Real Time Setup¶
Since a constant update rate of the RobotBackend
is important for stable control of the robot, a real-time capable operating
system should be used.
We are using Ubuntu with a PREEMPT_RT kernel patch. Below are instructions on how to install the patched kernel and how to configure the system to use it.
Install PREEMPT_RT Kernel¶
Alternative: The Low Latency Kernel¶
A potential alternative to the PREEMPT_RT patch is using the “lowlatency” kernel. It is easier to install and does not conflict with Nvidia drivers. On the other hand, the real-time capabilities it provides are weaker than those of the PREEMPT_RT kernel. Whether it is sufficient for you depends on your application (e.g. how much computational load you put on the system while the robot is running). We still recommend using the PREEMPT_RT kernel but you may test with the lowlatency kernel first.
To install on Ubuntu 20.04 (on other versions of Ubuntu adjust the version at the end):
sudo apt install linux-lowlatency-hwe-20.04
Configuration to use this kernel by default works the same as for the PREEMPT_RT kernel, see Boot with the PREEMPT_RT Kernel.
System Configuration¶
Normally root permission is needed to run real-time processes. To allow non-root users to run real-time processes, apply the configuration described in this section.
Create the “realtime” Group and Add Users¶
If you used our install script to install the PREEMPT_RT kernel, the “realtime” group is already created automatically. Otherwise, you can create it manually with this command:
sudo groupadd realtime
Then add users that should be able to run real-time applications to the group:
sudo usermod -aG realtime <username>
Set rtprio and memlock limits¶
Note: This step can be skipped if you used our install script to install the PREEMPT_RT kernel.
Create a file /etc/security/limits.d/99-realtime.conf with the following
content to increase the rtprio and memlock limit for users in the realtime
group:
@realtime - rtprio 99
@realtime - memlock unlimited
Set Permissions of /dev/cpu_dma_latency¶
The following command needs to be run after each reboot to allow non-root users to set the CPU DMA latency:
sudo chmod 0666 /dev/cpu_dma_latency
Alternatively you can set up a udev rule to do this automatically. For this,
simply create a file /etc/udev/rules.d/55-cpu_dma_latency.rules with the
following content:
KERNEL=="cpu_dma_latency", NAME="cpu_dma_latency", MODE="0666"
Building robot_interfaces for the Real-Time System¶
To be able to run the robot backend in a real-time thread, some options need to be set at build time. This is automatically done when building on a system that is running the PREEMPT_RT or lowlatency kernel. However, if you previously built while running a different kernel, you’ll need to rebuild.
You can also explicitly build with real-time settings on any kernel with the following command:
colcon build --cmake-args -DOS_VERSION=preempt-rt
Note
If you see the following output during initialisation of the robot, this means you are running a non-real-time build.
Warning this thread is not going to be real time.