Depth control

description of the file depth_controller.py

The class Depth_Control implements a PID controller in order to reach the depth desired.

ROS

Node

depth_controller

Topics

ROS topics subscribed	Message	Description
/BlueRov2/bar30	bluerov_ros_playground/Bar30	Absolute pressure in hPa
/Settings/set_depth	bluerov_ros_playground/Set_depth	Settings for the controller
/Settings/set_target	bluerov_ros_playground/Set_target	Depth to reach, vertical axis goes up

In this script, the axis Z for the depth goes up. To reach 1m depth the input must be -1

ROS topics published	Message	Description
/Command/depth	Uint16	Pwm send by the controller to the commander

Depth estimation

To estimate current depth the controller gets the absolute pressure from the Bar30 sensor and computes it with hydrostatic equation.

$$depth = -(p - p0) / (ρ * g)$$

Designation	Meaning	Value
depth	current depth	m
p	pressure measured	Pa
p0	surface pressure	99 000Pa
ρ	water density	1000 kg/m³
g	gravitational acceleration	9.81 kg/m²

PID controller

The PID controller uses the difference between depth calculated and depth desired, but also the derivative and the integrate values to compute a command in the interval [-400,+400]. This command is added to 1500, the pwm neutral of thrusters.

Saturation

A saturation method is added on pwm output before the publication on the command topic in case of a will to limit thruster power.

Send depth target

The depth target is defined by the publication of a Set_target message on the topic /Settings/set_target. This message is shared with the heading and the velocity controller to set their target

Several methods exist to publish the message : the GUI, command line, script, ....

Set_target message :		Units
float64	depth_desired	m
float64	heading_desired	deg
float64	velocity_desired	pwm range[-400,400]

In this script, the axis Z for the depth goes up. To reach 1m depth the input must be -1

Tune PID and set saturation

PID coefficients and maximum pwm for saturation of the command can be modified in real time by the Set_depth message published on the topic /Settings/set_depth.

These values send are not saved and the controller will get its initial value. To change permanently the values of those parameters you must change it in the code, in the __init__ method.

Set_depth message
bool	enable_depth_ctrl
uint16	pwm_max
uint32	KI
uint32	KP
uint32	KD

See Commander to know how enable_depth_ctrl is used