Module: forceAtSiteLTI

Executive Summary

The ForceAtSiteLTI module implements a three-input, three-output linear time-invariant force model by specializing LinearTimeInvariantSystem. It maps site-frame force vectors from ForceAtSiteMsgPayload into the LTI input and publishes filtered or shaped site-frame forces through the same payload type.

Module Description

The inherited LTI dynamics are

\[\dot{\mathbf{x}} = \mathbf{A}\mathbf{x} + \mathbf{B}\mathbf{u}, \qquad \mathbf{y} = \mathbf{C}\mathbf{x} + \mathbf{D}\mathbf{u}\]

with fixed dimensions for this subclass:

• input size: \(m=3\)

• output size: \(p=3\)

The input mapping is

\[\begin{split}\mathbf{u} = \begin{bmatrix} F_x \\ F_y \\ F_z \end{bmatrix}, \qquad \mathbf{u} = \texttt{inMsg().force_S}\end{split}\]

and output publication is

\[\texttt{outMsg.force_S} = \mathbf{y}_{0:3}\]

All quantities are expressed in the site frame \(S\), consistent with ForceAtSiteMsgPayload.

Message Interfaces

Module I/O Messages

Msg Variable Name	Msg Type	Description
inMsg	ForceAtSiteMsgPayload	Input 3D force vector in site frame. force_S maps to \(\mathbf{u}\).
outMsg	ForceAtSiteMsgPayload	Output 3D force vector in site frame. force_S is populated from the first three components of \(\mathbf{y}\).

Module Assumptions and Limitations

• The model is strictly 3D (input/output dimensions are always 3).

• The state dimension is inherited from the configured base-class matrices.

• If no state is configured, output uses direct feedthrough from \(\mathbf{D}\) when available.

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class ForceAtSiteLTI : public LinearTimeInvariantSystem

#include <forceAtSiteLTI.h>

Linear 3D force model based on LinearTimeInvariantSystem.

This class implements a three input, three output LTI system where the input and output are carried through ForceAtSiteMsgPayload messages.

Input: u = [Fx, Fy, Fz]^T in the site S frame, read from inMsg().force_S.

Output: y = [Fx, Fy, Fz]^T in the site S frame, written to outMsg.force_S.

The internal dynamics are defined by the A, B, C, D matrices stored in the base class LinearTimeInvariantSystem.

Public Functions

ForceAtSiteLTI() = default

Default constructor.

The system matrices must be configured by the user before running the simulation, either by direct calls to setA, setB, setC, setD or through convenience configuration helpers.

inline virtual size_t getInputSize() const override

Get the dimension of the input vector.

This model is strictly 3D, so the input dimension is always 3.

Returns:

Number of inputs (always 3).

inline virtual size_t getOutputSize() const override

Get the dimension of the output vector.

This model is strictly 3D, so the output dimension is always 3.

Returns:

Number of outputs (always 3).

virtual Eigen::VectorXd readInput(uint64_t CurrentSimNanos) override

Read the current input vector from the subscribed input message.

This method constructs a 3 by 1 Eigen::VectorXd whose elements are taken from the force_S array of the ForceAtSiteMsgPayload read via inMsg().

Parameters:

CurrentSimNanos – Current simulation time in nanoseconds. (Unused in this implementation.)

Returns:

Input vector u of size 3.

virtual void writeOutput(uint64_t CurrentSimNanos, const Eigen::VectorXd &y) override

Write the current output vector to the output message.

The first three elements of the output vector y are written to the force_S array of the ForceAtSiteMsgPayload and sent on outMsg.

Parameters:

• CurrentSimNanos – Current simulation time in nanoseconds.

• y – Output vector of size at least 3.

Public Members

Message<ForceAtSiteMsgPayload> outMsg

Output message carrying the 3D force command at the site.

The force_S array is populated from the first three elements of the output vector y computed by LinearTimeInvariantSystem.

ReadFunctor<ForceAtSiteMsgPayload> inMsg

Input message read functor providing the 3D force at the site.

The force_S array of the payload is mapped to the input vector u.