Read only engines?

[gfabbris]

Yesterday, @jwkim-anl and I had issues running the E4C incidence engine. It looks like that it's a read only engine, i.e. you can only read the incidence angle and azimuth, but not move the diffractometer angles.

@prjemian: could you check that this is indeed the case?

[prjemian]

Can you post a copy of the code you used to set this up?

[gfabbris]

This uses the VM IOC:

In [1]: import gi
   ...: gi.require_version('Hkl', '5.0')
   ...: # MUST come before `import hkl`
   ...: from hkl.diffract import E4CV
   ...: from ophyd import Component, PseudoSingle, EpicsMotor
   ...: 
   ...: class FourcIncidence(E4CV):
   ...:     incidence = Component(PseudoSingle, "")
   ...:     azimuth = Component(PseudoSingle, "")
   ...: 
   ...:     theta = Component(EpicsMotor, 'm41')
   ...:     chi = Component(EpicsMotor, 'm42')
   ...:     phi = Component(EpicsMotor, 'm43')
   ...:     tth = Component(EpicsMotor, 'm44')
   ...: 
   ...: fourc_inc = FourcIncidence("gp:", name="fouce_inc",
   ...:                            calc_kw={"engine": "incidence"})
   ...: 
   ...: RE(bps.mv(
   ...:     fourc_inc.theta, 10,
   ...:     fourc_inc.tth, 20,
   ...:     fourc_inc.chi, 0,
   ...:     fourc_inc.phi, 0
   ...:    ))
Out[1]: ()

In [2]: fourc_inc.engine._engine.parameters_values_set((0, 0, 1), 1)
Out[2]: 1

In [3]: fourc_inc.inverse(10, 0, 0, 20)
Out[3]: FourcIncidencePseudoPos(incidence=10.000000000000002, azimuth=90.0)

In [4]: fourc_inc.forward(10, 90)
---------------------------------------------------------------------------
Error                                     Traceback (most recent call last)
~/Apps/miniconda3/envs/bluesky_2021_1/lib/python3.8/site-packages/hkl/engine.py in pseudo_positions(self, values)
    230         try:
--> 231             geometry_list = self._engine.pseudo_axis_values_set(values,
    232                                                                 self._units)

Error: hkl-mode-incidence-error-quark: The "incidence" engine is readonly (1)

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
~/Apps/miniconda3/envs/bluesky_2021_1/lib/python3.8/site-packages/hkl/calc.py in forward_iter(self, start, end, max_iters, threshold, decision_fcn)
    536         try:
--> 537             self.engine.pseudo_positions = end
    538             return self.engine.solutions

~/Apps/miniconda3/envs/bluesky_2021_1/lib/python3.8/site-packages/hkl/engine.py in pseudo_positions(self, values)
    233         except GLib.GError as ex:
--> 234             raise ValueError('Calculation failed (%s)' % ex)
    235 

ValueError: Calculation failed (hkl-mode-incidence-error-quark: The "incidence" engine is readonly (1))

During handling of the above exception, another exception occurred:

UnreachableError                          Traceback (most recent call last)
<ipython-input-4-71eebfe36a58> in <module>
----> 1 fourc_inc.forward(10, 90)

~/Apps/miniconda3/envs/bluesky_2021_1/lib/python3.8/site-packages/ophyd/pseudopos.py in wrapped(self, *args, **kwargs)
    186             pos, new_kwargs = m(*args, **kwargs)
    187 
--> 188             return method(self, pos, **new_kwargs)
    189 
    190         return wrapped

~/Apps/miniconda3/envs/bluesky_2021_1/lib/python3.8/site-packages/hkl/diffract.py in forward(self, pseudo)
    264     @pseudo_position_argument
    265     def forward(self, pseudo):
--> 266         solutions = self.calc.forward_iter(start=self.position, end=pseudo,
    267                                            max_iters=100)
    268         logger.debug('pseudo to real: {}'.format(solutions))

~/Apps/miniconda3/envs/bluesky_2021_1/lib/python3.8/site-packages/hkl/calc.py in wrapped(self, *args, **kwargs)
     47             initial_pos = self.physical_positions
     48             try:
---> 49                 return func(self, *args, **kwargs)
     50             finally:
     51                 self.physical_positions = initial_pos

~/Apps/miniconda3/envs/bluesky_2021_1/lib/python3.8/site-packages/hkl/calc.py in forward_iter(self, start, end, max_iters, threshold, decision_fcn)
    538             return self.engine.solutions
    539         except ValueError:
--> 540             raise UnreachableError(
    541                 f"Unable to solve. iterations={iters}/{max_iters}\n"
    542                 f"Last valid position: {valid_pseudo}\n{valid_real} ",

UnreachableError: Unable to solve. iterations=100/100
Last valid position: None
None 

[gfabbris]

Note the "Error: hkl-mode-incidence-error-quark: The "incidence" engine is readonly (1)"

[prjemian]

Thanks, I'll look that one up.