In the current context of aircraft electrification, the aerospace industry needs to address increased design complexity and higher levels of integration, which can easily impact program cost and product time-to-market. System electrification, including electrified propulsion, being one of the major trends in the industry, comes with challenges related to electromagnetic compatibility (EMC). This article discusses EMC engineering challenges and solutions for an electrified unmanned aerial vehicle (UAV).

When it comes to complying with EMC requirements for aircraft development, there are different cases to consider: from high-intensity radiated field (HIRF), indirect effects of lightning (IEL) to the particular EMC challenges caused by electrification. These different use cases have one thing in common: one should guarantee that safety-critical systems do not fail due to EMC/EMI.

For an engineer, it is key to understand the critical components for EMC as early as possible in the product design cycle. Some components can be classified as ‘emitters’: these will emit EM noise either through conduction (going into the wiring system) or through radiation (going into the air). Other systems can be classified as ‘transfer paths’: once the EM noise has entered such systems, it can be easily transmitted throughout the entire system. An example of transfer paths is the electrical wire harness or the fuselage, depending on the material where metallic components will easily transmit energy due to electric current flowing through the system.

Finally, some electronic systems are to be classified as receptors, sometimes also referred to as ‘victims’. Examples of victims can be avionics systems, radar systems, sensors, etc. These victims are susceptible to EM interference either at the pins of the system (current of voltages) or susceptible through the application of EM fields and waves, directly applied to the electronic system. To avoid EMC issues, engineers have the option to either reduce the EM noise source or to optimize the transfer path, allowing minimal transfer of EM energy, or to increase the EM shielding at the receptor side. It is important to notice that this always applies to both conducted as well as radiated EM noise.

Picture 1: Electromagnetic compatibility in context of aviation
Figure 1: Aerospace regulatory standards related to EMC

Aircraft regulatory standards and compliance

The aircraft design must meet specific Federal Aviation Association (FAA) regulations and guidelines related to EMC to achieve safety compliance (Fig.1). High-intensity Radiated Field (HIRF) testing ensures all aspects of electrical wiring, installations, and aircraft-level systems are safe for operation so the aircraft can still fly. For example, at an airport with a large radar that sends a signal to the planes, the aircraft will still function properly even with an external field that could cause electromagnetic interference (EMI).

Another industry standard for aircraft compliance is IEL when lightning hits the aircraft. On average, a plane is hit by a lightning strike one to two times per year. When lightning strikes an airplane, an electrical current flows through the aircraft. Because of those high currents, electromagnetic energy can creep into the equipment, potentially causing electronic equipment failure. In the case of HIRF and IEL, the aircraft cables are critical to be protected because, especially at high frequencies, all this energy can easily get into the power cables and sensor cables and impact the systems and subsystems throughout the aircraft.

Simulating the electrified propulsion

In the context of electrified propulsion, power electronics play a crucial role in the generation of electricity. They are the cause of substantial disturbances (electromagnetic waves) scattering inside the aircraft, entering cables and electronic systems. All these systems can interfere with each other, which needs to be analyzed and verified in the design.

Figure 2: Schematic view of electrified propulsion for EMC purposes

UAVs must function properly in their electromagnetic environment to ensure safety and to maintain the systems’ operational performance. For HIRF compliance, the aircraft’s critical electronic systems must be able to withstand EM disturbances up to 40 GHz to normal operation. Figure 2 shows a UAV electric propulsion model, which comprises cabling and wiring, a battery (B), inverter power (I), electronics (E), DC cables from the battery to the inverter, and a motor (M) driving the fan or propeller.

In ideal operational conditions, DC energy flows from the battery into the inverter, where it is converted into a nice pure sine. In real conditions, the inverter will not only generate the AC signal, but will also generate higher-order harmonics and create energy up to several 10’s MHz or even 100’s MHz. This is due to the way the inverter operates: essentially implementing a switching scheme to generate the AC signal. These switching frequencies are clearly visible in the current spectrum of the AC and DC lines and all return paths. But even more, the rise and fall time of the switches of the inverter is so short, several 10’s of a nanosecond, making the inverter product EM energy up to several 100MHz.

Figure 3: Schematic view of the electrified proposal circuit
Figure 3: Schematic view of the electrified proposal circuit

Step 1 (Fig. 3) correlates the relationship between what is happening at the source (which is the inverter) to how the signal passes through the aircraft, how it then goes into the cables, and finally, into the electronics. Engineers must balance different requirements and possible solutions: either use highly shielded and expensive cables, which would also add weight to the UAV or rework routing to avoid EMC, or even change switching characteristics of the inverter. The latter could make EMC behavior better but could result in reduced motor efficiency. Balancing such requirements, including EMC, as early as possible in the design cycle is critical.

