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The realtime machine

At the end of this page the PC that runs your program can hold the 1 kHz loop’s 1 ms deadline, and you have checked the link to the robot before the first motion.

  • A wired Ethernet connection to the robot’s control unit. 172.16.0.2 is the address in Franka’s documented default setup; the examples take it as argv[1].
  • FCI mode unlocked in Desk and the brakes open.
  • The ports are fixed by the protocol, 1337 for the robot and 1338 for the Franka Hand, and only one FCI client may be connected at a time. How the FCI works has the rest.

A PREEMPT_RT kernel

The loop has a hard 1 ms deadline. The crate checks /sys/kernel/realtime, as libfranka does:

cat /sys/kernel/realtime    # 1 on a PREEMPT_RT kernel

Permission for SCHED_FIFO

Raising the control thread to SCHED_FIFO needs CAP_SYS_NICE or a nonzero RLIMIT_RTPRIO. As an unprivileged user:

ulimit -r 99

This works only once an rtprio limit is configured for the user, for example /etc/security/limits.d/99-realtime.conf containing <user> - rtprio 99 (needs pam_limits and a fresh login). Ubuntu’s realtime-kernel variant sets up a realtime group that already has rtprio and memlock.

RealtimeConfig

Robot::new takes a RealtimeConfig:

  • Enforce, the default as in libfranka: checks that the calling thread can be raised to SCHED_FIFO and that the kernel advertises realtime capabilities, and fails with FrankaError::Realtime and libfranka’s message otherwise.
  • Ignore: skips both checks and runs the loop at whatever priority the process has.

Against the simulator Ignore costs nothing. Against a robot it means the loop can miss its deadline under load, which the robot reports as a falling control_command_success_rate and, eventually, a communication_constraints_violation reflex.

Every example reads the choice from FRANKA_REALTIME (enforce, the default, or ignore), so one binary serves the realtime PC and the simulator:

FRANKA_REALTIME=enforce cargo run --release --example echo_robot_state -- 172.16.0.2
FRANKA_REALTIME=ignore  cargo run --release --example echo_robot_state -- 127.0.0.1

Only the control thread should be SCHED_FIFO. Do not start the whole process with chrt -f 80: with other busy threads in the same process (a Rerun recorder, for instance) that starved the loop. The crate raises the control thread itself; a target control loop takes realtime_priority when a lower priority is wanted next to other realtime threads. The realtime rules lists what the loop must and must not do.

Before any motion, run communication_test: after moving the arm to the ready pose it runs a zero-torque loop and reports how many robot states were lost and the min/avg/max control_command_success_rate, exiting non-zero when the average is below 0.9. It is the first thing to point at a new PC or network card.

cargo run --release --example communication_test -- 172.16.0.2

A degraded Ethernet cable does not show up in ping: on a real FER (2026-09-09) it appeared as a communication_constraints_violation reflex with a clean ping, and a packet capture of the 1 kHz stream is what diagnosed it.

Details

RobotImpl::new raises its own thread’s priority unconditionally (libfranka’s Robot::Impl does the same) and under Ignore only swallows a failure, so on a machine where ulimit -r is nonzero the control thread ends up at SCHED_FIFO even with Ignore.