FPGA Event-Based Drone Collision Avoidance

Real-time obstacle detection and evasion for drones using event cameras,
normal flow estimation, and FPGA-accelerated inference.

[VecKM Paper (ICCV 2025)]    [Bonazzi et al. (CVPRW 2025)]    [VecKM C++ Reference]

Abstract

Event cameras capture per-pixel brightness changes asynchronously at microsecond resolution, making them ideal for high-speed drone navigation. This codebase combines a vectorized kernel mixture (VecKM) normal flow estimator with a production-grade FPGA + ARM pipeline to detect looming objects, predict time-to-collision, and execute graded evasion maneuvers — all in real time.

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Interactive Figures 🖱️

Self-contained HTML figures with SVG vector graphics, hover tooltips, zoom/pan, and responsive layout. Click any image to open the interactive version — scroll to zoom, drag to pan, hover for details.

Fig. 1 — System Architecture

Dual-pipeline collision avoidance: Event Camera → FPGA Fabric (AER interface, ring buffer, spatial hash, systolic encoder) → ARM CPU (VecKM flow ~100Hz + CNN ~1kHz) → Motors

Fig. 2 — VecKM Normal Flow Estimation

3-step pipeline: k-NN event neighborhood → vectorized kernel mixture encoding (d=128) → MLP flow regression. Simulated diverging flow field with looming object collision cone.

Fig. 3 — Five-Level Graded Evasion Response

Graded danger response (NONE → CAUTION → WARNING → CRITICAL → EMERGENCY) with hysteresis state machine and recovery thresholds.

Fig. 4 — FPGA Pipeline Timing & Resources

Stage latency breakdown (~297 μs total, 100MHz), XCZU9EG resource utilization (DSP/BRAM/LUT/FF), dataflow architecture, throughput analysis.

Fig. 5 — TTC & Collision Prediction

Lee's τ time-to-collision (τ = R/Ṙ), TTC vs. distance curves, 4D Kalman filter tracker, safe bearing sector with evasion vector.

🖱️ Interact: Scroll = zoom | Drag = pan | Hover colored elements = annotation tooltips | Works on desktop + mobile

Generated by figures/generate_figures.py. PNG previews via figures/screenshot_figures.js.

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Normal Flow API

The underlying normal flow estimator can also be used standalone:

from models.inference import NormalFlowEstimator
from models.visualize import gen_flow_video

estimator = NormalFlowEstimator(training_set="UNION")
flow_predictions, flow_uncertainty = estimator.inference(events_t, undistorted_events_xy)
flow_predictions[flow_uncertainty > 0.3] = np.nan

gen_flow_video(events_t.numpy(), undistorted_events_xy.numpy(),
               flow_predictions.numpy(), './frames', './output.mp4', fps=30)

Variable	Description	Shape
events_t	Sorted event timestamps (seconds)	(n,) float64
undistorted_events_xy	Undistorted normalized coordinates (~[-1, 1])	(n, 2) float32
flow_predictions	Predicted normal flow (undist. normalized px/s)	(n, 2) float32
flow_uncertainty	Prediction uncertainty	(n,) float32 ≥ 0

Training sets: "UNION" (default, recommended), "MVSEC", "DSEC", "EVIMO".

Undistorted Coordinates

import cv2

def get_undistorted_events_xy(raw_events_xy, K, D):
    raw_events_xy = raw_events_xy.astype(np.float32)
    undistorted = cv2.undistortPoints(raw_events_xy.reshape(-1, 1, 2), K, D)
    return undistorted.reshape(-1, 2)

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Egomotion Estimation

SVM-based egomotion estimator using predicted normal flow and IMU. See ./egomotion.

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Architecture

Two complementary collision avoidance pipelines run in parallel:

