Glossary#

This page defines the terms of the documentation. Each page uses one term for one thing.

Project Terms#

ACRE: The Purdue Agronomy Center for Research and Education in Indiana. The simulator shows a surveyed tile of this farm.
ACRES: The Agricultural Control and Robotics Environment Simulator. ACRES is the name of the project.
ACRES Core: The simulator without Unreal Engine. It is a C++ library (libacres_core) with a Python module (acres_core). The short name is Core.
agent: One vehicle that a session controls. Agent 0 is the first vehicle of the session.
agents file: The JSON file of the option -Agents=. It has one object for each agent.
bag: A ROS 2 recording of topics. The ROS 2 tools write it in the MCAP format.
barrier: A marker that the simulator sends on each data channel at the end of a lockstep step.
batch: A set of ACRES Core environments that one call steps together.
calibration: The procedure that fits the parameters of a model to recorded data of the real vehicle or the real farm.
command replay: A replay that applies the recorded controls of a session log to the vehicle.
command row: The 18 drive-by-wire values of one environment for one control step of a batch.
control step: One call of the step function of a batch. The default length is 12 physics steps, 0.1 s.
Core server: The program core_sim of the package acres_core_sim. It gives ACRES Core the three sockets of the game.
crop patch: A block of plants of 1.52 m by 1 m. The crop model keeps the state of each crop patch.
DDS loopback fence: The environment settings that keep all ROS 2 traffic of the workstation on the address 127.0.0.1.
draft: The force of the soil on an implement against the direction of travel. The sensed draft is the horizontal force in the lower links of the hitch.
drive-by-wire: The Dataspeed system that controls the steering, the throttle, the brake and the gear of the Polaris with commands. The short name is DBW.
drive script: The JSON file of the option -DriveScript=. It contains timed controls, a path and camera movements.
driveline solve: The one implicit solution of all wheel speeds, the clutch torque and the engine speed in a physics step of the Maxxum.
editor: The Unreal Editor.
energy ledger: The account of the fuel energy for each physics step: where the energy goes.
engine-free model: A model in C++ that uses no Unreal types. The game and ACRES Core compile the same source file.
environment: One Polaris of a batch with its own farm marks and soil-water scenario.
episode: The time between two resets of the simulation. The sensor recorder also divides a recording into episodes.
episode clock: The message /sim/episode with the episode number, the physics step and the simulation state.
episode log: An MCAP file with the states of all agents at the physics rate, the farm events and the conditions.
episode replay: A replay that shows an episode log in the game. The vehicles follow the logged states.
equipment rig: The skeleton and the controls of a vehicle mesh or an implement mesh. The physics state moves the equipment rig.
farm event: One change of a crop or of the soil in the episode log.
farm stamp: One contact of a tyre or a vehicle body with the farm in the episode log.
farm traffic: The tractors, pickups, highway cars and workers that the game controls.
field: One of the 59 numbered crop fields of the tile, for example F39.
field-day kit: The set of test scripts that the lab runs on the real Polaris.
field setup: The folder with field-setup.json and terrain-edits.f32. It changes the crop, the soil and the terrain of single fields for a session.
free run: The mode in which the simulation follows the wall clock. It is the opposite of lockstep.
game: The Unreal application of ACRES. The project name in Unreal is Acres.
ground layer: The polygons of surface_polygons.json: roads, lanes, yards and verges with their surface class.
hardware shift: A set of offsets on the actuators and the sensors of one agent. It simulates a vehicle that differs from the model.
harvest cutter: The 3 m cutter with a bunker of the option -FarmHarvester. The short name is cutter.
hitch: The three-point hitch at the rear of the Maxxum. It carries a mounted implement.
hitch mode: The control mode of the hitch: position, draft or float.
implement: A machine that the Maxxum carries or pulls, for example a chisel plow.
implement model: The engine-free model of the eight implements of the Maxxum.
legacy tractor: The tractor model before the Maxxum equipment rig. The option -VehicleModel=legacy selects it for regression runs.
lockstep: The mode in which the simulation advances only when a client requests steps.
Maxxum: The Case IH Maxxum 150 tractor of the simulator.
menu: The front-end menu of the game. It edits the configuration of a session.
