What are the key differences between a hard real-time system and a soft real-time system and how each is employed in automation?

The key difference between a hard real-time system and a soft real-time system lies in the consequence of missing a deadline. In a hard real-time system, missing a deadline results in a critical failure of the system, potentially leading to catastrophic consequences, such as equipment damage, injury, or loss of life. Strict adherence to deadlines is absolutely mandatory. A soft real-time system, on the other hand, can tolerate occasional missed deadlines without causing critical failures. The system's performance may degrade, but it continues to operate. Hard real-time systems are employed in safety-critical automation applications where timing is paramount. For example, in an emergency shutdown system for a nuclear reactor or a robot performing a high-speed welding operation, any delay in responding to a critical event could have severe consequences. These systems require deterministic behavior, meaning that the execution time of tasks must be predictable and guaranteed. This often involves using specialized real-time operating systems (RTOS) and carefully designed hardware and software architectures. Soft real-time systems are used in applications where occasional delays are acceptable, such as process monitoring, data logging, or human-machine interfaces (HMIs). For example, a SCADA system that displays real-time data from a manufacturing plant can tolerate occasional delays in updating the display without causing a major disruption. While timeliness is still important, the consequences of missing a deadline are less severe than in hard real-time systems. Therefore, soft real-time systems can often use more general-purpose operating systems and hardware. The selection depends on the tolerance to risk associated with missing a deadline.