┌──────────────────────────────────────────────────────────────────────────┐
│                              DRONE MAIN LOOP                             │
│                                                                          │
│  Event Camera (AER) ──────►  FPGA Fabric ──────► ARM CPU ────► Motors    │
│       │                        │                      │                  │
│       │                     ┌──┴────────────┐   ┌─────┴──────────┐       │
│       │                     │    encoder    │   │ collision_pred │       │
│       │                     │  spatial_hash │   │  evasion_ctrl  │       │
│       │                     │   ring_buf    │   │  safety_wdog   │       │
│       │                     │ normalization │   │  PWM output    │       │
│       │                     └───────────────┘   └────────────────┘       │
│       │                                                                  │
│       │  PIPELINE 1: Flow-Based (VecKM, ~100Hz)                          │
│       │  ┌──────────────┐   ┌──────────────┐     ┌───────────────────┐   │
│       │  │ ObjectDetect  │──►│ CollisionPred │──►│ EvasionController │   │
│       │  │ (VecKM flow+  │   │ (TTC+threats) │   │ (graded velocity) │   │
│       │  │  clustering)  │   └──────────────┘    └───────────────────┘   │
│       │  └──────────────┘                                                │
│       │                                                                  │
│       │  PIPELINE 2: Direct CNN ( 2025, ~1kHz)                           │
│       │  ┌───────────────┐   ┌───────────────────┐                       │
│       │  │ EventFrameAgg │──►│ DirectActionPred  │                       │
│       │  │ (80x80 @1kHz) │   │ (DPU CNN 5-class) │                       │
│       │  └───────────────┘   └───────────────────┘                       │
│       │                                                                  │
│       │  AUXILIARY MODULES                                               │
│       │  ┌──────────────────┐ ┌────────────────┐ ┌────────────────────┐  │
│       │  │ContrastMaximizer │ │ DepthEstimator │ │ DenseTTCEstimator  │  │
│       │  │(CMax-SLAM, 2018) │ │ (E2Depth, 2020)│ │ (EVReflex, 2021)   │  │
│       │  └──────────────────┘ └────────────────┘ └────────────────────┘  │
│       │                                                                  │
└──────────────────────────────────────────────────────────────────────────┘

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Quick Start

# Install
git clone https://github.com/Enotrium/FPGA-Event-Based-encode
cd FPGA-Event-Based-encode
make install

# Convert pretrained weights (d=384 → d=128 for FPGA)
make convert

# Run all tests (no hardware needed)
make test

# Run end-to-end simulation with visualization
python test/test_fpga_simulator.py --visualize

# Run hardware-in-the-loop simulation (no physical drone needed)
make test-hil

# Run the drone demo
cd demo && python main.py

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Directory Layout

Directory	Purpose
fpga/	HLS/C++ FPGA modules: AER interface, ring buffer, normalization, spatial hash k-NN, systolic encoder array, PWM output, top-level pipeline, testbench
arm/	ARM C++ controller: collision predictor (TTC + clustering), evasion controller (potential fields + hysteresis), safety watchdog (RC failsafe, NaN guard, altitude ceiling), Kalman filter object tracker, MAVLink v2 PX4 bridge, main control loop
drone/	Python drone control package: dual-pipeline controller, VecKM object detector, collision predictor, evasion controller, dense TTC estimator, event frame aggregator, direct CNN action predictor, contrast maximizer, monocular depth estimator
models/	VecKM normal flow estimator: local geometry encoder, feature transform, inference API with ring buffer + FPGA mode, model parameters
train/	Training pipeline: dataset loaders, model definition, training loop, inference, visualization, FPGA weight conversion, d=128 training config
test/	Test suite: Python FPGA pipeline simulator (5 scenarios), ARM C++ collision prediction + evasion controller unit tests, golden model equivalence checks (FPGA ↔ Python), hardware-in-the-loop simulation (PX4 SITL/AirSim/built-in)
demo/	Demo: event data, frames, flow visualization
egomotion/	SVM-based egomotion estimation from normal flow + IMU
.github/	CI/CD: GitHub Actions workflow (Python + ARM C++ + FPGA testbench + build verification), CODEOWNERS

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Drone Modules

Module	Description	SoTA Reference
drone_controller.py	Main integration: dual-pipeline, IMU fusion, flight controller interface	—
object_detector.py	VecKM normal flow + spatio-flow DBSCAN clustering	EVDodgeNet, ICRA 2020
collision_predictor.py	Looming-based TTC (Lee's τ), Kalman tracking, safe-zone computation	Falanga et al., Sci. Robot. 2020
evasion_controller.py	5-level graded response with hysteresis, potential-field vectors	Sanket et al., ICRA 2020
ttc_dense.py	Dense per-event TTC from flow divergence, connected-component threats	EVReflex, IROS 2021 + EV-TTC, RA-L 2025
event_frame_aggregator.py	Event-to-frame accumulation (80×80, 1ms windows), temporal stacking	Bonazzi et al., CVPRW 2025
direct_action_predictor.py	DPU-optimized CNN → 5-class evasion (STAY/LEFT/RIGHT/UP/DOWN)	Bonazzi et al., CVPRW 2025
contrast_maximizer.py	IWE-based flow refinement via hypothesis testing	Gallego et al., CVPR 2018
depth_estimator.py	Lightweight U-Net monocular depth from event frames	E2Depth, 3DV 2020

