hydraulic, thermal, or chemical power. Hence, those systems can be classified according to their functional objectives either as: (i) specialized machines performing specific operations; or (ii) multipurpose and adjustable machines. Control systems are a set of technologies enabling algorithmic computing or signal processing devices to use signals emitted from analog or digital detecting, sensing, and communicating devices in order to perform automatic operations of systems or process actuation. Such systems are expected to perform them routinely and independently of human intervention with a performance superior to manual operation.
Thus, control systems aim to provide the necessary input signals to achieve the desired patterns of variations of specific process variables. Therefore, the functions of control systems are embedded in electromechanical systems (machine or product control).
Example 1.1
Figure 1.1 shows a typical 3D printing robot for customized cooking with speed- and temperature-controlled system which could be combined with monitoring indicators for cooking time and cooking stage, as well as a control panel allowing the selection of the final mixing of the product and cooking program. This system would require:
1 the angular position control of a pressure valve delivering semi-liquefied food (paste), the x-y axis position control of the carriage driving the extruder head (nozzle) made of two motors with a screw mechanism, the table angular speed and the z-axis position control;
2 the heater temperature control (nozzle level);
3 the remote pressure and force control for the valve in charge of injecting pressured food paste feed based on environmental (e.g. space mission) and biological conditions (e.g. lower gravity forces); and
4 the logic control for the discrete selection of ingredients.
Figure 1.1 Customized 3D food printer.
Such control design combination enhances the product or machine functionality while reducing operating and maintenance costs. This is done by integrating data processing and computing operations within a field device or machine (e.g. washing machine, navigation systems etc.). Among the commonly encountered automated machines or products are those with: (i) embedded control functions; (ii) dedicated control functions; or (iii) a control function limited to a couple of sensors and actuators involved.
A technical process is the sum of all interacting machines within that process transforming and/or storing material, energy, or information. Such technical processes can be classified according to their operational objectives as follows:
1 Transportation-related processes, such as material handling processes, energy flow processes, and information transmission processes.
2 Transformation-related processes, such as chemical processes, manufacturing processes, power generation, and storage processes.
Technical processes can be characterized according to functional objectives, such as:
1 Processes characterized by a continuous flow of material or energy (e.g. cement drying process, electric power distribution, paper production). Here, the process variables are physically-related variables with a continuous range of values, such as temperatures in a heating system. The process parameters are physical properties (e.g. power transmission network impedance, liquefied gas density). Process control consists of maintaining the process state on a determined level or trajectory. In this case, process dynamics models can be obtained through differential equations.
2 Processes characterized by discrete event operations representing different process states such as device activation or deactivation during the startup or shutdown of a turbine. Here, process variables are binary signals indicating the discrete status of devices or machines involved in process operations as well as change in logic devices (e.g. activating events resulting from ON/OFF switch positioning). The process discrete event models can be obtained through Boolean functions or logic flow charts.
3 Processes characterized by identifiable objects that are transformed, transported or stored, such as silicon-based wafer production, data processing and storage operations. Here, process variables indicate the state changes of objects and can have a continuous range of values (i.e. temperature of a slab in a clogging mill, size of a part in a store) or binary variables. Those variables can also be non-physical categories (i.e. type, design, application, depot number) assigned to the objects.
Example 1.2
Figure 1.2, depicting a salt-generating solar-based thermal power plant substation, illustrates an example of non-continuous processes (discrete event- or object-related). In this power plant, solar radiation is collected by thousands of sun-tracking mirrors (heliostats), which reflect it toward a single receiver atop a centrally located tower. Solar radiation is the electromagnetic radiation emitted by the sun. As such, it is necessary to:
1 Control the collector angle and position (sun tracker) to face the sun to collect the maximum solar radiation as well as to maintain peak power despite varying climate conditions. This is done by adjusting the operating setting based on measured voltage and current outputs of the array.
2 Logically control the energy storage by switching between charging/discharging operating modes based on climatic conditions (sun availability), battery charge status, load levels, and level of energy collected through solar irradiation by mirror arrays.
3 Control the temperature of the collector used to melt a salt. The hot molten salt is stored in a storage tank to generate steam and later used to drive the turbine and attached generator.
4 Control the flow of heated fluid circulating between the tank and the collector. This fluid with molten salt at a low temperature is pumped to the cold collector tower for the next thermal cycle. The operating temperature over this thermal cycle derives the quantity of energy to be extracted.
Figure 1.2 Steam-based power generation technical process schematic.
Digital control systems aim to coordinate the operations of several electrically-driven machines in order to meet specific operational objectives such as water purification, voltage control in an electrical power grid, or temperature regulation in a fermentation tank. Thus, it is usually necessary to ensure the integration of a large number of control system instruments (from data processing and computing units to measuring units). Figure 1.3 illustrates the generic components in the design of computer control systems with supervisory functions. An example of the relationship between a technical process and electromechanical system control is illustrated in Figure 1.4.
