Standardizing EV Alignment, the final step for commercializing Wireless Charging, SAE J2954

New: See video interview & demonstration of DIPS alignment and WPT in action

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SAE J2954 Wireless Power Transfer & Alignment Video with demonstration

Authors: Jesse Schneider, ZEV Station; Ky Sealy, WiTricity; Maximilian Hollenbach, ifak; Mike B?ttigheimer, Timo L?mmle, Simon Joos MAHLE Powertrain; Ivo Teerlinck, Toyota Motor; Bastian Rappholz, Siemens; Andreas Wendt, Electreon

Alignment Methodology is Key to WPT

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.

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.

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:

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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.

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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.

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.

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.

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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.

SAE J2954 Alignment Witness Tests

The UWB and DIPS alignment systems were integrated into vehicles chosen by the technology providers and setup in accordance to SAE J2954 requirements for parking spaces. Three independent witnesses chosen by the SAE J2954 taskforce conducted the tests scheduled by the taskforce’s alignment sub team. Due to the positive results and outcome of the taskforce’s review, only the DIPS test is shown exemplarily.

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Both systems were integrated into an electric vehicle (with a VA). The test setups were conducted in realistic environments.? For the exemplary case of DIPS, the testing was performed in an underground garage of a building. Three GAs were setup in adjacent parking spaces placed in accordance with the SAE J2954 Standard.

Test Plan

Testing consisted of three blocks: basic tests, environmental tests and other tests. Basic tests included approaches of the GA from different directions, at different speeds and from several distances. For the environmental tests the GA was either covered (minimum thickness 5mm) with materials or surrounded by them, depending on the case. Other tests included verification of the frequencies being used with a spectrum analyzer and measurement of the magnetic flux emitted by the DIPS.

Final Alignment Comparison Test Results

Both UWB and DIPS systems passed the basic requirements of the tests. The window shown below is illustrating the EV alignment tolerance limits related to EV parking within the SAE J2954 tolerance zone.

SAE J2954 UWB test results as documented

From the driver’s perspective, a key difference was the visualization inside the car. Inside the E-Smart vehicle equipped with the DIPS, the screen gave fast and intuitive information to the driver about steering direction and distance or error if the driver overshot the target. The responsiveness of the guidance allowed the driver to approach the GA at all requested velocities, enabled the driver to correct the angle of steering, and gave brake signals in time to reach the SAE J2954 tolerance zone. The visualization of the UWB ranging system inside the other car had sporadic behavior and offered limited help for the driver. Therefore, the UWB system’s directions during parking did not help to reach the tolerance zone.

Another difference between the two systems witnessed was the multi-GA scenario. The DIPS showed sufficient performance with active GAs on the adjacent parking spaces. It also enabled the driver to switch the GA (with visualization) by driving the car to another parking space, without any interaction.

Voting decision: DIPS is clear winner for the standard

The SAE J2954 WPT Taskforce had the following four pieces of information available to them in order to make the decision between the two alignment methodologies for the next SAE J2954 standard:

a)???? A draft SAE J2954 alignment section update in the standard based on the two technologies.

b)???? A detailed technology description for each of the UWB and DIPS methodologies

c)????? An open industry survey (subjective/ objective) with scoring results for both technologies

d)???? Detailed witness testing reports for each of the UWB and DIPS technologies

The SAE WPT Taskforce, with this comprehensive set of alignment data and information, carried out a decision poll with the following results:

As per the figure above, the SAE J2954 Wireless Power Transfer & Alignment Taskforce voted with 75% of the members in favor of DIPS as the common alignment method for public infrastructure in the light-duty vehicle standard. The choice was based on these test results, technology readiness level, and industry feedback. Therefore, the latest SAE J2954 standard published in August of 2024 includes DIPS as the common standardized alignment methodology for WPT.

“With over two decades of experience in wireless power transfer, ifak e.V. was enlisted by SAE to carry out the SAE J2954 alignment witness tests for their standard. We were honored to assist in this crucial step in helping to harmonize global standardization of WPT for EVs,” stated Maximilian Hollenbach, Research Associate, for ifak (Institut für Automation und Kommunikation e.V) of Magdeburg Germany. “As a result of the field testing, data taken and feedback from the SAE WPT-TF, it was found that the differential inductive positioning system (DIPS) has proven to be a mature technology capable of satisfying all requirements of the SAE J2954 alignment. DIPS gives fast and precise feedback to the driver while being resilient against numerous extreme environmental conditions.”

“The next generation of EV charging can be as simple as parking in a spot, with automated wireless charging using SAE J2954.” stated Jesse Schneider (ZEV Station), chair of the SAE WPT Task Force. “The SAE WPT Taskforce has extensively tested wireless power transfer and alignment from the lab to the field, allowing safe wireless power transfer up to 11kW with efficiencies up to ?93%.The new standard gives consistent alignment to an exact parking spot with the DIPS System. This will enable a seamless wireless charging & parking, even for autonomous vehicles in rain and snow. With the new J2954 standard, the team has solved the missing link, enabling the start of commercialization of wireless charging for EVs, a gamechanger.”

Though it took more than a decade to arrive at the point of decision to determine an alignment method for WPT, SAE has carried out a transparent and data-based consensus process leading to a clear direction for standardizing the alignment method as well as wireless charging. This groundbreaking publication of the SAE J2954 Standard will lay the framework for commercialization of light duty wireless charging worldwide for plug-in electric vehicles as well as autonomous vehicles.

Looking to the future, the SAE J2954 Taskforce is currently working on higher WPT power classes up to 500kW in the heavy-duty recommended practice SAE J2954/2 and Dynamic WPT in SAE J2954/3.