Hard Real-Time

A guarantee that every single execution finishes before its deadline — missing even once counts as a system failure.

Hard real-time is the strictest tier of real-time computing: a task must complete within its specified deadline every single time, and missing it even once is treated as an error, potentially leading to an accident. The looser tier, soft real-time, tolerates the occasional miss with just some degradation in quality — a video dropping a frame now and then, for instance. A robot's low-level motor current loop, joint torque control, and bus communication typically demand hard real-time, often with periods of 1 kHz or higher, because computing a cycle late can cause the motor to jitter or lose control entirely. Achieving hard real-time generally relies on a real-time operating system (RTOS), Linux patched with PREEMPT_RT, or Xenomai, combined with a deterministic bus such as EtherCAT; large-model inference doesn't meet hard-real-time requirements, so it's usually kept at a higher level, with real-time control reserved for a dedicated low-level controller.

ExampleA humanoid robot's joint drivers run a torque loop at 1 kHz, which must finish and send its output every 1 millisecond without fail — not something that can be run on an ordinary Python process.

Related

Real-Time Operating System (RTOS) · PREEMPT_RT · Xenomai · EtherCAT (Ethernet for Control Automation Technology) · Control Frequency · Jitter (Timing Jitter)

Sources

Real-time computing - Wikipedia