The Role of the Mode Valve
The A320 aileron actuator model was developed with all components necessary for simulating the active mode – when the actuator follows the commands from the Actuator Control Electronic. The next step for increasing the model realism is to run stand-by (damping) mode and to switch from one mode to the other. This switch is performed by the mode valve.
For the A320 aileron actuator, the mode valve is a four port, two position valve. It is positioned between the servo valve and the actuator. Two of the ports are connected to the control ports of the servo valve while the other two are connected to the cylinder chambers.
In active mode, the mode valve establishes connection between the servo valve control ports and the cylinder ports enabling nominal functioning of the system. In stand-by mode, the mode valve isolates the control ports of the servo valve and connects both cylinder chambers through a damping orifice and anti-cavitation valves – refilling the chambers in case the pressure goes below return pressure.
Figure 1 A320 aileron actuator-like mode valve – simplified ISO1219 view
Figure 1 shows an ISO1219 view of this valve – omitting the anti-cavitation valves. Figure 2 shows its location in the actuator model.
Figure 2 position of the mode valve in a A320 aileron actuator-like
Modeling the Mode Valve
Since all combinations of the directional control valves (DCV) cannot be provided off-the-shelf, Modelon Hydraulics Library provides a framework in which the user can easily custom-create DCVs of their choice. For the mode valve, a generic DCV with 4 ports and 2 positions can be extended and easily parametrized to fit our needs.
Users need to provide parameters that are mapping between the continuous normalized spool position and the normalized flow path opening. In our implementation, it has been decided here to move the spool position from 0 to 1. The flow path area (normalized) also varies from 0 to 1 where 1 corresponds to the area indicated by nominal flow and pressure drop value provided by the user.
The valve being generic assumes all six possible paths – i.e. PA, PB, AT, BT, PT and AB. For the A320 aileron mode valve, the only possible paths are PA, BT and AB. The relationship between the spool position and flow path areas can be provided per the diagram seen below.
Figure 3 Normalized flow paths v/s spool opening for a mode valve similar to the one of an A320 aileron actuator
The above mapping between spool position and flow paths can be written in the model through the below parameter assignments. The paths that are not connected at all need not be specified as they are zeros by default.
- open_A_B = [0, 0, 0.5, 1, 1]
- open_B_T = [1, 1, 0.5, 0, 0]
- open_P_A = [1, 1, 0.5, 0, 0]
- spool_x_axis = [0, 0.45, 0.5, 0.55, 1]
Note that the mode valve is controlled by a unique solenoid valve. This solenoid would provide either the supply or return pressure to the spool surface, moving it to one position or another. In this blog post, we decided to only control the valve with a Boolean input to keep it simple.
Simulation of Both Aileron Actuators on a Control Surface
In order to illustrate the mode switching, a scenario is run where the actuator is initially in active mode and at time 0.5 will switch to the damping mode. The actuator is facing a sine antagonist load which is easily faced by the hydraulic actuator in active mode but that drives the actuator in by-pass mode.
Figure 4 Active-Stand-by switch mode at time 0.5 s
A more realistic scenario would be to include two actuators on a single control surface, as it is the case on an A320 aircraft. However, it has been decided to include that in a variant based on the Boeing 777 – as its mode valve also includes a blocking mode.
In this blog, we increased the realism of existing actuator model and covered the parametrization of customizable generic valves available in the Modelon Hydraulics Library. We demonstrated the capability for the actuator to switch modes and to operate in a aileron-scope system.
The following blog post will discuss the export of the plant part of the aileron model in a Hardware In the Loop (HIL) platform and interfacing it with its controller.
Stay tuned to find out the remaining interesting parts of the series covering full system performance analysis and component detailed design.