menzel
f74f09de7b
at_each_point issues the line move and then sleeps before firing the
burst. The sleep was acc_time alone, which covers the acceleration ramp
and nothing else -- while the move still has to cross the scan server ->
device server -> EPICS -> controller path first. acc_time shrinks with
the scan velocity; that path does not. On a slow scan the burst
therefore began well before the stage moved, and the first points of
every line piled up at the line start.
Measured from the position readback of five commissioning scans, as the
distance the stage was behind the trigger grid once the lag stopped
growing:
scan exp_time v_cmd lag latency
254 20 ms 0.5 mm/s 29.7 pts 0.594 s
450 50 ms 0.1 mm/s 11.9 pts 0.595 s
324 15 ms 0.667 mm/s 41.5 pts 0.623 s
473 100 ms 0.065 mm/s 6.3 pts 0.630 s
411 50 ms 0.1 mm/s 12.7 pts 0.635 s
Constant to +-3.5% across a 6.7x range of exposure and a 10x range of
velocity, on two axes. Scan 473 is sampled 74 times per line and shows
the shape plainly: the lag appears in the first interval and then holds
at 6.3-6.4 points for the rest of the line while the velocity sits at
exactly the commanded 0.065 mm/s. A start-up offset, not a velocity
error, and the stage itself is blameless.
Deliberately not solved by polling for motion. Observing the readback
costs a round trip of this same ~0.6 s, so it would trade a systematic
offset for a jitter of similar size -- and a constant offset displaces
every line equally, where a varying one shears the image line by line
and cannot be undone afterwards. The reproducibility is the asset here,
not the enemy.
The value lives on ddg1 next to the shutter delay, with a setter in
USER_ACCESS, because that is where cont_grid already fetches its trigger
timing. It is not a property of the delay generator and the docstring
says so. Setting it to 0 reproduces exactly the previous timing without
a redeploy, which is the intended way back if this makes things worse.
The real fix is to trigger the DDG from the motor
(scan_type: hardware_triggered), taking the round trip out of the timing
chain rather than compensating for it. This is the stopgap until then,
and it rests on the latency staying constant -- which is worth
re-checking with the same measurement whenever the deployment changes.
Getting Started with Testing using pytest
BEC is using the pytest framework. It can be installed via
pip install pytest
in your python environment.
We note that pytest is part of the optional-dependencies [dev] of the plugin package.
Introduction
Tests in this package should be stored in the tests directory.
We suggest to sort tests of different submodules, i.e. scans or devices in the respective folder structure, and to folow a naming convention of <test_module_name.py>.
It is mandatory for test files to begin with test_ for pytest to discover them.
To run all tests, navigate to the directory of the plugin from the command line, and run the command
pytest -v --random-order ./tests
Note, the python environment needs to be active.
The additional arg -v allows pytest to run in verbose mode which provides more detailed information about the tests being run.
The argument --random-order instructs pytest to run the tests in random order, which is the default in the CI pipelines.
Test examples
Writing tests can be quite specific for the given function. We recommend writing tests as isolated as possible, i.e. try to test single functions instead of full classes. A very useful class to enable isolated testing is MagicMock. In addition, we also recommend to take a look at the How-to guides from pytest.