Standardizing EV Alignment SAE J2954, the final step for commercializing Wireless Charging (Part 1)
Published: September 18, 2024
Guest Post by Jesse Schneider, ZEV Station; Ky Sealy, WiTricity; Maximilian Hollenbach, ifak; Mike Böttigheimer, Timo Lämmle, Simon Joos MAHLE Powertrain; Ivo Teerlinck, Toyota Motor; Bastian Rappholz, Siemens; Andreas Wendt, Electreon
Wireless Power Transfer (WPT) is set to become an alternative to conductive charging and promises highly efficient charging of electric and plug-in-hybrid vehicles. For light duty vehicles, this has been fully specified up to 11kW in the latest 2024 SAE J2954. This missing link has been a standardized common alignment method for parking an EV for WPT.
For the new standard, two methodologies for alignment were evaluated: Differential Inductive Positioning System (DIPS) using an auxiliary magnetic field to align and Ultra-Wide Band (UWB) Ranging using Radio Frequency triangulation to align the electric vehicle to the wireless charger. Data and comparison of these alignment methodologies were collected in conjunction with analysis and input from the SAE J2954 WPT Taskforce. The objective is to highlight the benefits and shortcomings of each technology based on testing and to indicate a harmonized decision for one methodology. This was used to finalize the recently published SAE J2954 standard, with a standard alignment method, DIPS.
Alignment Methodology is Key to WPT
Wireless power transfer for electric vehicles works like an air-gapped loosely coupled transformer, operating within magnetic resonance for efficient operation. For WPT, EV Charging, this resonance is tuned to the 85kHz band, as specified in SAE J2954. A coil located inside a WPT ground assembly (GA) emits a magnetic field which is shaped by underlaying ferrites and shielding. A second coil, located inside the vehicle assembly (VA) is mounted to the underbody of a vehicle picks up the magnetic field and delivers the energy to the vehicle’s battery. While perfect alignment is not necessary, the alignment of the two coils should be within ± 75 mm side-to-side and ± 100 mm front-to-back as it is an important factor for ensuring efficient and safe operation at full power. Therefore, a single common alignment methodology is needed to ensure that all SAE J2954 compliant WPT systems are interoperable between manufacturers and provides a seamless experience to the end-users. This alignment method is useful for manual as well as automated and autonomous parking for WPT.
SAE J2954 Standardized Alignment Method “DIPS” for Wireless Charging (photo courtesy of MAHLE).
Scope of the SAE J2954 Alignment Witness Test
The testing, carried out in Stuttgart Germany was mainly focused on validating the WPT alignment with consideration for the following points:
• Parking alignment accuracy
• Repeatability
• Various approaches
• Electromagnetic (EM) field emissions
• Effect of environmental conditions, including metallic objects
• Parking performance
SAE J2954 DIPS Key Features
The following are the Differential Inductive Positioning System key features related to alignment of EVs for wireless power transfer.
- No line-of-sight dependency yields easier VA integration and platform development
- Operation with multiple WPT GAs in a parking lot is possible. No preselection by the driver is required.
- Robust even under worst-case environmental conditions: parking garage with metal in concrete, GA fully covered with wet leaves, ice, water, and other substances.
- One solution is possible for both static WPT (SAE J2954) and future dynamic WPT (SAE J2954/3).
- Compatible with WPT communication standards such as SAE J2847/6 or ISO 15118-20
- Use of near-field magnetics provides competitive advantage over a chip-based approach requiring far-field antennas
- Demonstrated robust technology readiness through public field testing.
SAE J2954 DIPS Technical Description
Concept
DIPS is an alignment methodology based on localized magnetic fields which are completely separate from the power transfer magnetic field. The DIPS GA includes five auxiliary coils used for alignment (shown in the red circles below)- each operating at a unique frequency and providing a defined magnetic flux. The yellow solenoid coil, which is wound around the ferrite and the litz wire of the power transfer coil, generates a horizontal field predominantly in x-direction (driving direction). The four red planar coils, which are placed above the litz wire, generate a vertical field above the GA power transfer coil.
Drawing of SAE J2954 DIPS Alignment on Ground & Vehicle Assemblies (courtesy of SAE J2954)
Many different solutions for the VA are possible given the GA DIPS system is standardized; one possibility for a DIPS VA position sensing solution is to use double crossed solenoidal coils wound around the ferrite at 45° angles to the driving direction (e.g., with a single turn each). For the steering information, the VA can evaluate the ratio between the induced voltages in coil A and coil B caused by the horizontal field of the GA. As an example, if the induced voltage in coil A is higher than in the orthogonal coil B, this means that the driver needs to steer left. This is the “differential” aspect of DIPS. For the braking / stopping information the magnitude of the signal received by the VA, which is based primarily on the flux from the GA’s solenoidal coil, can be used.
Parking Procedure - WPT Active Scan
Every parking event begins with the vehicle approaching a parking space. There can be one or several parking spaces available. The VA transmits the necessary frame for the active Wi-Fi scan known as a probe request as shown below. Therefore, the GA is notified that a vehicle is approaching, and this allows the DIPS magnetic field to be generated by the GA only when needed.
WPT Active Scan triangulation between two SAE J2954 parking spaces
The WPT Active Scan procedure can be accomplished without any additional resources in the vehicle since the already existing Wi-Fi modules in VA and GA can be reused. The primary purpose of the WPT Active Scan function is to allow the GAs to remain dormant and only enable the DIPS magnetic field when needed.
Parking Procedure - Alignment
Alignment assists the driver when approaching a charging location to align the VA within the defined allowed offsets over the GA. DIPS limits the range of operation to the parking space edge. Therefore, the VA starts the alignment process by evaluating the horizontal magnetic field of the GA as shown in the figure below. Once the four vertical field coils are within range of the VA detectors fine alignment takes place until the final position is reached.
Drawing of DIPS Fine Alignment process between Ground & Vehicle Assemblies
Parking Procedure - Pairing
Pairing aims to verify that the VA is communicating with the correct GA it is parked over as shown below. The VA obtains the necessary information for pairing, including the SSID of the Wi-Fi network of the GA and the EVSE-ID (ID of the GA) provided by the DIPS pairing method. Both IDs are modulated onto the auxiliary field coils and the VA can detect and evaluate this data. This evaluation can start during the approach and alignment into the parking space. Pairing is complete when the connection is set up and validated between the GA and VA via Wi-Fi.
Ground & Vehicle Assemblies, DIPS Pairing