Towards Contactless Energy Transfer for Robotic Actuation Systems

The high technological standards known today are not achievable without robotic actuation systems. Electromagnetic actuators are the driving force for most of these systems. In particular, rotary actuators are applied. A high-power density is established by the application of permanent magnets (PMs) on the stator. The rotor typically consists of a lightweight coil construction, such that a low inertia is achieved. Generally, the energy is supplied to the moving coils through a physical connection, namely a moving cable or sliding contact (slip ring technology). The disadvantages of these solutions are a restriction in the movement (moving cable) or friction and reliability issues (sliding contact). Alternatively, the energy can be transferred to the moving coils in a contactless manner.

Mature technology

Contactless energy transfer (CET) is a popular and mature technology. Applications include battery charging, robotics, and electrical machines. In a rotary application, a CET system typically employs a cylindrical transformer, which consists of a rotary and stationary side separated by an air gap. The energy is transferred by the linked magnetic field between the coils. The magnetic coupling is improved by integrating each coil into a magnetic core. The volume of the magnetic core is minimized by the application of a high electrical frequency.

Compelling alternative

In order to provide a compelling alternative to the existing technology, the volume added by the CET system to the actuator should be minimized. Consequently, an integrated solution is preferred. However, several challenges are imposed. Firstly, the integrated CET system induces losses in various actuator parts. Therefore, an accurate estimation of the losses is important, such that the efficiency and operating temperature are accurately predicted. Secondly, cross-coupling between the actuator and CET system might occur, which has the potential to compromise the functionality of either of the two systems.

Novel design methodology

In this research, a patent-pending integrated concept is proposed, which deals with the challenges related to the integration. The PMs of the actuator are arranged such that the magnetic field on one side of the array is enhanced, whereas the magnetic field on the other side is virtually zero (a quasi-Halbach configuration). Therefore, interference effects caused by the PMs are minimized and a hollow space is created in the center of the actuator, in which the CET system is integrated. Additionally, the actuator housing consists of aluminum, which shields the actuator from the CET system. The losses induced in the actuator housing are minimized by a bespoke magnetic core design, which is derived by the application of a novel design methodology. The volume of the magnetic core is minimized by the application of an electrical frequency in the megahertz range. For these reasons, a novel concept is established, in which the functionalities of the actuator and CET system are sufficiently decoupled.   

Integrated concept validated

Three unique experimental setups are realized in order to validate both the design methodology and the integrated concept. The main parts of the actuator (the PMs and coils) are progressively added to the setup, and the effect of each part on the CET system is analyzed. Additionally, the electromagnetic coupling between the two systems is quantified by a series of voltage measurements. The results have validated the integrated concept since the functionalities of both systems are not compromised by the integration of the CET system into the actuator. Therefore, the integrated concept can be used to improve the flexibility and durability of robotic actuation systems. Furthermore, the results have demonstrated that the design methodology is better suited for problems where the temperature and efficiency predictions are less critical, for example, initial sizing, which in turn can be further improved by more accurate models.

Title of PhD thesis:  ‘Towards Contactless Energy Transfer for Robotic Actuation Systems’

Supervisor: Elena Lomonova