add length_fraction param to body

Author: baggepinnen

docs/src/examples/pendulum.md

@@ -74,7 +74,7 @@ nothing # hide
 
 ![animation](pendulum.gif)
 
-By default, the world frame is indicated using the convention x: red, y: green, z: blue. The animation shows how the simple [`Body`](@ref) represents a point mass at a particular distance `r_cm` away from its mounting flange `frame_a`. To model a more physically motivated pendulum rod, we could have used a [`BodyShape`](@ref) component, which has two mounting flanges instead of one. The [`BodyShape`](@ref) component is shown in several of the examples available in the example sections of the documentation.
+By default, the world frame is indicated using the convention x: red, y: green, z: blue. The animation shows how the simple [`Body`](@ref) represents a point mass with inertial properties at a particular distance `r_cm` away from its mounting flange `frame_a`. The cylinder that is shown connecting the pivot point to the body is for visualization purposes only, it does not have any inertial properties. To model a more physically motivated pendulum rod, we could have used a [`BodyShape`](@ref) component, which has two mounting flanges instead of one. The [`BodyShape`](@ref) component is shown in several of the examples available in the example sections of the documentation.
 
 ## Adding damping
 To add damping to the pendulum such that the pendulum will eventually come to rest, we add a [`Damper`](@ref) to the revolute joint. The damping coefficient is given by `d`, and the damping force is proportional to the angular velocity of the joint. To add the damper to the revolute joint, we must create the joint with the keyword argument `axisflange = true`, this adds two internal flanges to the joint to which you can attach components from the `ModelingToolkitStandardLibrary.Mechanical.Rotational` module (1-dimensional components). We then connect one of the flanges of the damper to the axis flange of the joint, and the other damper flange to the support flange which is rigidly attached to the world.

ext/Render.jl

@@ -243,6 +243,7 @@ function render!(scene, ::typeof(Body), sys, sol, t)
     r_cm = get_fun(sol, collect(sys.r_cm))
     framefun = get_frame_fun(sol, sys.frame_a)
     radius = sol(sol.t[1], idxs=sys.radius) |> Float32
+    length_fraction = sol(sol.t[1], idxs=sys.length_fraction) |> Float32
     cylinder_radius = sol(sol.t[1], idxs=sys.cylinder_radius) |> Float32
     thing = @lift begin # Sphere
         Ta = framefun($t)
@@ -265,9 +266,8 @@ function render!(scene, ::typeof(Body), sys, sol, t)
         tip = Point3f(Ta[1:3, 4]) # Origin of frame a in world coords
 
         d = tip - point
-        d = d ./ norm(d)
         # d = Point3f(Ta[1:3, 1])
-        tip = point + 0.2f0d
+        tip = point + length_fraction*d
         extremity = tip
         origin = point
 

src/components.jl

@@ -240,6 +240,7 @@ Representing a body with 3 translational and 3 rotational degrees-of-freedom.
               w_a = 0,
               radius = 0.05,
               cylinder_radius = radius/2,
+              length_fraction = 1,
               air_resistance = 0.0,
               color = [1,0,0,1],
               quat=false,)
@@ -275,6 +276,7 @@ Representing a body with 3 translational and 3 rotational degrees-of-freedom.
         description = "Radius of the cylinder from frame to COM in animations",
     ]
     @parameters color[1:4] = color [description = "Color of the body in animations (RGBA)"]
+    @parameters length_fraction=length_fraction, [description = "Fraction of the length of the body that is the cylinder from frame to COM in animations"]
     # @parameters I[1:3, 1:3]=I [description="inertia tensor"]
 
     @parameters I_11=I_11 [description = "Element (1,1) of inertia tensor"]
@@ -350,7 +352,7 @@ Representing a body with 3 translational and 3 rotational degrees-of-freedom.
     # pars = [m;r_cm;radius;I_11;I_22;I_33;I_21;I_31;I_32;color]
 
     sys = ODESystem(eqs, t; name=:nothing, metadata = Dict(:isroot => isroot), systems = [frame_a])
-    add_params(sys, [radius; cylinder_radius; color]; name)
+    add_params(sys, [radius; cylinder_radius; color; length_fraction]; name)
 end