How NVH measurement technology is helping to shape the evolution of the automobile

Winterthur, June 2020 – Environmental concerns have led to increasingly strict CO2 regulations that are triggering disruptive changes in automotive development. The transformation from conventional combustion engines to electrified powertrains presents challenges for NVH engineering – and Kistler is responding with highly flexible and efficient measurement solutions.

NVH – which stands for 'noise, vibration and harshness” – includes all aspects of the acoustic and vibrational behavior of a vehicle. Increasing attention is focused on these issues, because optimizing the NVH behavior of a vehicle or powertrain helps to create reliable products with long lifetimes – but that's not all: it also characterizes how the occupants perceive the quality of a vehicle. These developments are turning NVH into a key factor in the evolution of the automobile – with the ultimate goal of raising comfort, driving pleasure and brand image to unprecedented levels.

From an NVH perspective, combustion engines and electric motors have fundamentally different characteristics. But even to non-engineers, it is obvious that the throaty sound universally associated with sporting performance and engine power is now giving way to a source of propulsion that sounds more like a whistle. A high-frequency tonal sound of this kind is totally new to end customers' ears, and they may perceive it as unpleasant. Another factor is the absence of what is known as the 'masking effect” of the combustion engine while it is running – so other sounds now become more noticeable: noises from the road and tires, or from wind and ventilation, are now more clearly audible in vehicles with an electric motor, and this can negatively impact users' perception of the comfort level.

How is e-mobility changing the approach to NVH?

Key tasks for NVH engineers include quantifying noise and vibrations generated by the powertrain, and understanding how they reach the passengers. An understanding of these aspects is fundamentally important so that vehicle design will match user expectations. It is customary engineering practice to divide all noise into two categories: airborne on the one hand, and structure-borne on the other. The first category includes the airborne noise originating from the powertrain that travels through the engine bay and mainly reaches the passengers through the air. Structure-borne sound, on the other hand, consists mostly of vibrations that are transmitted from the propulsion system via the bearings and mounting points to the vehicle frame, from where they reach the occupants. Separating these two types of contribution to noise is no easy matter – but if it is done during the vehicle's design phase, NVH performance will ultimately be improved.

In practice, it is frequently observed that the switch from a combustion engine to an electric motor leads to a drastic reduction of airborne noise transmission. This is due to the nature of the propulsion system, but the tonal behavior of the e-motor can lead to some localized issues at high frequencies. A reduction in the structure-borne contribution can also be expected, although to a lesser extent – and, more importantly, the lower noise level is less obvious in this case. The fact of the matter is that there are very major differences between the dynamic behavior of electric motors and combustion engines. This is why the force transmission path to the vehicle frame requires close attention, and why it needs to be measured and understood.

In addition to these factors, the transmission of structure-borne noise is also influenced by the vehicle's architecture – and here too, a transformation is in progress. Battery-powered electric vehicles carry relatively heavy battery packages that affect the vehicle's overall NVH footprint. OEMs are currently focusing their efforts on improvements to driving range and battery capacity, so the battery package is unlikely to become lighter in the foreseeable future. For these reasons, an assessment of vibration-induced influences plays a critical part in any effective optimization of NVH behavior. Above and beyond all this, problems with structure-borne sound transmission are difficult to overcome, and they occur in frequency ranges where acoustic solutions must be specifically designed if they are to achieve any effect at all.

As well as affecting vehicle architecture and development processes, all these changes and challenges influence the requirements for accurate and reliable measurements that must be met by the measuring chain. The primary need here is for more flexibility, and Kistler stands ready to support automobile manufacturers and vehicle developers with high-performance, easy-to-use measurement solutions for NVH applications.

Application-specific force measurement solutions

Accurate assessment of structure-borne transmission requires knowledge of the forces acting on the interface between the vibration source (such as the powertrain) and the vehicle's frame. The results are also key inputs for commonly used processes such as body-in-white optimization, transfer path analysis (TPA), and blocked force measurement. These applications impose specific geometrical constraints and unique operating load conditions that are largely beyond the capabilities of conventional force sensors.

To meet these demanding requirements, Kistler offers customized solutions based on piezoelectric (PE) measurement technology. They can be tailored precisely to each application, and they measure the interface forces with very high accuracy. Thanks to their high rigidity, PE sensors can attain high frequencies. Their measuring range is very broad, making it easy to capture dynamic forces with different orders of magnitude. Additional advantages such as overload-protected design and long lifetimes under cyclical loading make PE measurement technology a powerful instrument for NVH applications. In all these ways, PE measurement technology is ideally suited to meet the challenges presented by mobility – not only today, but also in tomorrow's world.

Backed by more than 60 years of experience in PE measurement technology, Kistler has the ability to provide complete solutions that can precisely measure up to six components (three forces and three torques) to match each customer’s geometrical and load requirements. From design and engineering through to production, commissioning and service: customers can turn to one single source for everything they need.

Overcoming challenges in acceleration measurement

As part of its automotive NVH portfolio, Kistler offers a wide choice of acceleration sensors for use in diverse vehicle and powertrain applications. Accelerometers based on Kistler’s unique PiezoStar crystal technology are especially suitable for high-temperature measurements on combustion-based powertrains. Thanks to the superior thermal stability of the 8766A sensor, for instance, engineers can benefit from reliable and accurate measurements – even under extreme conditions. As the transition to electric motors gathers pace, features such as ground isolation, lower acceleration measurement ranges and broader frequency ranges are becoming more relevant.

Sensors from Kistler such as the 8764B, 8763B and 8766A already offer these features, opening the way for applications with e-motors and providing dependable technology to meet a variety of measurement challenges. Heavy battery packs and lightweight vehicle designs also call for careful assessment of driving characteristics and ride stability. In these situations, capacitive MEMS sensors such as the 8316A and 8396A ensure highly accurate measurements even when very low-frequency vibrations are present.

Highly flexible data acquisition and analysis

Work to achieve new development objectives in NVH calls for efficient, out-of-the-box analytical processes. To support developers in this field, Kistler has recently launched KiSUITE Analysis, an intuitive and powerful data analysis platform that offers a high degree of freedom and flexibility for data evaluation and process customization. Designed to provide an environment that fosters innovation and creativity, this software package comprises a range of versatile and comprehensive NVH functions. KiSUITE Analysis complements the existing KiSUITE software from Kistler which already includes many application-oriented solutions for comprehensive vehicle and powertrain NVH analyses.

Kistler has also broadened its range of NVH hardware for data acquisition: KiNOVA Pro can now be customized with a variety of cards to expand the possible types of input and output, while the system's versatility is maintained thanks to the integrated standalone function.

Ready for the future of NVH?

These paradigm shifts are raising questions about shared engineering approaches and firmly-held beliefs. There needs to be a transformation in methods used for product development and performance measurement. One of the key challenges for today’s NVH engineers is to balance vehicle design with the acoustic comfort desired by end users ‒ and eventually, this open-ended process could yield new paradigms for vehicle comfort, driving experience and brand identity. Exciting times lie ahead for automotive and powertrain NVH, and Kistler is standing by to support its customers with customizable measurement solutions that deliver maximum efficiency.