fixed connections.md

Author: Brad Carman

docs/src/connectors/connections.md

@@ -136,8 +136,8 @@ using ModelingToolkitStandardLibrary
 const TV = ModelingToolkitStandardLibrary.Mechanical.Translational
 
 systems = @named begin
-    damping = TV.Damper(d = 1, flange_a.v = 1)
-    body = TV.Mass(m = 1, v = 1)
+    damping = TV.Damper(d = 1)
+    body = TV.Mass(m = 1)
     ground = TV.Fixed()
 end
 
@@ -155,7 +155,7 @@ nothing # hide
 As expected, we have a similar solution…
 
 ```@example connections
-prob = ODEProblem(sys, [], (0, 10.0), [])
+prob = ODEProblem(sys, [], (0, 10.0), [];   initialization_eqs=[sys.body.s ~ 0, sys.body.v ~ 1])
 sol_v = solve(prob)
 
 p1 = plot(sol_v, idxs = [body.v])
@@ -228,7 +228,7 @@ In this problem, we have a mass, spring, and damper which are connected to a fix
 The damper will connect the flange/flange 1 (`flange_a`) to the mass, and flange/flange 2 (`flange_b`) to the fixed point.  For both position- and velocity-based domains, we set the damping constant `d=1` and `va=1` and leave the default for `v_b_0` at 0.  For the position domain, we also need to set the initial positions for `flange_a` and `flange_b`.
 
 ```@example connections
-@named dv = TV.Damper(d = 1, flange_a.v = 1)
+@named dv = TV.Damper(d = 1)
 @named dp = TP.Damper(d = 1, va = 1, vb = 0.0, flange_a__s = 3, flange_b__s = 1)
 nothing # hide
 ```
@@ -238,7 +238,7 @@ nothing # hide
 The spring will connect the flange/flange 1 (`flange_a`) to the mass, and flange/flange 2 (`flange_b`) to the fixed point.  For both position- and velocity-based domains, we set the spring constant `k=1`.  The velocity domain then requires the initial velocity `va` and initial spring stretch `delta_s`.  The position domain instead needs the initial positions for `flange_a` and `flange_b` and the natural spring length `l`.
 
 ```@example connections
-@named sv = TV.Spring(k = 1, flange_a__v = 1, delta_s = 1)
+@named sv = TV.Spring(k = 1)
 @named sp = TP.Spring(k = 1, flange_a__s = 3, flange_b__s = 1, l = 1)
 nothing # hide
 ```
@@ -248,7 +248,7 @@ nothing # hide
 For both position- and velocity-based domains, we set the mass `m=1` and initial velocity `v=1`. Like the damper, the position domain requires the position initial conditions set as well.
 
 ```@example connections
-@named bv = TV.Mass(m = 1, v = 1)
+@named bv = TV.Mass(m = 1)
 @named bp = TP.Mass(m = 1, v = 1, s = 3)
 nothing # hide
 ```
@@ -270,7 +270,7 @@ As can be seen, the position-based domain requires more initial condition inform
 Let's define a quick function to simplify and solve the 2 different systems. Note, we will solve with a fixed time step and a set tolerance to compare the numerical differences.
 
 ```@example connections
-function simplify_and_solve(damping, spring, body, ground)
+function simplify_and_solve(damping, spring, body, ground; initialization_eqs = Equation[])
     eqs = [connect(spring.flange_a, body.flange, damping.flange_a)
            connect(spring.flange_b, damping.flange_b, ground.flange)]
 
@@ -280,8 +280,8 @@ function simplify_and_solve(damping, spring, body, ground)
 
     println.(full_equations(sys))
 
