Trawl door with controlled lift force

Trawl door whose lift and weight are set by controllers, to find the door properties that give a desired spread and bottom contact.

Collaboration diagram for Trawl door with controlled lift force:

Author

Karl-Johan Reite

The door is a point mass at the position state, acted on by the external forces (warp and bridles), a seafloor contact force, and a control force:

• the lift, sideways and outboard, from a PID controller on the spread error,
• the drag, aft (-x), the size of the lift divided by LiftDragRatio (a negative lift, inboard, also drags),
• the weight, down (+z), from a PID controller on the bottom contact force error.

The door is towed along +x: the drag and the lift do not follow the direction of travel. The integrals of both controllers are states. The mass is WeightMax / 9.81. The seafloor contact is environment::SeafloorForcesInterface::CoulombNonDirectionalForce on a sphere of diameter Size, with friction coefficient 0.6, vertical stiffness 10000 N/m and full Coulomb friction from 0.1 m/s.

The spread error is measured one of two ways:

• UseWidthMeasurement true: Distance is a measured width (for example net or door spread), and the error is the width minus DistanceGoal;
• otherwise: Distance is a reference position, and the door is to keep DistanceGoal from its y component, outboard.

Example configuration excerpt

<Lib LibName              = "fishery"
   SimObject              = "Trawl/TrawlDoorControlledLift"
   Name                   = "Door"
   LiftDragRatio          = "2"
   LiftMax                = "100000"
   LiftMin                = "20000"
   WeightMax              = "200000"
   WeightMin              = "50000"
   IsPort                 = "1"
   NumConnections         = "2"
   Size                   = "1"
   WeightcontrolPID       = "10,1,1"
   LiftcontrolPID         = "10,1,1"
   UseWidthMeasurement    = "0"
/>

Configuration parameters

Name	Width	Description
LiftDragRatio	1	Ratio between lift and drag.
LiftMax	1	Maximum lift force [N].
LiftMin	1	Minimum lift force [N].
WeightMax	1	Maximum weight in water [N]. Also sets the mass, WeightMax / 9.81.
WeightMin	1	Minimum weight in water [N].
IsPort	1	Is this the port door? (0 = starboard, 1 = port).
NumConnections	1	The number of external forces.
WeightcontrolPID	3	The proportional, integral and derivative gains of the weight controller.
LiftcontrolPID	3	The proportional, integral and derivative gains of the lift controller.
Size	1	Diameter of the sphere representing the door [m] (Default: 3).
UseWidthMeasurement	1	Is Distance a measured width (1) or a reference position (0)? (Default: 0).

Input Ports

Name	Width	Description
Force<N>	3	External force N, N = 1, 2, .., NumConnections [N].
BottomForceGoal	1	The desired vertical force from the seafloor [N].
DistanceGoal	1	The desired width, or the desired y distance from the reference position [m].
Distance	1 or 3	The measured width (UseWidthMeasurement = 1, 1 element), or the reference position (UseWidthMeasurement = 0, 3 elements).

Output Ports

Name	Width	Description
Pos<N>	3	Position, one port per connection N.
Vel<N>	3	Velocity, one port per connection N.
Lift	1	Lift force [N].
Drag	1	Drag force [N].
Weight	1	Weight in water [N].
BottomContact	1	Positive: the vertical contact force from the seafloor [N]. Negative: minus the height above the seafloor [m].

Initial conditions

Name	Width	Description
Pos	3	Position (Default: "0,0,0").
Vel	3	Velocity (Default: "0,0,0").

This SimObject is referred to as Trawl/TrawlDoorControlledLift