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.
A Ctrl+C during lsamrot/fsamroy/osamroy rotation leaves the cached
user_setpoint at the target even though the axis never finished moving
(stop() halts the motor mid-move). The next scan then wrongly concluded
"no rotation required" from setpoint equality alone, leaving LamNI's
air bearings unclamped (confirmed via galil show all) and the beamline
unable to scan until the setpoint was nudged manually.
lamni_rotation() now also requires
lgalil_is_air_off_and_orchestra_enabled() before skipping, since that
flag only goes true once the Galil #CENROT sequence (air-clamp plus
centering) has fully completed. flomni_rotation()/omny_rotation() use
a readback-vs-tolerance check instead, since those axes have no
air-bearing clamp step.
Along the way, fixed lgalil_is_air_off_and_orchestra_enabled() itself:
it did bool(socket_put_and_receive(...)) directly on the raw Galil
reply string, which is always truthy, so it could never report
anything but True. Confirmed via HW testing in LamNI.
menzel
x01dc
wakonig_k
appel_c