add SpringDamper component (#327)

* add SpringDamper component

* fix Fixed

* format

Author: baggepinnen

src/Mechanical/TranslationalModelica/TranslationalModelica.jl

@@ -10,7 +10,7 @@ using ...Blocks: RealInput, RealOutput
 export Flange
 include("utils.jl")
 
-export Fixed, Mass, Spring, Damper
+export Fixed, Mass, Spring, Damper, SpringDamper
 include("components.jl")
 
 export Force

src/Mechanical/TranslationalModelica/components.jl

@@ -13,11 +13,11 @@ Flange fixed in housing at a given position.
 """
 @mtkmodel Fixed begin
     @parameters begin
-        s0
+        s0 = 0
     end
 
     @components begin
-        flange = Flange(; s = 0.0)
+        flange = Flange()
     end
 
     @equations begin
@@ -113,3 +113,37 @@ Linear 1D translational damper
         lossPower ~ f * v_rel
     end
 end
+
+"""
+    SpringDamper(; name, c = 0.0, d = 0.0, s_rel0 = 0.0)
+
+Linear 1D translational spring and damper in parallel
+
+# Parameters:
+- `c`: [N/m] Spring constant
+- `d`: [N.s/m] Damping constant
+- `s_rel0`: Unstretched spring length
+
+# Connectors:
+- `flange_a: 1-dim. translational flange on one side of spring`
+- `flange_b: 1-dim. translational flange on opposite side of spring`
+
+# Variables:
+- `lossPower`: [W] Power dissipated by the damper
+- `f`: [N] Total force
+"""
+@mtkmodel SpringDamper begin
+    @extend flange_a, flange_b, s_rel, v_rel, f = pc = PartialCompliantWithRelativeStates()
+    @parameters begin
+        d = 0.0, [description = "Damping constant [Ns/m]"]
+        c = 0.0, [description = "Spring constant [N/m]"]
+        s_rel0 = 0.0, [description = "Unstretched spring length [m]"]
+    end
+    @variables begin
+        lossPower(t), [description = "Power dissipated by the damper [W]"]
+    end
+    @equations begin
+        f ~ c * (s_rel - s_rel0) + d * v_rel
+        lossPower ~ d * v_rel^2
+    end
+end

test/Mechanical/translational_modelica.jl

@@ -3,7 +3,8 @@ using ModelingToolkit: t_nounits as t, D_nounits as D
 
 using ModelingToolkitStandardLibrary.Blocks: Sine
 using ModelingToolkitStandardLibrary.Mechanical.TranslationalModelica: Damper, Spring, Mass,
-                                                                       Fixed, Force
+                                                                       Fixed, Force,
+                                                                       SpringDamper
 
 @testset "spring damper mass fixed" begin
     @mtkmodel SpringDamperMassFixed begin
@@ -56,3 +57,31 @@ end
     @test -lb≈ub atol=1e-2
     @test -0.11 < lb < -0.1
 end
+
+@testset "driven SpringDamper mass" begin
+    @mtkmodel DrivenSpringDamperMass2 begin
+        @components begin
+            springdamper = SpringDamper(; d = 1, c = 1, s_rel0 = 1)
+            mass = Mass(; m = 1, v = 1, s = 0)
+            fixed = Fixed(; s0 = 1)
+            force = Force()
+            source = Sine(frequency = 3, amplitude = 2)
+        end
+
+        @equations begin
+            connect(force.f, source.output)
+            connect(force.flange, mass.flange_a)
+            connect(springdamper.flange_a, mass.flange_b)
+            connect(springdamper.flange_b, fixed.flange)
+        end
+    end
+
+    @mtkbuild sys = DrivenSpringDamperMass2()
+
+    prob = ODEProblem(sys, [], (0, 20.0), [])
+    sol = solve(prob, Rodas4())
+
+    lb, ub = extrema(sol(15:0.05:20, idxs = sys.mass.v).u)
+    @test -lb≈ub atol=1e-2
+    @test -0.11 < lb < -0.1
+end