Linkage constraints raise unnecessary error when units of linked variables are equivalent.

[robfalck]

Description

When linking variables across phases with equivalent units, dymos will raise this error:

ValueError: traj: Linkage units were not specified but the units of var_a (NM) and var_b (nmi) are not the same. Units for this linkage constraint must be specified explicitly.

This is due to the fact that it's checking the unit string for equality (NM != nmi) rather than checking the conversion factor between the units (openmdao.utils.units.unit_conversion('NM', 'nmi') = (1.0, 0.0)).

Example

import openmdao.api as om

        import dymos as dm
        from dymos.examples.finite_burn_orbit_raise.finite_burn_eom import FiniteBurnODE

        traj = dm.Trajectory()
        p = om.Problem(model=om.Group())
        p.model.add_subsystem('traj', traj)

        p.driver = om.pyOptSparseDriver()
        p.driver.options['optimizer'] = 'IPOPT'

        p.driver.declare_coloring()

        traj.add_parameter('c', opt=False, val=1.5, units='DU/TU',
                           targets={'burn1': ['c'], 'coast': ['c'], 'burn2': ['c']})

        # First Phase (burn)

        burn1 = dm.Phase(ode_class=FiniteBurnODE,
                         transcription=dm.GaussLobatto(num_segments=10, order=3, compressed=True))

        burn1 = traj.add_phase('burn1', burn1)

        burn1.set_time_options(fix_initial=True, duration_bounds=(.5, 10), units='TU')
        burn1.add_state('r', fix_initial=True, fix_final=False, defect_scaler=100.0,
                        rate_source='r_dot', targets=['r'], units='nmi')
        burn1.add_state('theta', fix_initial=True, fix_final=False, defect_scaler=100.0,
                        rate_source='theta_dot', targets=['theta'], units='rad')
        burn1.add_state('vr', fix_initial=True, fix_final=False, defect_scaler=100.0,
                        rate_source='vr_dot', targets=['vr'], units='DU/TU')
        burn1.add_state('vt', fix_initial=True, fix_final=False, defect_scaler=100.0,
                        rate_source='vt_dot', targets=['vt'], units='DU/TU')
        burn1.add_state('accel', fix_initial=True, fix_final=False,
                        rate_source='at_dot', targets=['accel'], units='DU/TU**2')
        burn1.add_state('deltav', fix_initial=True, fix_final=False,
                        rate_source='deltav_dot', units='DU/TU')
        burn1.add_control('u1', rate_continuity=True, rate2_continuity=True, units='deg',
                          scaler=0.01,
                          rate_continuity_scaler=0.001, rate2_continuity_scaler=0.001,
                          lower=-30, upper=30, targets=['u1'])

        # Second Phase (Coast)
        coast = dm.Phase(ode_class=FiniteBurnODE,
                         transcription=dm.Radau(num_segments=10, order=3))

        traj.add_phase('coast', coast)

        coast.set_time_options(initial_bounds=(0.5, 20), duration_bounds=(.5, 10), duration_ref=10, units='TU')

        coast.add_state('r', fix_initial=False, fix_final=False, defect_scaler=100.0,
                        rate_source='r_dot', targets=['r'], units='nmi')
        coast.add_state('theta', fix_initial=False, fix_final=False, defect_scaler=100.0,
                        units='rad', rate_source='theta_dot', targets=['theta'])
        coast.add_state('vr', fix_initial=False, fix_final=False, defect_scaler=100.0,
                        rate_source='vr_dot', targets=['vr'], units='DU/TU')
        coast.add_state('vt', fix_initial=False, fix_final=False, defect_scaler=100.0,
                        rate_source='vt_dot', targets=['vt'], units='DU/TU')
        coast.add_state('accel', fix_initial=True, fix_final=False, ref=1.0E-12, defect_ref=1.0E-12,
                        rate_source='at_dot', targets=['accel'], units='DU/TU**2')
        coast.add_state('deltav', fix_initial=False, fix_final=False,
                        rate_source='deltav_dot', units='DU/TU')
        coast.add_parameter('u1', targets=['u1'], opt=False, val=0.0, units='deg')

        # Third Phase (burn)

        burn2 = dm.Phase(ode_class=FiniteBurnODE,
                         transcription=dm.GaussLobatto(num_segments=10, order=3, compressed=True))

        traj.add_phase('burn2', burn2)

        burn2.set_time_options(initial_bounds=(0.5, 20), duration_bounds=(.5, 10), initial_ref=10, units='TU')
        burn2.add_state('r', fix_initial=False, fix_final=True,
                        rate_source='r_dot', targets=['r'], units='NM')
        burn2.add_state('theta', fix_initial=False, fix_final=False,
                        rate_source='theta_dot', targets=['theta'], units='rad')
        burn2.add_state('vr', fix_initial=False, fix_final=True,
                        rate_source='vr_dot', targets=['vr'], units='DU/TU')
        burn2.add_state('vt', fix_initial=False, fix_final=True,
                        rate_source='vt_dot', targets=['vt'], units='DU/TU')
        burn2.add_state('accel', fix_initial=False, fix_final=False, defect_ref=1.0E-6,
                        rate_source='at_dot', targets=['accel'], units='DU/TU**2')
        burn2.add_state('deltav', fix_initial=False, fix_final=False,
                        rate_source='deltav_dot', units='DU/TU')
        burn2.add_control('u1', targets=['u1'], rate_continuity=True, rate2_continuity=True,
                          units='deg', scaler=0.01, lower=-30, upper=30)

        burn2.add_objective('deltav', loc='final', scaler=1.0)

        burn1.add_timeseries_output('pos_x')
        coast.add_timeseries_output('pos_x')
        burn2.add_timeseries_output('pos_x')

        burn1.add_timeseries_output('pos_y')
        coast.add_timeseries_output('pos_y')
        burn2.add_timeseries_output('pos_y')

        # Link Phases
        traj.link_phases(phases=['burn1', 'coast', 'burn2'],
                         vars=['time', 'r', 'vr', 'vt', 'deltav'])
        traj.link_phases(phases=['burn1', 'burn2'], vars=['accel'])

        # Finish Problem Setup
        p.model.linear_solver = om.DirectSolver()

        p.setup(check=True, force_alloc_complex=True)

Dymos Version

1.7.1-dev

Relevant environment information

No response