add parameter conversion helper

README.md

@@ -201,6 +201,17 @@ plot(t, [Ym Um], layout=(2,1), ylabel = ["y" "u"], legend=false)
 ```
 Once again, the output looks identical and is therefore omitted here.
 
+## Parameter conversion
+The form of the PID controller used in this package is often referred to as ["standard form"](https://en.wikipedia.org/wiki/Proportional%E2%80%93integral%E2%80%93derivative_controller#Standard_form). If you have PID parameters on "parallel form"
+```math
+K_p (br-y) + K_i (r-y)/s - K_d s y/(Tf s + 1)
+```
+you may convert these to the standard form
+```math
+K (b r - y + 1/T_i (r - y) - s T_d y/(1 + s T_d / N))
+```
+using the function `K, Ti, Td = parallel2standard(kp, ki, kd)` or, if a filter parameter is included, `K, Ti, Td, N = parallel2standard(kp, ki, kd, Tf)`. This function also accepts a vector of parameters in the same order, in which case a vector is returned.
+
 ## Details
 - The derivative term only acts on the (filtered) measurement and not the command signal. It is thus safe to pass step changes in the reference to the controller. The parameter $b$ can further be set to zero to avoid step changes in the control signal in response to step changes in the reference.
 - Bumpless transfer when updating `K` is realized by updating the state `I`. See the docs for `set_K!` for more details.

examples/juliac/juliac_pid.jl

@@ -46,5 +46,5 @@ end
 
 # compile using something like this, modified to suit your local paths
 # cd(@__DIR__)
-# run(`/home/fredrikb/repos/julia/julia --project --experimental /home/fredrikb/repos/julia/contrib/juliac.jl --output-lib juliac_pid --experimental --trim=unsafe-warn --compile-ccallable juliac_pid.jl`)
+# run(`/home/fredrikb/repos/julia/julia --project --experimental /home/fredrikb/repos/julia/contrib/juliac.jl --output-lib juliac_pid --trim=unsafe-warn --experimental --compile-ccallable juliac_pid.jl`)
 # run(`ls -ltrh`)

src/DiscretePIDs.jl

@@ -1,6 +1,6 @@
 module DiscretePIDs
 
-export DiscretePID, calculate_control!, set_K!, set_Td!, set_Ti!, reset_state!
+export DiscretePID, calculate_control!, set_K!, set_Td!, set_Ti!, reset_state!, parallel2standard
 
 using Printf
 
@@ -201,4 +201,41 @@ function reset_state!(pid::DiscretePID)
     nothing
 end
 
+"""
+    K, Ti, Td = parallel2standard(Kp, Ki, Kd)
+
+Convert parameters from form "parallel" form
+``K_p + K_i/s + K_d s``
+
+to "standard" form used in DiscretePID:
+``K(1 + 1/(T_i s) + T_d s)``
+
+You may provide either three arguments or an array with three elements in the same order.
+"""
+function parallel2standard(Kp, Ki, Kd)
+    Kp == 0 && throw(DomainError("Cannot convert to standard form when Kp=0"))
+    return (Kp, Kp / Ki, Kd / Kp)
+end
+
+"""
+    K, Ti, Td, N = parallel2standard(Kp, Ki, Kd, Tf)
+
+Convert parameters from form "parallel" form with first-order filter
+``K_p (br-y) + K_i (r-y)/s - K_d s y/(Tf s + 1)``
+
+to "standard" form used in DiscretePID:
+``K (br-y + (r-y)/(T_i s) - T_d s y/(T_d / N s + 1))``
+
+You may provide either four arguments or an array with four elements in the same order.
+"""
+function parallel2standard(Kp, Ki, Kd, Tf)
+    Kp, Ti, Td = parallel2standard(Kp, Ki, Kd)
+    N = Td / Tf
+    return (Kp, Ti, Td, N)
+end
+
+function parallel2standard(p)
+    return [parallel2standard(p...)...]
+end
+
 end

test/runtests.jl

@@ -183,4 +183,12 @@ res2 = lsim(P, ctrl, 3)
 @test DiscretePID(Ts=1f0) isa DiscretePID{Float32}
 @test DiscretePID(Ts=1.0) isa DiscretePID{Float64}
 
+kpkikdTf = rand(4)
+kp, ki, kd, Tf = kpkikdTf
+ps = parallel2standard(kpkikdTf)
+K,Ti,Td,N = ps
+
+@test ControlSystemsBase.pid(kp, ki, kd; Tf, form = :parallel, filter_order=1) ≈ ControlSystemsBase.pid(K, Ti, Td; Tf=Td/N, form = :standard, filter_order=1)
+
+
 end