Real Time System Introduction
A system is classified as real-time if it is required to complete the work on a time constraint.It does not mean that a system must meet some timing
deadlines.Typical examples of real-time systems include Air Traffic Control Systems, Networked Multimedia Systems, Command Control Systems,multimedia systems, virtual reality, games,stock market etc.
Hard real time system:If a job must never misses its deadline then the system is called hard real-time system. For a hard real-time system, every deadline must be hit. In a real hard real-time system, if the system fails to hit the deadline even once the system is said to have failed. E.g., flight control, manufacturing control
Soft real time system:If some deadlines can be missed occasionally acceptably with low probability then the system is called soft real time system. In a soft real time system, even if the system fails to meet the deadline one or more than once, the system is still not considered to have failed. For example, streaming audio-video, such as a temperature reading.
Digital Control
A/D => convert analog inputs to machine readable (digital) format
D/A => convert digital outputs to a form that can be input to a plant (analog)
y(t) => analog sensor reading
r(t) =>reference input
Digital control system is an implementation of real-time system that are embedded in sensors and actuators and functions as digital controller.The plant term in the block diagram refers to a controllers system, for example, an engine, a brake, an aircraft. The state of plant is monitored by sensors and can be change by actuators. The sensors read the current state of plant in analog form i.e y(t) and computes a control output based on difference between analog sensor reading and reference input (desired state).The computing is done in control-law computation of the controller. The output generated form it is then converted to analog signal to feed plant from actuator to being the plant closure to desired state.
The controllers in a complex monitor and control system are often organized in a hierarchy. There can be multiple control loops and the high-level controller interfaces with the operator and monitors the behavior of low-level controllers. The output of high-level controller acts as the reference input of low-level controller.
Example of Control Hierarchy
A hierarchical control system is a form of control system in which a set of devices and governing software is arranged in a hierarchical tree.
- Guidance and control :- Guidance is the process of calculating the changes in position, velocity, attitude, and/or rotation rates of a moving object required to follow a certain trajectory and/or attitude profile based on information about the object’s state of motion. A digital controller deals with some dynamical behavior of the physical plant, a second-
level controller typically performs guidance and path planning functions to achieve a higher level of goal. For example, the flight management system. - Real-Time Command and Control:- The controller at the highest level of a control hierarchy is a command and control system. An Air Traffic Control (ATC) system is an excellent example.The ATC system monitors the aircraft in its coverage area and the environment(e.g, weather condition) and generates and presents the information needed by the operators
(i.e., the air traffic controllers). Outputs from the ATC system include the assigned arrival times to metering fixes for individual aircraft. As stated earlier, these outputs are reference inputs to on-board flight management systems. Thus, the ATC system indirectly controls the embedded components in low levels of the control hierarchy.
Signal Processing
Most signal processing applications have some kind of real-time requirements. A signal processing application is typically a part of a larger system.
Radar Signal Processing
To search for objects of interest in its coverage area, the
radar scans the area by pointing its antenna in one direction at a time. During the time the antenna dwells in a direction, it first sends a short radio frequency pulse. It then collects and examines the echo signal returning to the antenna.