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Evasion Levels

Level	Danger	Behavior
NONE	< 0.15	Normal cruise at set speed
CAUTION	≥ 0.15	20% speed reduction, gentle lateral nudge
WARNING	≥ 0.35	50% speed reduction, strong lateral + vertical
CRITICAL	≥ 0.60	Full stop forward, aggressive dodge
EMERGENCY	≥ 0.85	Reverse thrust + maximum dodge + hard yaw

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Safety Features

• RC link failsafe: auto-disarm if no data received within 500ms
• Altitude ceiling: 120m FAA limit enforced
• NaN/Inf guard: velocity commands with NaN/Inf trigger immediate disarm
• Motor timeout: auto-disarm after 30s of hover (zero command)
• Velocity smoothing: EMA filter on all axes (α=0.3) for jerk-free flight
• Arming debounce: 2s hold-to-arm prevents accidental activation

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Production Engineering

This codebase has been hardened for production deployment with several key additions:

Component	File	Purpose
CI/CD	.github/workflows/ci.yml	Automated testing on every push: Python pipeline + ARM C++ unit tests + FPGA testbench + build verification
Build System	Makefile	Single-command interface: make test, make lint, make build, make ci, make convert, make clean
Security	SECURITY.md	10-attack-surface threat model, Zynq secure boot chain (RSA-4096/AES-256-GCM), MAVLink v2 signing, AXI memory protection, supply chain SBOM, adversarial robustness
Golden Model Tests	test/golden_model_test.py	FPGA ↔ Python equivalence verification: k-NN overlap, INT16 quantization error, ensemble consistency, complex arithmetic correctness, d=384→128 quality
HIL Simulation	test/hil_gazebo_bridge.py	Hardware-in-the-loop testing with PX4 SITL + Gazebo, AirSim, or built-in physics; 100Hz closed-loop collision avoidance
Kalman Tracker	arm/kalman_tracker.h	4D Kalman filter (x,y,vx,vy) with Mahalanobis-distance data association and track lifecycle management
MAVLink Bridge	arm/mavlink_bridge.h	MAVLink v2 offboard velocity control for PX4/ArduPilot with heartbeat, arming, and telemetry parsing
FPGA Synthesis	fpga/build.tcl	Vitis HLS synthesis script for XCZU9EG with csim/synth/cosim/export targets and full optimization directives
Onboarding	CONTRIBUTING.md	Development setup, code style guide, PR checklist, constant-mirroring policy

See PRODUCTION_READINESS.md for a detailed gap analysis and resolution status.

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Testing

# Python pipeline simulation (looming, lateral, noise, multi-object, throughput)
python test/test_fpga_simulator.py --visualize

# ARM C++ unit tests (collision predictor + evasion controller)
cd arm && make test && ./test_arm_cp

# HLS C-simulation (requires Vitis HLS)
cd fpga && vitis_hls -f build.tcl

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Configuration

All pipeline constants are defined in fpga/config.yaml and mirrored across:

• models/params.py — FPGAParams
• fpga/*.h — HLS compile-time constants
• arm/*.h — ARM run-time parameters

Key sections: pipeline, normalization, camera, fpga, collision_prediction, evasion_controller, safety, event_frame, direct_action, telemetry.

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Evaluated Datasets

Evaluated on MVSEC, DSEC, EVIMO, FPV, VECtor. Flow prediction videos for 42 scenes: Google Drive.

Precomputed undistorted coordinates: Google Drive.

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Train Your Own Estimator

See train/. For FPGA-optimized training (d=128):

python train/s1_train.py --params FPGAParams

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Citation

@article{yuan2024learning,
  title={Learning Normal Flow Directly From Event Neighborhoods},
  author={Yuan, Dehao and Burner, Levi and Wu, Jiayi and Liu, Minghui and
          Chen, Jingxi and Aloimonos, Yiannis and Ferm{\"u}ller, Cornelia},
  journal={arXiv preprint arXiv:2412.11284},
  year={2024}
}