adds Free component

Author: Brad Carman

src/Mechanical/Translational/components.jl

@@ -1,3 +1,22 @@
+"""
+    Free(; name)
+
+Use to close a system that has un-connected `MechanicalPort`'s.  
+
+# Connectors:
+
+  - `flange`: 1-dim. translational flange
+"""
+@component function Free(; name)
+    @named flange = MechanicalPort()
+    vars = []
+    eqs = [
+        flange.f ~ 0,
+    ]
+    return compose(ODESystem(eqs, t, vars, []; name, defaults = [flange.v => 0]),
+                   flange)
+end
+
 """
     Fixed(;name)
 

test/Mechanical/translational.jl

@@ -7,6 +7,27 @@ import ModelingToolkitStandardLibrary.Mechanical.TranslationalPosition as TP
 @parameters t
 D = Differential(t)
 
+@testset "Free" begin
+    function System(; name)
+        systems = @named begin
+            mass = TV.Mass(; m = 100, g = -10)
+            free = TV.Free()
+        end
+
+        eqs = [connect(mass.flange, free.flange)]
+
+        ODESystem(eqs, t, [], []; name, systems)
+    end
+
+    @named system = System()
+    s = complete(system)
+    sys = structural_simplify(system)
+    prob = ODEProblem(sys, [], (0, 0.1))
+    sol = solve(prob, Rosenbrock23())
+
+    @test sol[s.mass.flange.v][end]≈-0.1 * 10 atol=1e-3
+end
+
 @testset "spring damper mass fixed" begin
     @named dv = TV.Damper(d = 1, v_a_0 = 1)
     @named dp = TP.Damper(d = 1, v_a_0 = 1, s_a_0 = 3, s_b_0 = 1)