Книга: Real-Time Concepts for Embedded Systems

1.2.1 Real-Time Systems

1.2.1 Real-Time Systems

The environment of the real-time system creates the external events. These events are received by one or more components of the real-time system. The response of the real-time system is then injected into its environment through one or more of its components. Decomposition of the real-time system, as shown in Figure 1.5, leads to the general structure of real-time systems.

The structure of a real-time system, as shown in Figure 1.7, is a controlling system and at least one controlled system. The controlling system interacts with the controlled system in various ways. First, the interaction can be periodic, in which communication is initiated from the controlling system to the controlled system. In this case, the communication is predictable and occurs at predefined intervals. Second, the interaction can be aperiodic, in which communication is initiated from the controlled system to the controlling system. In this case, the communication is unpredictable and is determined by the random occurrences of external events in the environment of the controlled system. Finally, the communication can be a combination of both types. The controlling system must process and respond to the events and information generated by the controlled system in a guaranteed time frame.

Imagine a real-time weapons defense system whose role is to protect a naval destroyer by shooting down incoming missiles. The idea is to shred an incoming missile into pieces with bullets before it reaches the ship. The weapons system is comprised of a radar system, a command-and-decision (C&D) system, and weapons firing control system. The controlling system is the C&D system, whereas the controlled systems are the radar system and the weapons firing control system.

· The radar system scans and searches for potential targets. Coordinates of a potential target are sent to the C&D system periodically with high frequency after the target is acquired.

· The C&D system must first determine the threat level by threat classification and evaluation, based on the target information provided by the radar system. If a threat is imminent, the C&D system must, at a minimum, calculate the speed and flight path or trajectory, as well as estimate the impact location. Because a missile tends to drift off its flight path with the degree of drift dependent on the precision of its guidance system, the C&D system calculates an area (a box) around the flight path.

· The C&D system then activates the weapons firing control system closest to the anticipated impact location and guides the weapons system to fire continuously within the moving area or box until the target is destroyed. The weapons firing control system is comprised of large-caliber, multi-barrel, high-muzzle velocity, high-power machine guns.

In this weapons defense system example, the communication between the radar system and the C&D system is aperiodic, because the occurrence of a potential target is unpredictable and the potential target can appear at any time. The communication between the C&D system and the weapons firing control system is, however, periodic because the C&D system feeds the firing coordinates into the weapons control system periodically (with an extremely high frequency). Initial firing coordinates are based on a pre-computed flight path but are updated in real-time according to the actual location of the incoming missile.

Consider another example of a real-time system-the cruise missile guidance system. A cruise missile flies at subsonic speed. It can travel at about 10 meters above water, 30 meters above flat ground, and 100 meters above mountain terrains. A modern cruise missile can hit a target within a 50-meter range. All these capabilities are due to the high-precision, real-time guidance system built into the nose of a cruise missile. In a simplified view, the guidance system is comprised of the radar system (both forward-looking and look-down radars), the navigation system, and the divert-and-altitude-control system. The navigation system contains digital maps covering the missile flight path. The forward-looking radar scans and maps out the approaching terrains. This information is fed to the navigation system in real time. The navigation system must then recalculate flight coordinates to avoid terrain obstacles. The new coordinates are immediately fed to the divert-and-altitude-control system to adjust the flight path. The look-down radar periodically scans the ground terrain along its flight path. The scanned data is compared with the estimated section of the pre-recorded maps. Corrective adjustments are made to the flight coordinates and sent to the divert-and-altitude-control system if data comparison indicates that the missile has drifted off the intended flight path.

In this example, the controlling system is the navigation system. The controlled systems are the radar system and the divert-and-altitude-control system. We can observe both periodic and aperiodic communications in this example. The communication between the radars and the navigation system is aperiodic. The communication between the navigation system and the diver-and-altitude-control system is periodic.

Let us consider one more example of a real-time system-a DVD player. The DVD player must decode both the video and the audio streams from the disc simultaneously. While a movie is being played, the viewer can activate the on-screen display using a remote control. On-screen display is a user menu that allows the user to change parameters, such as the audio output format and language options. The DVD player is the controlling system, and the remote control is the controlled system. In this case, the remote control is viewed as a sensor because it feeds events, such as pause and language selection, into the DVD player.

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