Industrial Ethernet Guide - “Hard” and “Soft” Real-Time Control
The following is part of A Comprehensible Guide to Industrial Ethernet by Wilfried Voss.
“Hard” real-time (HRT) control systems are designed to support specific response times, and thus, they are highly deterministic, but they depend on stable cycle time. A missed deadline may post a safety hazard or other serious consequences.
“Soft” real-time (SRT) systems deliver a best-effort response, and there is only a preferred cycle time. In all consequence, late responses are tolerated, and errors in an SRT system, while not encouraged, are not as safety-critical as in HRT systems.
All real-time systems have a certain level of timing variance, called jitter. Jitter should be measurable to guarantee system performance.
The building blocks of a real-time system are so-called jobs, and each job is composed of certain sequential items:
- Release Time – The time when the job becomes available to the system
- Execution Time – The time to process the job.
- Response Time – The time between start of release and end of execution.
- Ready Time – The earliest time that execution can start.
- Deadline – The time at which execution must be finished.
Currently, there is no standard that would explicitly define the boundary between soft and hard real-time control through a set of firm parameters and values. A definition of, for instance, the level of determinism and cycle time is difficult because even a non-real-time system needs to respond within a certain amount of time.
There are, however, a few recommendations used in the industry to determine the need for a hard real-time system:
- Cycle Time: A cycle time of less than 10 milliseconds.
- Cycle Time Jitter: A jitter (cycle time variation) of 10 percent (or better) of the cycle time.
And yet again, speed is not as important as precision. The cycle time is of minor importance as long as it is constant.
The following lists some typical cycle times for various control applications:
- Low speed sensors (e.g. temperature, pressure) – Tens of milliseconds
- Drive control systems – Milliseconds
- Motion control (e.g. robotics) – Hundreds of microseconds
- Precision motion control – Tens of microseconds
- High-speed devices - Microseconds
Like any closed-box real-time systems, real-time communication networks can be hard or soft, depending on their requirements and abilities, and Ethernet offers many benefits (most prominently: speed) over other serial systems in regards to real-time control.
Note: In 2002 the IEEE 1588 Precision Time Protocol Standard addressed the need for deterministic responses by introducing a precision clock synchronization protocol for networked measurement and control systems, which has been adopted by some (but not all) Industrial Ethernet protocols.
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