Feedback control
Feedback control is an important technique that is widely used in the process industries.
Its main advantages are as follows.
However, feedback control also has certain inherent disadvantages:
Feedforward Control
The basic concept of feedforward control is to measure important disturbance variables and take corrective action before they upset the process.
Feedforward control has several disadvantages:
Example:
A boiler drum with a conventional feedback control system is shown in Fig.1 The level of the boiling liquid is measured and used to adjust the feedwater flow rate.
This control system tends to be quite sensitive to rapid changes in the disturbance variable, steam flow rate, as a result of the small liquid capacity of the boiler drum. Rapid disturbance changes can occur as a result of steam demands made by downstream processing units.
The feedforward control scheme in below Fig can provide better control of the liquid level. Here the steam flow rate is measured, and the feedforward controller adjusts the feedwater flow rate.
In practical applications, feedforward control is normally used in combination with feedback control.
Feedforward control is used to reduce the effects of measurable disturbances, while feedback trim compensates for inaccuracies in the process model, measurement error, and unmeasured disturbances.
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