sensors — Line Tension Sensing

The sensors package publishes /fishing_arm/tension as an interfaces/FishingTension message. A single node — load_cell_node — serves both hardware and simulation by switching its source parameter.

Source: software/src/sensors/sensors/load_cell_publisher.py
Python API: sensors
Config: config/sensor_params.yaml

Design principle: one node, two backends

The tension controller (control — Tension Controllers) always subscribes to /fishing_arm/tension. It never needs to know whether tension came from a physical HX711 or a MuJoCo simulation model. bringup sets the backend at launch:

source = 'sim_fts' if use_sim else 'hardware'

        flowchart TB
    LC["load_cell_node"]
    LC -->|"always"| TOP["/fishing_arm/tension<br/>FishingTension"]

    subgraph hw["source:=hardware"]
        HX["HX711 ADC<br/>read_raw_adc()"]
        CAL["calibration_offset · calibration_scale"]
        HX --> CAL --> LC
    end

    subgraph sim["source:=sim_fts"]
        TF["TF: rod_tip_link → fish_link<br/>line length L"]
        STR["Hooke's law:<br/>k(L-L0) + c dL/dt"]
        WRENCH["/tension_sensor_broadcaster/wrench<br/>(fallback)"]
        TF --> STR --> LC
        WRENCH -.->|"if TF unavailable"| LC
    end
    

Node: load_cell_node

Parameters (from sensor_params.yaml)

Parameter	Default	Purpose
calibration_offset	1245.0	HX711 zero-load ADC reading
calibration_scale	0.0023	N per ADC bit
use_moving_average	true	Enable sliding-window smoothing
window_size	10	Moving average length
publish_frequency	80.0	Timer rate (Hz)
tension_max_threshold	20.0	Log warning above this (N)
noise_floor	0.1	Zero out sub-threshold readings
frame_id	rod_tip_link	Message header frame
source	hardware	hardware or sim_fts (overridden by launch)
sim_line_stiffness	220.0	Must match MJCF tendon stiffness
sim_line_springlength	0.20	Rest length L₀ (m)
sim_line_damping	1.5	Damping on stretch rate
sim_rod_frame	rod_tip_link	TF frame at rod tip
sim_fish_frame	fish_link	TF frame at fish

Hardware path (source:=hardware)

1. read_raw_adc() reads the HX711 (currently a placeholder with simulated sine disturbance for development).

2. Apply (raw - calibration_offset) * calibration_scale → Newtons.

3. Zero out below noise_floor.

4. Optional moving average over window_size samples.

5. Publish FishingTension.

Warning

read_raw_adc() is still a stub. Replace with real SPI/serial HX711 driver before trusting hardware tension values.

Simulation path (source:=sim_fts)

Primary method — stretch model matching the MJCF tendon:

1. Lookup TF transform rod_tip_link → fish_link.

2. Compute line length L = ||translation||.

3. Stretch = max(L - sim_line_springlength, 0).

4. Tension = k * stretch + c * dL/dt (damping only on lengthening).

5. Fallback: if TF unavailable, subscribe to /tension_sensor_broadcaster/wrench and use force magnitude.

The stretch model is preferred because it matches the controller’s physical expectation and is independent of the FTS frame orientation.

ROS 2 interfaces

Publications

Topic	Type	Rate
/fishing_arm/tension	interfaces/FishingTension	publish_frequency (80 Hz)

Message fields:

• tension_newtons — measured tension (primary signal for controllers)

• target_tension_newtons — reserved (currently always 0.0; controllers use params instead)

Subscriptions (sim only)

Topic	Type	When
/tension_sensor_broadcaster/wrench	geometry_msgs/WrenchStamped	source==sim_fts, TF fallback

TF (sim only)

Uses tf2_ros.Buffer + TransformListener to compute inter-frame distance. Depends on robot_state_publisher publishing the tree that includes fish_link (sim URDF only).

Signal processing pipeline

Each timer tick (timer_callback):

raw reading → noise floor → moving average → FishingTension publish
                ↓
         threshold warning (log)

The moving average uses a fixed-length collections.deque — O(1) append and bounded memory.

Interactions

Package / node	Relationship
control — Tension Controllers	Subscribes to /fishing_arm/tension; E-stop on missing/stale data
bringup — Launch Orchestration	Sets source from use_sim; always includes sensor.launch.py
diagnostics — Recording & Bring-Up Tests	recorder and hardware_check subscribe to tension topic
description — Robot Model Assets	Defines rod_tip_link, fish_link frames; MJCF tendon params
tension_sensor_broadcaster	Sim-only ros2_control broadcaster → wrench topic
interfaces — Custom Messages	Defines FishingTension.msg

Physical layer diagrams

• Simulation chain: Simulation ROS 2 Graph (data flow section)

• Hardware chain: Hardware ROS 2 Graph (data flow section)

Launch

# Normally launched by bringup; standalone:
ros2 launch sensors sensor.launch.py source:=hardware
ros2 launch sensors sensor.launch.py source:=sim_fts