Basic Steps to Consider in Designing a Control System

Typically a control system should be designed to work together with an already existing process. The control design problem can be stated at local, supervisory or even plant-wide level.

If we consider the local level, the typical steps in designing the control are:

1. Define the components of the process to be controlled, manipulated, or which are fixed. Additional define which are the variables of interest.
2. Define the user control goals.
3. Get a draft model of the process and the attached signals.
4. Select which will be the manipulated and measured variables.
5. Choose a suitable control structure.
6. Translate into a control language (also suitable for the selected control structure) the user requirements.
7. Apply the controlled design methodology based on the decision taken in the previous steps (variables, models, goals).
8. Validate (by simulation or experimentally) the design and tune the controller parameters.
9. Define the controller implementation. In the case of digital controllers, select the hardware and software to fulfil the control requirements.
10. Install the control in the process.
11. Evaluate the controlled system performances.

Most of these activities should be performed iteratively. For example, if the designed controller does not match the requirements, either the control structure or the variables should be revisited. If the implementation of the controller introduces additional constraints like time delays, computation time, etc. the design should be reconsidered, taking into account the new requirements. If the controlled system is driven out of the region of the validation of the model, a new model should be acquired or the uncertainties should be reduced.

Once the user goals have been stated, some decisions should be made to design the controller:

• Which variables are selected as controlled, measured and manipulated and which ones are going to be treated as disturbances.
• Based on this choice, which are the achievable performances?
• To meet the control requirements, which is the most suitable control design approach?

Control Goals

From the set of usually unclear “user” control goals, mainly based on qualitative and economical requirements as well as operational constraints, the desirable controlled plant performances for control design should be derived. They may concern different properties such as:

• Reference tracking, to follow the changes in the set-points or references.
• Control decoupling, to better understand and tune the different sub processes or control variables.
• Disturbance rejection, to cope with non-manipulated external variables.
• Measurement noise rejection, to be able to use “imperfect” sensor and transmission systems.
• Robustness against changes in the plant (model) or expected disturbances.

Control Structures

The choice of the variables to be used as control variables as well as the information used to generate the control actions will determine the control structure.

The control structures to be considered are:

• Open loop vs. closed loop – In an open loop control structure, the control actions are generated based on external information: set-points or objectives, initial conditions, disturbances, operator data, etc. A good model of the process is needed and there is no option to cope with unexpected changes in the plant (either disturbances or plant changes). In contrast, closed loop control uses the information from the plant to generate the control. There are many options for dealing with disturbances, reference tracking and uncertainties. The main drawback of closed loop control is the requirement of the existence of errors to act on. Therefore a combination of both structures may allow better results.
• Single/multiple loop – In feedback controlled multiple-input-multiple-output (MIMO) systems, the vector of the input actions may computed altogether from the full set of measurements and available data or alternatively,  the information is split into blocks to determine each of the control actions. In this case, for each input, the remaining blocks of information can be considered as disturbances. This structure could also be designated as centralized/decentralized control.
• Two degree of freedom – The control action may be computed into two phases. Foremost, the control error is evaluated, and the control is based on the error. This is a feedback control action. Subsequently, an additional control action is computed based on external inputs. There are two degrees of freedom to design the controller and the design can be split to achieve tracking (references) and regulation (output feedback) performances.
• Multi-level control – In this control structure, groups of input (output) variables can be treated jointly to control a process variable. They will act locally, receiving commands (set-points) from the higher decision levels, and sending information back to these coordination levels.

Related: Principles of Control Systems

Performance Analysis

There are several criteria for defining “good nominal performance” in terms of the close-loop transfer matrices:

• Settling time – it is related to position of the system poles and its internal structure i.e. controllability and observability.
• Overshoot – although this can be checked for particular input/output combinations, it is not used during the design phase.
• Steady-state gain – position errors for reference tracking are established, in most loops by the DC gain of the sensitivity function. For disturbance rejection, the steady-state gain may be determined.
• Bandwidth – many control requirements may be cast in the frequency domain, in particular, ability to track references varying up to maximum rate and ability to reject disturbances whose frequency components are mainly concentrated on a particular band.

You may also read:

• Adaptive Control System
• Centralised vs. Decentralised Control Systems
• Process Time Lags and Stability of Automatic Control Systems
• Control Modes in Automation Systems
• The Performance Limits for PID Controllers