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1. The Excitement of Control Engineering

1.4 Types of Control-System Design

Control-system design in practice requires cyclic effort, in which one iterates through modeling, design, simulation, testing, and implementation.

Control-system design also takes several different forms, and each requires a slightly different approach.

One factor that affects the form that the effort takes is whether the system is part of a predominantly commercial mission. Examples where this is not the case include research, education and missions such as landing the first man on the moon. Although cost is always a consideration, these types of control design are mainly dictated by technical, pedagogical, reliability, and safety concerns.

On the other hand, if the control design is motivated commercially, one again gets different situations depending on whether the controller is a small subcomponent of a larger commercial product (such as the cruise controller or ABS in a car) or whether it is part of a manufacturing process (such as the motion controller in the robots assembling a car). In the first case, one must also consider the cost of including the controller in every product, which usually means that there is a major premium on cost and hence one is forced to use rather simple microcontrollers. In the second case, one can usually afford significantly more complex controllers, provided that they improve the manufacturing process in a way that significantly enhances the value of the manufactured product.

In all of these situations, the control engineer is further affected by where the control system is in its lifecycle:

  • initial grass roots design;
  • commissioning and tuning;
  • refinement and upgrades;
  • forensic studies.

1.4.1 Initial Grass Roots Design

In this phase, the control engineer is faced by a green-field or so-called grass roots project, and thus the designer can steer the development of a system from the beginning. This includes ensuring that the design of the overall system takes account of the subsequent control issues. All too often, systems and plants are designed on the basis of steady-state considerations alone. It is, then, small wonder that operational difficulties can appear down the track. It is our belief that control engineers should be an integral part of all design teams. The control engineer needs to interact with the design specifications and to ensure that dynamic as well as steady-state issues are considered.

1.4.2 Commissioning and Tuning

Once the basic architecture of a control system is in place, then the control engineer's job becomes one of tuning the control system to meet the required performance specifications as closely as possible. This phase requires a deep understanding of feedback principles to ensure that the tuning of the control system is carried out in an expedient, safe, and satisfactory fashion.

1.4.3 Refinement and Upgrades

Once a system is up and running, then the control engineer's job turns into one of maintenance and refinement. The motivation for refinement can come from many directions. They include the following:

  • internal forces--e.g., the availability of new sensors or actuators may open the door for improved performance;
  • external forces--e.g., market pressures or new environmental legislation may necessitate improved control performance.

1.4.4 "Forensic" Studies

Forensic investigations are often the role of control engineering consultants. Here, the aim is to suggest remedial actions that will rectify an observed control problem. In these studies, it is important that the control engineer take a holistic view, because successful control performance usually depends on satisfactory operation of many interconnected components. In our experience, poor control performance is as likely to be associated with basic plant design flaws, poor actuators, inadequate sensors, or computer problems as it is to be the result of poor control tuning. However, all of these issues can, and should be, part of the control engineer's domain. Indeed, it is often only the control engineer who has the necessary overview to resolve these complex issues successfully.