Embedded Control Systems: A Wide Concept

Embedded control systems are, from a very general perspective, control elements that, in a somewhat autonomous manner, interact with a physical system in order to have an automated control over it. The term “embedded” refers to the fact that they are placed in or nearby the physical system under control. Generally speaking, the interfaces between the physical and con- trol systems consist of a set of sensors, which provide information from the physical system to the embedded system, and a set of actuators capable, in general, of modifying the behavior of the physical system. 

Since most embedded systems are based on digital components, signals obtained from analog sensors must be transformed into equivalent digital mag- nitudes by means of the corresponding analog-to-digital converters (ADCs). Equivalently, analog actuators are managed from digital-to-analog convert- ers (DACs). In contrast, digital signals do not require such modifications. The success of smart sensor and actuator technologies allows such interfaces to be simplified, providing standardized communication buses as the interface between sensors/actuators and the core of the embedded control system. 

Without loss of generality regarding the earlier paragraphs, two particular cases are worth mentioning: communication and human interfaces. Although both would probably fit in the previously listed categories, their purposes and nature are quite specific. 

On one hand, communication interfaces allow an embedded system to be connected to other embedded systems or to computing elements, building up larger and more complex systems or infrastructures consisting of smaller interdependent physical subsystems, each one locally controlled by their own embedded subsystem (think, for instance, of a car or a manufacturing plant with lots of separate, but interconnected, subsystems). 

Communication interfaces are “natural” interfaces for embedded control systems since, in addition to their standardization, they take advantage from the distributed control system philosophy, providing scalability, modularity, and enhanced dependability—in terms of maintainability, fault tolerance, and availability as defined by Laprie (1985). 

On the other hand, human interfaces can be considered either like con- ventional sensors/actuators (in case they are simple elements such as but- tons, switches, or LEDs) or like simplified communication interfaces (in case they are elements such as serial links for connecting portable maintenance terminals or integrated in the global communication infrastructure in order to provide remote access). For instance, remote operation from users can be provided by a TCP/IP socket using either specific or standard protocols (like http for web access), which easily allows remote control to be performed from a web browser or a custom client application, the server being embedded in the control system. Nowadays, nobody gets surprised by the possibility of 

FIGURE 1.1

Generic block diagram of an embedded system.

using a web browser to access the control of a printer, a photocopy machine, a home router, or a webcam in a ski resort.

Figure 1.1 presents a general diagram of an embedded control system and its interaction with the physical system under control and other subsystems.

Systems based on analog sensors and actuators require signal conditioning operations, such as low-noise amplification, anti-aliasing filtering, or filter- ing for noise removal, to be applied to analog signals. Digital signal process- ing and computationally demanding operations are also usually required in this case. On the other hand, discrete sensors and actuators tend to make the embedded system more control dependent. Since they have to reflect states of the system, complexity in this case comes from the management of all state changes for all external events. As a matter of fact, medium- or large-size embedded systems usually require both types of sensors and actuators to be used. On top of that, in complex systems, different control subtasks have to be performed concurrently since the key to achieve successful designs is to apply the “divide and conquer” approach to the global system, in order to break down its functionality into smaller, simpler subsystems. 

As one might think, the previous paragraphs may serve as introductory section for a book on any type of embedded systems, these being based on microcontrollers, computers, application-specific integrated circuits (ASICs), or (of course) FPGAs. Therefore, since implementation platforms do not actually modify the earlier definitions and discussion significantly, one of the main objectives of this book is to show when and how FPGAs could (or should) be used for the efficient implementation of embedded control systems targeting industrial applications. Since each technology has its own advantages and limitations, decision criteria must be defined to select  the technology or technologies best suited to solve a given problem. Fairly speaking, the authors do not claim FPGAs to be used for any industrial con- trol system, but their intention is to help designers identify the cases where FPGA technology provides advantages (or is the only possibility) for the implementation of embedded systems in a particular application or applica- tion domain.