BGMod handbook

Causality

Who supplies effort, and who supplies flow?

The causal stroke marks the end receiving effort. That end supplies flow. In hydraulics, effort is pressure p and flow is volumetric flow rate Q. The half-arrow sets the positive power direction; it does not decide the stroke position.

Se sets pressure; I determines the common flow. C and R supply their pressure drops. Strokes are enlarged for clarity.

How does the system choose the stroke positions?

BGMod starts with the source and storage rules: C integrates flow to supply pressure; I integrates pressure difference to supply flow. It then propagates these requirements through 0/1 junctions and connected components, choosing consistent positions for the remaining strokes. If the requirements cannot be reconciled, analysis reports an error.

Component	Integral causal form	Stroke position
Se	Supplies prescribed effort.	Away from Se.
Sf	Supplies prescribed flow.	At Sf.
C / GasAccumulator	Integrates inflow; state determines effort.	Away from the storage component.
I / MechanicalLoad	Integrates effort balance; momentum determines flow.	At the inertia/load component.
R	Effort from flow or flow from effort.	Either, consistent with the network.
0 / 1	Exactly one effort-defining branch at 0; one flow-defining branch at 1.	Exactly one stroke at 0; exactly one bond without a stroke at 1.

Compound components add coupled constraints. A pump depends on pressure difference and cannot be treated as a freely invertible flow source at cutoff or on a flat table segment. Some networks require a joint algebraic solve; the report lists these groups explicitly. The strokes shown by analysis describe the initial operating mode, while schedule changes are checked during simulation.

A causal conflict is a modeling issue. Connecting two independent ideal pressure-defining storage elements directly to the same 0-junction overconstrains their states. For the bench, the accumulator is connected through an explicit line resistance. Add a resistance only when it represents the intended physics; do not use arbitrary hidden leakage to silence diagnostics.

TF inside the cylinder

BGMod uses hydraulic (p,Q) and mechanical (F,v) variables. With this choice the cylinder is an ideal transformer: Q = A·v and F = A·p, so pQ = Fv. Its internal graph has two TF elements with factors A_A and −A_B. These hydraulic–mechanical TFs are internal to the cylinder. Separate same-domain TF palette elements are available in all four physical domains. Gyrators (GY) are not available in the current library. Choose TF/GY from the actual variable mapping, rather than from the device name alone.