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Overview

The MIN parameter group is a collection of minimum safety and operational thresholds for various subsystems.

Key Concepts

1. Terrain Following (MIN_CM)

(ArduPlane) Defines the minimum clearance altitude (in centimeters) the autopilot will maintain above the ground when using Terrain Following (TERRAIN) logic.

2. Tracker Reversal (MIN_REVERSE_TIME)

(Antenna Tracker) For trackers that don't have continuous 360-degree rotation, they must occasionally "unwind." This parameter sets the minimum time (seconds) to continue the unwinding motion once a limit is hit, preventing rapid oscillation at the end-stop.

Developer Notes

  • Context: MIN_CM is associated with AP_Terrain. MIN_REVERSE_TIME is specific to AntennaTracker.

MIN_CM

cm
Default 20

MIN_CM: Rangefinder minimum distance

Description

Minimum distance in centimeters that the rangefinder can reliably read.

Values

  • Units: cm
  • Increment: 1
  • Default: 20

Description

This parameter defines the "blind spot" of the rangefinder.

  • Function: Any reading returned by the sensor that is less than this value will be treated as invalid (out of range).
  • Why it matters: Most acoustic (Sonar) and optical (Lidar) rangefinders cannot measure distance zero. They have a minimum range (e.g., 20cm). If the vehicle is on the ground and the sensor reads noise below this threshold, filtering out these values prevents the autopilot from thinking it's flying underground or getting bad altitude data.

Source Code

ardupilot/libraries/AP_RangeFinder/AP_RangeFinder_Params.cpp

View Source

MIN_REVERSE_TIME

s
Default 1
Range 0 20

Minimum time to apply a yaw reversal (MIN_REVERSE_TIME)

Description

This parameter is specific to Antenna Trackers with mechanical stops (non-continuous rotation). When the tracker hits its yaw limit (e.g., 360 degrees), it must stop and reverse direction ("unwind") to re-acquire the target. This parameter forces the unwinding motion to continue for a minimum duration, preventing the tracker from getting stuck toggling back and forth at the limit.

The Mathematics

Hysteresis Timer:
$$ \text{If } (\text{Limit Hit}) \implies \text{Reverse State} = \text{True} $$
$$ \text{If } (\text{Time in Reverse} < \text{MIN\_REVERSE\_TIME}) \implies \text{Keep Reversing} $$

The Engineer's View

Defined in AntennaTracker/Parameters.cpp. Handles the "Unwrap" logic for servos.

  • Lag Compensation: Mechanical systems have inertia and lag; this timer ensures the movement is significant enough to clear the stop.

Tuning & Behavior

  • Default Value: 1 s
  • Increase: If the tracker "chatters" or oscillates at the end-stop.
View Source
Moving Baseline Configuration (MB) Mission Sequencing Configuration (MIS)