Coming from Adams¶

If you model in MSC Adams, most of freedyn2 will be familiar – the same rigid bodies, joints, forces and HHT integration. What is different is how a model is made and looked at: a text file instead of a database, written by you or an assistant; a viewer that re-simulates on every save; variant studies from a slider.

Concepts side by side¶

Adams	freedyn2
Part, Ground	`bodies`, `ground`
Marker, hardpoint, construction frame	none needed: points and frames are given globally at t = 0 where they are used; shared ones are named `points` and `frames` (`{mirror: ...}` for left / right, `{in: part, local: ...}` for part coordinates)
CM marker, inertia tensor	`position` (center of mass), `orientation` (principal axes), `inertia` (principal moments)
Revolute, Translational, Cylindrical, Spherical, Fixed	joints `revolute`, `prismatic`, `cylindrical`, `spherical`, `fixed`
Planar, JPRIMs (inline, inplane, orientation, parallel axes, perpendicular)	`generic` with the directions to fix
Motion (displacement, angle)	`drivers` on a measure
Spring-damper (translational, rotational)	`spring_damper`, `torsion_spring`
Bushing	`bushing`
Single-component force / torque (SFORCE), VFORCE	`expression_force`, `expression_torque`
DX, DZ, VZ, AZ, DM, … in expressions	`measures`, named and used by name
STEP, IF, AKISPL, CUBSPL	`step()`, `if_else()`, `splines` (`akima`)
Design variables, parametric expressions	`parameters` and `=formulas` in any field
Tire (MF-Tyre / PAC), road (RDF, CRG)	`tire` (a built-in Magic Formula tire; `.tir` data sets through extensions), analytic road or OpenCRG
Requests, measures for post-processing	result channels, formula channels in the viewer
Accelerometer requests (ACCX, ACCY, ACCZ, WDTX …)	`sensors`: acceleration, specific force, angular velocity and acceleration at a point
Dynamic simulation, HHT / GSTIFF	`solver`: HHT on a Lie group, fixed step
Linear analysis (eigenvalues)	Eigenmodes and root locus in the viewer, `modes` / `locus` for the assistant
Design evaluation, DOE (one factor)	the fan (≋) in the viewer, `sweep` for the assistant
Adams PostProcessor	the viewer: plots, x-y, FFT, cursors, run comparison, Δ plots, report

What is different¶

Units are SI, always. An Adams model in mm, kg, N, s needs its lengths divided by 1000 and its stiffnesses converted; the checks warn when a model looks like mm.
No markers. Where Adams attaches a joint to marker I on part 1 and marker J on part 2, FreeDyn gives the joint one global location and orientation at t = 0; both bodies carry it from there. The same for springs (two global points) and forces.
Orientations are quaternions, not Euler angles (body 313 etc.): no singular angles. A rotation by φ about an axis n is [cos(φ/2), n sin(φ/2)]; formulas make it readable ([=cos(phi/2), 0, 0, =sin(phi/2)]).
The model is a file. Diff it, review it, keep it in git, let an assistant change it – and see every change in the viewer the moment it is saved.
Inertia as principal moments. Give the principal moments and turn the body frame to the principal axes (eigen-decomposition of the Adams inertia tensor).

Not (yet) in freedyn2¶

Flexible bodies, contacts (other than tires), couplers and gears, universal and constant-velocity joints, friction in joints, static and kinematic analyses (start from rest and let it settle, or prescribe the motion with drivers), co-simulation interfaces. An importer for Adams .adm (and .cmd with design variables) is planned; until then a model is quickest rebuilt by an assistant from the Adams listing.

A first translation¶

An Adams pendulum – a part on a revolute joint to ground, its I and J markers at the pivot with z along global y – becomes:

model: pendulum
gravity: [0, 0, -9.81]
bodies:
  link:                                  # PART/2, CM at (500, 0, 0) mm
    mass: 1.0
    inertia: [0.0008, 0.0834, 0.0834]    # IXX, IYY, IZZ about the CM, principal axes = global
    position: [0.5, 0, 0]
joints:
  pivot:                                 # JOINT/1, REVOLUTE, I = 2.3, J = 1.1
    type: revolute
    body1: link
    location: [0, 0, 0]
    orientation: [0.7071068, -0.7071068, 0, 0]   # z of the markers along global y
solver: {type: HHT, end_time: 2.0, step: 0.001}