Winches: Control

Control mode

Broadly speaking, there are two classes of control available for winches: length control and tension control. Within each class there are a number of modes, by which either the length or the tension of the winch can be specified, either directly or indirectly through its rate of change. For length-controlled winches, we can (knowing the length of the winch wire) calculate the tension in the winch wire at any time; conversely, for tension-controlled winches we can determine the corresponding length.

Length control

The length-control modes are:

Tension control

The different tension-control modes are:

Note: Tension-control changes are applied instantly at the start of each stage. This can apply a shock load which may, if it is large, affect the stability of the simulation.

Control type

During a simulation, winch control mode may be set in one of two ways: by whole simulation or by individual stage. Not all control modes are available to all control types, however, as described below.

Whole simulation

The only control modes available to the whole simulation control type are specified payout rate and specified tension. The payout rate or tension can be constant, may be chosen to vary with simulation time or be given by an external function.

By stage

When the winch is controlled on a stage-by-stage basis, for each stage of the simulation you choose from the winch control modes listed above. This control type enables you to switch between different control modes during a simulation: for example, you might choose to specify the winch wire length at the end of the first stage, and then zero tension change thereafter.

Note: The control mode remains fixed for the duration of each stage. Because there is a limit on the number of stages in an OrcaFlex simulation this can be restrictive.

Statics

Statics is a special case, for which non-zero rates of change of length or tension are not permissible. Consequently, the only control modes available in statics are specified length and specified tension.