Kistler's Multi-Axis Force and Torque Sensors in NVH Testing

Among manufacturers offering specialized solutions for this field, Kistler is a widely used brand in dynamic force measurement systems and automotive testing.

1. Why is measuring vibrations not enough to find the cause of the "vibration"?

For example, when driving at around 80 km/h, the driver feels the steering wheel vibrate. Engineers place accelerometer sensors on the steering wheel and capture a distinct vibration peak within a specific frequency range.

However, to accurately determine the excitation source and the contribution of each transmission, engineers need to measure the actual force and torque transmitted through each connection point in the system – engine mounts, cardan shaft joints, suspension bushings, or the contact point between the tire and the road surface.

Force and torque sensors will help us answer the following questions:

• What is the force transmitted through the motor's foot pedal?
• How do the forces acting on the suspension bushings differ in each direction?
• How does the load between the wheel and the road surface change?
• At what level does torque appear at the linkage point?
• Which transmission path contributes most to vibration in the cabin?

When multiple force and torque components need to be measured simultaneously at a single point, multi-axis force and torque sensors become a crucial part of the measurement system.

A six-axis sensor can measure: X-axis force, Y-axis force, Z-axis force, X-axis torque, Y-axis torque, and Z-axis torque. This allows engineers to determine the load state more comprehensively, rather than knowing only a single force value.

This is particularly important in automotive testing because the actual load on the engine, suspension, or wheels often doesn't act in a single direction.

2. Operating Principle

Most multi-axis torque sensors used in NVH testing utilize piezoelectric technology based on quartz crystals, thanks to their high mechanical stiffness 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 gauges: Utilize multiple resistance 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 crosstalk calibration at the factory—this is a clear technical difference between high-end and mainstream sensors.

3. 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.
• Force and torque measurement range: Must match the actual load at the measurement point - for example, a sensor for the engine mount has a completely different measurement range than a sensor for the stabilizer bar bushing or tire-road contact point.
• Sensor stiffness: 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 accelerometer sensors.
• Cross-axis interference: The percentage error where the measured signal level on one axis affects the measurement results on other axes - Kistler's high-quality piezoelectric sensors typically achieve cross-axis interference rates below ±3.5%.
• Natural frequency: It needs to be high enough to avoid signal distortion when measuring high-frequency dynamic forces – this is why quartz piezoelectric sensors are preferred in NVH and impact testing over traditional strain sensors.

4. Applications of multi-axis force and torque sensors in NVH testing

5. The Role of NVH in Network 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 measured transfer function 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.

6. Kistler's Outstanding 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.

Key technical specifications:

• Force measurement range Fz from -5 to 10 kN, shear force Fx/Fy from -5 to 5 kN
• Torque ±200 N·m
• The natural frequency for measuring force is approximately 18 kHz.
• Cross-axis interference is less than ±3.5%.
• Compact size (62 mm diameter, 90 mm height) and pre-calibrated, ready to use without further adjustment.

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 sensor measures three orthogonal force components Fx, Fy, and Fz at any given 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.

7. Common Errors When Using Multi-Axis Torque Sensors

• Ignoring the cross-calibration matrix: Using a sensor without properly verifying the cross-calibration data between axes 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 sensor: In TPA analysis, force and acceleration need to be measured synchronously with time - even small phase shifts can distort the transmission line analysis results.

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 accompanies customers throughout this entire process: advising on sensor selection, supplier selection, integration solutions, calibration, warranty, and maintenance.

Contact us now for advice on the most suitable Kistler force and torque sensor solutions for your NVH testing laboratory - Hotline: 0936093289 | Email: info@gpower.com.vn