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If I use an industrial robot with its hardware controller, should I install linux hard real-time like xenomai to use this kind of hardware controller?

If not, when is the case to use xenomai?

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In general, hard real-time OSes are used when there is a (usually periodic) task that is terminally critical, i.e. a process that needs to be run every few milliseconds, for example, and which, when queued, must be completed within a given time window to avoid a system failure. Note that "system failure" in this case doesn't have to mean that it will fry your CPU or break your robot, but instead that the resulting behaviour is considered a breach of the system's critical specifications, such as user safety or the like.

You may want to check out the criteria for different categories of real-time computing on Wikipedia

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  • $\begingroup$ Thank you for sharing concept of RT. I wondered if the robot has the hardware controller which implement PID or higher level controller and I use PC to communicate with that controller, should I install hard realtime OS to communicate with that controller? $\endgroup$ – sam Aug 18 '17 at 8:30
  • $\begingroup$ I guess that depends on the robot you use and its usage scenario - if you need pinpoint precision in order for the robot to complete its assignment according to specifications, then a real-time system is probably the way to go. If on the other hand you can live with some data points not arriving, e.g. because you use time-tracking and may interpolate accurately enough, then you don't require a (hard) real-time system $\endgroup$ – Sty Aug 18 '17 at 8:38
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You should find it useful to read this https://www.researchgate.net/project/A-Preview-of-Introduction-to-Fundamental-Principles-of-Dynamic-Real-Time-Systems. "Hard" real-time is a special case of the general case of "real-time." You can easily find that there are as many ad hoc "definitions" of all real-time terms as there are sources for those "definitions." That is a major disruption to both commerce and R&D in the real-time field. Such a thing is unimaginable in any other engineering or science field.

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hard RTOS is up to few microseconds to nanoseconds, soft RTOS is between miliseconds to microseconds.

It depends how fast you are getting data from the sensors and how fast you need the controller to react. Most cases ubuntu works fine and you can use ROS. In cases of hard RTOS I prefer micro kernels like QNX.

If really want to get the best out of your industrial robot just go for a micro-kernel like QNX, there are ways to install modified versions of ROS then you can work over built controllers by the community. There are lots of good things already developed that will spare a good time.

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