Continuous Scan Tutorial

This tutorial shows how to use Bliss in 3 steps:

• configuring data acquisition
• configuring data storage
• starting a continuous scan

First step: data acquisition configuration

A continuous scan in Bliss is the combination of acquisition objects, being either master or slaves, and data recorder objects.

Master devices are enabled with synchronization superpowers ; their responsibility is to start or trigger slaves to perform data acquisition at the right moment.

The link between masters and slaves can be either hardware, or emulated with software.

A scan can have multiple masters, and masters can also be slaves for other masters. There is no limit to imagination.

The set of masters and slaves acquisition objects is called an acquisition chain, it is the first thing to configure when defining a new Bliss scan:

from bliss.scanning.chain import AcquisitionChain

chain = AcquisitionChain()

Then, masters and slaves objects have to be created and added to the acquisition chain.

Master object creation

The SoftwarePositionTriggerMaster class is shipped with Bliss. It takes an Emotion Axis object, and turns it into a Bliss master, capable of triggering slaves at evenly spaced points between a start and an end position.

from bliss.scanning.acquisition.motor import SoftwarePositionTriggerMaster

emotion_master = SoftwarePositionTriggerMaster(m0, 5, 10, 10, time=5)

In the example above the SoftwarePositionTriggerMaster is configured to move m0 from 5 to 10, with 10 points. The optional time keyword argument specifies the time in seconds to go between the start and end positions (acquisition time). Motor speed is changed accordingly. Acceleration time is taken into account to ensure constant speed during acquisition. An extra step is also added after end position in order to guarantee the last point is exposed identically as the previous ones.

Considering an m0 Emotion axis with acceleration set to 100 mm.s^-2, and time for moving from 5 to 10 set to 5 seconds, velocity will be set to 1 mm.s^-1 and the effective move will be from 4.99 to 10.56.

Slave object creation

Bliss comes with the LimaAcquisitionMaster class, encapsulating a Tango Lima device for use within an acquisition chain:

params = { "acq_nb_frames": 10,
           "acq_expo_time": 0.3,
           "acq_trigger_mode": "INTERNAL_TRIGGER_MULTI" }
lima_acq_dev = LimaAcquisitionMaster(lima_dev, **params)

Acquisition is configured to take 10 frames of 300 milliseconds exposure time, each frame capture being triggered by software.

Adding master and slave to acquisition chain

The AcquisitionChain.add() method is used to associate a master with slaves or with another master, and to add a node in the chain.

chain.add(emotion_master, lima_acq_dev)

Internally, the AcquisitionChain.add() method builds a tree representing the different acquisition devices. Master acquisition devices are represented as nodes in a tree:

. chain(AcquisitionChain)
└── emotion_master(SoftwarePositionTriggerMaster)
    └── lima_dev(LimaAcquisitionMaster)

Second step: data storage configuration

Once a continuous scan is started, data is produced from the masters and slaves devices in the acquisition chain. A Scan object has to be created, in order to specify how data is saved and made accessible for other programs like Online Data Analysis.

A scan is identified by its name ; the full name is made of a prefix, plus a run number. The default prefix is scan, so the first scan is called scan_1, the second scan is called scan_2 and so on. The first argument to Scan is the scan name prefix.

In the same way the AcquisitionChain can be represented as a tree, the Scan saves data in a tree-like structure within the Redis cache. A scan node contains meta-data (scan_info), plus a data member. If data is too big, only a reference to the data is saved. For example, in the case of images, the file name is stored instead of the image bytes.

Scan objects can be placed inside a Container, in order to match data acquisition with data analysis logic. A Container is only identified by its name. Typically, a container will have a sample name, an each scan on this sample can be stored inside the container. Container objects can be nested without limitation.

??? no more data_manager…

from bliss.common.data_manager import Container, Scan

sample = Container('my_sample`')
scan = Scan(name='scan', chain=chain, parent=sample)

Launching a scan is done by calling run method :

scan.run()