Preset

Preset are uses to set environnement for a scan. Typically to control:

opening/closing of a shutter
detector cover
multiplexer
scan pause…

It’s a hook in scan iteration. This hook can be set in different levels for different needs. Either the need is to hook a whole scan with an inherited class of ScanPreset or the need is to hook a part of the acquisition chain with an object inherited from ChainPreset.

ScanPreset¶

This is the simplest one. This one has 3 callback methods:

prepare called before all devices preparation
start called before all devices starting
stop called after all devices are stopped.

DEMO [1]: from bliss.scanning.scan import ScanPreset
DEMO [2]: class Preset(ScanPreset):
             ...:     def prepare(self,scan):
             ...:         print(f"Preparing scan {scan.name}\n")
             ...:     def start(self,scan):
             ...:         print(f"Starting scan {scan.name}")
             ...:         print(f"Opening the shutter")
             ...:     def stop(self,scan):
             ...:         print(f"{scan.name} scan is stopped")
             ...:         print(f"Closing the shutter")
DEMO [3]: p = Preset()
DEMO [4]: s = loopscan(2,0.1,diode,run=False)
DEMO [5]: s.add_preset(p)
DEMO [6]: s.run()

Scan 12 Wed Mar 13 11:06:11 2019 /tmp/scans/demo/data.h5 demo user = seb
loopscan 2 0.1

           #         dt[s]         diode
Preparing scan loopscan

Starting scan loopscan
Opening the shutter
           0             0      -40.2222
           1      0.104891      -9.11111
Took 0:00:09.830967
loopscan scan is stopped
Closing the shutter

ChainPreset¶

This hook is linked to a top-master of the acquisition chain. So the callback method will be called during prepare, start and stop phases of this to top-master. It has exactly the same behaviour than the ScanPreset if the chain has only one top-master.

i.e: In a loopscan, the only top master is a timer, so here is the same simple example where you want to open a shutter at the beginning of the scan and closing it at the end.

DEMO [1]: from bliss.scanning.chain import ChainPreset
DEMO [1]: class Preset(ChainPreset):
             ...:     def prepare(self,acq_chain):
             ...:         print("Preparing")
             ...:     def start(self,acq_chain):
             ...:         print("Starting, Opening the shutter")
             ...:     def stop(self,acq_chain):
             ...:         print("Stopped, closing the shutter")
DEMO [1]: s = loopscan(2,0.1,diode,run=False)
DEMO [1]: p = Preset()
DEMO [1]: s.acq_chain.add_preset(p)
DEMO [1]: s.run()

Scan 13 Wed Mar 13 11:54:08 2019 /tmp/scans/demo/data.h5 demo user = seb
loopscan 2 0.1

           #         dt[s]         diode
Preparing
Starting, Opening the shutter
           0             0       24.1111
           1      0.105099       1.22222
Stopped, closing the shutter

Took 0:00:36.189189

ChainIterationPreset¶

Use this object when you want to set a hook on each iteration of a top-master. ChainIterationPreset is yield from ChainPreset instance by get_iterator method. i.e here is an example where you want to open/close the shutter for each point.

DEMO [1]: class Preset(ChainPreset):
              class Iterator(ChainIterationPreset):
                  def __init__(self,iteration_nb):
                      self.iteration = iteration_nb
                  def prepare(self):
                      print(f"Preparing iteration {self.iteration}")
                  def start(self):
                      print(f"Starting, Opening the shutter iter {self.iteration}")
                  def stop(self):
                      print(f"Stopped, closing the shutter, iter {self.iteration}")
              def get_iterator(self,acq_chain):
                  iteration_nb = 0
                  while True:
                      yield Preset.Iterator(iteration_nb)
                      iteration_nb += 1
DEMO [2]: p = Preset()
DEMO [3]: s = loopscan(2,0.1,diode,run=False)
DEMO [4]: s.acq_chain.add_preset(p)
DEMO [5]: s.run()

Scan 18 Wed Mar 13 14:15:40 2019 /tmp/scans/demo/data.h5 demo user = seb
loopscan 2 0.1

           #         dt[s]         diode
Preparing iteration 0
Starting, Opening the shutter iter 0
Stopped, closing the shutter, iter 0
Preparing iteration 1
Starting, Opening the shutter iter 1
           0             0       22.4444
           1       0.10728       13.5556
Stopped, closing the shutter, iter 1

Took 0:00:16.677241

In this example, you can see that data display and the chain iteration are executed by two separated greenlets and they are not synchronised. This is not a problem but can be confusing if you think that it’s sequencial.

To pause a scan¶

As Preset callbacks are executed synchronously they can easily pause it, just by not returning imediatly from prepare, start or stop callback methods. If for example you need to pause a scan if there is no beam, you just have to check in a loop the condition. As an example, wait to start a scan if the beam is not present.

class Preset(ScanPreset):
    def __init__(self,diode,beam_trigger_value):
        self._diode = diode
        self._beam_trigger_value

    def prepare(self,scan):
        beam_value = self._diode.read()
        while beam_value < self._beam_trigger_value:
            print("Waiting for beam")
            print(f"read {beam_value} expect {self._beam_trigger_value}",end='\r')
            gevent.sleep(1)
            beam_value = self._diode.read()