option: A command-line argument of the game. A switch has no value. A value option has the form -Name=value.
packaged game: The built executable of the game in the folder Packaged.
path replay: A replay in which the vehicle follows a recorded or planned path with a speed profile.
physics step: One step of the vehicle physics. The length is 1/120 s.
Polaris: The Polaris Ranger CREW XP 1000 utility vehicle of the Purdue lab, with the Dataspeed drive-by-wire.
preset: A set of menu values that the menu saved in Saved/Configs.
PTO: The power take-off of the Maxxum. It drives the machinery of an implement.
rate factor: The simulated time divided by the wall time in a free run of the Core server.
render-later: The method that records an episode log without images and makes the images later with an episode replay.
render tier: The quality level of the main view. The game selects it from the memory of the GPU.
rig map: The file Simulation/rigs/<id>.json that the equipment rig evaluates. A rig control is one named input of the rig map.
ROS 2 bridge: The node sim_bridge of the package acres_sim. It connects the local sockets of a simulator to ROS 2.
sensor overlay: A sensor file that the game puts on sensors.json for one vehicle, for example sensors_polaris.json.
sensor profile: The render state that the game pins while a sensor records. The profile calibrated is the validated profile. The low render tier uses the profile reduced-6gb. The source code names it the sensor render profile.
sensor rig: The set of sensors of a vehicle with their mounting poses.
sensor stream: The binary socket that sends the sensor data of a vehicle to the ROS 2 bridge. The option is -SensorStream=.
session: One run of the simulation, from the start to the end of the simulation.
session folder: The folder that contains the configuration and the logs of one session.
session log: The files that the game writes into the session folder during a session.
simulator control channel: The JSON socket that controls the simulation: pause, steps, resets, conditions and recording. The option is -SimControl=.
soil class: One of the seven texture classes of the soil library.
soil library: The table of soil classes with their mechanical parameters in AcresSoilModel.cpp.
soil unit: One SSURGO map unit with its hydraulic values. The soil-water model uses it.
steering centre: The steering wheel angle at which the Polaris moves straight.
stop hold: The brake pressure that the ULC applies when the Polaris does not move.
surface class: The class of the ground at a point: field, asphalt, concrete, gravel, grass lane, grass or dirt.
surface map: The function that gives the surface class at a point from the ground layer (AcresSurfaceMap).
surface template: One entry of the block surfaces in tractor.json. It gives the tyre parameters of a surface.
survey grid: The grid of the ACRE survey. One cell is 1.524 m (5 ft).
test stand: A program that runs a vehicle model without the game on a simple chassis. The model tests use it.
tile: The surveyed area of ACRE that the simulator contains.
ULC: The Universal Lat/Lon Controller of the Dataspeed drive-by-wire. It controls the speed of the Polaris.
vehicle: The Maxxum or the Polaris.
vehicle bridge: The JSON socket that sends commands to one vehicle and returns its reports. The option is -RlPort=. The source code names it the RL bridge.
vehicle PC: The computer on the real Polaris. ACRES only reads files from it.
watchdog: The command timeout of the drive-by-wire. A subsystem disengages when it gets no command for 0.1 s.
water grid: The grid of 4 m cells of the soil-water model.
weather replay file: An environment file with the weather of one real day (Calibration/Weather/Replay/env_<date>.json).
wetness index: A scale of the soil water: 0 at the wilting point, 1 at field capacity and 2 at saturation.
work panels: The HUD panels of the implement and the energy that the key I shows.
working depth: The depth of the tool points of an implement below the ground.
world frame: The ENU frame of the simulator in metres: x east, y north, z up. Its origin is the centre of the tile.

Abbreviations#

Abbreviation	Meaning
CAN	Controller Area Network
CDR	Common Data Representation, the wire format of ROS 2 messages
CVT	Continuously variable transmission
DBW	Drive-by-wire
DDS	Data Distribution Service, the middleware of ROS 2
ENU	East, north, up
FLU	Forward, left, up
GNSS	Global navigation satellite system
IMU	Inertial measurement unit
INS	Inertial navigation system
LiDAR	Light detection and ranging
MCAP	The container file format of ROS 2 bags and of the episode log
MFD	Mechanical front drive, the front-wheel drive of the Maxxum
PGN	Parameter group number of a J1939 message
PTO	Power take-off
QoS	Quality of service of a ROS 2 topic
RTK	Real-time kinematic positioning
SSURGO	The Soil Survey Geographic Database of the USDA
UTM	Universal Transverse Mercator