Real-Time Control

A control program that reads sensors, computes a command, and sends it to the motors on a fixed cycle, every cycle, on time.

Real-time control means a control program reads sensors, computes a command, and sends it to the motors on a fixed period (e.g., every 1 millisecond), and every single cycle must finish before its deadline. ‘Real-time’ here emphasizes being on time and predictable, not simply fast: a hard real-time system counts a single missed deadline as a failure, while a soft real-time system just degrades. Franka arms, for example, run the user's control callback through libfranka at 1 kHz, and the vendor requires the workstation to use a PREEMPT_RT real-time kernel and run the control program at real-time priority. A common division of labor in embodied-AI systems is: a large model such as a VLA infers a target at a few to a few dozen Hz, while low-level joint control tracks it inside a real-time loop at hundreds to thousands of Hz.

ExampleWhen doing torque control on a Franka FR3 with libfranka, the callback function is invoked every 1 millisecond, reading the robot's state and returning 7 joint torques; if the computer stalls and drops more than 20 consecutive packets, the control loop throws a communication_constraints_violation error and stops.

Also called

Real-Time Control Loop

Related

Control Frequency · Hard Real-Time · Real-Time Operating System (RTOS) · PREEMPT_RT · Jitter (Timing Jitter) · Control Latency

Sources

Wikipedia: Real-time computing
Franka FCI 文档：libfranka Overview（1 kHz 控制回路、丢包处理） (Chinese)
Franka FCI 文档：Setting up the Real-Time Kernel (Chinese)