Command and control

In contrast to a traditional ship’s propeller, ABB’s revolutionary marine propulsor, the ABB Dynafin, has multiple moving parts. To obtain optimal performance, these parts must be precisely orchestrated – even under the toughest oceanic conditions. For this task, a sophisticated control system is required.

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_{Ali Pekcan ABB BL Propulsion Helsinki, Finland ali.pekcan@fi.abb.com}

ABB Dynafin is a revolutionary marine propulsion system designed to extend the abilities of cycloidal-type propulsor concepts by introducing individual blade control with permanent magnet motors. The concept combines propulsion and steering, with blade trajectories inspired by the movement of a whale’s tail. Due to its effective pitch angle change of each blade, ABB Dynafin delivers much better hydrodynamic efficiency and maneuverability than traditional screw propellers.

It is essential that ABB Dynafin’s propulsion and steering elements are very carefully controlled so they follow exactly the required trajectories, even in very dynamic environments. For this, a precise control strategy is required.

Control via a “digital propeller design”

ABB Dynafin has two major parts: a main propulsion motor and a rotating lower part with blade motors →01. The main motor, which is fixed to the vessel hull, rotates the lower part and thus delivers the main thrust. Each blade on the lower part is driven by an individual blade motor, allowing the setting of the exact pitch angle required for adjusting the thrust magnitude and direction. The blade drives are powered through a slip ring unit, enabling the free rotation of the main propulsion motor. The slip ring also provides the physical link needed to communicate with the blade drive control units.

The principal control commands in a typical vessel are the rudder angle and propeller speed. In ABB Dynafin control, the captain can give the desired thrust magnitude and direction from the bridge’s remote control via common control interfaces (ABB Dynafin replicates the ABB -Azipod® experience in terms of compatibility with existing systems and extending and improving user experiences). This ability to effect a “digital propeller design” broadens the availability of standard modes of ship operation. For example, in addition to the standard modes – such as bollard pull, dynamic positioning, sea transit and maneuvering modes – new modes and trajectories can be created and supplied since each blade can be controlled individually. Each mode is implemented by optimizing the control parameters for that mode, resulting in different blade pitch angle trajectories and main wheel behavior. Behind this simple-sounding steering approach lies a sophisticated control system.

The ABB Dynafin control system

On a high level, the ABB Dynafin control system translates the speed and heading that the captain requests into an appropriate main wheel speed and a pitch angle for each blade. This part of the control – the thrust conversion module – takes place in the main control unit. The thrust conversion module then supplies the speed reference to the main propulsion drive and provides each blade drive control unit with the motion control parameters needed to achieve the required blade trajectory.

The low-level motion control on each blade control unit is designed to implement the trajectories defined by the motion control parameters designated by the thrust conversion. Motion control parameters define trajectories that each blade will follow during one rotation of the main wheel. These parameters are the eccentricity point (ECC), ie, the point to which all blades are perpendicular; the main wheel position and rotation speed; and the yaw angle. Yaw defines the rotation of the thrust vector effected by rotating the eccentricity point →01-02. The eccentricity parameter is used to calculate the pitch angle of the blades. To create a whale tail motion, the blades follow a trochoidal path where the eccentricity point is outside of the circumference of the rolling circle. In the case of ECC = 0, the blades will be rotating along the circumference with a pitch angle following the circle.

The main wheel speed reference from the thrust conversion is sent to the main propulsion drive. The actual speed and position of the main wheel are then measured by the main wheel encoder and sent to the main control unit. Tests have proven that the ABB Dynafin control algorithm achieves blade-tracking with the required precision.

The main wheel encoder measurements are also sent to the blade control units in each blade module so they can precisely maintain their position during the main wheel’s rotation. As mentioned, blade motion control is implemented in the blade control units separately for each blade drive. This separation allows fast communication with each drive and reduces the computational load on the main control unit. The low-level control executes in sub-millisecond cycle times. The blade position and angular speed are measured by encoders on the blades and sent directly to the blade control units. It is important to note that the number of blades has no effect on the overall control system and the control can be implemented in a modular way for a higher or lower number of blades.

