Control strategies are at the heart of chemical process control. However, a question arises...
CONTENTS
* Some examples of control strategies
What is a control strategy?
Ok, perhaps I went too far with the word strategy and words like: forms, ways or methods give the same idea and are more modest.
For short, a control strategy refers to a general plan to perform control of a variable or variables. At this point, the concept seems to be clear but; there is more.
A plan may sound like monolithic stuff while in practice the same control strategy may be a little different from one application to another one. On the other hand, some control strategies form a subset of more general ones. In this sense, a control strategy refers to a general plan.
Other terms, can be found in textbooks or listened in the field, such as: control method or control mode. The idea is barely the same as with control strategy. Sometimes, these terms can be used to discriminate among the different control options available.
The term control mode is more common to the expressions: manual control mode or automatic control mode. These are related to features of the controllers.
Some examples of control strategies
Well, there are some and an uncomplete list can be given here whitout order of importance,
- feedback control,
- feedforward control,
- on/off control,
- ratio control,
- cascade control,
- PID control and
- fuzzy logic control,
- among others.
One further control strategy could be artificial intelligence which may be already included in a controller software and whose implementation/installation may be quite different to the traditional options.
However, these can help a little more if some sort of order is introduced.
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| A chart for most known control strategies. Notice that some a subset of other ones. |
From my point of view, there are two major categories of control strategies:
Feedback and feedforward control
These two strategies may include as subset the following,
- on/off control,
- ratio control,
- PID control,
- fuzzy logic control.
You must be warned: the controller may use on/off, ratio, PID or fuzzy logic control in a feedback or feedforward; but, does it work? That is the key.
Remember that from the theoretical point of view, on/off, ratio, PID and fuzzy logic control may be just an equation or a set of instructions but; from the technical point of view these are to be embedded in a controller as a program in a microcontroller. On the other hand, feedback and feedforward are more related to the control loop.
Read more on control loops in: What is a control loop?
Cascade control
Next, feedback and feedforward can be mixed together, or not, to form another control strategy: cascade control. In other words, physical implementation of cascade control will require the combination of two control strategies so that weaknesses of single feedfoward or single feedback can be improved.
Cascade control not necessarily needs the combination of these two original strategies since you can use two feedback or two feedforward control strategies. This is a more complex form of control requiring more specialized hardware and programming since two controllers are required in which a set point of one of the controllers is manually set while the other one is continually changed.
Control strategy and control loop
These are two very close concepts. In some cases, these are used as synonyms.
Let us say, without loss of importance on each concept, that control strategy deals with a theoretical idea of how to perform control while control loop is more related to the physical installation/implementation of the control strategy. In other words, a feedback control strategy would be the same as a feedback control loop.
Control loop is also a term common to PID diagrams where several loops may appear and a systematic organization of the information (labels, tags, etc.) must be ensured. Also, there are open loops and closed loops; which are a further detail not included in a control strategy.
Check your knowledge on control strategies generalities
Follow the link below to test what you have learned from this post,
What you know about control strategies generalities
This is the end of the post. I hope you find it useful.
Other stuff of interest
- LE01 - AC and DC voltage measurement and continuity test
- The pros and cons of bulb thermometers
- The pros and cons of bimetallic thermometers
- Some examples of temperature instruments
- Minor losses - Formulas
- What is a process variable?
- What are the most important process variables?
- Time dependence of process variables
- A list of process variables
- PID controllers for temperature
- Valves - Gate design
Ildebrando.
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