-    prob = ODEProblem(sys, [], (0, 10.0), [])
-    sol = solve(prob; dt = 0.1, adaptive = false, reltol = 1e-9, abstol = 1e-9)
+    prob = ODEProblem(sys, [], (0, 10.0), []; initialization_eqs)
+    sol = solve(prob; abstol=1e-9, reltol=1e-9)
 
     return sol
 end
@@ -291,7 +291,12 @@ nothing # hide
 Now let's solve the velocity domain model
 
 ```@example connections
-solv = simplify_and_solve(dv, sv, bv, gv);
+initialization_eqs = [
+  bv.s ~ 3
+  bv.v ~ 1
+  sv.delta_s ~ 1
+]
+solv = simplify_and_solve(dv, sv, bv, gv; initialization_eqs);
 nothing # hide
 ```
 
@@ -346,12 +351,11 @@ By definition, the spring stretch is
 \Delta s = s - s_{b_0} - l
 ```
 
-Which means both systems are actually solving the same exact system.  We can plot the numerical difference between the 2 systems and see the result is negligible.
+Which means both systems are actually solving the same exact system.  We can plot the numerical difference between the 2 systems and see the result is negligible (much less than the tolerance of 1e-9).
 
 ```@example connections
 plot(title = "numerical difference: vel. vs. pos. domain", xlabel = "time [s]",
     ylabel = "solv[bv.v] .- solp[bp.v]")
 time = 0:0.1:10
 plot!(time, (solv(time)[bv.v] .- solp(time)[bp.v]), label = "")
-Plots.ylims!(-1e-15, 1e-15)
 ```

src/Mechanical/Translational/components.jl

@@ -65,9 +65,9 @@ Sliding mass with inertia
     @named flange = MechanicalPort()
 
     vars = @variables begin
-        s(t)
-        v(t)
-        f(t)
+        s(t), [guess=0]
+        v(t), [guess=0]
+        f(t), [guess=0]
     end
 
     eqs = [flange.v ~ v
@@ -98,22 +98,21 @@ Linear 1D translational spring
   - `flange_a`: 1-dim. translational flange on one side of spring
   - `flange_b`: 1-dim. translational flange on opposite side of spring
 """
-@component function Spring(; name, k, delta_s = 0.0, flange_a__v = 0.0, flange_b__v = 0.0)
-    Spring(REL; name, k, delta_s, flange_a__v, flange_b__v)
+@component function Spring(; name, k)
+    Spring(REL; name, k)
 end # default
 
-@component function Spring(::Val{:relative}; name, k, delta_s = 0.0, flange_a__v = 0.0,
-        flange_b__v = 0.0)
+@component function Spring(::Val{:relative}; name, k)
     pars = @parameters begin
         k = k
     end
     vars = @variables begin
-        delta_s(t) = delta_s
-        f(t)
+        delta_s(t), [guess = 0]
+        f(t), [guess=0]
     end
 
-    @named flange_a = MechanicalPort(; v = flange_a__v)
-    @named flange_b = MechanicalPort(; v = flange_b__v)
+    @named flange_a = MechanicalPort()
+    @named flange_b = MechanicalPort()
 
     eqs = [D(delta_s) ~ flange_a.v - flange_b.v
            f ~ k * delta_s
@@ -125,20 +124,19 @@ end # default
 end
 
 const ABS = Val(:absolute)
-@component function Spring(::Val{:absolute}; name, k, sa = 0, sb = 0, flange_a__v = 0.0,
-        flange_b__v = 0.0, l = 0)
+@component function Spring(::Val{:absolute}; name, k, l = 0)
     pars = @parameters begin
         k = k
         l = l
     end
     vars = @variables begin
-        sa(t) = sa
-        sb(t) = sb
-        f(t)
+        sa(t), [guess=0]
+        sb(t), [guess=0]
+        f(t), [guess=0]
     end
 
-    @named flange_a = MechanicalPort(; v = flange_a__v)
-    @named flange_b = MechanicalPort(; v = flange_b__v)
+    @named flange_a = MechanicalPort()
+    @named flange_b = MechanicalPort()
 
     eqs = [D(sa) ~ flange_a.v
            D(sb) ~ flange_b.v
@@ -169,8 +167,8 @@ Linear 1D translational damper
         d
     end
     @variables begin
-        v(t)
-        f(t)
+        v(t), [guess=0]
+        f(t), [guess=0]
     end
 
     @components begin