Measuring sound quality in an automotive production line

In a dealer’s showroom, the sound a seat makes sets the customer’s initial perception of vehicle quality. Car manufacturers have to get the sound and vibration behaviour exactly right in the new car showroom because power seat noises are primary subjective quality indicators.

Sci-Mech Technical Services – a company that designs, builds and integrates instrumentation and automation solutions that satisfy the requirements of fully automated assembly and test cells – was tasked with developing a cost-effective solution for production line testing of automotive seating products. The challenge: provide objective metrics for suggesting corrective actions to be combined with standardized sound quality metrics, so that subjective human impressions can be incorporated into the overall acceptability criteria.

Sound quality metrics
Sci-Mech makes acoustic measurements on the seat track in the sound-proof stations using National Instrument’s NI PXI-4462 24-bit dynamic signal acquisition (DSA) module with GRAS Type 40 AE .5-in. prepolarized free-field microphones with Type 26CA preamplifiers powered directly from the PXI-4462.

Noise, vibration and harshness (NVH) specialist and National Instruments Alliance Partner Signal.X Technologies, of Milford, Mich., worked with Sci-Mech and the customer to measure sound quality metrics using LabVIEW and the Sound and Vibration Measurement Suite. Metrics considered included Peak A-weighted SPL, Zwicker Stationary Loudness (Sones), pseudo time varying loudness, key harmonic detection, periodic and non-periodic impulse indicators, and rpm variation and drift.

A key requirement of the production environment is that metrics offer intuitive explanations of possible defects based on industry-standard formulations. It is notable that the metric set is a hybrid of objective and subjective parameters focused on ensuring a repeatable production process. As a set, the metrics can correlate to subjective human impressions of the sound of the seat track, as well as direct indication of specific defects that will fail one or more objective metrics. This approach to noise and vibration performance assessment with Signal.X technology ensures that only quality seats earn a passing grade.

Functional testing
The 15-station test line, which includes various platforms and systems, runs on a single Ethernet network. Because of the high digital channel counts required for a project as complex as the current seat cushion assembly and test line, a desktop PC alone could not support the required number of PCI boards. The team chose NI PXI remote chassis to perform the extensive data acquisition and communicate to the PC-based controller. They also used two NI PXI-6511 low-cost, industrial digital input modules, which provided 128 bank-isolated 24 V inputs.

With bank isolation, it was possible to connect the Turck sensors directly to save assembly time, reduce circuit complexity, and decrease cost. Two NI PXI-6512 low-cost, industrial digital l/O modules for the 128 bank-isolated digital outputs allowed the team to connect to the devices directly from the module’s breakout box without putting every signal through an optical isolator. With so many channels, this saved setup time as well as panel space.

To control the servo motors of the system, they used an NI 7340 four-axis motion controller and an NI 7774 four-axis motion control interface to a Rockwell Control Logix 5561 controller. Full driver integration with the NI Measurement & Automation (MAX) configuration utility offered an intuitive graphical user interface (GUI) for setup and troubleshooting.

To assist in the software design phase and reduce overall programming time, they used the LabVIEW State Diagram Toolkit to concurrently visualize program architecture and develop code. With the LabVIEW State Diagram Toolkit, it was possible to share machine operation concepts with Sci-Mech management and the client.

The team also used the NI Motion Assistant to set up the initial motion profiles. This tool helped save hours of development time and was very useful in the initial setup of the servo motors. It was possible to safely check the acceleration and deceleration ramp profiles and overall velocities of the automotive seat testing system before program implementation.

Benefits of the NI platform
Sci-Mech and Signal.X Technologies used LabVIEW and NI PXI hardware to deliver an advanced production line test system for seat tracks that incorporates sound quality as a part of the functional tests, ensuring the parts meet the highest quality standards with 100 percent testing.

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