Trawl door with controlled lift force and mass

Trawl door whose lift is set by a controller and whose mass follows a goal, to find the door properties that give a desired spread and mass.

Collaboration diagram for Trawl door with controlled lift force and mass:

Author

Karl-Johan Reite, Joakim Haugen

Trawl/TrawlDoorControlledLift with a mass state in place of its weight controller (ported from the 2.x TrawlDoorCtrlLiftMass, FISH-0103). 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 in water, down (+z), 9.81 × 0.87 × the mass: a steel door.

The mass is a state, which is also the door's inertia. It follows the input MassGoal, bounded by MassMin and MassMax, as a first-order lag with time constant 1 s: d(mass)/dt = goal − mass. It starts at (MassMin + MassMax) / 2 unless the input file sets it.

The door is towed along +x: the drag and the lift do not follow the direction of travel. The integral of the lift controller is a state. 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/TrawlDoorControlledLiftAndMass"
   Name                   = "Door"
   LiftDragRatio          = "2"
   LiftMax                = "100000"
   LiftMin                = "20000"
   MassMax                = "20000"
   MassMin                = "5000"
   IsPort                 = "1"
   NumConnections         = "2"
   Size                   = "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].
MassMax	1	Maximum mass [kg].
MassMin	1	Minimum mass [kg], positive.
IsPort	1	Is this the port door? (0 = starboard, 1 = port).
NumConnections	1	The number of external forces.
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].
MassGoal	1	The desired mass [kg].
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], 9.81 × 0.87 × the mass.
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").
Mass	1	Mass [kg] (Default: (MassMin + MassMax) / 2).

This SimObject is referred to as Trawl/TrawlDoorControlledLiftAndMass