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Fix pid_tf coefficients for Ki=0 with first-order derivative filter #1058

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Merged
baggepinnen merged 2 commits into from
May 16, 2026
Merged

Fix pid_tf coefficients for Ki=0 with first-order derivative filter #1058

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c23485c
Fix pid_tf coefficients for Ki=0 with first-order derivative filter
baggepinnen May 15, 2026
850b8e7
Add regression test for pid_tf Ki=0 filter_order=1 fix
baggepinnen May 16, 2026
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2 changes: 1 addition & 1 deletion lib/ControlSystemsBase/src/pid_design.jl
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Expand Up @@ -68,7 +68,7 @@ function pid_tf(param_p, param_i, param_d=zero(typeof(param_p)); form=:standard,
else
if Ki == 0
if filter_order == 1
tf([Kd*Tf + Kd, Kd], [Tf, 1])
return tf([Kd + Kp*Tf, Kp], [Tf, 1])
else
return tf([Kd, Kp], [Tf^2/(4d^2), Tf, 1])
end
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7 changes: 7 additions & 0 deletions lib/ControlSystemsBase/test/test_pid_design.jl
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Expand Up @@ -77,6 +77,13 @@ Ctf = pid(1.1, 1.2, 0, Tf=0.1, filter_order=1)
Css = pid(1.1, 1.2, 0, Tf=0.1, filter_order=1, state_space=true)
@test freqresptest(Ctf, Css) < 1e-10

# Regression for #1058: pid_tf for Ki=0 and filter_order=1 used to emit
# tf([Kd*Tf + Kd, Kd], [Tf, 1]) — Kp dropped and replaced with Kd.
Ctf = pid(1.1, 0, 1.5, Tf=0.1, filter_order=1, form=:parallel)
Css = pid(1.1, 0, 1.5, Tf=0.1, filter_order=1, form=:parallel, state_space=true)
@test freqresptest(Ctf, Css) < 1e-10
@test Ctf ≈ tf([1.5 + 1.1*0.1, 1.1], [0.1, 1])

# bodeplot([Ctf, Css])


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