In step 1, a pure circuital electrical system analysis is performed. That requires a sufficiently accurate model of the different components. Important to note is that standard functional models are not sufficient for those components. The fact is that parasitics need to be included in the circuital analysis, as those parasitics can significantly alter the electrical currents and voltages, especially at the higher frequencies, up to 100 MHz. But how can we obtain those parasitics?

In some cases, those parasitics come with the components and their individual SPICE models, from the component manufacturer. However, some of these parasitics are caused by the integration of those components into the entire invertor systems. Obtaining those parasitics can be done either experimentally or through simulation. Once a high-fidelity representation of the individual components is available, including the parasitics, the overall system-level analysis can be carried out using a transient circuit solver like SPICE or VHDL-AMS as supported in Xpedition AMS or SystemVision software from Siemens. Such analysis results in a spectrum of currents and voltages, for example, inside the high-voltage bus.

Figure 4: Workflow for a full 3D EMC analysis
Figure 4: Workflow for a full 3D EMC analysis

The sources resulting from Step 1, such as HV DC currents, are fed as a source into a coupled 3D electromagnetic simulation model. That model contains both a 3D representation of all critical components of the UAV and the wiring systems.

For these particular use cases, the solution is preferably based on a full-wave analysis because of the higher fidelity such analysis brings. The Simcenter 3D Electromagnetics simulation environment supports the Method of Moments (MoM) solver as well as fast multipole expansion (MLFMA). The full-wave analysis is carried out for the entire frequency band of interest: the switching frequency and the harmonics and the MHz frequencies generated through the invertor. Because the 3D simulation itself is linear, as opposed to the circuital analysis of step 1, one can easily split the entire spectrum into different frequency bands and run those on different nodes of an HPC cluster. Such a distributed solver will significantly boost computational performance.

EMC and EMI parameters can be derived from such analysis. What comes out of those 3D calculations are electrical signals (current and voltages) in the cables and electromagnetics 3D fields.

An end-to-end workflow

Siemens’ Xcelerator portfolio comprises electrical and electronic harness design and verification tools (Capital), electric and electronic circuit analysis (Xpedition AMS and SystemVision), 3D CAD modeling and cable routing tools (Siemens NX), and 3D electromagnetic simulation capabilities (Simcenter 3D). Design teams can establish end-to-end processes, with highly efficient digital threads within the processes allowing engineers to analyze designs for EMC, in minutes versus hours.

Figure 5: A comprehensive digital thread created with Siemens software tools supporting the EMC workflow

Capital contains information on wire harnesses linked to NX 3D design software and other CAD software. Capital also offers a database of information on the wires and cables within a harness. Simcenter 3D directly imports from Capital, providing efficient 3D simulation for sophisticated wire harnesses, including cable branches, connections, junctions, and so on. Roughly 90 percent of the information needed for EMC analysis is provided in Capital; the 10 percent extra data, specifically EMC-related, can be added via the Simcenter 3D user interface.

As Simcenter 3D is built on the NX foundation, it also allows users to carry out Design Space Exploration for EMC, directly taking CAD and routing information as an input.

Figure 6: Electronic systems and cabling inside a UAV

The engineer can zoom into the wire harness. If there is an EMC problem for which re-routing is a possible solution, the engineer can re-route the cables and automatically re-run the EMC analysis. Thanks to the digital thread, a change made in one domain (e.g., re-routing a cable) automatically applies in all relevant domains.

Conclusion

Aircraft electrification gives rise to new requirements for EMC. As the number of electric power sources in an aircraft increases, the risk for EMC-related failures of electronic equipment increases as well. Engineers will need to implement robust, efficient processes for high-fidelity EMC analysis. Such simulation tools have to be able to deal with complex 3D structures such as a complete aircraft structure, as well as the electric harness, being one of the critical components for EMC. Proper modeling of the electrified propulsion using circuital analysis, taking into account the switching scenarios of the invertor is key as well.

The Xcelerator portfolio provides the capabilities and the digital threads to address EMC problems in an efficient way.

For more information

Koen de Langhe
Director Product Management, 3D Simulation
Siemens Digital Industries Software

Koen De Langhe has 25 years of expertise in the area of CAE, primarily 3D mechanical analysis: structural dynamics, acoustics, NVH analysis and solutions for electromagnetics, including motors, transformers, EMC/EMI, antenna and radar.​ Within product management, Koen is responsible for setting the product strategy and roadmap for a range of 3D simulation products, and bringing existing products to the market and to customers.