This error signal is processed and applied to the control valve, which either opens or closes, proportionately, as the situation dictates. Another type of system is a chemical feed system that adds a chemical to a process based upon another parameter without relying on the use of feedback. The chemical is added according to a pre-determined formula, or proportionately. A good example would be the addition of chlorine to plant effluent. Within each system, several control settings are used to fine-tune the response of the control system.
Some controllers provide expanded capabilities such as logic operations, ramping and profiling.
This means that if a controller has a setting of 0.01seconds, a corrective change will occur, if required, every 0.01 seconds. This feature is useful for very fast processes. Derivative is not commonly used, but is useful for improving control response in fast processes. Derivative acts as an anticipatory response to a rapidly changing process. Help others by posting your comments, suggestions and experiences with water or wastewater treatment or any other concerns you may have on our On-Line Help Forum .