Multi-Axis Force and Torque Sensors in NVH Testing

This article analyzes the role, principles, and applications of multi-axis force-torque sensors in NVH testing, and introduces typical product lines from Kistler - one of the world's leading brands in this field.

Why is simply measuring vibrations not enough? 

In many NVH (Non-Vehicle Vibration) problems, measuring acceleration or sound only reveals the "phenomenon"—for example, steering wheel vibration at 80 km/h, or seat vibration at a certain engine speed range. But to understand "why" this phenomenon occurs, engineers need to measure the actual forces and torque transmitted through each connection point in the system—engine mounts, cardan shaft joints, suspension bushings, or the contact points between the tires and the road surface. 

Multi-axis force and torque sensors allow simultaneous measurement of multiple force components (Fx, Fy, Fz) and torque components (Mx, My, Mz) at a single point – providing a complete picture of the dynamic load acting on the structure, instead of just a single unidirectional force value like traditional force sensors. 

Operating Principle 

Most multi-axis torque sensors used in NVH testing utilize piezoelectric technology based on quartz crystals, thanks to their high mechanical rigidity and high natural frequency—key factors enabling dynamic measurements at high frequencies. 

• Multi-axis piezoelectric sensors: Multiple quartz discs are layered and oriented along different axes within a single sensor unit. When a force or torque is applied, each layer of quartz generates an electrical charge proportional to the force component along its sensitive axis. The electrical charge signal is transmitted through separate connectors for each force and torque group. 

• Multi-axis strain gauge sensors: Utilize multiple resistance bridges (Wheatstone bridges) positioned at different locations and angles, suitable for static or semi-dynamic measurements at a lower cost. 

Precise signal separation between axes requires highly symmetrical mechanical design and rigorous factory crosstalk calibration—this is a clear technical difference between high-end and mainstream sensors. 

Key Technical Specifications to Consider 

• Number of measurement axes (3 axes or 6 axes): 3-axis sensors only measure forces Fx, Fy, Fz; 6-axis sensors measure both force and moment (Fx, Fy, Fz, Mx, My, Mz) - necessary for complex load analysis in NVH. 

• Measuring range for force and torque: Must match the actual load at the measurement point - for example, a sensor for the engine mount has a completely different measuring range than a sensor for the stabilizer bar bushing or tire-road contact point. 

• Sensor rigidity: A sensor that is too soft compared to its original structure will alter the dynamic characteristics at the installation point, causing measurement data inaccuracies – similar to the mass load effect in an accelerometer. 

• Crosstalk: The percentage of error when a signal from one axis "interferes" with another axis - high-quality Kistler piezoelectric sensors typically achieve crosstalk below ±3.5%. 

• Natural frequency: This needs to be high enough to avoid signal distortion when measuring high-frequency dynamic forces – this is why quartz piezoelectric sensors are preferred over traditional strain gauges in NVH and crash testing. 

Applications in NVH Testing 

The Role in NVH (Transfer Path Analysis) 

Multi-axis torque sensors play a central role in Transfer Path Analysis (TPA) – a method for determining the excitation force at the source (e.g., engine mount) transmitted through the structure to the sensing point (e.g., steering wheel or vehicle floor) according to the contribution ratio of each transmission path. 

By combining force-torque data at the source point with the transfer function measured between points, engineers can determine which transmission path contributes most to the final perceived vibration, compare the effectiveness of different bearing or engine mount designs before building a prototype, and predict the impact of changing a component in the drivetrain without retesting the entire vehicle. 

