adding urdf docs

Author: simon-lc

docs/src/assets/rexhopper.png

@@ -1,12 +1,11 @@
 # Loading Mechanism via URDF
-Another way to build a mechanism is to directly load it from a URDF file. We can do this is the mechanism builder function. We illustrate this with the RExLab hopper.
+Another way to build a mechanism is to directly load it from a URDF file. We illustrate this with the RExLab hopper.
 
 ```julia
 function get_rexhopper(;
     timestep=0.01,
     gravity=[0.0; 0.0; -9.81],
     friction_coefficient=1.0,
-    contact_foot=true,
     contact_body=true,
     limits=true,
     model=:rexhopper,
@@ -16,16 +15,48 @@ function get_rexhopper(;
     damper=1.0,
     T=Float64)
 
+    # we define the path of the URDF file
     path = joinpath(@__DIR__, "../deps/$(String(model)).urdf")
+    # we load the URDF file into a Mechanism object
     mech = Mechanism(path, floating, T,
         gravity=gravity,
         timestep=timestep,
         spring=spring,
         damper=damper)
+    return mech
+end
+```
+Here, we have created a mechanism, that contains all the bodies and the joint and also the meshes loaded from the path contained in the URDF file. This is the resulting robot;
+![rexhopper](./assets/rexhopper.png)
 
-    # joint limits
-    joints = deepcopy(mech.joints)
+However, we are still missing the joint limits and the contact spheres. The following methods provides an example for adding joint limits and contact spheres. Our goal here was to best approximate the mesh shapes using as few spheres as possible.
+Parsing joint limits from the URDF to add them automatically to the mechanism should be easily feasible. That could be a great way to contribute and familiarize yourself with the code!
 
+```julia
+function get_rexhopper(;
+    timestep=0.01,
+    gravity=[0.0; 0.0; -9.81],
+    friction_coefficient=1.0,
+    contact_body=true,
+    limits=true,
+    model=:rexhopper,
+    floating=true,
+    contact_type=:nonlinear,
+    spring=0.0,
+    damper=1.0,
+    T=Float64)
+
+    # we define the path of the URDF file
+    path = joinpath(@__DIR__, "../deps/$(String(model)).urdf")
+    # we load the URDF file into a Mechanism object
+    mech = Mechanism(path, floating, T,
+        gravity=gravity,
+        timestep=timestep,
+        spring=spring,
+        damper=damper)
+
+    # we add joint limits to the mechanism
+    joints = deepcopy(mech.joints)
     if limits
         joint1 = get_joint(mech, :joint1)
         joint3 = get_joint(mech, :joint3)
@@ -40,13 +71,14 @@ function get_rexhopper(;
             damper=damper)
     end
 
-    if contact_foot
+    # we attach spherical collisions zones to certain bodies
+    if contact_body
         origin = Origin{T}()
         bodies = mech.bodies
         joints = mech.joints
 
         normal = [0.0; 0.0; 1.0]
-        models = []
+        contacts = []
 
         link3 = get_body(mech, :link3)
         link2 = get_body(mech, :link2)
@@ -55,22 +87,22 @@ function get_rexhopper(;
         base_radius = 0.125
         p = [0.1685; 0.0025; -0.0055]
         o = [0;0; foot_radius]
-        push!(models, contact_constraint(link3, normal,
+        push!(contacts, contact_constraint(link3, normal,
             friction_coefficient=friction_coefficient,
             contact_point=p,
             offset=o,
             contact_type=contact_type,
             name=:foot))
         p = [-0.10; -0.002; 0.01]
         o = [0;0; ankle_radius]
-        push!(models, contact_constraint(link3, normal,
+        push!(contacts, contact_constraint(link3, normal,
             friction_coefficient=friction_coefficient,
             contact_point=p,
             offset=o,
             contact_type=contact_type,
             name=:ankle3))
         p = [0.24; 0.007; 0.005]
-        push!(models, contact_constraint(link2, normal,
+        push!(contacts, contact_constraint(link2, normal,
             friction_coefficient=friction_coefficient,
             contact_point=p,
             offset=o,
@@ -79,15 +111,15 @@ function get_rexhopper(;
         base_link = get_body(mech, :base_link)
         p = [0.0; 0.0; 0.0]
         o = [0;0; base_radius]
-        push!(models, contact_constraint(base_link, normal,
+        push!(contacts, contact_constraint(base_link, normal,
             friction_coefficient=friction_coefficient,
             contact_point=p,
             offset=o,
             contact_type=contact_type,
             name=:torso))
 
         set_minimal_coordinates!(mech, get_joint(mech, :auto_generated_floating_joint), [0,0,1.0, 0,0,0])
-        mech = Mechanism(origin, bodies, joints, [models...],
+        mech = Mechanism(origin, bodies, joints, [contacts...],
             gravity=gravity,
             timestep=timestep,
             spring=spring,
@@ -96,3 +128,6 @@ function get_rexhopper(;
     return mech
 end
 ```
+
+This is the result, we added three spherical contacts to the robot. There is one coverinn the torso one covering the knee and one covering the foot, they are shown in red;
+![rexhopper_contact](./assets/rexhopper_contact.png)

docs/src/assets/rexhopper_contact.png

@@ -22,4 +22,4 @@ storage = simulate!(mechanism, 1.5, record=true, opts=SolverOptions(undercut=10.
     btol=1.0e-4, rtol=1.0e-4, verbose=false));
 
 # Visualize
-visualize(mechanism, storage, vis=vis, show_contact=true, build=false);
+visualize(mechanism, storage, vis=vis, show_contact=true, build=true);