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allow FirstOrder to operate as highpass instead of lowpass #77

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Merged
merged 1 commit into from
Jun 19, 2022
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allow FirstOrder to operate as highpass instead of lowpass #77

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15 changes: 11 additions & 4 deletions src/Blocks/continuous.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -61,15 +61,22 @@ function Derivative(; name, k=1, T, x_start=0.0)
end

"""
FirstOrder(; name, k=1, T, x_start=0.0)
FirstOrder(; name, k=1, T, x_start=0.0, lowpass=true)

A first-order filter with a single real pole in `s = -T` and gain `k`. The transfer function
A first-order filter with a single real pole in `s = -T` and gain `k`. If `lowpass=true` (default), the transfer function
is given by `Y(s)/U(s) = `
```
k
───────
sT + 1
```
and if `lowpass=false`, by
```
sT + 1 - k
──────────
sT + 1
```


# Parameters:
- `k`: Gain
Expand All @@ -82,15 +89,15 @@ sT + 1

See also [`SecondOrder`](@ref)
"""
function FirstOrder(; name, k=1, T, x_start=0.0)
function FirstOrder(; name, k=1, T, x_start=0.0, lowpass=true)
T > 0 || throw(ArgumentError("Time constant `T` has to be strictly positive"))
@named siso = SISO()
@unpack u, y = siso
sts = @variables x(t)=x_start
pars = @parameters T=T k=k
eqs = [
D(x) ~ (k*u - x) / T
y ~ x
lowpass ? y ~ x : y ~ k*u - x
]
extend(ODESystem(eqs, t, sts, pars; name=name), siso)
end
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11 changes: 10 additions & 1 deletion test/Blocks/continuous.jl
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Expand Up @@ -43,7 +43,7 @@ end
@testset "PT1" begin
pt1_func(t, k, T) = k * (1 - exp(-t / T)) # Known solution to first-order system

k, T = 1.0, 0.1
k, T = 1.2, 0.1
@named c = Constant(; k=1)
@named pt1 = FirstOrder(; k=k, T=T)
@named iosys = ODESystem(connect(c.output, pt1.input), t, systems=[pt1, c])
Expand All @@ -52,6 +52,15 @@ end
sol = solve(prob, Rodas4())
@test sol.retcode == :Success
@test sol[pt1.output.u] ≈ pt1_func.(sol.t, k, T) atol=1e-3

# Test highpass feature
@named pt1 = FirstOrder(; k=k, T=T, lowpass=false)
@named iosys = ODESystem(connect(c.output, pt1.input), t, systems=[pt1, c])
sys = structural_simplify(iosys)
prob = ODEProblem(sys, Pair[], (0.0, 100.0))
sol = solve(prob, Rodas4())
@test sol.retcode == :Success
@test sol[pt1.output.u] ≈ k .- pt1_func.(sol.t, k, T) atol=1e-3
end

@testset "PT2" begin
Expand Down