Fix pid_ss_2dof N-to-Tf conversion to match documented Tf = Td/N - #1067
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The docstring states that Tf defaults to Td/N, but the code computed Tf = kd/N using the parallel-form derivative gain kd (units gain*time rather than time). The two agree only when kp == 1. Compute Tf = kd/(kp*N) = Td/N instead, and throw a descriptive error when N is given with zero proportional gain, where Td is undefined. Also add tests pinning the Tf filter semantics of pid: with filter_order = 1 the derivative term is Kd*s/(Tf*s + 1), and with filter_order = 2 the whole controller is filtered by 1/((s*Tf)^2/(4d^2) + s*Tf + 1) with default d = 1/sqrt(2). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_014QR4FuuvxZU9a3UrTJjg5X
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #1067 +/- ##
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- Coverage 91.68% 91.63% -0.05%
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Files 42 42
Lines 5712 5714 +2
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- Hits 5237 5236 -1
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Problem
The
pid_2dof/pid_ss_2dofdocstring states thatTfdefaults toTd/N, but the code computedTf = kd/Nusing the parallel-form derivative gainkd(units gain·time rather than time). The two agree only whenkp == 1, so for any other proportional gain the derivative filter obtained fromNwas off by a factorkp.Fix
Compute
Tf = kd/(kp*N) = Td/N(withTd = kd/kp, the standard-form derivative time constant), matching the docstring. WhenNis given with zero proportional gain,Tdis undefined and a descriptiveArgumentErrorasks for an explicitTfinstead.This is technically breaking for callers who passed
Nwithkp ≠ 1and relied on the previous (undocumented) behavior.Tests
pid_ss_2dofNtest to the corrected relation and added a standard-form check that passingNequals passingTf = Td/Nwithkp ≠ 1, plus thekp == 0error case.Tffilter semantics ofpid: withfilter_order = 1the derivative term isKd·s/(Tf·s + 1); withfilter_order = 2the whole controller is filtered by1/((s·Tf)²/(4d²) + s·Tf + 1)with defaultd = 1/√2, in bothtfandssforms and in standard form.lib/ControlSystemsBase/test/test_pid_design.jlpasses (152/152).🤖 Generated with Claude Code
https://claude.ai/code/session_014QR4FuuvxZU9a3UrTJjg5X