Kistler Multi-Axis Torque Sensor Product Lines 

Kistler (Switzerland) is one of the world's leading brands of piezoelectric sensors for measuring force, torque, and acceleration, widely used in automotive development and testing, including NVH, durability, vehicle dynamics, and crash testing. Below are some typical product lines related to multi-axis force and torque measurement: 

Kistler Type 9306A - 6-axis torque/force sensor 

This is a 6-component piezoelectric sensor (Fx, Fy, Fz, Mx, My, Mz) integrated into a single unit, described as "a complete dynamometer in a sensor". Key specifications include: force measurement range Fz from -5 to 10 kN, shear force Fx/Fy from -5 to 5 kN, torque up to ±200 N·m, natural force measurement frequency of approximately 18 kHz, and crosstalk less than ±3.5%. It has a compact size (62 mm diameter, 90 mm height) and is pre-calibrated, ready to use without recalibration. The smaller 9306A31 variant is suitable for installations with limited space. 

Kistler Type 9347C (9317C/9327C/9347C/9367C/9377C series) - 3-component force link 

The force link series measures three orthogonal force components Fx, Fy, and Fz at any point of application. For example, the Type 9347C has a measuring range of Fz from -30 to 30 kN, Fx/Fy from -15 to 15 kN, a natural frequency along the Fz axis of approximately 10 kHz, and a stiffness of approximately 1,300 N/µm - suitable for tests requiring high dynamic response such as load analysis of machine bases or structural test benches. 

Kistler RoaDyn Integra Series - Wheel force transducer (6-component wheel force sensor) 

This specialized sensor is mounted directly on the wheel to measure the force and torque transmitted through the tire-road contact point under real-world operating conditions. It is used for Road Load Data Acquisition (RLDA) data collection, vehicle dynamics testing, and durability testing. The three versions, Integra 4, 5, and 6, measure force from 25 to 65 kN and torque from 4 to 12 kN·m, and can be fitted to wheel rims from 15 to 22 inches, suitable for passenger cars, SUVs, and commercial vehicles. 

Kistler Type 9350B - SmartCrash (electronic force sensor for crash testing) 

The piezoelectric force measuring element incorporates a built-in signal processing circuit, measuring three orthogonal force components Fx, Fy, and Fz with a very wide measurement range (Fx up to 500 kN; Fy and Fz up to 100 kN). It features TEDS (data logging, calibration, and sensor identification) functionality and is specifically designed for high-frequency impact force measurement in automotive crash tests. 

Common Errors When Using Multi-Axis Torque Sensors 

• Ignoring the crosstalk calibration matrix: Using a sensor without properly validating the crosstalk calibration data according to the actual installation configuration will cause cumulative errors between axes. 

• Installation changes that alter system stiffness: Sensors that are too stiff or too soft compared to the original components alter the dynamic characteristics at the measurement point, causing the results to not accurately reflect the actual system. 

• Choosing the wrong measurement range: A range that is too large for the actual load reduces signal resolution, while a range that is too small can easily overload and damage the sensor. 

• Signal asynchronization issues with the accelerometer: In TPA analysis, force and acceleration need to be measured synchronously with time - even small phase shifts can distort the transmission line analysis results. 

GPower Vietnam - Kistler Multi-Axis Force and Torque Sensor Solutions 

Selecting the right multi-axis torque sensor—from the number of measurement axes, load range, to the appropriate stiffness for the actual system—requires a deep understanding of both the equipment and the customer's specific engineering problem. GPower Vietnam is here to support you throughout this entire process. 

• - Sensor selection advice: Identify the right Kistler product line – from the 9306A for 6-axis composite measurement, the 9347C for 3-component force link, to the RoaDyn Integra for wheel torque measurement under real-world operating conditions. 

• - Supplier consultation: Direct connection with authorized Kistler suppliers, ensuring quality, clear origin, and full technical support. 

• - Integrated solution consulting: Ensuring the sensor operates synchronously with the DAQ system and accelerometer in the same test, serving NVH transmission analysis. 

• - Calibration, warranty, and maintenance: Regular calibration services ensure that the sensor maintains its accuracy over time. 

Contact GPower Vietnam today for advice on the most suitable Kistler force and torque sensor solutions for your NVH lab - Hotline: 0936093289 | Email: info@gpower.com.vn