vmplib (VEX Motion Profiling Library)

A C++17 library for 2D motion profiling on cubic Bézier paths, aimed at VEX robotics. Give it a path and a drivetrain, get back a time-parameterised trajectory: poses, linear velocity and angular velocity, one sample every dt.

It implements the methods described in

2D Motion Profiling for Competitive Robotics Read the PDF

Profiles are generated offline which can then be exported to your realtime code you run on your brain.

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What it does

• Cubic Bézier geometry. Position, derivatives, parametric speed and signed curvature anywhere on a segment.
• Arc-length parameterisation. 5-point Gauss–Legendre quadrature over composite panels for s(t), and a Newton–Raphson inverse for t(s).
• Two-pass velocity profiling. The velocity ceiling — top speed, the curvature limit , and any keyframes — is sampled up front, then swept backward.
• Keyframes. Pin a target speed to a point on the field. The point is projected onto the curve in 2D, and speeds between keyframes are interpolated in v², which makes each interval a constant-acceleration segment.
• Multi-segment paths. Velocity and the timestep grid stay continuous across segment joins; a step that overshoots the end of one segment carries its leftover arc length into the next.
• RAMSETE simulation. Every trajectory also comes back with what a RAMSETE follower actually achieves tracking it, which is how you tell whether a plan is trackable at all.

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Build

Needs GNU Make and a C++17 compiler (g++ or clang++).

git clone https://github.com/SerrialError/vmplib.git
cd vmplib
make            # builds bin/main
make test       # builds and runs the doctest suite
make clean

With devenv and direnv:

direnv allow    # auto-loads the shell on cd, or run `devenv shell`
build           # == make all
run --file examples/path-points.txt
clean           # == make clean

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Command-line use

./bin/main --file examples/path-points.txt

Flag	Default	Meaning
--file <path>	required	Path file to profile
--out <path>	output.txt	Where to write the result
--format desmos|code	desmos	Output style

--format desmos emits six lists you can paste straight into Desmos:

Label	Contents
X	planned poses, as (x, y)
L	planned linear velocity, as (t, v)
A	planned angular velocity, as (t, ω)
X_r, L_r, A_r	the same three for the RAMSETE-followed trajectory

--format code emits P and V as C++ initialiser lists, for pasting into robot code that replays a fixed trajectory.

Path file format

Plain text, one block per segment. This is the export format of path.jerryio, so a file saved from there works unmodified.

#PATH-START Path
#POINTS-START
-0.586, -0.410          <- four control points per cubic Bezier segment
-0.586, -0.201
-0.997,  0.335
-0.997,  0.544
#VELOCITIES-START
-0.700,  0.100, 0.3     <- x, y, target speed (m/s)
#PATH.JERRYIO-DATA {...}

#VELOCITIES-START may be empty. Each keyframe is an (x, y) point on the field plus the speed you want there; the point is projected onto the curve rather than matched by x alone, so paths that double back work correctly.

Units are default SI units throughout.

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Tuning

ProfileConfig defaults to the robot this project was originally tuned against. Override the fields rather than editing the source.

Field	Default
maxVelocity	1.8885 m/s
maxAccel	4.1220 m/s²
trackWidth	0.2951 m
ramseteB	2.0 m⁻²
ramseteZeta	0.7
dt	0.01 s

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Future plans

• CLI for path import/export
• Tighter integration with path.jerryio and other route planners

Contributions, issues, and pull requests are welcome.