and added damping at the transmitter should be considered only if the measurement is noisy and not removed by the analog input card hardware filter.