How do I define matrix parameters in a Dymos problem? - openmdao

Question

I'm trying to setup a dynamic optimization with dymos where I have an analysis upstream of my dymos trajectory. This upstream analysis computes some 2D-matrix K. I want to pass this matrix into my dymos problem. According to the documentation (and how I've done this in the past) is to add K as a paramter to the trajectory:
traj.add_parameter('K',targets={'phase0':['K'],opt=False,static_target=True).
However, this returns an error because static_target expects K to be a scalar. If I have static_target=False, this also returns an error because it expects K to have some dimension related to the number of nodes in the trajectory.
Is there something I'm missing here?
Is it sufficient to manually connect K to the trajectory via
p.model.connect('K','traj.phase0.rhs_disc.K') and
p.model.connect('K','traj.phase0.rhs_col.K')? Or will that create issues in how dymos works the problem.
It doesn't seem appropriate to vectorize K either.
Any suggestions are greatly appreciated.

Answer 1

In my opinion, the easiest way to connect parameters from trajectory to phase is to add the parameter to both the Trajectory and the phases in which it is to be used.
Consider a simple oscillator where the mass, spring constant, and dampening coefficient are given as a single size-3 input.
In this case, I used OpenMDAO's tags feature and a special dymos tag dymos.static_target so that dymos realizes the target isn't shaped with a different value at each node. I think its a bit easier to do it this way as opposed to having to add it later at the add_parameter call.
class OscillatorODEVectorParam(om.ExplicitComponent):
"""
A Dymos ODE for a damped harmonic oscillator.
"""
def initialize(self):
self.options.declare('num_nodes', types=int)
def setup(self):
nn = self.options['num_nodes']
# Inputs
self.add_input('x', shape=(nn,), desc='displacement', units='m')
self.add_input('v', shape=(nn,), desc='velocity', units='m/s')
self.add_input('constants', shape=(3,), units=None,
desc='a vector of mass, spring constant, and damping coefficient [m, k, c]',
tags=['dymos.static_target'])
self.add_output('v_dot', val=np.zeros(nn), desc='rate of change of velocity', units='m/s**2')
self.declare_coloring(wrt='*', method='fd')
def compute(self, inputs, outputs):
x = inputs['x']
v = inputs['v']
m, k, c = inputs['constants']
f_spring = -k * x
f_damper = -c * v
outputs['v_dot'] = (f_spring + f_damper) / m
To use the ODE, we have a problem with a single trajectory and in this case, as single phase.
Again, in my opinion, the clearest way to link parameters from the trajectory to phases is to add them in both places with the same name.
Dymos will perform some introspection and automatically link them up.
def test_ivp_driver_shaped_param(self):
import openmdao.api as om
import dymos as dm
import matplotlib.pyplot as plt
# plt.switch_backend('Agg') # disable plotting to the screen
from dymos.examples.oscillator.oscillator_ode import OscillatorODEVectorParam
# Instantiate an OpenMDAO Problem instance.
prob = om.Problem()
# We need an optimization driver. To solve this simple problem ScipyOptimizerDriver will work.
prob.driver = om.ScipyOptimizeDriver()
# Instantiate a Phase
phase = dm.Phase(ode_class=OscillatorODEVectorParam, transcription=dm.Radau(num_segments=10))
# Tell Dymos that the duration of the phase is bounded.
phase.set_time_options(fix_initial=True, fix_duration=True)
# Tell Dymos the states to be propagated using the given ODE.
phase.add_state('x', fix_initial=True, rate_source='v', targets=['x'], units='m')
phase.add_state('v', fix_initial=True, rate_source='v_dot', targets=['v'], units='m/s')
# The spring constant, damping coefficient, and mass are inputs to the system that are
# constant throughout the phase.
# Declare this parameter on phase and then we'll feed its value from the parent trajectory.
phase.add_parameter('constants', units=None)
# Since we're using an optimization driver, an objective is required. We'll minimize
# the final time in this case.
phase.add_objective('time', loc='final')
# Instantiate a Dymos Trajectory and add it to the Problem model.
traj = prob.model.add_subsystem('traj', dm.Trajectory())
traj.add_phase('phase0', phase)
# This parameter value will connect to any phase with a parameter named constants by default.
# This is the easiest way, in my opinion, to pass parameters from trajectory to phase.
traj.add_parameter('constants', units=None, opt=False)
# Setup the OpenMDAO problem
prob.setup()
# Assign values to the times and states
prob.set_val('traj.phase0.t_initial', 0.0)
prob.set_val('traj.phase0.t_duration', 15.0)
prob.set_val('traj.phase0.states:x', 10.0)
prob.set_val('traj.phase0.states:v', 0.0)
# m k c
prob.set_val('traj.parameters:constants', [1.0, 1.0, 0.5])
# Now we're using the optimization driver to iteratively run the model and vary the
# phase duration until the final y value is 0.
prob.run_driver()
# Perform an explicit simulation of our ODE from the initial conditions.
sim_out = traj.simulate(times_per_seg=50)
# Plot the state values obtained from the phase timeseries objects in the simulation output.
t_sol = prob.get_val('traj.phase0.timeseries.time')
t_sim = sim_out.get_val('traj.phase0.timeseries.time')
states = ['x', 'v']
fig, axes = plt.subplots(len(states), 1)
for i, state in enumerate(states):
sol = axes[i].plot(t_sol, prob.get_val(f'traj.phase0.timeseries.states:{state}'), 'o')
sim = axes[i].plot(t_sim, sim_out.get_val(f'traj.phase0.timeseries.states:{state}'), '-')
axes[i].set_ylabel(state)
axes[-1].set_xlabel('time (s)')
fig.legend((sol[0], sim[0]), ('solution', 'simulation'), 'lower right', ncol=2)
plt.tight_layout()
plt.show()