Analysis of transfer path from in-wheel motor to cabin noise

As developer of in-wheel powertrain, Elaphe Propulsion Technologies Ltd. has to make sure that it’s powertrain does not emit excessive vibrations, that will result in structure-borne noise. Since vehicle suspension is typically not considered to also perform as a motor insulation system, there is a lack of knowledge on specific vibration insulation properties, especially in higher frequencies which are excited by electrical powertrain.

Elaphe, based in the Slovenian capital, Ljubljana, is a company focusing on innovation, research and development of in-wheel motors for electric vehicles. Elaphe has produced several generations of in-wheel motor prototypes that are being tested on more vehicles. These vehicles and motors have served as showcases for customers and shown the way to meet standards and regulations of the automotive industry. Elaphe is on the way to meet serial production demands by Tier 1 suppliers and OEMs.

Figure 1. Sensor mounting around motor and suspension components.

Control of motor vibration is very important since it results in structure-borne motor noise, which can compromise passenger comfort. When developing motors to emit low vibration levels, some degree of vibration insulation is needed. To understand what can be solved by the vehicle design and what needs to be solved on the motor level, transfer path analysis on mule vehicles during operation was performed.

Elaphe used its mule cars to study the role of vehicle suspension systems and to determine how vibrations from the in-wheel motors are transferred to the vehicle. The results are used in further product development as well as in communication with customers, since in several cases the overall experience did not meet the customer expectations.

The equipment used was Dewesoft Sirius slices connected to various sensors - accelerometers, microphones, human controlled inputs - and vehicle CAN. One Sirius was used for regular NVH measurements, which were performed on more than a dozen of vehicles in house and at clients to evaluate NVH performance. For extended testing 3 synchronized Sirius slices powered by battery were used on a single mule vehicle, as modifications of the powertrain and suspension tuning had been performed.

Figure 2. Three Sirius units powered by a battery pack.

Typically, one Sirius was used for regular NVH measurements, which were performed on more than a dozen of vehicles in house and at clients to valuate NVH performance. For extended testing 3 synchronized Sirius slices powered by battery were used on a single mule vehicle, as nodifications of the powertrain and suspension tuning had been performed.

All the measurement equipment had to be mounted and connected on the vehicle in secure and reliable way, since the vehicle was operational. Several sensors had to be mounted below the chassis and also on the unstrung mass and suspension components. The acquisition system had to be powered off shore. While Dewesoft enables direct power supply from the vehicle low voltage system, for the most sensitive measurements Dewesoft battery source was used, not to introduce any line noise from low quality vehicle power supply or portable power converters.

Figure 3. Synchronized acquisition with several Sirius units; accelerometers, microphones, triggers, CAN and real time math channels.

Measurements of the noise and vibration levels on the in-wheel motors, on the chassis and in the cabin, during on track operation, were recorded. The data collected were post processed to reveal the amount and spectra of intrinsic vibration that penetrated the vehicle resulting in cabin noise.

Elaphe now has a much better understanding of the most problematic frequencies and working points that need to be addressed on the motor level.  They also discovered issues on vehicles, that need to be addressed in order to improve customer experience. The measurements enabled Elaphe to set the guidelines on how to address the NVH issues, they have already resolved many of them and are working on the remaining.

Elaphe made significant changes to motor control algorithms, and started collaborations with companies specialized in suspension system development. The measurements were used to evaluate the progress and Elaphe managed to eliminate practically all powertrain contributions to the cabin noise.

This overall test gave them confidence in product quality and an advantage in communication with customers, since they can present viability of their solutions, and all the improvements that were identified and already performed. It also places Elaphe in better position in the in-wheel market as development partner due to obtained know how on the system level.

Figure 4. Correlation matrix between vibrations on different vehicle locations/components and cabin noise with respect to noise frequency.

“Fast learning curve, ability to use equipment, we are familiar with from other measurements and support from Dewesoft, mostly by lending additional equipment for the most demanding measurements, enabled us to identify and characterize the critical operating conditions, “says Martin Strojnik, Head of Development at Elaphe. “Based on this Elaphe could set an efficient strategy for NVH reduction and monitoring of the progress.”

This progress is one of the prerequisites for the product acceptance in the automotive market. It brought changes not only to motor design, but also to power electronics. It raised awareness towards proper vehicle design and further research on the vehicle level.