Pylinkage

Pylinkage is a comprehensive Python library for planar linkage mechanisms. It provides tools to:

• Define linkages using joints (Crank, Revolute, Linear, etc.)
• Simulate kinematic motion with high-performance numba-compiled solvers
• Optimize geometry using Particle Swarm Optimization (PSO)
• Synthesize linkages from motion requirements (Burmester theory, Freudenstein's equation)
• Analyze symbolically using SymPy for closed-form expressions
• Visualize with multiple backends (Matplotlib, Plotly, SVG)

📚 Full Documentation — Complete tutorials, API reference, and examples.

Related Projects

• pylinkage-editor — Visual linkage design tool with an easy-to-use interface. Draw mechanisms interactively, run synthesis from the GUI, and export results.
• leggedsnake — Dynamic walking simulation built on pylinkage. Adds pymunk physics, genetic algorithm optimization, and walking-specific fitness evaluation.

Installation

pip install pylinkage            # Core only (~35 MB): define, simulate, and build linkages
pip install pylinkage[full]      # Everything (~400 MB): all optional backends included

Install only what you need:

Extra	What it adds
numba	JIT-compiled solvers (1.5-2.5M steps/sec)
scipy	Differential evolution optimizer, synthesis solvers
pso	Particle Swarm Optimization via pyswarms
symbolic	SymPy-based closed-form expressions and gradient optimization
viz	Matplotlib visualization and animation
plotly	Interactive HTML visualization
svg	Publication-quality SVG export via drawsvg

Extras can be combined: pip install pylinkage[viz,scipy,pso]

For development:

git clone https://github.com/HugoFara/pylinkage.git
cd pylinkage
uv sync  # or pip install -e ".[full,dev]"

Quick Start

Define and Visualize a Four-Bar Linkage

Using the component-based API (recommended). Visualization requires pip install pylinkage[viz].

from pylinkage.components import Ground
from pylinkage.actuators import Crank
from pylinkage.dyads import RRRDyad
from pylinkage.simulation import Linkage
from pylinkage.visualizer import show_linkage  # requires viz extra

# Define ground pivots
O1 = Ground(0, 0, name="O1")
O2 = Ground(3, 0, name="O2")

# Create crank (motor-driven input)
crank = Crank(anchor=O1, radius=1.0, angular_velocity=0.31, name="crank")

# Create rocker via RRR dyad (circle-circle intersection)
rocker = RRRDyad(
    anchor1=crank.output,
    anchor2=O2,
    distance1=3.0,
    distance2=1.0,
    name="rocker"
)

my_linkage = Linkage([O1, O2, crank, rocker], name="Four-Bar")
show_linkage(my_linkage)

Alternative: Links-First Builder

For a more mechanical engineering-oriented approach, use MechanismBuilder to define links with their lengths first, then connect them:

from pylinkage.mechanism import MechanismBuilder

# Define links by their lengths, then connect with joints
mechanism = (
    MechanismBuilder("four-bar")
    .add_ground_link("ground", ports={"O1": (0, 0), "O2": (4, 0)})
    .add_driver_link("crank", length=1.0, motor_port="O1", omega=0.1)
    .add_link("coupler", length=3.5)
    .add_link("rocker", length=3.0)
    .connect("crank.tip", "coupler.0")
    .connect("coupler.1", "rocker.0")
    .connect("rocker.1", "ground.O2")
    .build()
)

# Joint positions are computed automatically from link lengths
for positions in mechanism.step():
    print(positions)

Synthesize a Linkage from Requirements

Requires pip install pylinkage[scipy]. Design a four-bar where the coupler passes through specific points:

from pylinkage.synthesis import path_generation

# Find linkages where coupler traces through these points
points = [(0, 1), (1, 2), (2, 1.5), (3, 0)]
result = path_generation(points)

for linkage in result.solutions:
    pl.show_linkage(linkage)

Optimize with PSO

Requires pip install pylinkage[pso].

@pl.kinematic_minimization
def fitness(loci, **_):
    # Define your objective based on joint trajectories
    tip_locus = tuple(x[-1] for x in loci)
    return pl.bounding_box(tip_locus)[0]  # Minimize min_y

bounds = pl.generate_bounds(my_linkage.get_num_constraints())
score, position, coords = pl.particle_swarm_optimization(
    eval_func=fitness, linkage=my_linkage, bounds=bounds, order_relation=min
)[0]

Symbolic Analysis

Requires pip install pylinkage[symbolic]. Get closed-form trajectory expressions:

from pylinkage.symbolic import fourbar_symbolic, compute_trajectory_numeric
import numpy as np

linkage = fourbar_symbolic(ground_length=4, crank_length=1, coupler_length=3, rocker_length=3)
params = {"L1": 1.0, "L2": 3.0, "L3": 3.0}
trajectories = compute_trajectory_numeric(linkage, params, np.linspace(0, 2*np.pi, 100))

Features Overview

Module	Purpose	Extras needed
pylinkage.components	Base components: Ground, Component	—
pylinkage.actuators	Motor drivers: Crank, LinearActuator	—
pylinkage.dyads	Assur groups: RRRDyad, RRPDyad, FixedDyad	—
pylinkage.simulation	Linkage class for simulation via step() / step_fast()	—
pylinkage.mechanism	Low-level Links+Joints model and MechanismBuilder	—
pylinkage.assur	Assur group decomposition and graph representation	—
pylinkage.hypergraph	Hierarchical component-based linkage definition	—
pylinkage.solver	High-performance numba-compiled simulation backend	numba
pylinkage.optimization	PSO, differential evolution, grid search	pso, scipy
pylinkage.synthesis	Classical synthesis: function/path/motion generation	scipy
pylinkage.symbolic	SymPy-based symbolic computation and gradient optimization	symbolic
pylinkage.visualizer	Matplotlib, Plotly, and SVG visualization backends	viz, plotly, svg

Architecture

Level 0: Geometry       → Pure math primitives (numba-accelerated when installed)
Level 1: Solver         → Assur group solvers (numba-accelerated when installed)
Level 2: Hypergraph     → Abstract graph structures for linkage topology
Level 3: Assur          → Formal kinematic theory (DyadRRR, DyadRRP)
Level 4: User API       → Joint classes + Linkage orchestration
Level 5: Applications   → Optimization, Synthesis, Symbolic, Visualization

Performance: With the numba extra, step_fast() achieves 1.5-2.5M steps/sec (4-7x faster than step()). Without numba, the same code runs in pure Python/NumPy.

Requirements

• Python ≥ 3.10
• Core: numpy, tqdm
• Optional (via extras): numba, scipy, sympy, pyswarms, matplotlib, plotly, drawsvg

Contributing

Contributions welcome! Please see CONTRIBUTING.md and respect the CODE_OF_CONDUCT.md.