REAL-TIME CONTROL SYSTEMS
Definition and functions
There are several different definitions of a real-time control system in the literature and documents concerning control and automation. In some industrial markets, vendors and engineers claim that they have made real-time instruments without, however, defining what they mean by the term. For example, some washing machines claim to have real-time double cylinder drivers.
Real-time control should be one criterion of control operations to be fulfilled by industrial control systems. A control operation is a series of events or actions occurring within system hardware and software to give a specific result. A real-time control system is a system in which the correctness of
a result depends not only on its logical correctness but also on the time interval in which the result is made available. This is to say that time is one of the most important aspects of control systems. In general, the following three standards give the definition of a real-time control operation, and an industrial control system in which all the control operations occur in real-time qualifies as a real-time control system:
(1) Reliable operation execution
By reliable operation execution we mean two things: the operation execution must be stable, and it must be repeatable. Because a control operation is executed with both the system hardware and software, stable and repeatable operations represent the capability of the hardware, the software, and their mutual compatibility in this respect. Stable operation requires that the system must produce exactly the same result for any given set of input data or events within the same time frame. If a control operation provides different results with the same inputs, this operation is unstable; if a control operation produces the same result for the same inputs but exhibits variability in the time frame it takes from input to output, this operation cannot be considered to be real-time. Obviously, a precondition of stable operations is that this control system must support repeatable execution of any control operation with the same hardware and software environments.
(2) Determined operation deadline
Any control operation needs time to execute. At the instant of getting an input or recognizing an event, the system must start an operation; at the instant this operation is completed, the system must generate an output or give a response. As mentioned earlier, a key factor to any real-time control system is the time interval an operation takes between a new input or a new event, and the instant the response or update is given. Therefore, the real-time criterion requires that this time interval must be determined, and be within a confirmed deadline for executing any control operation.
(3) Predictable operation result
Finally, the result for any control operation must be predictable. This means there is no uncertainty which could affect the result of any control operation. For a real-time control system, it is not acceptable for the result of the control operation to be dependent upon factors that are unknown. Any factors that have an effect on the output state, for a given input state, must be known and controllable.
Although a real-time control operation should satisfy these three criteria, it is essential to identify a time frame for real-time operation because this factor is crucial in an industrial control system environment. For example, missing the deadline of critical servo-level periodic tasks could result in losing data or crashing control cycles, which could also lead to some loss of efficiency at best, or at worst could cause serious damage to equipment or cause human injury. However, it should be noted that real-time control is not the same as rapid control. Fast control aims at getting the results as quickly as possible, while real-time control aims at getting the results at a determined point of time within defined time tolerances, or deadlines.
These systems originated in the early twentieth century, with the need to solve two main types of control problem; event response, and closed-loop control systems. Event response requires that a system should respond to a stimulus in a determined amount of time, for instance in operations an automotive airbag system. Closed-loop control systems process feedback continuously in order to adjust an output; an automotive cruise control system is an example of a closed-loop control system. Both of these types of systems require the completion of an operation within a specific deadline. This type of performance is referred to as determinism.
To understand a real-time control system, we need to clarify several relevant terms:
The ready time of a task is the time instant when the task becomes available to the system, which means the system has built up the task object in the memory of the microprocessor chipset and inserted this object into the operating system kernel’s task queue.
The start time is the earliest time the task can start executing (always greater than or equal to the ready time).
The completion time is the time instant at which the task normally finishes running.
The execution time of a task is the time interval taken by the system for the whole processing of this task between the start time and the completion time.
The response time is the interval between the ready time and the completion of the execution. The deadline is the time when the task’s execution must be stopped.
If the completion time of a task is not earlier than the deadline, the task is late.
All real-time systems have a certain level of jitter. Jitter is a variance in the actual timing of the above times. In a real-time system, jitter should be within a given interval so that system performance can still be guaranteed.
Figure 1.5 is an illustration of these temporal task terminologies.
In their application, real-time control systems are divided into two classes; soft and hard. These two types of real-time control are distinguished in terms of the deadline of control operations.
In hard real-time systems, only those operations which can meet the prescribed deadlines are valid; the completion of an operation after its deadline is considered useless, and if the operations keep on missing the prescribed deadlines, the entire system will eventually fail. Hard real-time systems are typically supported by physical hardware in embedded control when it is imperative that an event is reacted to within a strict deadline. For example, nuclear power stations and car airbags must turn-on or turn-off immediately, when the corresponding commands arrive, otherwise they will physically damage their surroundings or threaten as human life. Another example is medical instruments such as heart pacemakers. A heart pacemaker is a battery-powered electronic device about the size of a matchbox which produces electrical current in order to cause the heart to beat in a regular and reliable manner. The delayed operation of a heart pacemaker could result in loss of human life.
Soft real-time means that the control system can tolerate some deadline missing with only decreased service quality or system malfunction resulting. Thus, soft real-time systems are typically
those used where there is some issue of concurrent access, and there is a need to keep a number of connected systems up to date with changing situations. For example, when pressing a cell phone button to answer an incoming call, the connection must be established soon after the button has been pressed, but the deadline is not mission-critical and small delays can be tolerated.
In conclusion, by a real-time control operation we mean those tasks or processes running in computerised industrial control systems or devices which satisfy three criteria: reliable operation execution, set operation deadline, and predictable operation results. The time deadline is crucial for real-time control operations. If the operation must meet the deadline, otherwise risking fatal errors or undesirable consequences, it is called a hard real-time operation. In contrast, if the meeting of the deadline for this operation is desirable but not essential, it is called a soft real-time operation. All computerized industrial control systems which provide real-time operations are defined as real-time control systems.