hp-sim5 simulates the physics of Hangprinter. Lines, spools, motors, and firmware are all simulated in great detail.

Simulation makes experimentation fast and cheap. Structured experiments enable us to improve Hangprinter.

Example usecases:

• Automate and Validate Hardware Design
• Digital Twin
• Automate and Validate Software Design

Try the live apps:

• A family of Slideprinters (2d Hangprinters) is currently deployed here: hp-sim.
• A flipper game that tests the Cable Physics engine here: flipper.

hp-sim5 includes a Cable Joints library and XPBD physics engine inspired and coded from the work of Matthias Müller.

hp-sim5 includes two implementations of both the physics engine and its Cable Joints library; one JavaScript and one in Python. They are equivalent so for simplicity we call sometimes refer to them as "the physics engine".

The physics engine is the heart of hp-sim5, and lives in the src directory. For a deeper dive into the physics engine and the flipper demo see README_adv.md.

hp-sim5 also includes lots of other code (subrepos, or "sub-projects") to make the best use of the simulation within the Hangprinter Project:

• klipper (fork)
• ReprapFirmware (fork)
• flex-compensation-dev
• forward-transform-dev
• autocal

hp-sim5 has added quite a bit to each of its sub-projects:

• Has flex compensation from flex-compensation-dev
• Has forward transform from forward-transform-dev
• Much improved support for Hangprinter and other cable robots. On par with ReprapFirmware (except the closed-loop parts, such as autocal and force/torque mode).
• Learn more in this Klipper pull request: klipper/pull/7093

• Has flex compensation from flex-compensation-dev
• Has forward transform from forward-transform-dev
• Has host control mode support similar to Klipper.
• Learn more in RRF/README.md.

• Our own repo solely devoted to developing flex compensation for all cable robots.
• Provides two algorithms for flex compensation, called QP and Tikhonov. Both are implemented in our ReprapFirmware and Klipper forks.
• Supports any anchor configuration.
• Learn more in flex-compensation-dev/hangprinter-flex-compensation/README.md.

• Our own repo devoted to developing forward transforms for all cable robots.
• Provides state-of-the-art forward transform for a wide range of cable robots.
• Tested on simulated data for five setups called Slideprinter, Hangprinter v3, Hangprinter v4, CubeCorners, and SkyCam.
• Implements three approaches:
  • "Pott", based on "On the forward kinematics of cable-driven parallel robots", Pott & Schmidt (2015).
  • "Nice", based on "Kinematics and statics of cable-driven parallel robots by interval-analysis-based methods", Berti (2015)
  • "Quadratic", based on "Fast and Reliable Iterative Cable-Driven Parallel Robot Forward Kinematics: A Quadratic Approximation Approach", by Mahnke & Caverly (2025)
• The quadratic approach is generally best, and was chosen for ReprapFirmware and Klipper.
• Learn more in forward-transform-dev/hangprinter-forward-transform/README.md.

• Our own directory for developing automatic calibration.
• Provides a uniquely user friendly way to find anchor positions using torque/force mode + encoders only.
• Very high level of abstraction, enabled by
  • Self-tuning of torques/forces to use during calibration
  • Self-calibrating encoder noise
  • Feature based optimizations, exploiting geometric patterns rather than collecting random samples
  • Active learning, actively searching for the best places to collect the next data
  • Outlier-robust filtering on two levels. If a few data points are bad, they will be detected and discarded thanks to its GNC-IRLS style pointwise loss.
• As a result, it will try very hard to find the anchors, without requiring any human guidance.
• Compatible with all cable driven robots, real and sim. (Currently only been tested on simulated Slideprinters).
• Learn more in autocal/README.md and autocal/README_elliptical_feature_calibration.md.

1. Install Node.js
2. In this repository run:

   npm install        # only needed the first time
   npx vite           # Needed every time to serve the html and js

3. Open http://localhost:5173/hp-sim5/hp-sim in your browser. There's also http://localhost:5173/hp-sim5/flipper for the flipper demo.
4. Hack away!

These commands cover hp-sim5-specific checks. Subrepos (like RRF, klipper, autocal, etc.) often have their own internal test suites; run those from within each subrepo when you need deeper coverage.

npx test
python -m pytest

Also check out scripts/run_ci_tests.sh.

python -m pytest autocal/tests
python -m pytest autocal/tests -k active_calibrate
python -m pytest autocal/tests/test_active_calibrate_golden.py

tests/run_all_rrf_http_endpoint_tests.sh

cmake --build RRF/build --target rrf_simulator -j
node autocal/control/tests/e2e/collect_single_sweep.e2e.test.mjs --sim
node autocal/control/tests/e2e/find_minimum_moving_force.e2e.test.mjs --sim
node autocal/control/tests/e2e/find_edge_force.e2e.test.mjs --sim
node autocal/control/tests/e2e/wait_for_stable_encoders.e2e.test.mjs --sim
node autocal/control/tests/e2e/return_to_origin_one_at_a_time.e2e.test.mjs --sim
node autocal/control/tests/e2e/return_to_origin_all_at_once.e2e.test.mjs --sim
node autocal/control/tests/e2e/calibrate_encoder_noise.e2e.test.mjs --sim

npx vite
# open tests/html/*.html in the browser for cable_joints 2D/3D visual tests

RRF/tests/run_draw_squares_determinism_test.sh
RRF/tests/run_logo_determinism_test.sh
RRF/tests/run_logo_slideprinter_determinism_test.sh

hp-sim5 is part of an effort to automate the Hangprinter Project. We want to automate everything except the actual users, and digitize everything except the finished working machines and their output.