Overview

The near-silent operation of electric vehicles has significantly increased the sensitivity of occupants to low-frequency vibrations. During vehicle launch and creep conditions, a low-speed judder was observed, adversely affecting perceived vehicle refinement and driving comfort.

The investigation identified electromagnetic torque ripple from the Permanent Magnet Synchronous Motor (PMSM) as the primary excitation source. This periodic torque variation excited the driveline and powertrain mounting system, resulting in vehicle body vibrations at very low speeds.

A comprehensive engineering workflow combining vehicle testing, multibody dynamics (MBD) modelling, simulation correlation, and design optimization was employed to identify the root cause and develop an effective mitigation strategy.

The Challenge

Customers reported a noticeable judder during:

• Vehicle launch
• Low-speed acceleration
• Creep operation
• Stop-and-go traffic
• Regenerative torque transitions

Although the vibration occurred only at low vehicle speeds, it had a significant impact on perceived quality (PQ) and vehicle refinement.

The engineering objective was to:

• Identify the excitation mechanism.
• Separate motor-induced excitation from structural resonance.
• Correlate simulation with physical vehicle measurements.
• Develop robust countermeasures without affecting overall NVH performance or vehicle dynamics.

Test-Based Root Cause Investigation

A detailed vehicle test campaign was conducted using instrumented prototype vehicles.

Measurements included:

• Motor electromagnetic torque
• Inverter torque command
• Driveshaft torque
• Powertrain mount accelerations
• Body floor acceleration
• Steering column vibration
• Seat rail acceleration
• Cabin sound pressure levels

Order tracking and frequency-domain analysis revealed that the dominant vibration corresponded to the torque ripple frequency generated by the PMSM at very low motor speeds.

The excitation propagated through the driveline into the powertrain mounting system, where structural compliance amplified the vibration felt by vehicle occupants.

Multibody Dynamics Modelling

To accurately capture the system dynamics, a high-fidelity multibody dynamics model of the complete electric powertrain was developed.

The model represented:

• Permanent Magnet Synchronous Motor (PMSM)
• Single-speed reduction gearbox
• Differential assembly
• Driveshafts with torsional flexibility
• Constant velocity joints
• Powertrain mounting system
• Suspension compliance
• Vehicle body interfaces
• Non-linear mount stiffness and damping
• Flexible driveline components

Measured motor torque ripple was incorporated as a time-varying excitation input, enabling realistic simulation of transient operating conditions.

The MBD model successfully reproduced the interaction between electromagnetic excitation and structural dynamics that could not be captured using simplified analytical models.

Model Correlation

A key objective of the project was to establish confidence in the virtual model through correlation with physical test data.

Correlation activities included:

• Time-domain acceleration comparison
• Frequency response analysis
• Mount reaction force validation
• Powertrain displacement comparison
• Driveline torsional response
• Modal behaviour verification

Excellent agreement between simulation and test confirmed that the model accurately predicted the low-speed judder phenomenon.

The validated model then became the primary engineering tool for evaluating design improvements and reducing the need for repeated physical prototype testing.

Root Cause Analysis

Simulation and experimental results demonstrated that the judder originated from the interaction between electromagnetic torque ripple and the natural dynamics of the driveline system.

The primary contributing factors were:

• PMSM torque ripple at low motor speeds
• Torsional compliance of the driveshafts
• Powertrain rigid-body motion
• Mount stiffness characteristics
• Structural resonance near excitation frequencies

Rather than a single component issue, the judder resulted from the coupled behaviour of the electric motor, driveline, mounts, and vehicle structure.

This systems-level understanding enabled targeted design improvements rather than isolated component modifications.

Mitigation Strategy

Several virtual design studies were performed to reduce vibration transmission while maintaining durability and vehicle performance.

The optimization included:

• Powertrain mount stiffness refinement
• Mount damping optimization
• Driveline torsional stiffness assessment
• Isolation tuning of the mounting system
• Evaluation of alternative mount orientations
• Sensitivity analysis of drivetrain parameters

The simulation environment enabled rapid evaluation of multiple design iterations before physical implementation.

Results

The optimized design delivered significant improvements in vehicle refinement:

• Reduced low-speed judder during vehicle launch
• Lower vibration transmitted to the passenger compartment
• Improved isolation of motor torque ripple
• Reduced dynamic motion of the powertrain
• Better correlation between simulation and vehicle testing
• Reduced prototype development time through virtual validation
• Enhanced customer-perceived quality and low-speed drivability

The final solution achieved the desired NVH targets without compromising powertrain durability or vehicle performance.

Technologies Used

• Multibody Dynamics (MBD)
• Flexible Body Modelling
• NVH Simulation
• Frequency Domain Analysis
• Order Tracking
• Time History Correlation
• Powertrain Mount Optimization
• Driveline Torsional Analysis
• Modal Analysis
• Vehicle Test Correlation

Engineering Impact

This project demonstrates the effectiveness of simulation-led engineering in solving complex electric vehicle NVH challenges. By integrating physical testing with high-fidelity multibody dynamics modelling, the root cause of low-speed judder was accurately identified as the interaction between PMSM torque ripple and the driveline dynamic response.

The validated virtual model enabled rapid design optimization, improved simulation confidence, and reduced development time while delivering a measurable improvement in vehicle refinement and customer experience.