A cylinder's force pressure lags when its chamber is joined to another chamber by a wide edge

What. Each actuator reads its chamber's pressure from the node's own implicit response to the step's volume change (_force_pressure in alloy/plugins/fluid/actuators.py): P + (P' - P)·κ/(κ + dt·Σg). That response holds every neighbour node at its step-start pressure, which is right for a reservoir or a large accumulator. When the edge goes to another small chamber and dt·Σg ≫ κ, the force reads close to the step-start pressure. The oil column's stiffness then acts a step late, and its ringing grows.

A second effect of the same split: the network adds a step's source inflow after the force is taken. While a pump is filling a chamber, the node's end-of-step pressure therefore reads up to K·Q·dt/V above the pressure the piston acts with.

Number. GA's forklift lift line (SCR-WP8-12 F2): a pump node of 0.5 L joined by a 1e-7 m³/(s·Pa) hose to the free-lift and main cylinders (0.1 L dead volume each), at the 1.25 ms task step. - Unladen carriage: the lift pressure swung between -3 and +5 MPa gauge, and the carriage crept 10 mm while held. - Gauge while flowing (one shared node instead, 0.1 L dead volume per cylinder): in mid free lift the node read 8.0 MPa while the piston acted with 5.97 MPa (34 % high). With the line's 1 L in the dead volumes it reads about 0.7 MPa high. Holding pressures read true.

Why. Per step, each actuator writes its chambers' volumes and takes its force from the pressure each node would reach on its own; the network then steps every node together (implicit in the conductances) and adds the sources' inflow. A neighbour held at its old pressure is right for a reservoir but not for a small chamber, whose pressure moves with the first one's within the step; and the inflow arrives after the force was taken.

Workaround. - For a common line (one pump teed to several cylinders), declare the node Node(shared=True): one oil volume and one pressure, the chamber of every actuator on it, with the coupling implicit through the node. - For pressure readouts while oil flows, use piston_pressure (the gauge pressure each actuator's force acted with, A and B sides) instead of the node's pressure.

Planned fix. Not scheduled (WP13). A network-implicit force pressure: the coupled response of the connected nodes to the actuators' volume changes, and the step's known source inflow added to the force's prediction.