Merge pull request #28 from JuliaComputing/SDP

add SpringDamperParallel component

docs/src/examples/spring_damper_system.md

@@ -70,4 +70,7 @@ plot(
 )
 ```
 
-This example has two parallel spring-mass parts, the first body (`body1`) is attached directly to the spring, with no joint in parallel with the spring. In this situation, we have to set `isroot=true` for `body1` to indicate that we want to use the body variables as state. The second body (`body2`) is attached to the spring with a joint in parallel with the spring, so we can use the joint variables as state, hence `isroot=false` for `body2`.
+This example has two parallel spring-mass parts, the first body (`body1`) is attached directly to the spring, with no joint in parallel with the spring. In this situation, we have to set `isroot=true` for `body1` to indicate that we want to use the body variables as state. The second body (`body2`) is attached to the spring with a joint in parallel with the spring, so we can use the joint variables as state, hence `isroot=false` for `body2`.
+
+
+In this example we used separate springs and dampers, see also the component [`SpringDamperParallel`](@ref) which combines the two in one component.

src/Multibody.jl

@@ -92,7 +92,7 @@ export Revolute, Prismatic, Spherical, Universal, GearConstraint, RollingWheelJo
  RollingWheel, FreeMotion
 include("joints.jl")
 
-export Spring, Damper, Torque, Force
+export Spring, Damper, SpringDamperParallel, Torque, Force
 include("forces.jl")
 
 export PartialCutForceBaseSensor, BasicCutForce, BasicCutTorque, CutTorque, CutForce

src/forces.jl

@@ -276,6 +276,8 @@ additional equations to handle the mass are removed.
 - `lengthFraction`: Location of spring mass with respect to `frame_a` as a fraction of the distance from `frame_a` to `frame_b` (=0: at `frame_a`; =1: at `frame_b`)
 - `s_unstretched`: Length of the spring when it is unstretched
 - `kwargs`: are passed to `LineForceWithMass`
+
+See also [`SpringDamperParallel`](@ref)
 """
 function Spring(; c, name, m = 0, lengthFraction = 0.5, s_unstretched = 0, kwargs...)
  @named ptf = PartialTwoFrames()
@@ -346,6 +348,8 @@ and `D(s)` is the time derivative of `s`.
 
 # Arguments:
 - `d`: Damping coefficient
+
+See also [`SpringDamperParallel`](@ref)
 """
 function Damper(; d, name, kwargs...)
  @named plf = PartialLineForce(; kwargs...)
@@ -356,3 +360,30 @@ function Damper(; d, name, kwargs...)
  ]
  extend(ODESystem(eqs, t; name), plf)
 end
+
+"""
+ SpringDamperParallel(; name, c, d, s_unstretched)
+
+Linear spring and linear damper in parallel acting as line force between `frame_a` and `frame_b`. A force `f` is exerted on the origin of `frame_b` and with opposite sign on the origin of `frame_a` along the line from the origin of `frame_a` to the origin of `frame_b` according to the equation:
+```math
+f = c (s - s_unstretched) + d*D(s)
+```
+where `c`, `s_unstretched` and `d` are parameters, `s` is the distance between the origin of `frame_a` and the origin of `frame_b` and `D(s)` is the time derivative of `s`.
+"""
+function SpringDamperParallel(; name, c, d, s_unstretched)
+ @named plf = PartialLineForce(; kwargs...)
+ @unpack s, f = plf
+
+ @parameters c=c [description = "spring constant", bounds = (0, Inf)]
+ @parameters d=d [description = "damping constant", bounds = (0, Inf)]
+ @parameters s_unstretched=s_unstretched [
+ description = "unstretched length of spring",
+ bounds = (0, Inf),
+ ]
+
+ eqs = [f_d ~ d * D(s)
+ f ~ c * (s - s_unstretched) + f_d
+ # lossPower ~ f_d*der(s)
+ ]
+ extend(ODESystem(eqs, t; name), plf)
+end