PoC: Blocking read implementation

[urfeex]

This PR is mainly a testing PR for implementing a non-sleeping ros2_control node that follows the hardware's pace.

It requires a non-sleeping controller manager, e.g. the one from ros-controls/ros2_control#3213.

Basically, this tries to solve the issue that the control loop on the ROS machine and the one on the robot can have a phase shift and might be slightly different. To illustrate things, I added a simple controller that moves a joint on a fixed sine wave pattern with fixed sampling times in between. This is for illustration purposes and will not get merged.

With a non-blocking read with a controller_manager running at a fixed 500Hz we can observe that every couple of minutes, the two loops (controller_manager and robot) come to a point, where it can happen that the robot didn't receive a new command since the last control cycle.

I've used a preempt-rt patched kernel for that test. The upper curve shows the total amount of control cycles without a new command coming in to the robot. For the test I used a position interface.

We can see that control runs quite stable up to a point where the two loops get "out of sync". There, the robot doesn't get a new package for the next control loop. In our case we do linear extrapolation in that case. In the next cycle, we get a new command correcting this extrapolation. This is especially visible on the acceleration domain. After a while, this stabilizes again only to reoccur a few minutes later.

With the controller manager not sleeping and the hardware interface doing a blocking read, the same plot looks exactly like the left half of the plot above, but for a very long time (tested for ~1h). The number of missed packages stays at 0 given a direct network connection without any disturbances.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 52 lines in your changes missing coverage. Please review.
✅ Project coverage is 4.92%. Comparing base (1b121b7) to head (4468554).
⚠️ Report is 568 commits behind head on main.

Files with missing lines	Patch %	Lines
ur_controllers/src/sine_controller.cpp	0.00%	52 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #1760      +/-   ##
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+ Coverage   3.59%   4.92%   +1.33%     
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  Files         13      37      +24     
  Lines        947    4307    +3360     
  Branches     152     502     +350     
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+ Hits          34     212     +178     
- Misses       843    4093    +3250     
+ Partials      70       2      -68     

Flag	Coverage Δ
unittests	4.92% <0.00%> (+1.33%)	⬆️

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[gavanderhoorn]

🎉 no more set-rate-to-slightly-higher-than-500hz-just-so-we-don't-miss-any-updates