# Mechanism type in RigidBodyDynamics.jl defaulting to Mechanism type in ConstrainedDynamics.jl

**URL:** https://discourse.julialang.org/t/mechanism-type-in-rigidbodydynamics-jl-defaulting-to-mechanism-type-in-constraineddynamics-jl/82525
**Category:** General Usage
**Created:** [June 9, 2022, 8:22pm UTC](https://discourse.julialang.org/t/mechanism-type-in-rigidbodydynamics-jl-defaulting-to-mechanism-type-in-constraineddynamics-jl/82525 "2022-06-09T20:22:32Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![jpro](https://avatars.discourse-cdn.com/v4/letter/j/9fc29f/32.png) [@jpro](https://discourse.julialang.org/u/jpro)
#### Post date: [June 9, 2022, 8:22pm UTC](https://discourse.julialang.org/t/mechanism-type-in-rigidbodydynamics-jl-defaulting-to-mechanism-type-in-constraineddynamics-jl/82525/1 "2022-06-09T20:22:32Z")

</div>

I’m new to the RigidBodyDynamics package and wanted to follow this tutorial.

[Quickstart - double pendulum](https://juliarobotics.org/RigidBodyDynamics.jl/stable/generated/1.%20Quickstart%20-%20double%20pendulum/1.%20Quickstart%20-%20double%20pendulum/)

When I call the Mechanism type I get errors. Here is my code along with the errors. It appears based on the error that it is expecting a type defined by the “mechaism\_struct” in the ConstrainedDynamics.jl package, and thus is expecting additional parameters to be defined (bodes, delta t).

```julia
using RigidBodyDynamics

using StaticArrays

using LinearAlgebra

g = -9.81 # gravitational acceleration in z-direction

world = RigidBody{Float64}("world")

doublependulum = Mechanism(world; gravity = SVector(0, 0, g))

```

```julia
ERROR: MethodError: no method matching Mechanism(::RigidBody{Float64}; gravity=[0.0, 0.0, -9.81])
Closest candidates are:
  Mechanism(::Origin{T}, ::Array{Body{T}, 1}; Δt, g) where T at C:\Users\JuliaProctor\.julia\packages\ConstrainedDynamics\yq8xZ\src\main_components\mechanism_struct.jl:146 got unsupported keyword argument "gravity"
  Mechanism(::Origin{T}, ::Array{Body{T}, 1}, ::Vector{<:EqualityConstraint{T}}; Δt, g) where T at C:\Users\JuliaProctor\.julia\packages\ConstrainedDynamics\yq8xZ\src\main_components\mechanism_struct.jl:127 got unsupported keyword argument "gravity" Mechanism(::Origin{T}, ::Array{Body{T}, 1}, ::Vector{<:EqualityConstraint{T}}, ::Vector{<:InequalityConstraint{T}}; Δt, g) where T at C:\Users\JuliaProctor\.julia\packages\ConstrainedDynamics\yq8xZ\src\main_components\mechanism_struct.jl:39 got unsupported keyword argument "gravity"

```

How is this happening if I’m not even using the ConstrainedDynamics package in this script? Any ideas to resolve this issue are much appreciated.

---

<div class="post-metadata">

### Author: ![contradict](https://avatars.discourse-cdn.com/v4/letter/c/ac91a4/32.png) [@contradict](https://discourse.julialang.org/u/contradict)
#### Post date: [June 10, 2022, 3:40pm UTC](https://discourse.julialang.org/t/mechanism-type-in-rigidbodydynamics-jl-defaulting-to-mechanism-type-in-constraineddynamics-jl/82525/2 "2022-06-10T15:40:04Z")

</div>

Works fine here, perhaps you have an old version of something?

```julia
julia> doublependulum = Mechanism(world; gravity = SVector(0, 0, g))
Spanning tree:
Vertex: world (root)
No non-tree joints.

(jl_UkHAQX) pkg> st
      Status `/tmp/jl_UkHAQX/Project.toml`
  [366cf18f] RigidBodyDynamics v2.3.2
  [90137ffa] StaticArrays v1.4.6

```