Driving force challenges

The low-level control is based on the blade pitch function in the form of a mathematical model and on knowledge of disturbances (hydrodynamic loads, frictional loads, acceleration changes, etc.) To achieve high performance, the ABB Dynafin propulsion system must follow the predefined blade pitch function with high accuracy. However, several challenges arise when attempting to control blade motion:

• The blade’s pivot point is typically not aligned with its principal axis of inertia. This misalignment induces a centrifugal torque during wheel rotation, complicating blade control.
• Many blade pitch functions aimed at achieving high efficiency require both high acceleration and rapid acceleration changes for blade motion, which pose difficulties for the blade motors and drives to handle effectively.
• Certain blade pitch functions involve changes in blade rotational direction, necessitating blade motors to compensate for frictional torque.
• Hydrodynamic loads applied to the blades introduce tracking errors in blade pitch functions. Failure to accurately follow specified blade pitch functions can degrade propeller performance, increase torque on wheel motors and reduce overall efficiency.

The ABB Dynafin control algorithms are designed to cope with these challenges.

The torque reference from the blade control is generated for each blade drive using a feed-forward loop →03. Motion control parameters (RPM, ECC, yaw and main wheel position) are input to the blade motion control block, where a position reference is calculated as well as the necessary feed-forward model outputs for disturbance compensation. The cascaded blade-drive control block calculates the final torque required for high-precision tracking of the blade motion. Employing this model-based torque feed-forward compensation provides an accurate torque value to compensate for centrifugal torque, acceleration torque, friction torque and hydrodynamic torque, all of which are difficult for the feedback control. Finally, control software handles the limits and transition of trajectories in the final stage for smooth transitions and affordable torque profiles.

Testing with hardware

The ABB Dynafin team has developed a hardware-in-the-loop (HIL) test platform to verify control performance, requirements analysis, failure modes and effects analysis (FMEA), and hardware suitability for the final product →04.

In HIL testing, the system under test is connected to a simulation or model of its environment. The system’s response to various inputs is then measured and compared to expected results. HIL simulation allows control system evaluation in a controlled environment before deployment in the actual marine environment.

In the case of ABB Dynafin, HIL testing involves connecting all the physical components from the control network topology – such as the main control unit, blade control units, interface cards and I/O modules – to a simulation that mimics the load conditions and various operating scenarios. The motor suppliers provide motor model parameters. Hydrodynamic loads are generated using computational fluid dynamics (CFD) simulations corresponding to the trajectories under test. The power stage of the drives is also modeled.

Initially, a two-blade system was created to verify various aspects of the drives, control units, redundancy schemes, blade tracking, etc. Since the control network topology is now frozen, the HIL test system will be expanded to a five-blade setup for the second stage of the testing.

Redundancy and failsafe

At sea, vessel reliability and the safety of the crew are of paramount importance. For this reason, the control network topology has been designed to follow standard requirements, ensuring redundancy and high performance. An extensive design FMEA has been carried out for each design component and, according to “single failure criteria,” active components have been duplicated for redundancy. The FMEA outcome shows ABB Dynafin fulfills the rules established by the International Maritime Organization (IMO) and the International Convention for the Safety of Life at Sea (the SOLAS Convention). For example, in the case of a failure, vessel steering is ensured by switching the failed unit to rudder mode, where the main wheel is stopped and all active blades are used as rudders for steering.

ABB’s Dynafin provides all-round best-in-class performance, efficiency, safety and reliability – all enabled by a sophisticated control system that overcomes the many challenges faced by a marine propulsor composed of multiple moving parts that is exposed to the harsh conditions found at sea. The modular nature of the control system simplifies the extension of ABB Dynafin to cover many more marine applications. 

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