quadEM electrometers (TetrAMM, FX4, …) in continuous mode

Objective

Show the recommended pattern for using an EPICS quadEM electrometer (such as the TetrAMM or the FX4) with bluesky when the device is operated as a continuous beam-position monitor (BPM).

Why no apstools device?

apstools does not provide its own quadEM device class. Support already exists upstream in ophyd.quadem (QuadEM, TetrAMM, NSLS_EM, APS_EM), so the recommended pattern is to subclass the appropriate ophyd class directly for beamline-specific behavior rather than to depend on an apstools wrapper.

This is the same conclusion reached for the TetrAMM (issue #878) and the FX4 (issue #1185): if the device can be represented cleanly by ophyd.quadem.QuadEM, keep the customization in beamline code.

The triggering problem

ophyd.quadem.QuadEM.__init__() sets these staging signals:

self.stage_sigs.update(
    [("acquire", 0), ("acquire_mode", 2)]  # stop acquiring, single mode
)

For a detector that is triggered once per data point this is correct. But when the electrometer is used as a continuous BPM, these defaults cause a scan such as

RE(bp.rel_scan([tetramm], motor, -1, 1, 11))

to stall: the trigger presses Acquire in single mode and the trigger status object never reports done, so the scan hangs between points (issue #1023).

Recommended beamline pattern

For continuous / BPM operation, subclass the ophyd device, clear the staging signals so the acquisition mode is left untouched, and override trigger() to press Acquire and immediately mark the status as finished so a subsequent .read() returns the current values:

from ophyd import TetrAMM
from ophyd.status import Status


class ContinuousTetrAMM(TetrAMM):
    """TetrAMM as a continuous beam-position monitor.

    * Do not change the acquire mode when staging.
    * ``trigger()`` ensures acquisition is running, then reports done
      immediately so ``read()`` returns the current values.
    """

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        # Leave the device in whatever (continuous) mode it is already in.
        self.stage_sigs = {}

    def trigger(self):
        # Make sure the detector is acquiring, then report done.
        self.acquire.put(1)
        status = Status(self)
        status.set_finished()
        return status


tetramm = ContinuousTetrAMM("ioc_prefix:", name="tetramm")
tetramm.wait_for_connection()

The same pattern applies to any other quadEM-family device by subclassing the appropriate ophyd class (for example a local QuadEM subclass with port_name="FX4" for the FX4; see issue #1185).

bluesky

Use the device in a plan like any other detector:

RE(bp.rel_scan([tetramm], motor, -1, 1, 11))