Switch Diffraction Modes#

A diffraction mode is a ConstraintSet that describes which motor stages are free, which are held at declared values, and which are related to others (e.g. bisecting). See Concepts for background, and Work with Constraints and Diffraction Modes for the full constraint framework.

List available modes#

import ad_hoc_diffractometer as ahd

g = ahd.presets.fourcv()
print(list(g.modes.keys()))
# ['bisecting', 'fixed_chi', 'fixed_phi', 'fixed_omega',
#  'fixed_psi', 'double_diffraction']

Get and set the active mode#

# The default mode is set by the factory
print(g.mode_name)   # 'bisecting'

# Switch to a different mode
g.mode_name = "fixed_chi"
print(g.mode_name)   # 'fixed_chi'

# Clear the active mode (all stages free — forward() will raise)
g.mode_name = None

Mode reference#

Bisecting mode#

The bisecting BisectConstraint drives the co-axial sample stage to half the detector angle, placing the sample symmetrically between the incident and diffracted beams.

g.mode_name = "bisecting"
solutions = g.forward(1, 0, 0)
# omega = ttheta / 2 in every solution

Fixed-angle modes#

Fixed-angle modes hold one sample stage at its declared constraint value during a single forward() call. The value is part of the ConstraintSet definition and is constant only for the duration of that call.

# fixed_chi: factory default holds chi at 90°
g.mode_name = "fixed_chi"
solutions = g.forward(1, 0, 0)
# sol["chi"] == 90.0 for every solution

To use a different value, construct a new ConstraintSet and assign it. The constraint persists until replaced — only reassign when the value changes:

from ad_hoc_diffractometer import ConstraintSet, SampleConstraint

# Call 1: chi = 45°
g.modes["my_chi"] = ConstraintSet([SampleConstraint("chi", 45.0)])
g.mode_name = "my_chi"
sols_45 = g.forward(1, 0, 0)   # chi = 45° this call

# Call 2: chi = 60°
g.modes["my_chi"] = ConstraintSet([SampleConstraint("chi", 60.0)])
sols_60 = g.forward(1, 0, 0)   # chi = 60° this call

See Work with Constraints and Diffraction Modes for the full run-time pattern.

fixed_omega#

Holds omega at 0° (any geometry where omega is a sample stage).

g.mode_name = "fixed_omega"
solutions = g.forward(1, 0, 0)
# sol["omega"] == 0.0 for every solution

Inspect a mode’s constraints#

g = ahd.presets.fourcv()
cs = g.modes["fixed_chi"]

print(cs)
# ConstraintSet([SampleConstraint('chi', 90.0)])

print(cs.computed)
# ['omega', 'phi', 'ttheta']

print(cs.constant_stages)
# ['chi']

print(cs.is_fully_constrained(g))
# True  (N - 3 = 1 constraint for N = 4 DOF)

print(cs.is_implemented(g))
# True

Check if a mode is implemented#

Some modes require a prerequisite on the geometry. For example, fixed_psi requires g.azimuthal_reference to be set:

g.mode_name = "fixed_psi"
print(g.modes["fixed_psi"].is_implemented(g))  # False (no azimuthal_reference)

g.azimuthal_reference = (0, 0, 1)
print(g.modes["fixed_psi"].is_implemented(g))  # True

# forward() raises NotImplementedError for unimplemented modes

Clear the active mode#

g.mode_name = None   # all stages free; forward() raises NotImplementedError