The selected propulsion configuration for the WAM-V uses two aft-facing outboard motors. This maximizes the useful forward thrust from these motors, but yields an underactuated dynamic system. In such an underactuated system, control algorithms do have authority in the surge and yaw axes, but have no authority in the sway axis. This property becomes an issue when designing station-keeping controllers which benefit from sway authority. The ability to control the thrust angle of the motors (that is, to vary their angle relative to the WAM-V body) allows toggling between high thrust and sway authority modes.
There are many ways to rotate a motor, but the solution here must meet a few criteria. The mechanism should be waterproof, strong, reliable, and compatible with the EP-5 outboard motors. During brainstorming, several concepts were considered. The two leading ones were gear-based and linear actuator-based mechanisms (shown below).
While both options here could have met all design criteria, the linear actuator option was selected for its ease of manufacturability.
The actual selected actuator was the PA-10 24V 12” stroke linear actuator from progressive automations. This actuator provides a maximum of 450lbf and a 12” stroke. Additionally, it has a built in hall effect sensor which can be used to implement feedback control. With the actuator selected, only the static mechanical portions of the system needed to be designed.
The two static components serve to connect the linear actuator to the buoyancy bod and the motor. The motor-side connection also acts as a lever, the length of which is a key design parameter affecting both the kinematics and dynamics of the system. In the interest of ease of manufacturability, both sections use only prefabricated 8020 extrusion and waterjetted components.
The design of the motor-side fixture (”motor lever”) began with a study of the key design parameters, namely the lever arm length. Because the actuator stroke length is fixed, the lever length determines the overall actuation angle. From the controls team, an actuation angle of 30 degrees was set as the design target. In terms of dynamics, a longer lever increases the torque applied by the actuator, but requires the part transmitting that torque be stronger.
The required torque to maintain a motor angle is a key constraint to the problem. If the required torque is comparable to the available torque (for reasonable lever lengths), then the design problem requires trading kinematic performance for dynamic performance. Fortunately, the estimated required torque is relatively small, so the design problem was simplified to merely a kinematics issue.
Yaw kinematics for the final configuration. (generic outboard motor shown)
Yaw kinematics for the final configuration. (generic outboard motor shown)
For a given layout, a maximum torque is determined. This torque is used to size the physical lever part. This component is waterjetted from 1/2” 1018 steel, chosen for its high stiffness and yield strength. The part was designed and evaluated using Solidworks’ FEA package
The pod-side mount merely needed to be designed for stiffness and to interface with the buoyancy pod. The resulting design was relatively quickly manufactured. A comparison between the CAD model and realized design shows that several stiffness improving parts were not required.
