JuliaControl/ControlSystems.jl

Differentiable control design

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#791 opened on Jan 4, 2023

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help wanted

Description

A number of functions in this package and in RobustAndOptimalControl.jl fail when trying to compute gradients through them using ForwardDiff or Zygote. This issue tries to summarize the current status

Problematic functions

Function AD package Comment
c2d ForwardDiff :zoh requires exp!(::Matrix{Dual}). ForwardDiffChainRules buggy, but manual implementation possible. #844
feedback Zygote try/catch and @warn
are All Handled through ImplicitDiff #844
hinfnorm ForwardDiff Non-smooth. Works for Zygote. See tests for comments and comment. ForwardDiff in #844 but not for MIMO
svd/qr/schur ForwardDiff DifferentiableFactorizations.jl may be helpful
hessenberg ForwardDiff, ReverseDiff GenericLinearAlgebra.Hessenberg has different fieldnames from LinearAlgebra.Hessenberg

c2d example failing

foo(x) = sum(exp(reshape(x, 2, 2)))
v = randn(4)
using ForwardDiff
ForwardDiff.gradient(foo, v)

ERROR: MethodError: no method matching exp(::Matrix{ForwardDiff.Dual...

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Contributor guide

Tech stack
None
Domain
performancedeveloper experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Juliaautomatic differentiationcontrol theorylinear algebra
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Investigate implementing custom differentiation rules for the problematic functions listed in the table, particularly c2d with ForwardDiff using manual implementations as suggested in PR #844. For feedback with Zygote, resolve try/catch and @warn issues. Consider using DifferentiableFactorizations.jl for svd/qr/schur. Test compatibility with ForwardDiff and Zygote after fixes. See linked PRs and test files for guidance.