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POEM ID: 099
Title: InputResidsComp
authors: robfalck (Rob Falck)
Competing POEMs:
Related POEMs:
Associated implementation PR: #3295

Status:

  • Active
  • Requesting decision
  • Accepted
  • Rejected
  • Integrated

Motivation

A while ago, OpenMDAO added the capability to allow implicit components to have differing numbers of residual variables and outputs, so long as the total size of the residuals is equal to the total size of the outputs (POEM_069).

Since then, one pattern that seemed to recur as a result of this change was to create an ImplicitComponent that simply took any input given and used it as a residual value for the system.

Rather than reimplementing this capability repeatedly, the decision has been made to add it as an OpenMDAO component.

Proposed Solution

InputResidsComp is an ImplicitComponent for which overrides add_input, allowing a residual ref value to be provided in addition to the typical input metadata.

Implicit outputs are added with the add_output method. They are not added automatically as they are with something like the BalanceComp.

Required Changes to OpenMDAO

OpenMDAO currently allows inputs and outputs to be shaped based upon connections or relative to other inputs and outputs through shape_by_conn, copy_shape, and compute_shape areguments. Residuals do not currently support these arguments.

This is similar to the issues that drove the need for a setup_partials method which allowed partials to be declared in final_setup, once the sizes of all inputs and outputs ad been resolved.

As of a result, OpenMDAO components will get a setup_residuals method which will allow add_residual to be called once sizes have been resolved.

  • It is an error to define setup_residuals in an ExplicitComponent

Example

The following example shows how InputResidsComp can be used in a situation where the number of inputs/residuals and outputs differ, but their total sizes are the same.

def test_input_resids_comp_copy_shape(self):
    p = om.Problem()

    p.model.add_subsystem('exec_comp',
                            om.ExecComp(['res_a = a - x[0]',
                                        'res_b = b - x[1:]'],
                                        a={'shape': (1,)},
                                        b={'shape': (2,)},
                                        res_a={'shape': (1,)},
                                        res_b={'shape': (2,)},
                                        x={'shape':3}),
                                        promotes_inputs=['*'],
                                        promotes_outputs=['*'])

    resid_comp = p.model.add_subsystem('resid_comp',
                                        om.InputResidsComp(),
                                        promotes_inputs=['*'],
                                        promotes_outputs=['*'])

    resid_comp.add_output('x', shape=(3,))
    resid_comp.add_input('res_a', shape_by_conn=True, ref=1.0)
    resid_comp.add_input('res_b', shape_by_conn=True, ref=1.0)

    p.model.nonlinear_solver = om.NewtonSolver(solve_subsystems=False)
    p.model.linear_solver = om.DirectSolver()

    p.setup()

    p.set_val('a', 3.0)
    p.set_val('b', [4.0, 5.0])

    p.run_model()