Le temps réel en automatisation industrielle : ce que cela signifie en pratique

What Real-Time Actually Means

In industrial automation, real-time does not mean "very fast", it means deterministic. A real-time system guarantees that a task will execute within a specified time window, every time, without exception. A servo control loop running at 1ms cycle time must receive its position command exactly every 1ms ±10 microseconds, regardless of other system activity.

Hard Real-Time vs Soft Real-Time

• Hard real-time, missing a deadline is a system failure with physical consequences (motor over-speed, collision, unsafe condition). Requires a certified RTOS.
• Soft real-time, occasional deadline misses cause degraded performance but not safety failures. SCADA data updates, HMI refreshes, and data logging are soft real-time tasks.

Windows Is Not Real-Time

Standard Windows (even Windows 10 IoT) is not a real-time OS. Background processes, antivirus scans, OS updates, driver interrupts, can delay task execution by tens of milliseconds. For motion control and safety PLC applications, a dedicated RTOS (VxWorks, QNX, or Linux with PREEMPT_RT patch) is required for the control layer, with Windows handling only the HMI and data presentation layer.

EtherCAT and Real-Time Fieldbus

EtherCAT achieves 31.25-microsecond deterministic cycle times over standard Ethernet hardware, enabling PC-based soft motion controllers to achieve hard real-time servo control without a dedicated DSP. EtherCAT master software (TwinCAT, CODESYS, etc.) runs in the real-time partition of a PC-based control system, with Windows or Linux providing the operator HMI layer.